diff --git a/yarn-project/end-to-end/src/automine/delivery/interactive_handshake_responder.ts b/yarn-project/end-to-end/src/automine/delivery/interactive_handshake_responder.ts
new file mode 100644
index 000000000000..e46d0fa2b4d5
--- /dev/null
+++ b/yarn-project/end-to-end/src/automine/delivery/interactive_handshake_responder.ts
@@ -0,0 +1,125 @@
+import { AztecAddress, type CompleteAddress } from '@aztec/aztec.js/addresses';
+import { DomainSeparator } from '@aztec/constants';
+import { poseidon2HashWithSeparator } from '@aztec/foundation/crypto/poseidon';
+import { Schnorr, type SchnorrSignature } from '@aztec/foundation/crypto/schnorr';
+import { Fq, Fr } from '@aztec/foundation/curves/bn254';
+import type { GrumpkinScalar } from '@aztec/foundation/curves/grumpkin';
+import type { CustomRequest } from '@aztec/pxe/config';
+import {
+  INTERACTIVE_HANDSHAKE_REQUEST_KIND,
+  STANDARD_HANDSHAKE_REGISTRY_ADDRESS,
+} from '@aztec/standard-contracts/handshake-registry/constants';
+import { type PublicKeys, derivePublicKeyFromSecretKey } from '@aztec/stdlib/keys';
+
+/**
+ * The decoded payload of the registry's interactive-handshake signature request. Note it never carries the sender,
+ * so the recipient authorizes the handshake without learning who initiated it.
+ */
+export type InteractiveHandshakeRequest = {
+  /** The account whose authorization is being requested. */
+  recipient: AztecAddress;
+  chainId: Fr;
+  version: Fr;
+  /** The x-coordinate of the handshake's ephemeral public key (its y-coordinate is positive by construction). */
+  ephPkX: Fr;
+};
+
+/**
+ * The recipient's signed authorization for an interactive handshake. Mirrors the registry contract's
+ * `RecipientSignature` struct field for field.
+ */
+export type RecipientSignature = {
+  /** The recipient's master public keys, bound in-circuit to the recipient's address via `partialAddress`. */
+  publicKeys: PublicKeys;
+  partialAddress: Fr;
+  /** The x-coordinate of the recipient's master message-signing public key. */
+  mspkX: Fr;
+  /** Whether that key's y-coordinate is positive, so the circuit can reconstruct the point from `mspkX`. */
+  mspkYIsPositive: boolean;
+  /** The schnorr signature over the handshake message. */
+  signature: SchnorrSignature;
+};
+
+/**
+ * Parses and validates the registry's interactive-handshake signature request.
+ *
+ * @throws If the request kind is not {@link INTERACTIVE_HANDSHAKE_REQUEST_KIND}, the issuing contract is not the
+ * standard HandshakeRegistry, or the payload does not have the expected shape.
+ */
+export function parseInteractiveHandshakeRequest(request: CustomRequest): InteractiveHandshakeRequest {
+  if (!request.kind.equals(INTERACTIVE_HANDSHAKE_REQUEST_KIND)) {
+    throw new Error(`Not an interactive-handshake signature request: unexpected kind ${request.kind}`);
+  }
+  if (!request.contractAddress.equals(STANDARD_HANDSHAKE_REGISTRY_ADDRESS)) {
+    throw new Error(
+      `Interactive-handshake signature request issued by ${request.contractAddress}, expected the standard HandshakeRegistry at ${STANDARD_HANDSHAKE_REGISTRY_ADDRESS}`,
