diff --git a/l1-contracts/scripts/constants-codegen/solidity.json b/l1-contracts/scripts/constants-codegen/solidity.json
index 2046ce24d44a..2fcd7cc212c4 100644
--- a/l1-contracts/scripts/constants-codegen/solidity.json
+++ b/l1-contracts/scripts/constants-codegen/solidity.json
@@ -8,7 +8,5 @@
   "EMPTY_EPOCH_OUT_HASH",
   "FEE_JUICE_ADDRESS",
   "BLS12_POINT_COMPRESSED_BYTES",
-  "ROOT_ROLLUP_PUBLIC_INPUTS_LENGTH",
-  "NUM_MSGS_PER_BASE_PARITY",
-  "NUM_BASE_PARITY_PER_ROOT_PARITY"
+  "ROOT_ROLLUP_PUBLIC_INPUTS_LENGTH"
 ]
diff --git a/l1-contracts/src/core/libraries/crypto/FrontierLib.sol b/l1-contracts/src/core/libraries/crypto/FrontierLib.sol
deleted file mode 100644
index 6fd66130a15a..000000000000
--- a/l1-contracts/src/core/libraries/crypto/FrontierLib.sol
+++ /dev/null
@@ -1,91 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright 2024 Aztec Labs.
-pragma solidity >=0.8.27;
-
-import {Hash} from "@aztec/core/libraries/crypto/Hash.sol";
-
-/**
- * @title FrontierLib
- * @author Aztec Labs
- * @notice Library for managing frontier trees.
- */
-library FrontierLib {
-  struct Forest {
-    mapping(uint256 index => bytes32 zero) zeros;
-  }
-
-  struct Tree {
-    uint256 nextIndex;
-    mapping(uint256 => bytes32) frontier;
-  }
-
-  function initialize(Forest storage _self, uint256 _height) internal {
-    _self.zeros[0] = bytes32(0);
-    for (uint256 i = 1; i <= _height; i++) {
-      _self.zeros[i] = Hash.sha256ToField(bytes.concat(_self.zeros[i - 1], _self.zeros[i - 1]));
-    }
-  }
-
-  function insertLeaf(Tree storage _self, bytes32 _leaf) internal returns (uint256) {
-    uint256 index = _self.nextIndex;
-    uint256 level = computeLevel(index);
-    bytes32 right = _leaf;
-    for (uint256 i = 0; i < level; i++) {
-      right = Hash.sha256ToField(bytes.concat(_self.frontier[i], right));
-    }
-    _self.frontier[level] = right;
-
-    _self.nextIndex++;
-
-    return index;
-  }
-
-  function root(Tree storage _self, Forest storage _forest, uint256 _height, uint256 _size)
-    internal
-    view
-    returns (bytes32)
-  {
-    uint256 next = _self.nextIndex;
-    if (next == 0) {
-      return _forest.zeros[_height];
-    }
-    if (next == _size) {
-      return _self.frontier[_height];
-    }
-
-    uint256 index = next - 1;
-    uint256 level = computeLevel(index);
-
-    // We should start at the highest frontier level with a left leaf
-    bytes32 temp = _self.frontier[level];
-
-    uint256 bits = index >> level;
-    for (uint256 i = level; i < _height; i++) {
-      bool isRight = bits & 1 == 1;
-      if (isRight) {
-        temp = Hash.sha256ToField(bytes.concat(_self.frontier[i], temp));
-      } else {
-        temp = Hash.sha256ToField(bytes.concat(temp, _forest.zeros[i]));
-      }
-      bits >>= 1;
-    }
-
-    return temp;
-  }
-
-  function isFull(Tree storage _self, uint256 _size) internal view returns (bool) {
-    return _self.nextIndex == _size;
-  }
-
-  function computeLevel(uint256 _leafIndex) internal pure returns (uint256) {
-    // The number of trailing ones is how many times in a row we are the right child.
-    // e.g., each time this happens we go another layer up to update the parent.
-    uint256 count = 0;
-    uint256 index = _leafIndex;
-    while (index & 1 == 1) {
-      count++;
-      index >>= 1;
-    }
-    return count;
-  }
-}
diff --git a/l1-contracts/src/core/libraries/rollup/ProposedHeaderLib.sol b/l1-contracts/src/core/libraries/rollup/ProposedHeaderLib.sol
index 5a456dda77bb..374752f646f9 100644
--- a/l1-contracts/src/core/libraries/rollup/ProposedHeaderLib.sol
+++ b/l1-contracts/src/core/libraries/rollup/ProposedHeaderLib.sol
@@ -21,7 +21,6 @@ struct ProposedHeader {
   bytes32 lastArchiveRoot;
   bytes32 blockHeadersHash;
   bytes32 blobsHash;
-  bytes32 inHash;
   bytes32 inboxRollingHash;
   bytes32 outHash;
   Slot slotNumber;
@@ -56,7 +55,6 @@ library ProposedHeaderLib {
         _header.lastArchiveRoot,
         _header.blockHeadersHash,
         _header.blobsHash,
-        _header.inHash,
         _header.inboxRollingHash,
         _header.outHash,
         _header.slotNumber,
diff --git a/l1-contracts/test/Parity.t.sol b/l1-contracts/test/Parity.t.sol
deleted file mode 100644
index 8ed33bf99e75..000000000000
--- a/l1-contracts/test/Parity.t.sol
+++ /dev/null
@@ -1,80 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright 2024 Aztec Labs.
-pragma solidity >=0.8.27;
-
-import {Test} from "forge-std/Test.sol";
-
-import {FrontierMerkle} from "./harnesses/Frontier.sol";
-import {Constants} from "@aztec/core/libraries/ConstantsGen.sol";
-
-contract ParityTest is Test {
-  function setUp() public {}
-
-  // Checks whether sha root matches output of base parity circuit
-  function testRootMatchesBaseParity() public {
-    uint248[256] memory msgs;
-    for (uint248 i = 0; i < msgs.length; i++) {
-      msgs[i] = i;
-    }
-
-    // We can't use Constants.NUM_MSGS_PER_BASE_PARITY directly when defining the array so we do the check here to
-    // ensure it does not get outdated.
-    assertEq(msgs.length, Constants.NUM_MSGS_PER_BASE_PARITY, "NUM_MSGS_PER_BASE_PARITY changed, update msgs.");
-
-    uint256 treeHeight = 8; // log_2(NUM_MSGS_PER_BASE_PARITY)
-      // We don't have log_2 directly accessible in solidity so I just do the following check here to ensure
-      // the hardcoded value is not outdated.
-    assertEq(
-      2 ** treeHeight,
-      Constants.NUM_MSGS_PER_BASE_PARITY,
-      "Base parity circuit subtree height changed, update treeHeight."
-    );
-
-    FrontierMerkle frontier = new FrontierMerkle(treeHeight);
-
-    for (uint256 i = 0; i < msgs.length; i++) {
-      frontier.insertLeaf(bytes32(bytes.concat(new bytes(1), bytes31(msgs[i]))));
-    }
-    // matches noir-protocol-circuits/crates/parity-lib/src/tests/parity_base_tests.nr
-    bytes32 expectedRoot = 0x00279d4d4dd5bcb9b1a4e742640588b917102f9f8bc97a6c95706ca4e7a8a76b;
-    assertEq(frontier.root(), expectedRoot, "Root does not match base parity circuit root");
-  }
-
-  // Checks whether sha root matches output of root parity circuit
-  function testRootMatchesRootParity() public {
-    // sha256 roots coming out of base parity circuits
-    // matches noir-protocol-circuits/crates/parity-lib/src/root/root_parity_inputs.nr
-    uint248[4] memory baseRoots = [
-      0xb3a3fc1968999f2c2d798b900bdf0de41311be2a4d20496a7e792a521fc8ab,
-      0x43f78e0ebc9633ce336a8c086064d898c32fb5d7d6011f5427459c0b8d14e9,
-      0x024259b6404280addcc9319bc5a32c9a5d56af5c93b2f941fa326064fbe963,
-      0x53042d820859d80c474d4694e03778f8dc0ac88fc1c3a97b4369c1096e904a
-    ];
-
-    // We can't use Constants.NUM_BASE_PARITY_PER_ROOT_PARITY directly when defining the array so we do the check here
-    // to ensure it does not get outdated.
-    assertEq(
-      baseRoots.length,
-      Constants.NUM_BASE_PARITY_PER_ROOT_PARITY,
-      "NUM_BASE_PARITY_PER_ROOT_PARITY changed, update baseRoots."
-    );
-
-    uint256 treeHeight = 2; // log_2(NUM_BASE_PARITY_PER_ROOT_PARITY)
-      // We don't have log_2 directly accessible in solidity so I just do the following check here to ensure
-      // the hardcoded value is not outdated.
-    assertEq(
-      2 ** treeHeight,
-      Constants.NUM_BASE_PARITY_PER_ROOT_PARITY,
-      "Root parity circuit subtree height changed, update treeHeight."
-    );
-
-    FrontierMerkle frontier = new FrontierMerkle(treeHeight);
-
-    for (uint256 i = 0; i < baseRoots.length; i++) {
-      frontier.insertLeaf(bytes32(bytes.concat(new bytes(1), bytes31(baseRoots[i]))));
-    }
-
-    bytes32 expectedRoot = 0x00a0c56543aa73140e5ca27231eee3107bd4e11d62164feb411d77c9d9b2da47;
-    assertEq(frontier.root(), expectedRoot, "Root does not match root parity circuit root");
-  }
-}
diff --git a/l1-contracts/test/RollupFieldRange.t.sol b/l1-contracts/test/RollupFieldRange.t.sol
index 9763efa0ae4e..a3d05d087ddd 100644
--- a/l1-contracts/test/RollupFieldRange.t.sol
+++ b/l1-contracts/test/RollupFieldRange.t.sol
@@ -164,8 +164,6 @@ contract RollupFieldRangeTest is RollupBase {
     vm.warp(max(block.timestamp, Timestamp.unwrap(ts)));
 
     _populateInbox(full.populate.sender, full.populate.recipient, full.populate.l1ToL2Content);
-    // The header's inHash field is an unconstrained pass-through hint post-flip; propose does not check it.
-    header.inHash = bytes32(0);
     header.gasFees.feePerL2Gas = SafeCast.toUint128(rollup.getManaMinFeeAt(ts, true));
 
     // Every field the range check guards, set to the maximal in-range value.
diff --git a/l1-contracts/test/base/DecoderBase.sol b/l1-contracts/test/base/DecoderBase.sol
index 7abcb375c1aa..21f1431d7978 100644
--- a/l1-contracts/test/base/DecoderBase.sol
+++ b/l1-contracts/test/base/DecoderBase.sol
@@ -46,7 +46,6 @@ contract DecoderBase is TestBase {
     address coinbase;
     bytes32 feeRecipient;
     GasFees gasFees;
-    bytes32 inHash;
     bytes32 inboxRollingHash;
     bytes32 lastArchiveRoot;
     bytes32 outHash;
@@ -104,7 +103,6 @@ contract DecoderBase is TestBase {
           lastArchiveRoot: full.checkpoint.header.lastArchiveRoot,
           blockHeadersHash: full.checkpoint.header.blockHeadersHash,
           blobsHash: full.checkpoint.header.blobsHash,
-          inHash: full.checkpoint.header.inHash,
           inboxRollingHash: full.checkpoint.header.inboxRollingHash,
           outHash: full.checkpoint.header.outHash,
           slotNumber: Slot.wrap(full.checkpoint.header.slotNumber),
diff --git a/l1-contracts/test/base/RollupBase.sol b/l1-contracts/test/base/RollupBase.sol
index f7080e4cc617..cdf6ee4014a2 100644
--- a/l1-contracts/test/base/RollupBase.sol
+++ b/l1-contracts/test/base/RollupBase.sol
@@ -171,8 +171,6 @@ contract RollupBase is DecoderBase {
 
     // We jump to the time of the block, always past the L1 block the messages above landed in.
     vm.warp(max(block.timestamp + 1, Timestamp.unwrap(full.checkpoint.header.timestamp)));
-    // The header's inHash field is an unconstrained pass-through hint post-flip; propose does not check it.
-    full.checkpoint.header.inHash = bytes32(0);
     // Streaming Inbox: reference the newest bucket so the checkpoint consumes all messages
     // seeded above and the mandatory-consumption assert is trivially satisfied (a wrong ref could only revert).
     uint256 bucketHint = rollup.getInbox().getCurrentBucketSeq();
diff --git a/l1-contracts/test/escape-hatch/integration/EscapeHatchIntegrationBase.sol b/l1-contracts/test/escape-hatch/integration/EscapeHatchIntegrationBase.sol
index 5e6cd5fea89e..ea22ac757e75 100644
--- a/l1-contracts/test/escape-hatch/integration/EscapeHatchIntegrationBase.sol
+++ b/l1-contracts/test/escape-hatch/integration/EscapeHatchIntegrationBase.sol
@@ -130,18 +130,17 @@ abstract contract EscapeHatchIntegrationBase is ValidatorSelectionTestBase {
    * @dev Uses:
    *   - archive: GENESIS_ARCHIVE_ROOT
    *   - oracleInput: zero
-   *   - header fields from fixture for blockHeadersHash/blobsHash/inHash/outHash, rest overridden
+   *   - header fields from fixture for blockHeadersHash/blobsHash/outHash, rest overridden
    */
   function _buildProposeArgs(address _proposer) internal view returns (ProposeArgs memory args, bytes memory blobs) {
     bytes32 archive = bytes32(Constants.GENESIS_ARCHIVE_ROOT);
     Slot slotNumber = rollup.getCurrentSlot();
 
