diff --git a/CLAUDE.md b/CLAUDE.md
index 17ac1b34..35631302 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -395,7 +395,7 @@ aggregation at interval 2).
 
 ## Resources
 
-**Specs:** `leanSpec/src/lean_spec/` (Python reference implementation)
+**Specs:** `leanSpec/src/lean_spec/spec/` (Python reference implementation; fork logic under `forks/<fork>/`, e.g. `forks/lstar/`)
 **Devnet:** `lean-quickstart` (github.com/blockblaz/lean-quickstart)
 **Releases:** See `RELEASE.md` for release process documentation
 
diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 2c5a707d..ce8fc3a0 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -13,3 +13,7 @@
 - [Metrics](./metrics.md)
 - [Checkpoint Sync](./checkpoint_sync.md)
 - [Fork Choice Visualization](./fork_choice_visualization.md)
+
+# Development
+
+- [Spec Deviations](./spec_deviations.md)
diff --git a/docs/spec_deviations.md b/docs/spec_deviations.md
new file mode 100644
index 00000000..453534b1
--- /dev/null
+++ b/docs/spec_deviations.md
@@ -0,0 +1,23 @@
+# Spec Deviations
+
+ethlambda diverges from the [leanSpec](https://github.com/leanEthereum/leanSpec)
+reference in a few places, mainly for performance reasons. This page lists those
+deviations; each will be fleshed out with rationale, implementation notes, and
+trade-offs over time.
+
+## Asynchronous signature aggregation with early stop
+
+Aggregation runs off the main BlockChainServer actor loop and stops early once it runs out of time.
+
+- **ethlambda:** at interval 2 the actor snapshots the current slot's raw gossip signatures (`snapshot_current_slot_aggregation_inputs`, `crates/blockchain/src/aggregation.rs`) and spawns a `tokio::task::spawn_blocking` worker (`run_aggregation_worker`, `aggregation.rs`). The snapshot deliberately covers only current-slot raw gossip signatures, skipping existing-proof reuse and stale-slot groups so the interval-2 budget is never spent re-aggregating past slots (proof reuse is handled later, at block build). The worker streams each finished group back as an `AggregateProduced` message; the actor loop is never blocked on XMSS work.
+- **Early stop:** a `send_after(AGGREGATION_DEADLINE, ...)` timer cancels the session after 750ms (`AGGREGATION_DEADLINE`, `aggregation.rs`); the interval is 800ms (`MILLISECONDS_PER_INTERVAL`, `crates/blockchain/src/lib.rs`), leaving ~50ms for publish/propagation. The worker checks `cancel.is_cancelled()` before each job (`aggregation.rs`); in-flight jobs finish, remaining jobs are dropped.
+- **leanSpec:** `aggregate()` is called inline and synchronously from `tick_interval` at interval 2. It processes every group with no time budget, no worker, no cancellation.
+- **Equivalence:** the per-group XMSS aggregation logic is the same as leanSpec's; the deviations are scheduling (off the actor loop), input scope (current-slot raw gossip only), and a partial-result bound. On cancellation the worker emits only the groups that finished, so a slot may pack fewer aggregates than the synchronous path would; any such subset still yields a valid block, affecting how many votes are included rather than signature validity.
+
+## Attestation scoring on block building
+
+Attestations are scored and selected when packing a block, rather than greedily included as they arrive.
+
+- **ethlambda:** `select_attestations` (`crates/blockchain/src/block_builder.rs`) ranks candidate `AttestationData` entries by tier `Finalize > Justify > Build` (`enum Tier`, `block_builder.rs`). The within-tier order is tier-dependent (`EntryScore::ordering_key`, `block_builder.rs`): `Finalize`/`Justify` entries already cross 2/3, so newer chain progress leads (target slot, attestation slot, then new-voter count); `Build` entries only add marginal voters, so coverage leads (new-voter count, target slot, then attestation slot). `data_root` is the final deterministic tiebreak in both tiers. Each round picks the best candidate against a projected post-state, up to `MAX_ATTESTATIONS_DATA = 8` distinct `AttestationData` entries (`crates/common/types/src/block.rs`); a winning entry may carry several proofs, which a later compaction step (`compact_attestations`, `block_builder.rs`) merges back to one per `AttestationData`.
+- **leanSpec:** `build_block` iterates entries sorted by `target.slot` (oldest first) and includes the first ones that pass filters (greedy, no scoring), re-running the loop as a fixed point when justification/finalization advances. Same cap: `MAX_ATTESTATIONS_DATA = 8`.
+- **Equivalence:** both produce a valid block; ethlambda prioritizes attestations that advance finality/justification rather than processing in slot order.