+    );
+  }
+  if (request.payload.length !== 4) {
+    throw new Error(`Interactive-handshake signature request payload has ${request.payload.length} fields, expected 4`);
+  }
+
+  const [recipient, chainId, version, ephPkX] = request.payload;
+  return { recipient: new AztecAddress(recipient), chainId, version, ephPkX };
+}
+
+/**
+ * Produces the recipient's signed authorization for an interactive handshake, signing with the master
+ * message-signing secret key.
+ */
+export async function signInteractiveHandshake(
+  request: InteractiveHandshakeRequest,
+  completeAddress: CompleteAddress,
+  masterMessageSigningSecretKey: GrumpkinScalar,
+): Promise<RecipientSignature> {
+  const mspk = await derivePublicKeyFromSecretKey(masterMessageSigningSecretKey);
+  const [mspkX, mspkYIsPositive] = mspk.toXAndSign();
+
+  const message = await computeInteractiveHandshakeSignatureMessage({
+    chainId: request.chainId,
+    version: request.version,
+    registry: STANDARD_HANDSHAKE_REGISTRY_ADDRESS,
+    ephPkX: request.ephPkX,
+  });
+  const signature = await new Schnorr().constructSignature(message, masterMessageSigningSecretKey);
+
+  return {
+    publicKeys: completeAddress.publicKeys,
+    partialAddress: completeAddress.partialAddress,
+    mspkX,
+    mspkYIsPositive,
+    signature,
+  };
+}
+
+/** Serializes a {@link RecipientSignature} to the field layout the registry's in-circuit deserialization expects. */
+export function recipientSignatureToFields(recipientSignature: RecipientSignature): Fr[] {
+  const s = Fq.fromBuffer(recipientSignature.signature.s);
+  const e = Fq.fromBuffer(recipientSignature.signature.e);
+  return [
+    ...recipientSignature.publicKeys.toFields(),
+    recipientSignature.partialAddress,
+    recipientSignature.mspkX,
+    new Fr(recipientSignature.mspkYIsPositive),
+    s.lo,
+    s.hi,
+    e.lo,
+    e.hi,
+  ];
+}
+
+/**
+ * The message an interactive-handshake authorization signs: the handshake's ephemeral key and chain context under
+ * `DomainSeparator.INTERACTIVE_HANDSHAKE_SIGNATURE`, exactly as the registry recomputes it in-circuit.
+ */
+function computeInteractiveHandshakeSignatureMessage(args: {
+  chainId: Fr;
+  version: Fr;
+  registry: AztecAddress;
+  ephPkX: Fr;
+}): Promise<Fr> {
+  return poseidon2HashWithSeparator(
+    [args.chainId, args.version, args.registry, args.ephPkX],
+    DomainSeparator.INTERACTIVE_HANDSHAKE_SIGNATURE,
+  );
+}
diff --git a/yarn-project/end-to-end/src/automine/delivery/onchain.test.ts b/yarn-project/end-to-end/src/automine/delivery/onchain.test.ts
index cdab97abcf31..0e0dfaeb5039 100644
--- a/yarn-project/end-to-end/src/automine/delivery/onchain.test.ts
+++ b/yarn-project/end-to-end/src/automine/delivery/onchain.test.ts
@@ -1,5 +1,15 @@
+import type { InitialAccountData } from '@aztec/accounts/testing';
+import type { CompleteAddress } from '@aztec/aztec.js/addresses';
 import { Point } from '@aztec/foundation/curves/grumpkin';
+import type { ResolveCustomRequest } from '@aztec/pxe/config';
+import { deriveKeys } from '@aztec/stdlib/keys';
 