-    // Build header fresh, only copying blockHeadersHash/blobsHash/inHash/outHash from fixture
+    // Build header fresh, only copying blockHeadersHash/blobsHash/outHash from fixture
     ProposedHeader memory header = ProposedHeader({
       lastArchiveRoot: archive,
       blockHeadersHash: full.checkpoint.header.blockHeadersHash,
       blobsHash: full.checkpoint.header.blobsHash,
-      inHash: full.checkpoint.header.inHash,
       inboxRollingHash: full.checkpoint.header.inboxRollingHash,
       outHash: full.checkpoint.header.outHash,
       slotNumber: slotNumber,
diff --git a/l1-contracts/test/fixtures/empty_checkpoint_1.json b/l1-contracts/test/fixtures/empty_checkpoint_1.json
index 1f829f1b819b..eb94728b3ccd 100644
--- a/l1-contracts/test/fixtures/empty_checkpoint_1.json
+++ b/l1-contracts/test/fixtures/empty_checkpoint_1.json
@@ -34,7 +34,6 @@
       "lastArchiveRoot": "0x177a4955b31ecaafad999753938a44e526b54c5ba5d536688227f85f15cfbdf5",
       "blockHeadersHash": "0x2e3e0911389bc48fa8126a93273d016cc7dc08019f8ffc5f1f5ae7d90745eaa2",
       "blobsHash": "0x00e5b752fe6bc2154155ff3a979c4c5fa91d3ac0d716169ac521e1560fd83b2b",
-      "inHash": "0x00de7b349d2306334734e4f58b1302a6ed5a6c796a706f6597a5641b6d468223",
       "inboxRollingHash": "0x00cad6497cf81748167158f149ba70c31f34c68b0ae1b156117de63e073d14b5",
       "outHash": "0x00c95e0ceb41951039e1592745ec2faea9866f6eaf01bf189a4463b4143af093",
       "slotNumber": 99,
@@ -48,7 +47,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x0002fc455005d993521be4e6f8ed92d1e0b50e58b21445881c87d02ed51a1e4c",
+    "headerHash": "0x00230a553b85ec7931eded45ec1ea096e47789226758b16dca0403f5ab53f3d6",
     "numTxs": 0
   }
 }
diff --git a/l1-contracts/test/fixtures/empty_checkpoint_2.json b/l1-contracts/test/fixtures/empty_checkpoint_2.json
index d2dcd961b92f..5ee1a899f2f8 100644
--- a/l1-contracts/test/fixtures/empty_checkpoint_2.json
+++ b/l1-contracts/test/fixtures/empty_checkpoint_2.json
@@ -34,7 +34,6 @@
       "lastArchiveRoot": "0x09d45c1e01b8596153838a068ddb470ead95e6e1e151f40b4d48664c1e311af6",
       "blockHeadersHash": "0x0b3bda1754ca30707b8c0bbe72760c68e574cf23309e7e4fd7cabea36b4078da",
       "blobsHash": "0x000e9acabf609c9c113078ecb383ba6310573ce246958b605452132617d2c960",
-      "inHash": "0x006504de282a40084bb8098456a915c645d53482d351db52fa9433b6cd638763",
       "inboxRollingHash": "0x0080dbf2bd9576e81fd99fc0f9d35bfd7fc15a6221d0b6e1e92a1884f14d0c84",
       "outHash": "0x00c95e0ceb41951039e1592745ec2faea9866f6eaf01bf189a4463b4143af093",
       "slotNumber": 102,
@@ -48,7 +47,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x00120ecaf7a72f61adf3c8ee180acd930401221668a4792a8f91584755840d16",
+    "headerHash": "0x00a184a160ada9af4ba75961ba596043d76f56c75b2e7e9652a0c162a7bf59b9",
     "numTxs": 0
   }
 }
diff --git a/l1-contracts/test/fixtures/mixed_checkpoint_1.json b/l1-contracts/test/fixtures/mixed_checkpoint_1.json
index b1604aceb038..fda840d182d1 100644
--- a/l1-contracts/test/fixtures/mixed_checkpoint_1.json
+++ b/l1-contracts/test/fixtures/mixed_checkpoint_1.json
@@ -67,7 +67,6 @@
       "lastArchiveRoot": "0x177a4955b31ecaafad999753938a44e526b54c5ba5d536688227f85f15cfbdf5",
       "blockHeadersHash": "0x087b6f59388fa4207876ee1b50521ff838d6eed422d0ad07ff996393f32a77e6",
       "blobsHash": "0x001bef3ff3f657c565ff86d5072186f7f2bfddb8a31ca0714562c164fe954d84",
-      "inHash": "0x00de7b349d2306334734e4f58b1302a6ed5a6c796a706f6597a5641b6d468223",
       "inboxRollingHash": "0x00cad6497cf81748167158f149ba70c31f34c68b0ae1b156117de63e073d14b5",
       "outHash": "0x00cffdbb0e7f5e164d314d781d38ae31230910a3bae4c34e7df6222df71b1539",
       "slotNumber": 99,
@@ -81,7 +80,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x00c1431d117825d25f8169e35eb70ed4b16c0fc4b3cac85506852c240d03740e",
+    "headerHash": "0x00f1c11fddfca8ff6073d1715c8b886d501facad96b7b032fb440462849697b8",
     "numTxs": 4
   }
 }
diff --git a/l1-contracts/test/fixtures/mixed_checkpoint_2.json b/l1-contracts/test/fixtures/mixed_checkpoint_2.json
index d2125d5786a4..2fdaa2de220d 100644
--- a/l1-contracts/test/fixtures/mixed_checkpoint_2.json
+++ b/l1-contracts/test/fixtures/mixed_checkpoint_2.json
@@ -67,7 +67,6 @@
       "lastArchiveRoot": "0x0c87f76b6c5cc918111d2acc975bf7f2133f5c3cc23c8cf6cc1c3e78f5c84a3e",
       "blockHeadersHash": "0x144dbe32a03df9ccc672207c93cd22099eb91e44a71e1676148cd3c6c6c98b9e",
       "blobsHash": "0x008bd0b669b942b57ccf85d3401214db65cde3608afa0b2ae0e57f35ec60d72e",
-      "inHash": "0x006504de282a40084bb8098456a915c645d53482d351db52fa9433b6cd638763",
       "inboxRollingHash": "0x0080dbf2bd9576e81fd99fc0f9d35bfd7fc15a6221d0b6e1e92a1884f14d0c84",
       "outHash": "0x008a85da85a596471f2e5fe402fde332723da8d24b6e7affd60d16c7cb7e9020",
       "slotNumber": 102,
@@ -81,7 +80,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x00ab5faa05cb38faa2cd1fc62c40007f64594c9fdc3bb8b7b478a351a7c7dc5a",
+    "headerHash": "0x00e0c076d07c17d50e3aefe9ca264ba7a4894fab4956272bfbea600e9e34bb0f",
     "numTxs": 4
   }
 }
diff --git a/l1-contracts/test/fixtures/single_tx_checkpoint_1.json b/l1-contracts/test/fixtures/single_tx_checkpoint_1.json
index 6d409f05d521..1969d1dfa7f7 100644
--- a/l1-contracts/test/fixtures/single_tx_checkpoint_1.json
+++ b/l1-contracts/test/fixtures/single_tx_checkpoint_1.json
@@ -43,7 +43,6 @@
       "lastArchiveRoot": "0x177a4955b31ecaafad999753938a44e526b54c5ba5d536688227f85f15cfbdf5",
       "blockHeadersHash": "0x07db5c24565ad9a2c9d39ef7d9a4446e9742d6090567ff28aef9a45f4738a5cb",
       "blobsHash": "0x00ec2400a7cfc9d975cb0802980d49387588738160a0cf0301f07e1abad6456c",
-      "inHash": "0x00de7b349d2306334734e4f58b1302a6ed5a6c796a706f6597a5641b6d468223",
       "inboxRollingHash": "0x00cad6497cf81748167158f149ba70c31f34c68b0ae1b156117de63e073d14b5",
       "outHash": "0x007c92c6cf05665e1c02a305370a4d38bcb8b555261c6b39c862f8c067d6bcb7",
       "slotNumber": 99,
@@ -57,7 +56,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x0072a35395a79027db315321cbdaba5bc0b070979c3395b7369ec902ca48d9b2",
+    "headerHash": "0x0018784f81057f236b2da6a1e9bc20cb7c931343cbecb2035b605dc8ca5dd3cb",
     "numTxs": 1
   }
 }
diff --git a/l1-contracts/test/fixtures/single_tx_checkpoint_2.json b/l1-contracts/test/fixtures/single_tx_checkpoint_2.json
index dc1244dcd73c..5487087aed88 100644
--- a/l1-contracts/test/fixtures/single_tx_checkpoint_2.json
+++ b/l1-contracts/test/fixtures/single_tx_checkpoint_2.json
@@ -43,7 +43,6 @@
       "lastArchiveRoot": "0x0f198bb13b63a3a8791a8b9994167bbe1554811a106fda0acb5e6c6539b1713e",
       "blockHeadersHash": "0x12813725f2d16ce92088d2401ffa4a53ce6061bf75b77320ca7b8ef6c5145adf",
       "blobsHash": "0x002b8ae4c9f405529e2b689b829852ad52f77acdac57d1dbac3dabea1760affc",
-      "inHash": "0x006504de282a40084bb8098456a915c645d53482d351db52fa9433b6cd638763",
       "inboxRollingHash": "0x0080dbf2bd9576e81fd99fc0f9d35bfd7fc15a6221d0b6e1e92a1884f14d0c84",
       "outHash": "0x0007eac1d76cddf92b28b8f11cd292f199f35dfc588376092986575cef487f59",
       "slotNumber": 102,
@@ -57,7 +56,7 @@
       "totalManaUsed": 0,
       "accumulatedFees": 0
     },
-    "headerHash": "0x000b3c0bf27d3123a81b66570e4b63734cb06bbfd260a70fabbbfd41dec13255",
+    "headerHash": "0x00782cfd7fa9f4e407c734be2832e8afb818d2ef8a0a11d288a3df34bcae0047",
     "numTxs": 1
   }
 }
diff --git a/l1-contracts/test/harnesses/Frontier.sol b/l1-contracts/test/harnesses/Frontier.sol
deleted file mode 100644
index 719e5692c8b5..000000000000
--- a/l1-contracts/test/harnesses/Frontier.sol
+++ /dev/null
@@ -1,42 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright 2024 Aztec Labs.
-pragma solidity >=0.8.27;
-
-import {FrontierLib} from "@aztec/core/libraries/crypto/FrontierLib.sol";
-
-import {Ownable} from "@oz/access/Ownable.sol";
-
-// This truncates each hash and hash preimage to 31 bytes to follow Noir.
-// It follows the logic in /noir-protocol-circuits/crates/parity-lib/src/utils/sha256_merkle_tree.nr
-// TODO(Miranda): Possibly nuke this contract, and use a generic version which can either use
-// regular sha256 or sha256ToField when emulating circuits
-contract FrontierMerkle is Ownable {
-  using FrontierLib for FrontierLib.Tree;
-  using FrontierLib for FrontierLib.Forest;
-
-  uint256 public immutable HEIGHT;
-  uint256 public immutable SIZE;
-
-  // Practically immutable value as we only set it in the constructor.
-  FrontierLib.Forest internal forest;
-
-  FrontierLib.Tree internal tree;
-
-  constructor(uint256 _height) Ownable(msg.sender) {
-    HEIGHT = _height;
-    SIZE = 2 ** _height;
-    forest.initialize(_height);
-  }
-
-  function insertLeaf(bytes32 _leaf) external onlyOwner returns (uint256) {
-    return tree.insertLeaf(_leaf);
-  }
-
-  function root() external view returns (bytes32) {
-    return tree.root(forest, HEIGHT, SIZE);
-  }
-
-  function isFull() external view returns (bool) {
-    return tree.isFull(SIZE);
-  }
-}
diff --git a/l1-contracts/test/merkle/Frontier.t.sol b/l1-contracts/test/merkle/Frontier.t.sol
deleted file mode 100644
index b6b8e3844a90..000000000000
--- a/l1-contracts/test/merkle/Frontier.t.sol
+++ /dev/null
@@ -1,30 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright 2024 Aztec Labs.
-pragma solidity >=0.8.27;
-
-import {Test} from "forge-std/Test.sol";
-
-import {NaiveMerkle} from "./Naive.sol";
-import {FrontierMerkle} from "../harnesses/Frontier.sol";
-
-contract FrontierTest is Test {
-  function setUp() public {
-    // Pause gas metering as calculating the root on each insert is expensive
-    vm.pauseGasMetering();
-  }
-
-  function testFrontier() public {
-    uint256 depth = 10;
-
-    NaiveMerkle merkle = new NaiveMerkle(depth);
-    FrontierMerkle frontier = new FrontierMerkle(depth);
-
-    uint256 upper = frontier.SIZE();
-    for (uint256 i = 0; i < upper; i++) {
-      bytes32 leaf = sha256(abi.encode(i + 1));
-      merkle.insertLeaf(leaf);
-      frontier.insertLeaf(leaf);
-      assertEq(merkle.computeRoot(), frontier.root(), "Frontier Roots should be equal");
-    }
-  }
-}
diff --git a/l1-contracts/test/validator-selection/ValidatorSelection.t.sol b/l1-contracts/test/validator-selection/ValidatorSelection.t.sol
index 80820932ec1b..a968be6d803f 100644
--- a/l1-contracts/test/validator-selection/ValidatorSelection.t.sol
+++ b/l1-contracts/test/validator-selection/ValidatorSelection.t.sol
@@ -546,8 +546,6 @@ contract ValidatorSelectionTest is ValidatorSelectionTestBase {
 
     {
       uint128 manaMinFee = SafeCast.toUint128(rollup.getManaMinFeeAt(Timestamp.wrap(block.timestamp), true));
-      // The header's inHash field is an unconstrained pass-through hint post-flip; propose does not check it.
-      header.inHash = bytes32(0);
       header.gasFees.feePerL2Gas = manaMinFee;
     }
 
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/block_rollup_public_inputs.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/block_rollup_public_inputs.nr
index d78a718b3269..e014d61946b2 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/block_rollup_public_inputs.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/block_rollup_public_inputs.nr
@@ -40,11 +40,13 @@ pub struct BlockRollupPublicInputs {
     // stored in the checkpoint header, enabling validation of the blocks included in a checkpoint given their headers.
     pub block_headers_hash: Field,
 
-    // Whether this block range starts at the first block of its checkpoint. Only the first block root sets this true;
-    // merges propagate it from the left rollup and `validate_consecutive_block_rollups` asserts the right rollup's is
-    // false, so it can only reach the checkpoint root through the leftmost leaf. It replaces `in_hash`'s former
-    // structural role: the checkpoint root asserts the merged value is true, and it drives the block-end blob-absorb
-    // flag (the l1-to-l2 tree root is absorbed only for the first block).
+    // Whether this block range starts at the first block of its checkpoint. Only the first block root variants set it
+    // true; merges propagate it from the left rollup (`merge_block_rollups`), and
+    // `validate_consecutive_block_rollups` asserts the right rollup of every merge is not a first block. The flag is
+    // therefore a proof that the range's leftmost leaf is a first-variant block root, which is what lets the
+    // checkpoint root assert its leftmost block is one (`checkpoint_root_inputs_validator`) even though it cannot see
+    // the leaf VK indices. Since only first variants may be fully empty (no txs and no messages), that also confines
+    // a fully-empty filler block to the first position of a checkpoint.
     pub is_first_block: bool,
 
     // Poseidon2 message-bundle sponge threaded across the blocks of the checkpoint. The leftmost block starts from the
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/parity_public_inputs.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/parity_public_inputs.nr
index a2260af3e24e..1914ae700516 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/parity_public_inputs.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/abis/parity_public_inputs.nr
@@ -3,12 +3,6 @@ use types::traits::{Deserialize, Empty, Serialize};
 
