diff --git a/Makefile b/Makefile
index 43c97943..31c38aaa 100644
--- a/Makefile
+++ b/Makefile
@@ -6,6 +6,8 @@ FEATURES = bitcoin dogecoin litecoin zcash
 
 build: $(addprefix build-,$(FEATURES))
 
+build-local: $(addprefix build-local-,$(FEATURES))
+
 clippy: $(addprefix clippy-,$(FEATURES))
 
 fmt: $(addprefix fmt-,$(FEATURES))
@@ -19,6 +21,18 @@ $(foreach feature,$(FEATURES), \
 	) \
 )
 
+ifeq ($(shell uname),Darwin)
+LLVM_PATH = /opt/homebrew/opt/llvm@19/bin
+LOCAL_ENV = PATH="$(LLVM_PATH):$(PATH)" CC=$(LLVM_PATH)/clang AR=$(LLVM_PATH)/llvm-ar
+endif
+
+$(foreach feature,$(FEATURES), \
+	$(eval build-local-$(feature): ; \
+		$(LOCAL_ENV) cargo near build non-reproducible-wasm --no-default-features --features "$(feature)" --manifest-path $(NEAR_MANIFEST) && \
+		mkdir -p res && mv ./contract/target/near/btc_light_client_contract.wasm ./res/$(feature).wasm \
+	) \
+)
+
 $(foreach feature,$(FEATURES), \
 	$(eval clippy-$(feature): ; cargo clippy --no-default-features --features "$(feature)" --manifest-path $(NEAR_MANIFEST) -- $(LINT_OPTIONS)) \
 )
diff --git a/contract/AGENTS.md b/contract/AGENTS.md
new file mode 100644
index 00000000..ceb2b988
--- /dev/null
+++ b/contract/AGENTS.md
@@ -0,0 +1 @@
+CLAUDE.md
diff --git a/contract/CLAUDE.md b/contract/CLAUDE.md
new file mode 100644
index 00000000..8ef954d2
--- /dev/null
+++ b/contract/CLAUDE.md
@@ -0,0 +1,87 @@
+# BTC Light Client Contract
+
+## Project Overview
+
+A Bitcoin/Litecoin/Dogecoin/Zcash SPV light client implemented as a NEAR Protocol smart contract. It verifies and stores block headers on-chain, enabling trustless verification of Bitcoin (and other UTXO chain) transactions without running a full node. Relayers can submit blocks and they are verified on-chain via proof-of-work validation, difficulty adjustment checks, and chain selection rules.
+
+## Build & Test
+
+Requires: Rust 1.86.0, `cargo-near`, `wasm32-unknown-unknown` target.
+
+```bash
+# Build specific chain variant (reproducible via cargo-near)
+make build-bitcoin       # default, feature: bitcoin
+make build-litecoin      # feature: litecoin
+make build-dogecoin      # feature: dogecoin
+make build-zcash         # feature: zcash
+
+# Build all chain variants locally (non-reproducible)
+make build-local         # all variants
+make build-local-bitcoin # single variant
+
+# Run all tests (all features)
+make test
+
+# Lint (strict: -D warnings -D clippy::pedantic)
+make clippy
+
+# Format check
+make fmt
+```
+
+## Architecture & Key Concepts
+
+### On-Chain State (`BtcLightClient` in `contract/src/lib.rs`)
+
+- **headers_pool**: `LookupMap<H256, ExtendedHeader>` — all stored headers (main chain + forks)
+- **mainchain_height_to_header** / **mainchain_header_to_height**: bidirectional main chain index
+- **mainchain_tip_blockhash**: current chain tip
+- **gc_threshold**: max number of mainchain blocks to keep in storage. When the mainchain grows beyond this, the oldest mainchain blocks are pruned. GC runs automatically during `submit_blocks()` (with `batch_size` = number of submitted headers) and can also be triggered manually via `run_mainchain_gc(batch_size)`. Only mainchain blocks are deleted; fork/sidechain blocks are not affected
+
+### Block Submission Flow
+
+1. Call `submit_blocks(headers)` with one or more block headers
+2. For each header, the contract:
+   - Looks up the previous block in storage (rejects unattached blocks)
+   - **Verifies PoW**: block hash ≤ target derived from `bits` field
+   - **Checks difficulty adjustment**: validates `bits` matches the chain's retarget algorithm
+   - Computes accumulated `chain_work`
+3. If the block extends the main chain tip → appended directly
+4. If it's a fork → stored separately; if fork's `chain_work` > main chain → **automatic reorg**
+
+### Chain Reorganization
+
+When a fork accumulates more work than the main chain:
+1. Walk both chains back to common ancestor
+2. Demote old main chain blocks (remove from height index)
+3. Promote fork blocks to main chain
+4. Update tip pointer
+
+**Caveat**: If mainchain blocks near the fork point have been garbage collected, reorg will fail — the contract panics with `PrevBlockNotFound` when it cannot walk the chain back to the common ancestor. This means GC depth must be set conservatively relative to expected fork lengths
+
+### Transaction Inclusion Verification
+
+`verify_transaction_inclusion(ProofArgs)` — SPV proof: given a tx hash, block hash, and merkle proof, verifies the transaction is in the block by recomputing the merkle root.
+
+**Important**: This function is vulnerable to the standard Bitcoin merkle tree second-preimage attack — it may return `true` for an internal node hash rather than a real transaction hash. Block headers do not contain the transaction count, so proof depth cannot be validated on-chain. Callers MUST validate that the `tx_id` corresponds to a valid transaction (e.g., by verifying raw transaction data) before trusting the inclusion proof.
+
+
+### Garbage Collection
+
+`run_mainchain_gc(batch_size)` removes the oldest mainchain blocks from storage when the mainchain exceeds `gc_threshold`. Only mainchain blocks are pruned; fork/sidechain blocks remain. The `batch_size` parameter limits how many blocks are removed per call to bound gas usage.
+
+## Important Build Flags
+
+The release profile sets `overflow-checks = true` in `Cargo.toml`. This ensures all arithmetic overflows/underflows panic safely instead of wrapping. Do not remove this flag — several view functions rely on it for safe behavior with untrusted inputs.
+
+## Chain-Specific Implementation Details
+
+Each chain lives in its own module and is selected at compile time via mutually exclusive feature flags (`--no-default-features --features "<flag>"`):
+
+| Chain | Module | PoW Hash | Difficulty Algorithm | Special |
+|-------|--------|----------|---------------------|---------|
+| Bitcoin | `bitcoin.rs` | SHA-256d | Every 2016 blocks, 14-day target | — |
+| Litecoin | `litecoin.rs` | Scrypt | Every 2016 blocks, 3.5-day target | Right-shift before multiply |
+| Dogecoin | `dogecoin.rs` | Scrypt | Per-block Digishield (after 145k) | AuxPoW merged mining |
+| Zcash | `zcash.rs` | Equihash (200,9) | 17-block averaging window, MTP | Blossom hardfork handling |
+