+import type { TestWallet } from '../../test-wallet/test_wallet.js';
+import {
+  parseInteractiveHandshakeRequest,
+  recipientSignatureToFields,
+  signInteractiveHandshake,
+} from './interactive_handshake_responder.js';
 import { buildMessageDeliveryTest } from './onchain_delivery_harness.js';
 
 describe('onchain delivery', () => {
@@ -34,8 +44,8 @@ describe('onchain delivery', () => {
     },
   });
 
-  // With the recipient registering the sender,
-  // the recipient PXE reconstructs the address-derived tag and discovers the delivery.
+  // With the recipient registering the sender, the recipient PXE reconstructs the address-derived tag
+  // and discovers the delivery.
   buildMessageDeliveryTest({
     strategy: 'address-derived',
     mode: 'unconstrained',
@@ -44,4 +54,47 @@ describe('onchain delivery', () => {
       await recipientWallet.registerSender(senderAddress);
     },
   });
+
+  buildMessageDeliveryTest({
+    strategy: 'interactive handshake',
+    mode: 'constrained',
+    senderHook: () => Promise.resolve({ type: 'interactive-handshake' }),
+    customRequestResponder: interactiveHandshakeResponder,
+  });
+
+  buildMessageDeliveryTest({
+    strategy: 'interactive handshake',
+    mode: 'unconstrained',
+    senderHook: () => Promise.resolve({ type: 'interactive-handshake' }),
+    customRequestResponder: interactiveHandshakeResponder,
+  });
+
+  // Serves the registry's interactive-handshake signature request for the recipient: registers the handshake on the
+  // recipient PXE, then answers with the signed response.
+  function interactiveHandshakeResponder(
+    recipientWallet: TestWallet,
+    recipientAccount: InitialAccountData,
+    recipientCompleteAddress: CompleteAddress,
+  ): ResolveCustomRequest {
+    return async request => {
+      const parsed = parseInteractiveHandshakeRequest(request);
+
+      // Register before signing.
+      await recipientWallet.registerTaggingSecretSource({
+        kind: 'handshake',
+        recipient: parsed.recipient,
+        ephPk: parsed.ephPkX,
+      });
+
+      // The master message-signing secret key is deliberately never held by PXE or the key store; the wallet
+      // derives it client-side from the account secret.
+      const { masterMessageSigningSecretKey } = await deriveKeys(recipientAccount.secret);
+      const recipientSignature = await signInteractiveHandshake(
+        parsed,
+        recipientCompleteAddress,
+        masterMessageSigningSecretKey,
+      );
+      return recipientSignatureToFields(recipientSignature);
+    };
+  }
 });
diff --git a/yarn-project/end-to-end/src/automine/delivery/onchain_delivery_harness.ts b/yarn-project/end-to-end/src/automine/delivery/onchain_delivery_harness.ts
index d10ae9196aea..6e00fca5969e 100644
--- a/yarn-project/end-to-end/src/automine/delivery/onchain_delivery_harness.ts
+++ b/yarn-project/end-to-end/src/automine/delivery/onchain_delivery_harness.ts
@@ -1,11 +1,12 @@
 import { type InitialAccountData, generateSchnorrAccounts } from '@aztec/accounts/testing';
 import type { FieldLike } from '@aztec/aztec.js/abi';
 import { NO_FROM } from '@aztec/aztec.js/account';
-import type { AztecAddress } from '@aztec/aztec.js/addresses';
+import type { AztecAddress, CompleteAddress } from '@aztec/aztec.js/addresses';
 import type { AztecNode } from '@aztec/aztec.js/node';
+import type { AccountManager } from '@aztec/aztec.js/wallet';
 import { BlockNumber } from '@aztec/foundation/branded-types';
 import { type DeliveryEvent, OnchainDeliveryTestContract } from '@aztec/noir-test-contracts.js/OnchainDeliveryTest';
-import type { PXECreationOptions } from '@aztec/pxe/server';
+import type { CustomRequest, ResolveCustomRequest, ResolveTaggingSecretStrategy } from '@aztec/pxe/config';
 import type { AztecNodeDebug } from '@aztec/stdlib/interfaces/client';
 
 import { jest } from '@jest/globals';
@@ -14,9 +15,13 @@ import { AUTOMINE_E2E_OPTS } from '../../fixtures/fixtures.js';
 import { ensureHandshakeRegistryPublished, setup, setupPXEAndGetWallet } from '../../fixtures/setup.js';
 import { TestWallet } from '../../test-wallet/test_wallet.js';
 