 #[derive(Deserialize, Eq, Serialize)]
 pub struct ParityPublicInputs {
-    // The L1 `in_hash`: the sha256 frontier root of the checkpoint's L1-to-L2 messages, checked on L1 against
-    // `inbox.consume()`. It is an UNCONSTRAINED pass-through: `InboxParity` echoes the value the prover supplies and
-    // does not recompute it from the messages (the frontier tree is gone from the parity body). Until the Fast Inbox
-    // flip moves the L1 anchor to the rolling hash, `in_hash` remains the authoritative L1 check, so the prover must
-    // supply the true frontier root; the in-circuit tie between it and the processed messages is deferred to the flip.
-    pub in_hash: Field,
     // Rolling hash of the Inbox message chain before absorbing this checkpoint's messages.
     pub start_rolling_hash: Field,
     // Rolling hash of the Inbox message chain after absorbing the `num_msgs` real messages. Each link is
@@ -29,7 +23,6 @@ pub struct ParityPublicInputs {
 impl Empty for ParityPublicInputs {
     fn empty() -> Self {
         ParityPublicInputs {
-            in_hash: 0,
             start_rolling_hash: 0,
             end_rolling_hash: 0,
             start_sponge: L1ToL2MessageSponge::empty(),
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/checkpoint_root_single_block_rollup.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/checkpoint_root_single_block_rollup.nr
index 79a4ef5f2f1a..3265df9d7c37 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/checkpoint_root_single_block_rollup.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/checkpoint_root_single_block_rollup.nr
@@ -21,8 +21,10 @@ use types::{
 //
 // That `is_first_block` assertion is the only thing keeping a message-only block from standing as a checkpoint's sole
 // block. If it is ever relaxed (e.g. replaced by a "leftmost block's start_msg_sponge is empty" check), this entry
-// must be dropped in the same change, or a transaction-less single-block checkpoint — one whose blob omits the
-// first-block l1-to-l2 root — would silently become provable.
+// must be dropped in the same change, because barring the message-only variant from standing alone is what keeps
+// each checkpoint shape to a single valid encoding: a sole transaction-less block carrying a message bundle is
+// already provable as an empty-tx-first block, which accepts a non-empty bundle. It is also what keeps the rule
+// confining fully-empty blocks to the first position enforceable.
 global ALLOWED_PREVIOUS_VK_INDICES: [u32; 4] = [
     BLOCK_ROOT_FIRST_ROLLUP_VK_INDEX,
     BLOCK_ROOT_SINGLE_TX_FIRST_ROLLUP_VK_INDEX,
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/components/checkpoint_rollup_public_inputs_composer.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/components/checkpoint_rollup_public_inputs_composer.nr
index 56aa0e132933..b1f4bf8df971 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/components/checkpoint_rollup_public_inputs_composer.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/components/checkpoint_rollup_public_inputs_composer.nr
@@ -21,8 +21,8 @@ use types::{
 
 pub struct CheckpointRollupPublicInputsComposer<let NumBlobs: u32> {
     merged_rollup: BlockRollupPublicInputs,
-    // Public inputs of the inbox parity proof (verified by the caller). The checkpoint's `in_hash` and
-    // `inbox_rolling_hash` are sourced from it, and its message sponge is checked against the blocks' accumulated one.
+    // Public inputs of the inbox parity proof (verified by the caller). The checkpoint's `inbox_rolling_hash` is
+    // sourced from it, and its message sponge is checked against the blocks' accumulated one.
     parity: ParityPublicInputs,
     // The below are all hints:
     previous_out_hash: AppendOnlyTreeSnapshot,
@@ -117,8 +117,8 @@ impl<let NumBlobs: u32> CheckpointRollupPublicInputsComposer<NumBlobs> {
             new_archive: merged_rollup.new_archive,
             previous_out_hash: self.previous_out_hash,
             new_out_hash,
-            // Sourced from the inbox parity proof, which commits to the same message list behind the L1-checked
-            // `in_hash`. Checkpoint merges assert continuity across checkpoints (`right.start == left.end`).
+            // Sourced from the inbox parity proof, which commits to the checkpoint's message list. Checkpoint merges
+            // assert continuity across checkpoints (`right.start == left.end`).
             start_inbox_rolling_hash: self.parity.start_rolling_hash,
             end_inbox_rolling_hash: self.parity.end_rolling_hash,
             checkpoint_header_hashes,
@@ -145,10 +145,7 @@ impl<let NumBlobs: u32> CheckpointRollupPublicInputsComposer<NumBlobs> {
             last_archive_root: merged_rollup.previous_archive.root,
             block_headers_hash: merged_rollup.block_headers_hash,
             blobs_hash: self.blobs_hash,
-            // `in_hash` (the sha256 frontier root, checked on L1) and `inbox_rolling_hash` are sourced from the inbox
-            // parity proof. `in_hash` is an unconstrained pass-through there (see `ParityPublicInputs::in_hash`); it
-            // remains the authoritative L1 check until the Fast Inbox flip.
-            in_hash: self.parity.in_hash,
+            // Sourced from the inbox parity proof: the checkpoint's only inbox commitment (AZIP-22 Fast Inbox).
             inbox_rolling_hash: self.parity.end_rolling_hash,
             epoch_out_hash,
             slot_number: constants.slot_number,
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/tests/mod.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/tests/mod.nr
index 38b92d5163e4..9dec29e65cba 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/tests/mod.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/checkpoint_root/tests/mod.nr
@@ -154,8 +154,8 @@ impl TestBuilder {
         let start_block_number = fixture_builder.start_block_number;
         if !is_first_block(left_rollup_vk_index)
             & (left_rollup_vk_index != BLOCK_MERGE_ROLLUP_VK_INDEX) {
-            // Change the start_block_number to be smaller than the left rollup's start_block_number so that the in_hash
-            // won't be set on the left rollup.
+            // Change the start_block_number to be smaller than the left rollup's start_block_number so that the
+            // first-block designation is not on the left rollup.
             fixture_builder.start_block_number = start_block_number - 1;
         }
         let mut left_rollup = fixture_builder.get_merged_block_rollup_public_inputs(
@@ -163,8 +163,8 @@ impl TestBuilder {
             start_block_number + num_left_blocks - 1,
         );
         if is_first_block(right_rollup_vk_index) {
-            // Change the start_block_number to be the right rollup's start_block_number so that the in_hash will be set
-            // on the right rollup.
+            // Change the start_block_number to be the right rollup's start_block_number so that the first-block
+            // designation is on the right rollup.
             fixture_builder.start_block_number = start_block_number + num_left_blocks;
         }
         let mut right_rollup = fixture_builder.get_merged_block_rollup_public_inputs(
@@ -192,8 +192,8 @@ impl TestBuilder {
         left_rollup.start_state = hints.previous_block_header.state;
 
         // The InboxParity proof commits to the same checkpoint messages the block roots absorbed. Its message sponge
-        // (empty to the checkpoint's accumulated value) matches the blocks' merged sponge; its rolling hash and in_hash
-        // feed the checkpoint header. Built at the base slot the block fixtures use for their sponge.
+        // (empty to the checkpoint's accumulated value) matches the blocks' merged sponge; its rolling hash feeds the
+        // checkpoint header. Built at the base slot the block fixtures use for their sponge.
         let inbox_parity = fixture_builder.get_parity_public_inputs(fixture_builder.start_slot_number);
 
         Self {
@@ -445,7 +445,6 @@ impl TestBuilder {
             last_archive_root: left.previous_archive.root,
             block_headers_hash,
             blobs_hash: self.hints.blobs_hash,
-            in_hash: self.inbox_parity.in_hash,
             inbox_rolling_hash: self.inbox_parity.end_rolling_hash,
             epoch_out_hash,
             slot_number: left.constants.slot_number,
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/inbox_parity.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/inbox_parity.nr
index 83f99510b696..c8784c827a6f 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/inbox_parity.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/inbox_parity.nr
@@ -18,10 +18,6 @@ pub struct InboxParityPrivateInputs<let S: u32> {
     pub(crate) start_rolling_hash: Field,
     // Message-bundle sponge before this checkpoint's messages (empty at checkpoint start).
     pub(crate) start_sponge: L1ToL2MessageSponge,
-    // The L1 `in_hash` (sha256 frontier root of this checkpoint's messages), passed through UNCONSTRAINED to the
-    // public inputs. See `ParityPublicInputs::in_hash`: the frontier tree is gone from this body, so the prover
-    // supplies the value and the circuit does not recompute or bind it to `msgs`.
-    pub(crate) in_hash: Field,
     pub(crate) vk_tree_root: Field,
     pub(crate) prover_id: Field,
 }
@@ -35,7 +31,6 @@ pub struct InboxParityPrivateInputs<let S: u32> {
 /// - Absorbs the same real messages into the message sponge (real-count absorb; the checkpoint root asserts this
 ///   equals the sponge the block roots accumulate)
 /// - Asserts the padding lanes past `num_msgs` are zero so they cannot silently enter either accumulator
-/// - Passes `in_hash` through unconstrained (see `InboxParityPrivateInputs::in_hash`)
 ///
 /// The output feeds the Checkpoint Root circuits, which verify this proof against the `{64, 256, 1024}` VK ladder.
 ///
@@ -51,7 +46,6 @@ pub fn execute<let S: u32>(inputs: InboxParityPrivateInputs<S>) -> ParityPublicI
     end_sponge.absorb(inputs.msgs, inputs.num_msgs);
 
     ParityPublicInputs {
-        in_hash: inputs.in_hash,
         start_rolling_hash: inputs.start_rolling_hash,
         end_rolling_hash,
         start_sponge: inputs.start_sponge,
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/tests/inbox_parity_tests.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/tests/inbox_parity_tests.nr
index 3ec816a2eb0d..89c20b998519 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/tests/inbox_parity_tests.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/parity/tests/inbox_parity_tests.nr
@@ -18,14 +18,11 @@ fn public_inputs_match_expected() {
         num_msgs: S,
         start_rolling_hash: 0,
         start_sponge: L1ToL2MessageSponge::empty(),
-        in_hash: 0x1234,
         vk_tree_root: 42,
         prover_id: 7,
     };
     let public_inputs = inbox_parity::execute(private_inputs);
 
-    // `in_hash` is echoed through unconstrained.
-    assert_eq(public_inputs.in_hash, 0x1234);
     assert_eq(public_inputs.vk_tree_root, 42);
     assert_eq(public_inputs.prover_id, 7);
 
@@ -57,7 +54,6 @@ fn rolling_hash_and_sponge_ignore_padding_and_thread_start() {
         num_msgs: 3,
         start_rolling_hash: start,
         start_sponge,
-        in_hash: 0,
         vk_tree_root: 42,
         prover_id: 7,
     };
@@ -88,7 +84,6 @@ fn sponge_matches_block_root_real_count_absorb() {
         num_msgs: 2,
         start_rolling_hash: 0,
         start_sponge: L1ToL2MessageSponge::empty(),
-        in_hash: 0,
         vk_tree_root: 42,
         prover_id: 7,
     };
@@ -113,7 +108,6 @@ fn non_zero_padding_lane_fails() {
         num_msgs: 2,
         start_rolling_hash: 0,
         start_sponge: L1ToL2MessageSponge::empty(),
-        in_hash: 0,
         vk_tree_root: 42,
         prover_id: 7,
     };
@@ -127,7 +121,6 @@ fn num_msgs_greater_than_size_fails() {
         num_msgs: S + 1,
         start_rolling_hash: 0,
         start_sponge: L1ToL2MessageSponge::empty(),
-        in_hash: 0,
         vk_tree_root: 42,
         prover_id: 7,
     };
diff --git a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/tests/rollup_fixture_builder.nr b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/tests/rollup_fixture_builder.nr
index 129c55ae4061..c96feaf9d36e 100644
--- a/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/tests/rollup_fixture_builder.nr
+++ b/noir-projects/noir-protocol-circuits/crates/rollup-lib/src/tests/rollup_fixture_builder.nr
@@ -48,7 +48,7 @@ pub struct RollupFixtureBuilder {
     pub protocol_contracts_hash: Field,
     pub prover_id: Field,
 
-    // Used to identify the first block, which will have an empty start_sponge_blob, and non-empty in_hash.
+    // Used to identify the first block, which will have an empty start_sponge_blob.
     pub start_block_number: u32,
 
     // Used to identify the first checkpoint, which will have an empty start_blob_accumulator.
@@ -458,7 +458,6 @@ impl RollupFixtureBuilder {
 
     pub fn get_parity_public_inputs(self, slot_number: Field) -> ParityPublicInputs {
         ParityPublicInputs {
-            in_hash: slot_number * 85831493,
             start_rolling_hash: self.get_inbox_rolling_hash(slot_number),
             end_rolling_hash: self.get_inbox_rolling_hash(slot_number + 1),
             // The message sponge resets per checkpoint, so it starts empty and ends at the checkpoint's accumulated
diff --git a/noir-projects/noir-protocol-circuits/crates/types/src/abis/checkpoint_header.nr b/noir-projects/noir-protocol-circuits/crates/types/src/abis/checkpoint_header.nr
index 59cb06c87a07..ca150f0d819f 100644
--- a/noir-projects/noir-protocol-circuits/crates/types/src/abis/checkpoint_header.nr
+++ b/noir-projects/noir-protocol-circuits/crates/types/src/abis/checkpoint_header.nr
@@ -14,10 +14,9 @@ pub struct CheckpointHeader {
     pub last_archive_root: Field,
     pub block_headers_hash: Field,
     pub blobs_hash: Field,
-    pub in_hash: Field,
-    // Inbox rolling-hash chain value after consuming all L1-to-L2 messages bundled into this checkpoint. It is the
-    // dual of `in_hash` (AZIP-22 Fast Inbox): the truncated-to-field sha256 chain the L1 Inbox accumulates. Currently a
-    // pass-through commitment; `in_hash` remains the authoritative L1 check until the Fast Inbox flip.
+    // Inbox rolling-hash chain value after consuming all L1-to-L2 messages bundled into this checkpoint (AZIP-22 Fast
+    // Inbox): the truncated-to-field sha256 chain the L1 Inbox accumulates. This is the checkpoint's only inbox
+    // commitment.
     pub inbox_rolling_hash: Field,
     // The root of the epoch out hash balanced tree. The out hash of the first checkpoint in the epoch is inserted at
     // index 0, the second at index 1, and so on.
@@ -43,7 +42,6 @@ impl Empty for CheckpointHeader {
             last_archive_root: 0,
             block_headers_hash: 0,
             blobs_hash: 0,
-            in_hash: 0,
             inbox_rolling_hash: 0,
             epoch_out_hash: 0,
             slot_number: 0,
@@ -62,7 +60,6 @@ impl CheckpointHeader {
         let last_archive_root_bytes: [u8; 32] = self.last_archive_root.to_be_bytes();
         let block_headers_hash_bytes: [u8; 32] = self.block_headers_hash.to_be_bytes();
         let blobs_hash_bytes: [u8; 32] = self.blobs_hash.to_be_bytes();
-        let in_hash_bytes: [u8; 32] = self.in_hash.to_be_bytes();
         let inbox_rolling_hash_bytes: [u8; 32] = self.inbox_rolling_hash.to_be_bytes();
         let epoch_out_hash_bytes: [u8; 32] = self.epoch_out_hash.to_be_bytes();
         let slot_number_bytes: [u8; 32] = self.slot_number.to_be_bytes();
@@ -83,7 +80,6 @@ impl CheckpointHeader {
         last_archive_root_bytes
             .concat(block_headers_hash_bytes)
             .concat(blobs_hash_bytes)
-            .concat(in_hash_bytes)
             .concat(inbox_rolling_hash_bytes)
             .concat(epoch_out_hash_bytes)
             .concat(slot_number_bytes)
@@ -108,7 +104,7 @@ fn empty_checkpoint_header_hash_matches_ts() {
     let header = CheckpointHeader::empty();
 
