Phase 1 inserts positions into sent_positions after sending, but the dedup set is only consulted in phase 2. If send_replay_message fails partway through phase 1 (e.g. timeout), the error propagates and the client is disconnected — so partial sends in phase 1 are fine.

However, sentinel entries (pos == None) are sent to the client but cannot be deduplicated in phase 2 since they have no position key. If the same sentinel payload is also in the broadcast channel, the client will receive it twice. This may be acceptable if sentinels are rare/non-existent in practice today, but it's worth a comment documenting the behavior.

last_position is updated before calling (self.handler)(...). If the handler panics (unwinding), the position is committed but the message was never processed downstream. On reconnect, the subscriber would skip this message.

Consider updating last_position after the handler returns, or document that the handler is assumed to be infallible for position-tracking purposes.

The fallback block always passes flashblock_index = 0 here. If a fallback block is published after flashblock index > 0 was already published for the same block number, reconnecting clients that resume from (block_number, 0) would skip this fallback since entries_after uses strictly-greater-than comparison and (N, 0) > (N, 0) is false.

Is the intent that the fallback block replaces/supersedes all prior flashblocks for the same block? If so, this may need a different position or the ring buffer lookup semantics would need to include the cutoff position itself. If fallback blocks are only published when no prior flashblocks exist for that block number, this is fine — but worth a comment clarifying the invariant.

The old publish() returned an error when there were no receivers (io::ErrorKind::ConnectionAborted). The new code silently discards the SendError here. This is a deliberate behavioral change (the ring buffer preserves the message for future replay), but it means callers can no longer distinguish "published to live subscribers" from "stored in ring buffer only."

If any caller relied on the error to detect zero-subscriber scenarios, this is a silent regression. Worth documenting in the method's doc comment that a successful return does not guarantee any live subscriber received the message.

This is a breaking change to the public API: publish() previously returned io::Result<usize> and now returns Result<usize, serde_json::Error>. The two current call sites in payload.rs handle this fine (one uses .map_err(PayloadBuilderError::other), the other uses .wrap_err(...)), but any out-of-tree consumers matching on io::Error will break at compile time.

Also, the old behavior returned io::Error(ConnectionAborted) when no subscribers were connected; now SendError is silently discarded (line 115). This is intentional for the ring-buffer-based replay model, but worth documenting that Ok(size) no longer implies any live subscriber received the message.

Phase 1 inserts positions into sent_positions before calling send_replay_message, but the insert happens regardless of send success since ? would propagate the error and disconnect. This ordering is fine.

However, sentinel entries (pos == None) are sent to the client but cannot be tracked in sent_positions. Today the publisher always passes Some(position) to ring_buffer.push(), so sentinels never exist in practice. But the RingBuffer API advertises sentinel support as a core feature (README, doc comments, tests). If sentinels are never used, consider simplifying the ring buffer to require positions unconditionally (push(position: I, value: V)) — this removes the Option wrapping, eliminates the sentinel code path in replay, and makes the API harder to misuse.

parse_flashblock_position fully deserializes the top-level index and nested metadata.block_number on every incoming text message. serde_json::from_str must validate the entire JSON even though we only need two fields — for a 500 KB flashblock payload this is non-trivial work.

Consider using serde_json::from_str with #[serde(rename_all = ...)] and #[serde(default)] on a purpose-built struct (which you already do), but also consider serde_json::from_slice with RawValue to short-circuit parsing, or simply string-search for the two numeric fields before falling back to full parse. This is on the hot path for every received flashblock.

Also: the doc comment references FlashblocksPayloadV1 but this is in a different crate (base-alloy-flashblocks). If the wire format changes (e.g., index is renamed or metadata becomes a typed struct with different field names), this parser will silently stop tracking positions, degrading reconnection to full re-sync without any warning. Consider adding an integration test or at minimum a debug_assert! / warning log when position extraction fails for messages that "look like" flashblock payloads.

receiver.resubscribe() is called here, before tokio::spawn. The new receiver starts accumulating messages from this instant. The spawned task then performs the handshake (up to 10s), during which messages continue to arrive on this receiver.

This is correct for the two-phase replay protocol — those messages will be deduplicated in phase 2. But under high publish rates with a slow handshake, the broadcast receiver could lag and lose messages before BroadcastLoop::run() even starts. The receiver's buffer is bounded by channel_capacity (default 100). If >100 flashblocks are published during a 10s handshake, the receiver lags and phase 2 will return ReplayError::Lagged, disconnecting the client before it receives any data.

Consider creating the receiver inside the spawned task, after the handshake completes. The ring buffer snapshot in phase 1 already covers messages published before the receiver is created, so no messages would be lost.

The _position is discarded in the live broadcast path. If the subscriber lags and receives RecvError::Lagged (line 103), it logs a warning and continues — but the client has a gap in its stream with no way to detect it application-level.

Consider sending the position as part of the WebSocket message (or in a wrapper) so downstream clients can detect gaps independently, rather than relying solely on the server-side lag counter. Alternatively, disconnect lagged clients so they can reconnect with replay, which would be more consistent with the replay-on-reconnect model.