-// The wallet hook that selects a message's tagging-secret source. Derived from the exported PXE options
-// rather than importing the hook type, which `@aztec/pxe/server` does not re-export.
-export type SenderHook = NonNullable<NonNullable<PXECreationOptions['hooks']>['resolveTaggingSecretStrategy']>;
+// Builds the hook serving custom requests issued during the sender's simulations. Installed on the sender PXE at
+// creation but built only once the recipient exists, since serving typically needs the recipient's wallet and keys.
+export type CustomRequestResponder = (
+  recipient: TestWallet,
+  recipientAccount: InitialAccountData,
+  recipientCompleteAddress: CompleteAddress,
+) => ResolveCustomRequest;
 
 export type Mode = 'constrained' | 'unconstrained';
 
@@ -40,19 +45,22 @@ export function buildMessageDeliveryTest(opts: {
   strategy: string;
   mode: DeliveryMode;
   // Required: every cell states its source explicitly rather than leaning on the PXE default (covered by unit tests).
-  senderHook: SenderHook;
+  senderHook: ResolveTaggingSecretStrategy;
   // Recipient-side setup the source requires (e.g. registering a raw arbitrary secret); runs once after deployment.
   recipientRegistration?: (
     recipient: TestWallet,
     recipientAddress: AztecAddress,
     sender: AztecAddress,
   ) => Promise<void>;
+  // Serves the custom requests issued during the sender's simulations (e.g. the registry's interactive-handshake
+  // signature request).
+  customRequestResponder?: CustomRequestResponder;
   // Extra `it()`s to register in this cell's suite, e.g. assertions against state a custom `senderHook` recorded.
   // Called inside the same `describe`, after the two baseline assertions below, so it shares their `beforeAll`
   // instead of depending on Jest's cross-`describe` execution order.
   additionalTests?: () => void;
 }) {
-  const { strategy, mode, senderHook, recipientRegistration, additionalTests } = opts;
+  const { strategy, mode, senderHook, recipientRegistration, customRequestResponder, additionalTests } = opts;
   const description = `${strategy} x ${formatMode(mode)}`;
 
   describe(description, () => {
@@ -84,11 +92,14 @@ export function buildMessageDeliveryTest(opts: {
         ? contractSender.methods.emit_note(recipient, value)
         : contractSender.methods.emit_note_unconstrained(recipient, value);
 
+    let additionallyFundedAccounts: InitialAccountData[];
+    let recipientAccount: AccountManager | undefined;
+    let customRequestCount = 0;
+
     beforeAll(async () => {
       // The sender PXE holds the sender and carries this cell's tagging-secret-strategy hook. The recipient is funded
       // at genesis here but created and deployed on the isolated recipient PXE below, so it carries no sender state
       // from other cells.
-      let additionallyFundedAccounts: InitialAccountData[];
       ({
         aztecNode,
         additionallyFundedAccounts,
@@ -98,7 +109,26 @@ export function buildMessageDeliveryTest(opts: {
       } = await setup(1, {
         ...AUTOMINE_E2E_OPTS,
         additionallyFundedAccounts: await generateSchnorrAccounts(1, 'schnorr'),
-        pxeCreationOptions: { hooks: { resolveTaggingSecretStrategy: senderHook } },
+        pxeCreationOptions: {
+          hooks: {
+            resolveTaggingSecretStrategy: senderHook,
+            resolveCustomRequest: async (request: CustomRequest) => {
+              if (!customRequestResponder) {
+                throw new Error('A custom request arrived but this test cell has no customRequestResponder configured');
+              }
+              if (!recipientAccount) {
+                throw new Error('A custom request arrived before the recipient wallet was created');
+              }
+              customRequestCount++;
+              const respond = customRequestResponder(
+                walletRecipient,
+                additionallyFundedAccounts[0],
+                await recipientAccount.getCompleteAddress(),
+              );
+              return respond(request);
+            },
+          },
+        },
       }));
 