     // Generated from checkpoint_header.test.ts
-    let empty_checkpoint_header_hash_from_ts = 0x0008c3e5bbea4fba57201a69d4bf70a0d255df921fdef4924c22da087f9338c2;
+    let empty_checkpoint_header_hash_from_ts = 0x002e384af86a480f952aa16443fd29646a9063865e62d7c403fc7ed697bb7712;
 
     assert_eq(header.hash(), empty_checkpoint_header_hash_from_ts);
 }
@@ -119,7 +115,6 @@ fn checkpoint_header_hash_matches_ts() {
         last_archive_root: 123,
         block_headers_hash: 456,
         blobs_hash: 77,
-        in_hash: 88,
         inbox_rolling_hash: 89,
         epoch_out_hash: 99,
         slot_number: 1234,
@@ -137,7 +132,7 @@ fn checkpoint_header_hash_matches_ts() {
     assert_eq(deserialized, header);
 
     // Generated from checkpoint_header.test.ts
-    let checkpoint_header_hash_from_ts = 0x00519b87177a8a5e4edd03f4d820aec6d402497ef1ab70e2ecd4d4c39b339611;
+    let checkpoint_header_hash_from_ts = 0x00751391e842cd7b2014478255dd3309df86327197a0feb03f0af1b758f62ba5;
 
     assert_eq(header.hash(), checkpoint_header_hash_from_ts);
 }
@@ -150,7 +145,6 @@ fn checkpoint_header_hash_large_values_matches_ts() {
         last_archive_root: MAX_FIELD_VALUE - 123,
         block_headers_hash: MAX_FIELD_VALUE - 456,
         blobs_hash: MAX_FIELD_VALUE - 77,
-        in_hash: MAX_FIELD_VALUE - 88,
         inbox_rolling_hash: MAX_FIELD_VALUE - 89,
         epoch_out_hash: MAX_FIELD_VALUE - 99,
         slot_number: MAX_FIELD_VALUE - 1234,
@@ -170,7 +164,7 @@ fn checkpoint_header_hash_large_values_matches_ts() {
     assert_eq(deserialized, header);
 
     // Generated from checkpoint_header.test.ts
-    let checkpoint_header_hash_large_values_from_ts = 0x00d64307fa93c32ae46a4c3b6a1911d31994daa8d11d50201f0086a6cbaa9bac;
+    let checkpoint_header_hash_large_values_from_ts = 0x005bd09725c6e77a4a28a7ccdaf7875ba5882431ca3c82e62db96e8a12769ce5;
 
     assert_eq(header.hash(), checkpoint_header_hash_large_values_from_ts);
 }
diff --git a/noir-projects/noir-protocol-circuits/crates/types/src/constants.nr b/noir-projects/noir-protocol-circuits/crates/types/src/constants.nr
index 262adeec56b3..58dbd4831614 100644
--- a/noir-projects/noir-protocol-circuits/crates/types/src/constants.nr
+++ b/noir-projects/noir-protocol-circuits/crates/types/src/constants.nr
@@ -61,12 +61,10 @@ pub global NOTE_HASH_SUBTREE_ROOT_SIBLING_PATH_LENGTH: u32 =
     NOTE_HASH_TREE_HEIGHT - NOTE_HASH_SUBTREE_HEIGHT;
 pub global NULLIFIER_SUBTREE_ROOT_SIBLING_PATH_LENGTH: u32 =
     NULLIFIER_TREE_HEIGHT - NULLIFIER_SUBTREE_HEIGHT;
-pub global L1_TO_L2_MSG_SUBTREE_ROOT_SIBLING_PATH_LENGTH: u32 =
-    L1_TO_L2_MSG_TREE_HEIGHT - L1_TO_L2_MSG_SUBTREE_HEIGHT;
 // Cap on L1-to-L2 messages bundled into a single L2 block. One quarter of the per-checkpoint
 // cap: a checkpoint drains its Inbox consumption across up to four message-bearing blocks.
 pub global MAX_L1_TO_L2_MSGS_PER_BLOCK: u32 = 256;
-// Cap on L1-to-L2 messages consumed by a single checkpoint. Semantically today's NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP.
+// Cap on L1-to-L2 messages consumed by a single checkpoint.
 pub global MAX_L1_TO_L2_MSGS_PER_CHECKPOINT: u32 = 1024;
 // Minimum bucket age, in seconds, at the start of a checkpoint's build frame for its consumption to be mandatory under
 // the streaming Inbox censorship cutoff. One L1 slot: validators cannot be required to act on
@@ -109,9 +107,6 @@ pub global MAX_NULLIFIER_READ_REQUESTS_PER_CALL: u32 = 16;
 pub global MAX_KEY_VALIDATION_REQUESTS_PER_CALL: u32 = MAX_PRIVATE_LOGS_PER_CALL;
 // docs:end:constants
 
-// ROLLUP CONTRACT CONSTANTS - constants used only in l1-contracts
-pub global NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP: u32 = 1 << L1_TO_L2_MSG_SUBTREE_HEIGHT;
-
 // VK TREE CONSTANTS
 pub global PRIVATE_KERNEL_INIT_VK_INDEX: u32 = 0;
 pub global PRIVATE_KERNEL_INNER_VK_INDEX: u32 = 1;
@@ -453,7 +448,6 @@ pub global BLOCK_HEADER_LENGTH: u32 = APPEND_ONLY_TREE_SNAPSHOT_LENGTH
 pub global CHECKPOINT_HEADER_LENGTH: u32 = 1 /* last_archive_root */
     + 1 /* block_headers_hash */
     + 1 /* blobs_hash */
-    + 1 /* in_hash */
     + 1 /* inbox_rolling_hash */
     + 1 /* out_hash */
     + 1 /* slot_number */
@@ -675,12 +669,6 @@ pub global INBOX_PARITY_SIZE_SMALL: u32 = 64;
 pub global INBOX_PARITY_SIZE_MEDIUM: u32 = 256;
 pub global INBOX_PARITY_SIZE_LARGE: u32 = 1024;
 
-// Fan-in of the sha256 `in_hash` frontier tree (NUM_BASE_PARITY_PER_ROOT_PARITY * NUM_MSGS_PER_BASE_PARITY ==
-// NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP). The circuits no longer build this tree, but the constants are still consumed
-// by the generated Solidity `Constants` and the L1 `Parity.t.sol` test that checks the frontier-root construction.
-pub global NUM_MSGS_PER_BASE_PARITY: u32 = 256;
-pub global NUM_BASE_PARITY_PER_ROOT_PARITY: u32 = 4;
-
 // Lengths of the different types of proofs in fields
 pub global RECURSIVE_PROOF_LENGTH: u32 = 410;
 pub global NESTED_RECURSIVE_PROOF_LENGTH: u32 = RECURSIVE_PROOF_LENGTH;
@@ -1443,7 +1431,7 @@ mod test {
     use crate::traits::ToField;
     use super::{
         GLOBAL_INDEX_CONTRACT_MIN_REVERTIBLE_SIDE_EFFECT_COUNTER_OFFSET, INBOX_PARITY_SIZE_LARGE,
-        MAX_ETH_ADDRESS_VALUE, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
+        MAX_ETH_ADDRESS_VALUE, MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
         TOTAL_COUNTED_SIDE_EFFECTS_PER_CALL,
     };
 
@@ -1490,7 +1478,7 @@ mod test {
     #[test]
     fn parity_covers_all_l1_to_l2_msgs() {
         // The largest InboxParity rung must be able to hold a full checkpoint's worth of messages.
-        assert_eq(INBOX_PARITY_SIZE_LARGE, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP);
+        assert_eq(INBOX_PARITY_SIZE_LARGE, MAX_L1_TO_L2_MSGS_PER_CHECKPOINT);
     }
 
     #[test]
diff --git a/yarn-project/archiver/src/l1/calldata_retriever.test.ts b/yarn-project/archiver/src/l1/calldata_retriever.test.ts
index cc589173dbf9..ce2fa5c3f3c3 100644
--- a/yarn-project/archiver/src/l1/calldata_retriever.test.ts
+++ b/yarn-project/archiver/src/l1/calldata_retriever.test.ts
@@ -1248,7 +1248,7 @@ describe('CalldataRetriever', () => {
       expect(result.blockHash).toBe(tx.blockHash);
 
       // Verify all components are properly decoded
-      expect(result.header.inHash).toBeInstanceOf(Fr);
+      expect(result.header.inboxRollingHash).toBeInstanceOf(Fr);
       expect(result.header.gasFees).toBeInstanceOf(GasFees);
 
       // Verify instrumentation was called
@@ -1322,7 +1322,7 @@ describe('CalldataRetriever', () => {
       expect(result.blockHash).toBe(blockHash);
 
       // Verify all components are properly decoded
-      expect(result.header.inHash).toBeInstanceOf(Fr);
+      expect(result.header.inboxRollingHash).toBeInstanceOf(Fr);
       expect(result.header.gasFees).toBeInstanceOf(GasFees);
 
       // Verify proxy implementation was checked
diff --git a/yarn-project/archiver/src/test/mock_structs.ts b/yarn-project/archiver/src/test/mock_structs.ts
index 5cc54cfe9bad..7cd23455029e 100644
--- a/yarn-project/archiver/src/test/mock_structs.ts
+++ b/yarn-project/archiver/src/test/mock_structs.ts
@@ -1,8 +1,4 @@
-import {
-  MAX_NOTE_HASHES_PER_TX,
-  NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
-  PRIVATE_LOG_SIZE_IN_FIELDS,
-} from '@aztec/constants';
+import { MAX_L1_TO_L2_MSGS_PER_CHECKPOINT, MAX_NOTE_HASHES_PER_TX, PRIVATE_LOG_SIZE_IN_FIELDS } from '@aztec/constants';
 import { makeTuple } from '@aztec/foundation/array';
 import { BlockNumber, CheckpointNumber, IndexWithinCheckpoint } from '@aztec/foundation/branded-types';
 import { Buffer16, Buffer32 } from '@aztec/foundation/buffer';
@@ -104,12 +100,12 @@ export function makeInboxMessagesWithFullBlocks(
   opts: { initialCheckpointNumber?: CheckpointNumber } = {},
 ): InboxMessage[] {
   const { initialCheckpointNumber = CheckpointNumber(13) } = opts;
-  return makeInboxMessages(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP * blockCount, {
+  return makeInboxMessages(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT * blockCount, {
     // Keep the compact global index from makeInboxMessages; only spread the (now-vestigial) checkpoint assignment
     // across blocks so multi-block coverage still exercises differing checkpoint numbers.
     overrideFn: (msg, i) => {
       const checkpointNumber = CheckpointNumber(
-        initialCheckpointNumber + Math.floor(i / NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP),
+        initialCheckpointNumber + Math.floor(i / MAX_L1_TO_L2_MSGS_PER_CHECKPOINT),
       );
       return { ...msg, checkpointNumber };
     },
diff --git a/yarn-project/ethereum/src/contracts/rollup.ts b/yarn-project/ethereum/src/contracts/rollup.ts
index f3f39b1020ef..ba41242c3e5f 100644
--- a/yarn-project/ethereum/src/contracts/rollup.ts
+++ b/yarn-project/ethereum/src/contracts/rollup.ts
@@ -79,7 +79,6 @@ export type ViemHeader = {
   lastArchiveRoot: `0x${string}`;
   blockHeadersHash: `0x${string}`;
   blobsHash: `0x${string}`;
-  inHash: `0x${string}`;
   inboxRollingHash: `0x${string}`;
   outHash: `0x${string}`;
   slotNumber: bigint;
diff --git a/yarn-project/ivc-integration/src/base_parity_inputs.test.ts b/yarn-project/ivc-integration/src/base_parity_inputs.test.ts
index 2d5ea50b7092..278ed58ca3a7 100644
--- a/yarn-project/ivc-integration/src/base_parity_inputs.test.ts
+++ b/yarn-project/ivc-integration/src/base_parity_inputs.test.ts
@@ -15,7 +15,7 @@ import { createLogger } from '@aztec/foundation/log';
 import { Noir } from '@aztec/noir-noir_js';
 import { ServerCircuitArtifacts } from '@aztec/noir-protocol-circuits-types/server';
 import { getVKTreeRoot } from '@aztec/noir-protocol-circuits-types/vk-tree';
-import { L1ToL2MessageSponge, computeInHashFromL1ToL2Messages } from '@aztec/stdlib/messaging';
+import { L1ToL2MessageSponge } from '@aztec/stdlib/messaging';
 import { InboxParityPrivateInputs } from '@aztec/stdlib/parity';
 
 import { jest } from '@jest/globals';
@@ -47,7 +47,6 @@ describe('Inbox Parity Benchmark Inputs', () => {
       l1ToL2Messages,
       Fr.ZERO,
       L1ToL2MessageSponge.empty(),
-      computeInHashFromL1ToL2Messages(l1ToL2Messages),
       vkTreeRoot,
       Fr.random(),
     );
@@ -75,8 +74,6 @@ describe('Inbox Parity Benchmark Inputs', () => {
         num_absorbed: startSponge.numAbsorbed,
       },
       // eslint-disable-next-line camelcase
-      in_hash: inputs.inHash.toString(),
-      // eslint-disable-next-line camelcase
       vk_tree_root: inputs.vkTreeRoot.toString(),
       // eslint-disable-next-line camelcase
       prover_id: inputs.proverId.toString(),
diff --git a/yarn-project/ivc-integration/src/bb_js_debug.test.ts b/yarn-project/ivc-integration/src/bb_js_debug.test.ts
index f478e61dd182..9ce6333f5fcb 100644
--- a/yarn-project/ivc-integration/src/bb_js_debug.test.ts
+++ b/yarn-project/ivc-integration/src/bb_js_debug.test.ts
@@ -12,7 +12,7 @@ import { createLogger } from '@aztec/foundation/log';
 import { Noir } from '@aztec/noir-noir_js';
 import { ServerCircuitArtifacts } from '@aztec/noir-protocol-circuits-types/server';
 import { getVKTreeRoot } from '@aztec/noir-protocol-circuits-types/vk-tree';
-import { L1ToL2MessageSponge, computeInHashFromL1ToL2Messages } from '@aztec/stdlib/messaging';
+import { L1ToL2MessageSponge } from '@aztec/stdlib/messaging';
 import { InboxParityPrivateInputs } from '@aztec/stdlib/parity';
 
 import { jest } from '@jest/globals';
@@ -62,7 +62,6 @@ describe('BB.js Debug Wrapper', () => {
       l1ToL2Messages,
       Fr.ZERO,
       L1ToL2MessageSponge.empty(),
-      computeInHashFromL1ToL2Messages(l1ToL2Messages),
       vkTreeRoot,
       Fr.random(),
     );
@@ -88,8 +87,6 @@ describe('BB.js Debug Wrapper', () => {
         num_absorbed: startSponge.numAbsorbed,
       },
       // eslint-disable-next-line camelcase
-      in_hash: inboxParityInputs.inHash.toString(),
-      // eslint-disable-next-line camelcase
       vk_tree_root: inboxParityInputs.vkTreeRoot.toString(),
       // eslint-disable-next-line camelcase
       prover_id: inboxParityInputs.proverId.toString(),
diff --git a/yarn-project/noir-protocol-circuits-types/src/conversion/server.ts b/yarn-project/noir-protocol-circuits-types/src/conversion/server.ts
index f37702498807..0e32f920b309 100644
--- a/yarn-project/noir-protocol-circuits-types/src/conversion/server.ts
+++ b/yarn-project/noir-protocol-circuits-types/src/conversion/server.ts
@@ -481,7 +481,6 @@ export function mapAvmProofDataToNoir(
 
 function mapParityPublicInputsToNoir(parityPublicInputs: ParityPublicInputs): ParityPublicInputsNoir {
   return {
-    in_hash: mapFieldToNoir(parityPublicInputs.inHash),
     start_rolling_hash: mapFieldToNoir(parityPublicInputs.startRollingHash),
     end_rolling_hash: mapFieldToNoir(parityPublicInputs.endRollingHash),
     start_sponge: mapL1ToL2MessageSpongeToNoir(parityPublicInputs.startSponge),
@@ -520,7 +519,6 @@ export function mapRootRollupPublicInputsFromNoir(
  */
 export function mapParityPublicInputsFromNoir(parityPublicInputs: ParityPublicInputsNoir): ParityPublicInputs {
   return new ParityPublicInputs(
-    mapFieldFromNoir(parityPublicInputs.in_hash),
     mapFieldFromNoir(parityPublicInputs.start_rolling_hash),
     mapFieldFromNoir(parityPublicInputs.end_rolling_hash),
     mapL1ToL2MessageSpongeFromNoir(parityPublicInputs.start_sponge),
@@ -739,7 +737,6 @@ export function mapInboxParityPrivateInputsToNoir(inputs: InboxParityPrivateInpu
     num_msgs: mapNumberToNoir(inputs.numMessages),
     start_rolling_hash: mapFieldToNoir(inputs.startRollingHash),
     start_sponge: mapL1ToL2MessageSpongeToNoir(inputs.startSponge),
-    in_hash: mapFieldToNoir(inputs.inHash),
     vk_tree_root: mapFieldToNoir(inputs.vkTreeRoot),
     prover_id: mapFieldToNoir(inputs.proverId),
   };
diff --git a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
index b50ef0313ed0..e2481445632b 100644
--- a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
+++ b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
@@ -248,7 +248,7 @@ export class LightweightCheckpointBuilder {
     this.blocks.push(block);
 
     // Accumulate the streaming bundle now that the block is fully built, so a mid-build throw above leaves the
-    // checkpoint's message list (and thus its inHash/rolling hash) consistent with the blocks actually built.
+    // checkpoint's message list (and thus its rolling hash) consistent with the blocks actually built.
     this.l1ToL2Messages.push(...l1ToL2Messages);
 
     const [msSpongeAbsorb] = await elapsed(() => this.spongeBlob.absorb(blockBlobFields));
@@ -283,9 +283,6 @@ export class LightweightCheckpointBuilder {
     const blobs = await getBlobsPerL1Block(this.blobFields);
     const blobsHash = computeBlobsHashFromBlobs(blobs);
 
-    // Legacy inHash is dead post-flip; the checkpoint header carries zero (AZIP-22 Fast Inbox). The consensus
-    // rolling hash over the consumed messages is the authoritative Inbox commitment.
-    const inHash = Fr.ZERO;
     const inboxRollingHash = accumulateInboxRollingHash(this.previousInboxRollingHash, this.l1ToL2Messages);
 
     const { slotNumber, coinbase, feeRecipient, gasFees } = this.constants;
@@ -303,7 +300,6 @@ export class LightweightCheckpointBuilder {
     const header = CheckpointHeader.from({
       lastArchiveRoot: this.lastArchives[0].root,
       blobsHash,
-      inHash,
       inboxRollingHash,
       epochOutHash,
       blockHeadersHash,
diff --git a/yarn-project/prover-client/src/orchestrator/checkpoint-proving-state.ts b/yarn-project/prover-client/src/orchestrator/checkpoint-proving-state.ts
index 04697e6e6249..98d3d9946ac3 100644
--- a/yarn-project/prover-client/src/orchestrator/checkpoint-proving-state.ts
+++ b/yarn-project/prover-client/src/orchestrator/checkpoint-proving-state.ts
@@ -161,15 +161,13 @@ export class CheckpointProvingState {
   /**
    * Builds the checkpoint's single InboxParity input. The circuit is sized to the smallest ladder rung that fits the
    * message count; the rolling hash starts from the previous checkpoint's end and the message sponge starts empty (it
-   * resets per checkpoint). `in_hash` (the L1 frontier root) is supplied as an unconstrained pass-through hint.
+   * resets per checkpoint).
    */
   public getInboxParityInputs(): InboxParityPrivateInputs {
     return InboxParityPrivateInputs.fromMessages(
       this.l1ToL2Messages,
       this.startInboxRollingHash,
       L1ToL2MessageSponge.empty(),
-      // Legacy in_hash is dead post-flip; the InboxParity pass-through hint carries zero (AZIP-22 Fast Inbox).
-      Fr.ZERO,
       this.constants.vkTreeRoot,
       this.constants.proverId,
     );
diff --git a/yarn-project/prover-client/src/test/bb_prover_parity.test.ts b/yarn-project/prover-client/src/test/bb_prover_parity.test.ts
index fbb920407ead..4848af4efd85 100644
--- a/yarn-project/prover-client/src/test/bb_prover_parity.test.ts
+++ b/yarn-project/prover-client/src/test/bb_prover_parity.test.ts
@@ -5,7 +5,7 @@ import { createLogger } from '@aztec/foundation/log';
 import type { ServerProtocolArtifact } from '@aztec/noir-protocol-circuits-types/server';
 import { ServerCircuitVks } from '@aztec/noir-protocol-circuits-types/server/vks';
 import { getVKTreeRoot } from '@aztec/noir-protocol-circuits-types/vk-tree';
-import { L1ToL2MessageSponge, computeInHashFromL1ToL2Messages } from '@aztec/stdlib/messaging';
+import { L1ToL2MessageSponge } from '@aztec/stdlib/messaging';
 import { INBOX_PARITY_SIZES, InboxParityPrivateInputs, type InboxParitySize } from '@aztec/stdlib/parity';
 
 import { TestContext } from '../mocks/test_context.js';
@@ -48,15 +48,11 @@ describe('prover/bb_prover/parity', () => {
       // Fill the rung with real messages so `numMessages === size` (the largest circuit for that rung).
       const messages = Array.from({ length: size }, () => Fr.random());
       const proverId = Fr.random();
-      // The in_hash is a sha256 frontier root (top byte zeroed to fit the field), which `ParityPublicInputs` enforces;
-      // compute it from the messages rather than using a raw random field.
-      const inHash = computeInHashFromL1ToL2Messages(messages);
 
       const inputs = InboxParityPrivateInputs.fromMessages(
         messages,
         Fr.ZERO,
         L1ToL2MessageSponge.empty(),
-        inHash,
         getVKTreeRoot(),
         proverId,
       );
diff --git a/yarn-project/pxe/src/storage/backwards_compatibility_tests/__snapshots__/L2TipsKVStore.json b/yarn-project/pxe/src/storage/backwards_compatibility_tests/__snapshots__/L2TipsKVStore.json
index e83ff396e83e..c919b85f457b 100644
--- a/yarn-project/pxe/src/storage/backwards_compatibility_tests/__snapshots__/L2TipsKVStore.json
+++ b/yarn-project/pxe/src/storage/backwards_compatibility_tests/__snapshots__/L2TipsKVStore.json
@@ -16,7 +16,7 @@
   "pxe_l2_tip_checkpoints": [
     {
       "key": "utf8:checkpointed",
-      "value": "{\"number\":47,\"hash\":\"0x009bac3aae6ac2c7945a4686ec07381ac88bbd323cbfaccd6dffea9b254ae820\"}"
+      "value": "{\"number\":47,\"hash\":\"0x00407c60c560276f14936ea39b073d155bc34a485e8a83c0439c9b8ef83f737e\"}"
     },
     {
       "key": "utf8:proven",
diff --git a/yarn-project/pxe/src/storage/backwards_compatibility_tests/schema_tests.ts b/yarn-project/pxe/src/storage/backwards_compatibility_tests/schema_tests.ts
index a9a75a77ffbc..c1802c49f19b 100644
--- a/yarn-project/pxe/src/storage/backwards_compatibility_tests/schema_tests.ts
+++ b/yarn-project/pxe/src/storage/backwards_compatibility_tests/schema_tests.ts
@@ -276,7 +276,6 @@ export const SCHEMA_TESTS: readonly SchemaTest[] = [
             new Fr(5n),
             new Fr(7n),
             new Fr(11n),
-            new Fr(13n),
             new Fr(15n),
             new Fr(17n),
             SlotNumber(19),
diff --git a/yarn-project/sequencer-client/src/publisher/l1_publisher.integration.test.ts b/yarn-project/sequencer-client/src/publisher/l1_publisher.integration.test.ts
index 1d15418c5d42..05f09d1745e5 100644
--- a/yarn-project/sequencer-client/src/publisher/l1_publisher.integration.test.ts
+++ b/yarn-project/sequencer-client/src/publisher/l1_publisher.integration.test.ts
@@ -14,6 +14,8 @@ import {
 } from '@aztec/blob-lib';
 import {
   GENESIS_ARCHIVE_ROOT,
+  INBOX_LAG_SECONDS,
+  MAX_L1_TO_L2_MSGS_PER_BLOCK,
   MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   MAX_NULLIFIERS_PER_TX,
   MAX_PROCESSABLE_L2_GAS,
@@ -92,6 +94,7 @@ import { type PrivateKeyAccount, privateKeyToAccount } from 'viem/accounts';
 import { foundry } from 'viem/chains';
 
 import { type SequencerClientConfig, getConfigEnvVars } from '../config.js';
+import { selectInboxBucketForBlock } from '../sequencer/inbox_bucket_selector.js';
 import { sendL1ToL2Message } from './l1_to_l2_messaging.js';
 import { SequencerPublisherMetrics } from './sequencer-publisher-metrics.js';
 import { SequencerPublisher } from './sequencer-publisher.js';
@@ -473,7 +476,7 @@ describe('L1Publisher integration', () => {
 
   /**
    * Build a checkpoint with a single block using the LightweightCheckpointBuilder.
-   * This properly computes all checkpoint header fields (blobsHash, blockHeadersHash, inHash, inboxRollingHash,
+   * This properly computes all checkpoint header fields (blobsHash, blockHeadersHash, inboxRollingHash,
    * epochOutHash, etc.). `previousInboxRollingHash` is the previous checkpoint's rolling hash (zero at genesis), so
    * the header's `inboxRollingHash` continues the on-chain Inbox chain over `l1ToL2Messages`.
    */
@@ -564,17 +567,23 @@ describe('L1Publisher integration', () => {
         '0x1647b194c649f5dd01d7c832f89b0f496043c9150797923ea89e93d5ac619a93',
       );
 
-      // Streaming Inbox consumption (AZIP-22 Fast Inbox): each checkpoint consumes every message sent so far, so its
-      // header rolling hash continues the previous checkpoint's and matches the Inbox's current bucket. Consuming
-      // through the newest bucket trivially satisfies the mandatory-consumption assert. A checkpoint's messages are
-      // sent one per L1 block, so they span several Inbox buckets; the checkpoint's consumed bucket is the newest,
-      // whose sequence we read straight off the deployed Inbox and mirror into messageSource so the propose bucket
-      // hint can be looked up by the checkpoint's cumulative message count.
+      // Streaming Inbox consumption (AZIP-22 Fast Inbox): the L1 Rollup only lets a checkpoint consume Inbox buckets
+      // that have aged past the censorship cutoff (`toTimestamp(slot - 1) - INBOX_LAG_SECONDS`), measured in L1 time,
+      // not whole checkpoints. Each checkpoint mirrors the real Inbox buckets into messageSource, then reuses the
+      // production `selectInboxBucketForBlock` (which mirrors `ProposeLib.validateInboxConsumption`) to pick exactly
+      // the buckets it must consume, deriving the consumed bundle, the propose bucket hint, and the header rolling
+      // hash from that one selection so header, world state, and L1 agree by construction.
       const inbox = getContract({
         address: getAddress(l1ContractAddresses.inboxAddress.toString()),
         abi: InboxAbi,
         client: l1Client,
       });
+      // Every message sent to the Inbox, in insertion order, so each bucket's leaves can be mirrored into messageSource.
+      const allSentMessages: Fr[] = [];
+      let mirroredThroughSeq = 0n;
+      let mirroredThroughTotal = 0n;
+      // The last Inbox bucket this checkpoint chain has consumed through; genesis sentinel to start.
+      let parent = { seq: 0n, totalMsgCount: 0n };
       let previousInboxRollingHash = Fr.ZERO;
       const blobFieldsPerCheckpoint: Fr[][] = [];
       // The below batched blob is used for testing different epochs with 1..numberOfConsecutiveBlocks blocks on L1.
@@ -586,10 +595,32 @@ describe('L1Publisher integration', () => {
         // and causes a chain prune
         const l1ToL2Content = range(Math.min(16, MAX_L1_TO_L2_MSGS_PER_CHECKPOINT), 128 * i + 1 + 0x400).map(fr);
 
-        const currentL1ToL2Messages: Fr[] = [];
+        const sentThisCheckpoint: Fr[] = [];
         for (let j = 0; j < l1ToL2Content.length; j++) {
-          currentL1ToL2Messages.push(await sendToL2(l1ToL2Content[j], recipientAddress));
+          sentThisCheckpoint.push(await sendToL2(l1ToL2Content[j], recipientAddress));
         }
+        allSentMessages.push(...sentThisCheckpoint);
+
+        // Mirror the Inbox's new buckets (seq, timestamp, rolling hash, totals) and their leaves into messageSource,
+        // so the selector, the world-state synchronizer, and L1 all read the same bucket state.
+        const currentBucketSeq = await inbox.read.getCurrentBucketSeq();
+        for (let seq = mirroredThroughSeq + 1n; seq <= currentBucketSeq; seq++) {
+          const bucket = await inbox.read.getBucket([seq]);
+          const bucketMessages = allSentMessages.slice(Number(mirroredThroughTotal), Number(bucket.totalMsgCount));
+          messageSource.setInboxBucket(
+            {
+              seq,
+              inboxRollingHash: Fr.fromString(bucket.rollingHash),
+              totalMsgCount: bucket.totalMsgCount,
+              timestamp: bucket.timestamp,
+              msgCount: Number(bucket.msgCount),
+              lastMessageIndex: bucket.totalMsgCount - 1n,
+            },
+            bucketMessages,
+          );
+          mirroredThroughTotal = bucket.totalMsgCount;
+        }
+        mirroredThroughSeq = currentBucketSeq;
 
         // Ensure that each transaction has unique (non-intersecting nullifier values)
         const totalNullifiersPerBlock = 4 * MAX_NULLIFIERS_PER_TX;
@@ -612,6 +643,25 @@ describe('L1Publisher integration', () => {
           new GasFees(0, await rollup.getManaMinFeeAt(timestamp, true)),
         );
 