       ({ wallet: walletRecipient, teardown: teardownRecipient } = await setupPXEAndGetWallet(
@@ -108,7 +138,7 @@ export function buildMessageDeliveryTest(opts: {
         undefined,
         'pxe-recipient',
       ));
-      const recipientAccount = await walletRecipient.createSchnorrAccount(
+      recipientAccount = await walletRecipient.createSchnorrAccount(
         additionallyFundedAccounts[0].secret,
         additionallyFundedAccounts[0].salt,
         additionallyFundedAccounts[0].signingKey,
@@ -172,6 +202,12 @@ export function buildMessageDeliveryTest(opts: {
       expect(readNotes).toEqual(noteValues);
     });
 
+    if (customRequestResponder) {
+      it('the custom request hook fires exactly once, on the send that bootstraps the tagging secret', () => {
+        expect(customRequestCount).toBe(1);
+      });
+    }
+
     additionalTests?.();
   });
 }
diff --git a/yarn-project/standard-contracts/src/handshake-registry/constants.ts b/yarn-project/standard-contracts/src/handshake-registry/constants.ts
index 48b4cd153728..0573b2b125d5 100644
--- a/yarn-project/standard-contracts/src/handshake-registry/constants.ts
+++ b/yarn-project/standard-contracts/src/handshake-registry/constants.ts
@@ -1,6 +1,8 @@
 // Lightweight metadata leaf export for browser bundles: importing from
 // `@aztec/standard-contracts/handshake-registry/constants` avoids dragging in the
 // `HandshakeRegistry.json` static import.
+import { sha256ToField } from '@aztec/foundation/crypto/sha256';
+import type { Fr } from '@aztec/foundation/curves/bn254';
 import type { AztecAddress } from '@aztec/stdlib/aztec-address';
 
 import { StandardContractAddress, StandardContractClassId, StandardContractSalt } from '../standard_contract_data.js';
@@ -8,3 +10,14 @@ import { StandardContractAddress, StandardContractClassId, StandardContractSalt
 export const STANDARD_HANDSHAKE_REGISTRY_ADDRESS: AztecAddress = StandardContractAddress.HandshakeRegistry;
 export const STANDARD_HANDSHAKE_REGISTRY_CLASS_ID = StandardContractClassId.HandshakeRegistry;
 export const STANDARD_HANDSHAKE_REGISTRY_SALT = StandardContractSalt.HandshakeRegistry;
+
+/**
+ * Request kind under which the HandshakeRegistry asks for a recipient's interactive-handshake signature through the
+ * `resolveCustomRequest` hook. Mirrors `INTERACTIVE_HANDSHAKE_REQUEST_KIND` in the registry contract.
+ */
+// TODO: remove this mirrored constant and read the value from the HandshakeRegistry artifact once the contract
+// global can be `#[abi]`-exported. Fixed upstream but not yet released:
+// https://github.com/noir-lang/noir/pull/12714 and https://github.com/noir-lang/noir/issues/12620.
+export const INTERACTIVE_HANDSHAKE_REQUEST_KIND: Fr = sha256ToField([
+  Buffer.from('HANDSHAKE_REGISTRY::INTERACTIVE_HANDSHAKE_REQUEST'),
+]);
diff --git a/yarn-project/standard-contracts/src/handshake-registry/index.ts b/yarn-project/standard-contracts/src/handshake-registry/index.ts
index f10e7186a45f..2cc367647d2e 100644
--- a/yarn-project/standard-contracts/src/handshake-registry/index.ts
+++ b/yarn-project/standard-contracts/src/handshake-registry/index.ts
@@ -6,6 +6,7 @@ import { makeStandardContract } from '../make_standard_contract.js';
 import type { StandardContract } from '../standard_contract.js';
 
 export {
+  INTERACTIVE_HANDSHAKE_REQUEST_KIND,
   STANDARD_HANDSHAKE_REGISTRY_ADDRESS,
   STANDARD_HANDSHAKE_REGISTRY_CLASS_ID,
   STANDARD_HANDSHAKE_REGISTRY_SALT,