+        // Reuse the production streaming selector to pick the buckets this single-block (hence last-block) checkpoint
+        // must consume, then derive the consumed bundle, the propose bucket hint, and the rolling-hash cursor from
+        // that one selection so the header, world state, and L1 all agree.
+        const buildFrameStart = await rollup.getTimestampForSlot(SlotNumber(slot - 1));
+        const cutoffTimestamp = buildFrameStart - BigInt(INBOX_LAG_SECONDS);
+        const selection = await selectInboxBucketForBlock({
+          messageSource,
+          now: buildFrameStart,
+          lagSeconds: BigInt(INBOX_LAG_SECONDS),
+          parent,
+          checkpointStartTotalMsgCount: parent.totalMsgCount,
+          perBlockCap: MAX_L1_TO_L2_MSGS_PER_BLOCK,
+          perCheckpointCap: MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
+          isLastBlock: true,
+          cutoffTimestamp,
+        });
+        const currentL1ToL2Messages = selection.consume ? selection.bundle : [];
+        const bucketHint = selection.consume ? selection.bucket.seq : parent.seq;
+
         const checkpoint = await buildCheckpoint(
           globalVariables,
           txs,
@@ -621,24 +671,9 @@ describe('L1Publisher integration', () => {
         );
         previousInboxRollingHash = checkpoint.header.inboxRollingHash;
         const block = checkpoint.blocks[0];
-
-        // Mirror the Inbox's newest bucket into messageSource so the world-state synchronizer can rebuild this
-        // block's consumed bundle when it syncs the block back on the next iteration, and so the propose bucket hint
-        // resolves to the sequence whose rolling hash the checkpoint header committed to. One bucket per block keyed
-        // by its cumulative L1->L2 tree leaf count -- exactly the value world-state looks the bucket up by.
-        const currentBucketSeq = await inbox.read.getCurrentBucketSeq();
-        const cumulativeMsgCount = BigInt(block.header.state.l1ToL2MessageTree.nextAvailableLeafIndex);
-        messageSource.setInboxBucket(
-          {
-            seq: currentBucketSeq,
-            inboxRollingHash: checkpoint.header.inboxRollingHash,
-            totalMsgCount: cumulativeMsgCount,
-            timestamp,
-            msgCount: currentL1ToL2Messages.length,
-            lastMessageIndex: cumulativeMsgCount - 1n,
-          },
-          currentL1ToL2Messages,
-        );
+        if (selection.consume) {
+          parent = { seq: selection.bucket.seq, totalMsgCount: selection.bucket.totalMsgCount };
+        }
 
         const totalManaUsed = txs.reduce((acc, tx) => acc.add(new Fr(tx.gasUsed.billedGas.l2Gas)), Fr.ZERO);
         expect(totalManaUsed.toBigInt()).toEqual(block.header.totalManaUsed.toBigInt());
@@ -667,8 +702,6 @@ describe('L1Publisher integration', () => {
           deployerAccount.address,
         );
 
-        // The checkpoint consumed everything sent so far, so its consumed bucket is the Inbox's newest.
-        const bucketHint = (await messageSource.getInboxBucketByTotalMsgCount(cumulativeMsgCount))!.seq;
         await publisher.enqueueProposeCheckpoint(
           checkpoint,
           CommitteeAttestationsAndSigners.empty(getSignatureContext()),
diff --git a/yarn-project/sequencer-client/src/publisher/write_json.ts b/yarn-project/sequencer-client/src/publisher/write_json.ts
index 401a94908a6f..e3a2b74e1603 100644
--- a/yarn-project/sequencer-client/src/publisher/write_json.ts
+++ b/yarn-project/sequencer-client/src/publisher/write_json.ts
@@ -55,7 +55,6 @@ export async function writeJson(
         lastArchiveRoot: asHex(checkpointHeader.lastArchiveRoot),
         blockHeadersHash: asHex(checkpointHeader.blockHeadersHash),
         blobsHash: asHex(checkpointHeader.blobsHash),
-        inHash: asHex(checkpointHeader.inHash),
         inboxRollingHash: asHex(checkpointHeader.inboxRollingHash),
         outHash: asHex(checkpointHeader.epochOutHash),
         slotNumber: Number(checkpointHeader.slotNumber),
diff --git a/yarn-project/sequencer-client/src/sequencer/sequencer.test.ts b/yarn-project/sequencer-client/src/sequencer/sequencer.test.ts
index 95a866e36a3e..2f77bc1c6cc2 100644
--- a/yarn-project/sequencer-client/src/sequencer/sequencer.test.ts
+++ b/yarn-project/sequencer-client/src/sequencer/sequencer.test.ts
@@ -1,4 +1,4 @@
-import { NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP } from '@aztec/constants';
+import { MAX_L1_TO_L2_MSGS_PER_CHECKPOINT } from '@aztec/constants';
 import { type EpochCache, type EpochCommitteeInfo, PROPOSER_PIPELINING_SLOT_OFFSET } from '@aztec/epoch-cache';
 import { NoCommitteeError, type RollupContract } from '@aztec/ethereum/contracts';
 import {
@@ -349,7 +349,7 @@ describe('sequencer', () => {
     });
 
     l1ToL2MessageSource = mock<L1ToL2MessageSource>({
-      getL1ToL2Messages: () => Promise.resolve(Array(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP).fill(Fr.ZERO)),
+      getL1ToL2Messages: () => Promise.resolve(Array(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT).fill(Fr.ZERO)),
       getL2Tips: mockFn().mockResolvedValue({
         proposed: { number: lastBlockNumber, hash },
         checkpointed: {
diff --git a/yarn-project/sequencer-client/src/test/utils.ts b/yarn-project/sequencer-client/src/test/utils.ts
index 92845b668f07..5bb506d4f729 100644
--- a/yarn-project/sequencer-client/src/test/utils.ts
+++ b/yarn-project/sequencer-client/src/test/utils.ts
@@ -114,7 +114,7 @@ export function createMockSignatures(signer: Secp256k1Signer): CommitteeAttestat
 
 /**
  * Creates a CheckpointHeader from an L2Block for testing purposes.
- * Uses mock values for blockHeadersHash, blobsHash and inHash since L2Block doesn't have these fields.
+ * Uses mock values for blockHeadersHash and blobsHash since L2Block doesn't have these fields.
  */
 function createCheckpointHeaderFromBlock(block: L2Block): CheckpointHeader {
   const gv = block.header.globalVariables;
@@ -122,7 +122,6 @@ function createCheckpointHeaderFromBlock(block: L2Block): CheckpointHeader {
     block.header.lastArchive.root,
     Fr.random(), // blockHeadersHash - mock value for testing
     Fr.random(), // blobsHash - mock value for testing
-    Fr.random(), // inHash - mock value for testing
     Fr.random(), // inboxRollingHash - mock value for testing
     Fr.random(), // outHash - mock value for testing
     gv.slotNumber,
diff --git a/yarn-project/stdlib/src/messaging/in_hash.ts b/yarn-project/stdlib/src/messaging/in_hash.ts
index 52833bf937fe..fd5191abb5c1 100644
--- a/yarn-project/stdlib/src/messaging/in_hash.ts
+++ b/yarn-project/stdlib/src/messaging/in_hash.ts
@@ -1,4 +1,4 @@
-import { NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP } from '@aztec/constants';
+import { MAX_L1_TO_L2_MSGS_PER_CHECKPOINT } from '@aztec/constants';
 import { padArrayEnd } from '@aztec/foundation/collection';
 import { Fr } from '@aztec/foundation/curves/bn254';
 import { computeBalancedShaRoot } from '@aztec/foundation/trees';
@@ -7,6 +7,6 @@ import { computeBalancedShaRoot } from '@aztec/foundation/trees';
  * Computes the inHash for a checkpoint (or the first block in a checkpoint) given its l1 to l2 messages.
  */
 export function computeInHashFromL1ToL2Messages(unpaddedL1ToL2Messages: Fr[]): Fr {
-  const l1ToL2Messages = padArrayEnd<Fr, number>(unpaddedL1ToL2Messages, Fr.ZERO, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP);
+  const l1ToL2Messages = padArrayEnd<Fr, number>(unpaddedL1ToL2Messages, Fr.ZERO, MAX_L1_TO_L2_MSGS_PER_CHECKPOINT);
   return new Fr(computeBalancedShaRoot(l1ToL2Messages.map(msg => msg.toBuffer())));
 }
diff --git a/yarn-project/stdlib/src/messaging/inbox_leaf.ts b/yarn-project/stdlib/src/messaging/inbox_leaf.ts
index 21d703d06150..397c29a07efa 100644
--- a/yarn-project/stdlib/src/messaging/inbox_leaf.ts
+++ b/yarn-project/stdlib/src/messaging/inbox_leaf.ts
@@ -1,4 +1,4 @@
-import { INITIAL_CHECKPOINT_NUMBER, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP } from '@aztec/constants';
+import { INITIAL_CHECKPOINT_NUMBER, MAX_L1_TO_L2_MSGS_PER_CHECKPOINT } from '@aztec/constants';
 import { toBigIntBE } from '@aztec/foundation/bigint-buffer';
 import { CheckpointNumber } from '@aztec/foundation/branded-types';
 import { Fr } from '@aztec/foundation/curves/bn254';
@@ -24,7 +24,7 @@ export class InboxLeaf {
   }
 
   static smallestIndexForCheckpoint(checkpointNumber: CheckpointNumber): bigint {
-    return BigInt(checkpointNumber - INITIAL_CHECKPOINT_NUMBER) * BigInt(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP);
+    return BigInt(checkpointNumber - INITIAL_CHECKPOINT_NUMBER) * BigInt(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT);
   }
 
   /**
@@ -33,12 +33,12 @@ export class InboxLeaf {
    */
   static indexRangeForCheckpoint(checkpointNumber: CheckpointNumber): [bigint, bigint] {
     const start = this.smallestIndexForCheckpoint(checkpointNumber);
-    const end = start + BigInt(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP);
+    const end = start + BigInt(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT);
     return [start, end];
   }
 
   /** Returns the checkpoint number for a given leaf index */
   static checkpointNumberFromIndex(index: bigint): CheckpointNumber {
-    return CheckpointNumber(Number(index / BigInt(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP)) + INITIAL_CHECKPOINT_NUMBER);
+    return CheckpointNumber(Number(index / BigInt(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT)) + INITIAL_CHECKPOINT_NUMBER);
   }
 }
diff --git a/yarn-project/stdlib/src/p2p/checkpoint_proposal.ts b/yarn-project/stdlib/src/p2p/checkpoint_proposal.ts
index 126e24b56dc4..4c5f0f402927 100644
--- a/yarn-project/stdlib/src/p2p/checkpoint_proposal.ts
+++ b/yarn-project/stdlib/src/p2p/checkpoint_proposal.ts
@@ -110,14 +110,8 @@ export class CheckpointProposal extends Gossipable implements Signable {
   ) {
     super();
 
-    // Check that last block properties match those of the checkpoint.
-    if (lastBlock && 'inHash' in lastBlock && !lastBlock.inHash.equals(checkpointHeader.inHash)) {
-      throw new Error(
-        `CheckpointProposal lastBlock inHash ${lastBlock.inHash} does not match checkpoint inHash ${checkpointHeader.inHash}`,
-      );
-    }
-    // The last block's bucket reference (AZIP-22 Fast Inbox) commits to the same rolling hash as the checkpoint header,
-    // mirroring the inHash cross-check above. Optional pre-flip: only enforced when the reference is set.
+    // Check that last block properties match those of the checkpoint. The last block's bucket reference (AZIP-22 Fast
+    // Inbox) commits to the same rolling hash as the checkpoint header. Only enforced when the reference is set.
     if (lastBlock?.bucketRef && !lastBlock.bucketRef.inboxRollingHash.equals(checkpointHeader.inboxRollingHash)) {
       throw new Error(
         `CheckpointProposal lastBlock bucketRef rolling hash ${lastBlock.bucketRef.inboxRollingHash} does not match checkpoint inboxRollingHash ${checkpointHeader.inboxRollingHash}`,
@@ -147,7 +141,6 @@ export class CheckpointProposal extends Gossipable implements Signable {
 
   /**
    * Extract a BlockProposal from the last block info.
-   * Uses inHash from checkpointHeader.contentCommitment.inHash
    */
   getBlockProposal(): BlockProposal | undefined {
     if (!this.lastBlock) {
@@ -157,7 +150,7 @@ export class CheckpointProposal extends Gossipable implements Signable {
     return new BlockProposal(
       this.lastBlock.blockHeader,
       this.lastBlock.indexWithinCheckpoint,
-      this.checkpointHeader.inHash,
+      Fr.ZERO,
       this.archive,
       this.lastBlock.txHashes,
       this.lastBlock.signature,
diff --git a/yarn-project/stdlib/src/p2p/wire_compat_fixtures.ts b/yarn-project/stdlib/src/p2p/wire_compat_fixtures.ts
index 61f5720243ce..ca38bc7724f0 100644
--- a/yarn-project/stdlib/src/p2p/wire_compat_fixtures.ts
+++ b/yarn-project/stdlib/src/p2p/wire_compat_fixtures.ts
@@ -1,13 +1,14 @@
-// Golden wire fixtures captured from the pre-Fast-Inbox serialization format (AZIP-22, A-1381). They pin the exact
-// bytes a legacy peer produces and consumes, so the optional bucket-reference tail added to proposals stays wire
-// compatible: an unset proposal must serialize to these bytes, and decoding these bytes must yield no bucket reference.
+// Golden wire fixtures that pin the exact bytes a peer produces and consumes, so the optional bucket-reference tail
+// added to proposals stays wire compatible: an unset proposal must serialize to these bytes, and decoding these bytes
+// must yield no bucket reference. The block-proposal fixtures predate the bucket-reference change (AZIP-22, A-1381);
+// the checkpoint-proposal fixture was refreshed for the checkpoint-header format change that dropped `inHash`.
 //
 // Both fixtures come from a deterministic proposal built with:
 //   BlockHeader.empty(), IndexWithinCheckpoint(3), inHash=Fr(42), archiveRoot=Fr(99),
 //   txHashes=[TxHash.fromField(Fr(7)), TxHash.fromField(Fr(8))], Signature.empty(), EMPTY_COORDINATION_SIGNATURE_CONTEXT
 // (checkpoint: CheckpointHeader.empty(), archive=Fr(123), feeAssetPriceModifier=0, empty signature/context, and a
 // lastBlock with BlockHeader.empty(), IndexWithinCheckpoint(4), txHashes=[TxHash.fromField(Fr(7))], empty signature).
-// DO NOT regenerate from current code — the whole point is that these bytes predate the change.
+// Regenerate the checkpoint fixture only when the checkpoint serialization format changes on purpose.
 
 /** Legacy `BlockProposal.toBuffer()` bytes (no signedTxs, no bucket reference). */
 export const LEGACY_BLOCK_PROPOSAL_HEX =
@@ -15,7 +16,7 @@ export const LEGACY_BLOCK_PROPOSAL_HEX =
 
 /** Legacy `CheckpointProposal.toBuffer()` bytes (lastBlock without signedTxs or bucket reference). */
 export const LEGACY_CHECKPOINT_PROPOSAL_HEX =
-  '0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000007b0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000700000000';
+  '0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000007b0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000700000000';
 
 /** Legacy `BlockProposal.getPayloadToSign()` bytes (no bucket reference). */
 export const LEGACY_BLOCK_PROPOSAL_PAYLOAD_HEX =
diff --git a/yarn-project/stdlib/src/parity/inbox_parity_private_inputs.ts b/yarn-project/stdlib/src/parity/inbox_parity_private_inputs.ts
index 70639ab9fa16..fb799887d425 100644
--- a/yarn-project/stdlib/src/parity/inbox_parity_private_inputs.ts
+++ b/yarn-project/stdlib/src/parity/inbox_parity_private_inputs.ts
@@ -43,8 +43,6 @@ export class InboxParityPrivateInputs {
     public readonly startRollingHash: Fr,
     /** Message-bundle sponge before this checkpoint's messages (empty at checkpoint start). */
     public readonly startSponge: L1ToL2MessageSponge,
-    /** The L1 `in_hash` (sha256 frontier root), passed through unconstrained by the circuit. */
-    public readonly inHash: Fr,
     /** Root of the VK tree. */
     public readonly vkTreeRoot: Fr,
     /** Prover identity committed to by the circuit, for sybil protection. */
@@ -63,7 +61,6 @@ export class InboxParityPrivateInputs {
     messages: Fr[],
     startRollingHash: Fr,
     startSponge: L1ToL2MessageSponge,
-    inHash: Fr,
     vkTreeRoot: Fr,
     proverId: Fr,
   ): InboxParityPrivateInputs {
@@ -75,7 +72,6 @@ export class InboxParityPrivateInputs {
       messages.length,
       startRollingHash,
       startSponge,
-      inHash,
       vkTreeRoot,
       proverId,
     );
@@ -89,7 +85,6 @@ export class InboxParityPrivateInputs {
       new Fr(this.numMessages),
       this.startRollingHash,
       this.startSponge,
-      this.inHash,
       this.vkTreeRoot,
       this.proverId,
     );
@@ -117,7 +112,6 @@ export class InboxParityPrivateInputs {
       reader.readObject(L1ToL2MessageSponge),
       Fr.fromBuffer(reader),
       Fr.fromBuffer(reader),
-      Fr.fromBuffer(reader),
     );
   }
 
diff --git a/yarn-project/stdlib/src/parity/parity_public_inputs.ts b/yarn-project/stdlib/src/parity/parity_public_inputs.ts
index 40a9d12b48fd..2c07ab721ad1 100644
--- a/yarn-project/stdlib/src/parity/parity_public_inputs.ts
+++ b/yarn-project/stdlib/src/parity/parity_public_inputs.ts
@@ -8,11 +8,6 @@ import { L1ToL2MessageSponge } from '../messaging/l1_to_l2_message_sponge.js';
 
 export class ParityPublicInputs {
   constructor(
-    /**
-     * The L1 `in_hash` (sha256 frontier root of the checkpoint's messages). Unconstrained pass-through: InboxParity
-     * echoes the value the prover supplies; it stays the authoritative L1 check until the Fast Inbox flip.
-     */
-    public inHash: Fr,
     /** Inbox rolling hash before absorbing this checkpoint's messages. */
     public startRollingHash: Fr,
     /** Inbox rolling hash after absorbing the `numMsgs` real messages. */
@@ -27,11 +22,7 @@ export class ParityPublicInputs {
     public vkTreeRoot: Fr,
     /** Prover identity committed to by the circuit, for sybil protection. */
     public proverId: Fr,
-  ) {
-    if (inHash.toBuffer()[0] != 0) {
-      throw new Error(`inHash buffer must be 31 bytes. Got 32 bytes`);
-    }
-  }
+  ) {}
 
   /**
    * Serializes the inputs to a buffer.
@@ -39,7 +30,6 @@ export class ParityPublicInputs {
    */
   toBuffer() {
     return serializeToBuffer(
-      this.inHash,
       this.startRollingHash,
       this.endRollingHash,
       this.startSponge,
@@ -79,7 +69,6 @@ export class ParityPublicInputs {
    */
   static getFields(fields: FieldsOf<ParityPublicInputs>) {
     return [
-      fields.inHash,
       fields.startRollingHash,
       fields.endRollingHash,
       fields.startSponge,
@@ -98,7 +87,6 @@ export class ParityPublicInputs {
   static fromBuffer(buffer: Buffer | BufferReader) {
     const reader = BufferReader.asReader(buffer);
     return new ParityPublicInputs(
-      reader.readObject(Fr),
       reader.readObject(Fr),
       reader.readObject(Fr),
       reader.readObject(L1ToL2MessageSponge),
diff --git a/yarn-project/stdlib/src/rollup/checkpoint_header.test.ts b/yarn-project/stdlib/src/rollup/checkpoint_header.test.ts
index 5c29c4ce8017..764a57fdab9c 100644
--- a/yarn-project/stdlib/src/rollup/checkpoint_header.test.ts
+++ b/yarn-project/stdlib/src/rollup/checkpoint_header.test.ts
@@ -22,7 +22,7 @@ describe('CheckpointHeader', () => {
     const header = CheckpointHeader.empty();
     const hash = header.hash().toString();
 
-    expect(hash).toMatchInlineSnapshot(`"0x0008c3e5bbea4fba57201a69d4bf70a0d255df921fdef4924c22da087f9338c2"`);
+    expect(hash).toMatchInlineSnapshot(`"0x002e384af86a480f952aa16443fd29646a9063865e62d7c403fc7ed697bb7712"`);
 
     // Run with AZTEC_GENERATE_TEST_DATA=1 to update noir test data
     updateInlineTestData(
@@ -37,7 +37,6 @@ describe('CheckpointHeader', () => {
       lastArchiveRoot: new Fr(123),
       blockHeadersHash: new Fr(456),
       blobsHash: new Fr(77),
-      inHash: new Fr(88),
       inboxRollingHash: new Fr(89),
       epochOutHash: new Fr(99),
       slotNumber: SlotNumber(1234),
@@ -50,7 +49,7 @@ describe('CheckpointHeader', () => {
     });
     const hash = header.hash().toString();
 
-    expect(hash).toMatchInlineSnapshot(`"0x00519b87177a8a5e4edd03f4d820aec6d402497ef1ab70e2ecd4d4c39b339611"`);
+    expect(hash).toMatchInlineSnapshot(`"0x00751391e842cd7b2014478255dd3309df86327197a0feb03f0af1b758f62ba5"`);
 
     // Run with AZTEC_GENERATE_TEST_DATA=1 to update noir test data
     updateInlineTestData(
@@ -65,7 +64,6 @@ describe('CheckpointHeader', () => {
       lastArchiveRoot: new Fr(MAX_FIELD_VALUE - 123n),
       blockHeadersHash: new Fr(MAX_FIELD_VALUE - 456n),
       blobsHash: new Fr(MAX_FIELD_VALUE - 77n),
-      inHash: new Fr(MAX_FIELD_VALUE - 88n),
       inboxRollingHash: new Fr(MAX_FIELD_VALUE - 89n),
       epochOutHash: new Fr(MAX_FIELD_VALUE - 99n),
       slotNumber: SlotNumber(1234),
@@ -81,7 +79,7 @@ describe('CheckpointHeader', () => {
 
     const hash = header.hash().toString();
 
-    expect(hash).toMatchInlineSnapshot(`"0x00d64307fa93c32ae46a4c3b6a1911d31994daa8d11d50201f0086a6cbaa9bac"`);
+    expect(hash).toMatchInlineSnapshot(`"0x005bd09725c6e77a4a28a7ccdaf7875ba5882431ca3c82e62db96e8a12769ce5"`);
 
     // Run with AZTEC_GENERATE_TEST_DATA=1 to update noir test data
     updateInlineTestData(
diff --git a/yarn-project/stdlib/src/rollup/checkpoint_header.ts b/yarn-project/stdlib/src/rollup/checkpoint_header.ts
index 32cbc58874e0..bd59f7746a97 100644
--- a/yarn-project/stdlib/src/rollup/checkpoint_header.ts
+++ b/yarn-project/stdlib/src/rollup/checkpoint_header.ts
@@ -30,12 +30,10 @@ export class CheckpointHeader {
     public blockHeadersHash: Fr,
     /** Hash of the blobs in the checkpoint. */
     public blobsHash: Fr,
-    /** Root of the l1 to l2 messages subtree. */
-    public inHash: Fr,
     /**
-     * Inbox rolling-hash chain value after consuming all L1-to-L2 messages bundled into this checkpoint. The dual of
-     * `inHash` (AZIP-22 Fast Inbox): the truncated-to-field sha256 chain the L1 Inbox accumulates. Currently a
-     * pass-through commitment; `inHash` remains the authoritative L1 check until the Fast Inbox flip.
+     * Inbox rolling-hash chain value after consuming all L1-to-L2 messages bundled into this checkpoint (AZIP-22 Fast
+     * Inbox): the truncated-to-field sha256 chain the L1 Inbox accumulates. This is the checkpoint's only inbox
+     * commitment.
      */
     public inboxRollingHash: Fr,
     /**
@@ -68,7 +66,6 @@ export class CheckpointHeader {
         lastArchiveRoot: schemas.Fr,
         blockHeadersHash: schemas.Fr,
         blobsHash: schemas.Fr,
-        inHash: schemas.Fr,
         inboxRollingHash: schemas.Fr,
         epochOutHash: schemas.Fr,
         slotNumber: schemas.SlotNumber,
@@ -87,7 +84,6 @@ export class CheckpointHeader {
       fields.lastArchiveRoot,
       fields.blockHeadersHash,
       fields.blobsHash,
-      fields.inHash,
       fields.inboxRollingHash,
       fields.epochOutHash,
       fields.slotNumber,
@@ -113,7 +109,6 @@ export class CheckpointHeader {
       reader.readObject(Fr),
       reader.readObject(Fr),
       reader.readObject(Fr),
-      reader.readObject(Fr),
       SlotNumber(Fr.fromBuffer(reader).toNumber()),
       reader.readUInt64(),
       reader.readObject(EthAddress),
@@ -129,7 +124,6 @@ export class CheckpointHeader {
       this.lastArchiveRoot.equals(other.lastArchiveRoot) &&
       this.blockHeadersHash.equals(other.blockHeadersHash) &&
       this.blobsHash.equals(other.blobsHash) &&
-      this.inHash.equals(other.inHash) &&
       this.inboxRollingHash.equals(other.inboxRollingHash) &&
       this.epochOutHash.equals(other.epochOutHash) &&
       this.slotNumber === other.slotNumber &&
@@ -159,7 +153,6 @@ export class CheckpointHeader {
       this.lastArchiveRoot,
       this.blockHeadersHash,
       this.blobsHash,
-      this.inHash,
       this.inboxRollingHash,
       this.epochOutHash,
       new Fr(this.slotNumber),
@@ -181,7 +174,6 @@ export class CheckpointHeader {
       lastArchiveRoot: Fr.ZERO,
       blockHeadersHash: Fr.ZERO,
       blobsHash: Fr.ZERO,
-      inHash: Fr.ZERO,
       inboxRollingHash: Fr.ZERO,
       epochOutHash: Fr.ZERO,
       slotNumber: SlotNumber.ZERO,
@@ -200,7 +192,6 @@ export class CheckpointHeader {
       lastArchiveRoot: Fr.random(),
       blockHeadersHash: Fr.random(),
       blobsHash: Fr.random(),
-      inHash: Fr.random(),
       inboxRollingHash: Fr.random(),
       epochOutHash: Fr.random(),
       slotNumber: SlotNumber(Math.floor(Math.random() * 1000) + 1),
@@ -219,7 +210,6 @@ export class CheckpointHeader {
       this.lastArchiveRoot.isZero() &&
       this.blockHeadersHash.isZero() &&
       this.blobsHash.isZero() &&
-      this.inHash.isZero() &&
       this.inboxRollingHash.isZero() &&
       this.epochOutHash.isZero() &&
       this.slotNumber === 0 &&
@@ -249,7 +239,6 @@ export class CheckpointHeader {
       Fr.fromString(header.lastArchiveRoot),
       Fr.fromString(header.blockHeadersHash),
       Fr.fromString(header.blobsHash),
-      Fr.fromString(header.inHash),
       Fr.fromString(header.inboxRollingHash),
       Fr.fromString(header.outHash),
       SlotNumber.fromBigInt(header.slotNumber),
@@ -275,7 +264,6 @@ export class CheckpointHeader {
       lastArchiveRoot: this.lastArchiveRoot.toString(),
       blockHeadersHash: this.blockHeadersHash.toString(),
       blobsHash: this.blobsHash.toString(),
-      inHash: this.inHash.toString(),
       inboxRollingHash: this.inboxRollingHash.toString(),
       outHash: this.epochOutHash.toString(),
       slotNumber: BigInt(this.slotNumber),
@@ -296,7 +284,6 @@ export class CheckpointHeader {
       lastArchive: this.lastArchiveRoot.toString(),
       blockHeadersHash: this.blockHeadersHash.toString(),
       blobsHash: this.blobsHash.toString(),
-      inHash: this.inHash.toString(),
       inboxRollingHash: this.inboxRollingHash.toString(),
       epochOutHash: this.epochOutHash.toString(),
       slotNumber: this.slotNumber,
@@ -314,7 +301,6 @@ export class CheckpointHeader {
   lastArchiveRoot: ${this.lastArchiveRoot.toString()},
   blockHeadersHash: ${this.blockHeadersHash.toString()},
   blobsHash: ${inspect(this.blobsHash)},
-  inHash: ${inspect(this.inHash)},
   inboxRollingHash: ${inspect(this.inboxRollingHash)},
   epochOutHash: ${inspect(this.epochOutHash)},
   slotNumber: ${this.slotNumber},
diff --git a/yarn-project/stdlib/src/rollup/checkpoint_root_rollup_private_inputs.ts b/yarn-project/stdlib/src/rollup/checkpoint_root_rollup_private_inputs.ts
index 7838c30ee375..01f908d77221 100644
--- a/yarn-project/stdlib/src/rollup/checkpoint_root_rollup_private_inputs.ts
+++ b/yarn-project/stdlib/src/rollup/checkpoint_root_rollup_private_inputs.ts
@@ -119,8 +119,8 @@ export class CheckpointRootRollupPrivateInputs {
       RollupHonkProofData<BlockRollupPublicInputs>,
     ],
     /**
-     * Inbox parity proof over the checkpoint's L1-to-L2 messages (AZIP-22 Fast Inbox): it commits to the same message
-     * list behind the L1-checked `inHash`, and its message sponge is checked against the blocks' accumulated one.
+     * Inbox parity proof over the checkpoint's L1-to-L2 messages (AZIP-22 Fast Inbox): it commits to the checkpoint's
+     * message list, and its message sponge is checked against the blocks' accumulated one.
      */
     public inboxParity: UltraHonkProofData<ParityPublicInputs>,
     public hints: CheckpointRootRollupHints,
@@ -168,8 +168,8 @@ export class CheckpointRootSingleBlockRollupPrivateInputs {
   constructor(
     public previousRollup: RollupHonkProofData<BlockRollupPublicInputs>,
     /**
-     * Inbox parity proof over the checkpoint's L1-to-L2 messages (AZIP-22 Fast Inbox): it commits to the same message
-     * list behind the L1-checked `inHash`, and its message sponge is checked against the block's accumulated one.
+     * Inbox parity proof over the checkpoint's L1-to-L2 messages (AZIP-22 Fast Inbox): it commits to the checkpoint's
+     * message list, and its message sponge is checked against the block's accumulated one.
      */
     public inboxParity: UltraHonkProofData<ParityPublicInputs>,
     public hints: CheckpointRootRollupHints,
diff --git a/yarn-project/stdlib/src/tests/factories.ts b/yarn-project/stdlib/src/tests/factories.ts
index 06f5fcdd5e00..20b8ca8a0af0 100644
--- a/yarn-project/stdlib/src/tests/factories.ts
+++ b/yarn-project/stdlib/src/tests/factories.ts
@@ -17,6 +17,7 @@ import {
   MAX_ENQUEUED_CALLS_PER_TX,
   MAX_KEY_VALIDATION_REQUESTS_PER_CALL,
   MAX_L1_TO_L2_MSGS_PER_BLOCK,
+  MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   MAX_L2_TO_L1_MSGS_PER_CALL,
   MAX_L2_TO_L1_MSGS_PER_TX,
   MAX_NOTE_HASHES_PER_CALL,
@@ -36,7 +37,6 @@ import {
   NOTE_HASH_SUBTREE_ROOT_SIBLING_PATH_LENGTH,
   NULLIFIER_SUBTREE_ROOT_SIBLING_PATH_LENGTH,
   NULLIFIER_TREE_HEIGHT,
-  NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
   PRIVATE_LOG_SIZE_IN_FIELDS,
   PUBLIC_DATA_TREE_HEIGHT,
   RECURSIVE_ROLLUP_HONK_PROOF_LENGTH,
@@ -821,7 +821,6 @@ export function makeCheckpointRollupPublicInputs(seed = 0) {
 
 export function makeParityPublicInputs(seed = 0): ParityPublicInputs {
   return new ParityPublicInputs(
-    new Fr(BigInt(seed + 0x200)),
     new Fr(BigInt(seed + 0x400)),
     new Fr(BigInt(seed + 0x500)),
     makeL1ToL2MessageSponge(seed + 0x550),
@@ -842,7 +841,6 @@ export function makeInboxParityPrivateInputs(seed = 0): InboxParityPrivateInputs
     numMsgs,
     new Fr(seed + 0x3500),
     makeL1ToL2MessageSponge(seed + 0x3800),
-    new Fr(seed + 0x3900),
     new Fr(seed + 0x4000),
     new Fr(seed + 0x5000),
   );
@@ -888,7 +886,6 @@ export function makeCheckpointHeader(seed = 0, overrides: Partial<FieldsOf<Check
     lastArchiveRoot: fr(seed + 0x100),
     blockHeadersHash: fr(seed + 0x150),
     blobsHash: fr(seed + 0x200),
-    inHash: fr(seed + 0x210),
     inboxRollingHash: fr(seed + 0x215),
     epochOutHash: fr(seed + 0x220),
     slotNumber: SlotNumber(seed + 0x300),
@@ -909,14 +906,14 @@ export function makeCheckpointHeader(seed = 0, overrides: Partial<FieldsOf<Check
  */
 export function makeStateReference(seed = 0): StateReference {
   return new StateReference(
-    makeAppendOnlyTreeSnapshot(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP * seed),
+    makeAppendOnlyTreeSnapshot(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT * seed),
     makePartialStateReference(seed + 1),
   );
 }
 
 function makeTreeSnapshots(seed = 0) {
   return new TreeSnapshots(
-    makeAppendOnlyTreeSnapshot(seed * NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP),
+    makeAppendOnlyTreeSnapshot(seed * MAX_L1_TO_L2_MSGS_PER_CHECKPOINT),
     makeAppendOnlyTreeSnapshot((seed + 0x10) * MAX_NOTE_HASHES_PER_TX),
     makeAppendOnlyTreeSnapshot((seed + 0x20) * MAX_NULLIFIERS_PER_TX),
     makeAppendOnlyTreeSnapshot((seed + 0x30) * MAX_PUBLIC_DATA_UPDATE_REQUESTS_PER_TX),
diff --git a/yarn-project/stdlib/src/tests/mocks.ts b/yarn-project/stdlib/src/tests/mocks.ts
index 6a7df40b724c..8c3f6a990aa0 100644
--- a/yarn-project/stdlib/src/tests/mocks.ts
+++ b/yarn-project/stdlib/src/tests/mocks.ts
@@ -50,7 +50,6 @@ import {
 import { PrivateToAvmAccumulatedData } from '../kernel/private_to_avm_accumulated_data.js';
 import { PrivateToPublicAccumulatedDataBuilder } from '../kernel/private_to_public_accumulated_data_builder.js';
 import { PublicCallRequestArrayLengths } from '../kernel/public_call_request.js';
-import { computeInHashFromL1ToL2Messages } from '../messaging/in_hash.js';
 import { InboxBucketRef } from '../messaging/inbox_bucket.js';
 import { BlockProposal } from '../p2p/block_proposal.js';
 import { CheckpointAttestation } from '../p2p/checkpoint_attestation.js';
@@ -477,11 +476,9 @@ export async function mockCheckpointAndMessages(
   }
 
   const messages = blocksAndMessages[0].messages;
-  const inHash = computeInHashFromL1ToL2Messages(messages);
   const firstBlockLastArchive = blocksAndMessages[0].block.header.lastArchive;
   const checkpoint = await Checkpoint.random(checkpointNumber, {
     numBlocks: 0,
-    inHash,
     ...options,
     ...globals,
     lastArchive: firstBlockLastArchive,
@@ -633,7 +630,7 @@ export const makeCheckpointProposal = async (options?: MakeCheckpointProposalOpt
     ? await makeBlockProposal({
         blockHeader: options.lastBlock.blockHeader,
         indexWithinCheckpoint: options.lastBlock.indexWithinCheckpoint ?? IndexWithinCheckpoint(4),
-        inHash: checkpointHeader.inHash,
+        inHash: Fr.ZERO,
         archiveRoot,
         txHashes: options.lastBlock.txHashes,
         txs: options.lastBlock.txs,
diff --git a/yarn-project/txe/src/state_machine/index.ts b/yarn-project/txe/src/state_machine/index.ts
index c79587e921c4..94caba792da6 100644
--- a/yarn-project/txe/src/state_machine/index.ts
+++ b/yarn-project/txe/src/state_machine/index.ts
@@ -114,7 +114,6 @@ export class TXEStateMachine {
       block.archive,
       CheckpointHeader.from({
         lastArchiveRoot: block.header.lastArchive.root,
-        inHash: Fr.ZERO,
         inboxRollingHash: Fr.ZERO,
         blobsHash: Fr.ZERO,
         blockHeadersHash: Fr.ZERO,
diff --git a/yarn-project/validator-client/src/duties/validation_service.test.ts b/yarn-project/validator-client/src/duties/validation_service.test.ts
index 1cecf73317ac..d78c67628e4d 100644
--- a/yarn-project/validator-client/src/duties/validation_service.test.ts
+++ b/yarn-project/validator-client/src/duties/validation_service.test.ts
@@ -89,13 +89,13 @@ describe('ValidationService', () => {
     const archive = Fr.random();
     const checkpointHeader = makeCheckpointHeader(1);
 
-    // Create the block proposal first (as the sequencer would), using the checkpoint's inHash
-    // so that getSender() can verify the block proposal sender matches
+    // Create the block proposal first (as the sequencer would) so that getSender() can verify the block proposal
+    // sender matches. The block-level inHash is dead post-flip (AZIP-22 Fast Inbox), so it carries zero.
     const blockProposal = await service.createBlockProposal(
       blockHeader,
       CheckpointNumber(1),
       indexWithinCheckpoint,
-      checkpointHeader.inHash,
+      Fr.ZERO,
       archive,
       txs,
       addresses[0],
diff --git a/yarn-project/validator-client/src/validator.test.ts b/yarn-project/validator-client/src/validator.test.ts
index 1af680ff3218..0a0ae849ea4a 100644
--- a/yarn-project/validator-client/src/validator.test.ts
+++ b/yarn-project/validator-client/src/validator.test.ts
@@ -593,7 +593,7 @@ describe('ValidatorClient', () => {
       const emptyInHash = computeInHashFromL1ToL2Messages([]);
       const checkpointProposal = await makeCheckpointProposal({
         signer,
-        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber, inHash: emptyInHash }),
+        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber }),
         archiveRoot: Fr.random(),
         lastBlock: {
           blockHeader: makeBlockHeader(1, { blockNumber, slotNumber: proposal.slotNumber }),
@@ -669,9 +669,8 @@ describe('ValidatorClient', () => {
       validatorClient.updateConfig({ skipPushProposedBlocksToArchiver: false });
       validatorClient.getProposalHandler().register(p2pClient, true);
 
-      const emptyInHash = computeInHashFromL1ToL2Messages([]);
       const checkpointProposal = await makeCheckpointProposal({
-        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber, inHash: emptyInHash }),
+        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber }),
         archiveRoot: Fr.random(),
         lastBlock: {
           blockHeader: makeBlockHeader(1, { blockNumber, slotNumber: proposal.slotNumber }),
@@ -680,7 +679,7 @@ describe('ValidatorClient', () => {
         },
       });
       const equivocatedCheckpointProposal = await makeCheckpointProposal({
-        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber, inHash: emptyInHash }),
+        checkpointHeader: makeCheckpointHeader(1, { slotNumber: proposal.slotNumber }),
         archiveRoot: Fr.random(),
         lastBlock: {
           blockHeader: makeBlockHeader(1, { blockNumber, slotNumber: proposal.slotNumber }),
diff --git a/yarn-project/world-state/src/native/native_world_state.test.ts b/yarn-project/world-state/src/native/native_world_state.test.ts
index aeceac469d4e..e205170c8676 100644
--- a/yarn-project/world-state/src/native/native_world_state.test.ts
+++ b/yarn-project/world-state/src/native/native_world_state.test.ts
@@ -1,12 +1,12 @@
 import {
   ARCHIVE_HEIGHT,
   L1_TO_L2_MSG_TREE_HEIGHT,
+  MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   MAX_NOTE_HASHES_PER_TX,
   MAX_NULLIFIERS_PER_TX,
   MAX_PUBLIC_DATA_UPDATE_REQUESTS_PER_TX,
   NOTE_HASH_TREE_HEIGHT,
   NULLIFIER_TREE_HEIGHT,
-  NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
   PUBLIC_DATA_TREE_HEIGHT,
 } from '@aztec/constants';
 import { BlockNumber, SlotNumber } from '@aztec/foundation/branded-types';
@@ -170,7 +170,7 @@ describe('NativeWorldState', () => {
 
       const status = await ws.handleL2BlockAndMessages(block, messages);
 
-      // Non-first blocks append their bundle exactly as given (no padding to NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP).
+      // Non-first blocks append their bundle exactly as given (no padding to MAX_L1_TO_L2_MSGS_PER_CHECKPOINT).
       expect(status.meta.messageTreeMeta.size).toBe(BigInt(numMessages));
     });
   });
@@ -1508,8 +1508,8 @@ describe('NativeWorldState', () => {
 
         expect(status.meta.messageTreeMeta).toMatchObject({
           depth: L1_TO_L2_MSG_TREE_HEIGHT,
-          size: BigInt(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP * (i + 1)),
-          committedSize: BigInt(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP * (i + 1)),
+          size: BigInt(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT * (i + 1)),
+          committedSize: BigInt(MAX_L1_TO_L2_MSGS_PER_CHECKPOINT * (i + 1)),
           initialSize: BigInt(0),
           oldestHistoricBlock: 1,
           unfinalizedBlockHeight: i + 1,
diff --git a/yarn-project/world-state/src/test/utils.ts b/yarn-project/world-state/src/test/utils.ts
index 8870091b3951..4ce5c907bc44 100644
--- a/yarn-project/world-state/src/test/utils.ts
+++ b/yarn-project/world-state/src/test/utils.ts
@@ -1,8 +1,8 @@
 import {
+  MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   MAX_NOTE_HASHES_PER_TX,
   MAX_NULLIFIERS_PER_TX,
   NULLIFIER_SUBTREE_HEIGHT,
-  NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
 } from '@aztec/constants';
 import { asyncMap } from '@aztec/foundation/async-map';
 import { BlockNumber, type CheckpointNumber, IndexWithinCheckpoint } from '@aztec/foundation/branded-types';
@@ -78,7 +78,7 @@ export function mockBlock(
   size: number,
   fork: MerkleTreeWriteOperations,
   maxEffects: number | undefined = 1000, // Defaults to the maximum tx effects.
-  numL1ToL2Messages: number = NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
+  numL1ToL2Messages: number = MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   isFirstBlockInCheckpoint: boolean = true,
 ) {
   return mockBlockWithIndex(blockNum, isFirstBlockInCheckpoint ? 0 : 1, size, fork, numL1ToL2Messages, maxEffects);
@@ -94,7 +94,7 @@ export async function mockBlockWithIndex(
   indexWithinCheckpoint: number,
   size: number,
   fork: MerkleTreeWriteOperations,
-  numL1ToL2Messages: number = NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
+  numL1ToL2Messages: number = MAX_L1_TO_L2_MSGS_PER_CHECKPOINT,
   maxEffects: number | undefined = 1000, // Defaults to the maximum tx effects.
 ) {
   const block = await L2Block.random(blockNum, {
diff --git a/yarn-project/world-state/src/world-state-db/merkle_tree_db.ts b/yarn-project/world-state/src/world-state-db/merkle_tree_db.ts
index 1dabb0aacd30..f1cb5cb06f70 100644
--- a/yarn-project/world-state/src/world-state-db/merkle_tree_db.ts
+++ b/yarn-project/world-state/src/world-state-db/merkle_tree_db.ts
@@ -34,7 +34,7 @@ export interface MerkleTreeAdminDatabase extends ForkMerkleTreeOperations, Reado
    * Handles a single L2 block: inserts its note hashes, nullifiers, public data writes, and the block's L1-to-L2
    * message bundle into the merkle trees. Any block may carry a message bundle and transition the L1-to-L2 message
    * tree, not just the first block of a checkpoint. A first-in-checkpoint bundle is padded to
-   * NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP to match how the circuits build the tree; a non-first bundle is appended
+   * MAX_L1_TO_L2_MSGS_PER_CHECKPOINT to match how the circuits build the tree; a non-first bundle is appended
    * exactly as given. Padding is a transitional concern of this method that moves entirely to the caller at the flip.
    * @param block - The L2 block to handle.
    * @param l1ToL2Messages - The L1 to L2 messages for the block.