fix: useChat status stays ready during stream resumption

[threepointone]

Problem

useChat status stayed "ready" during stream resumption after page refresh — isLoading was false, no abort button or thinking indicator appeared.

Four root causes:

1. addEventListener race

The transport registered its own addEventListener listener for CF_AGENT_STREAM_RESUMING detection, but onAgentMessage (also via addEventListener) always handled the message first. The fallback path ran — bypassing the AI SDK pipeline entirely. Chunks flowed through onAgentMessage → setMessages directly, so useChat never saw them and never set status to "streaming".

2. Transport instance instability

useMemo created new transport instances across renders and Strict Mode cycles. When _pk changed (async queries, socket recreation), the resolver was stranded on the old transport while onAgentMessage called handleStreamResuming on the new one — it never found the resolver.

3. Chat recreation on _pk change

Using agent._pk as the useChat id caused the AI SDK to recreate the Chat when the socket changed (shouldRecreateChat: chatRef.current.id !== options.id). This abandoned the in-flight makeRequest (including resume). The resume effect wouldn't re-fire because its deps are [resume, chatRef] — chatRef is the same ref object, so the effect never re-runs.

4. Double STREAM_RESUMING

The server sends CF_AGENT_STREAM_RESUMING from both onConnect (line 348) and the RESUME_REQUEST handler (line 551). Without deduplication, the second message triggered a duplicate ACK and double chunk replay.

Fix

addEventListener race → synchronous callback

Replaced the transport's addEventListener-based detection with handleStreamResuming() — a public method that onAgentMessage calls directly:

Server → WebSocket → PartySocket → onAgentMessage
                                        ↓
                              customTransport.handleStreamResuming()
                                        ↓ (true)
                              _resumeResolver → ACK + ReadableStream → useChat pipeline
                                        ↓ (false)
                              localRequestIdsRef check → fallback (cross-tab)

Transport instability → true singleton

The transport is now a true singleton (useRef, created once, never recreated). transport.agent is updated every render to point at the latest socket. The resolver survives _pk changes because the transport instance never changes — both reconnectToStream (via ChatStore) and handleStreamResuming (via onAgentMessage) always operate on the same instance.

The agent property was changed from private to public, and reconnectToStream's resolver uses this.agent (not a captured local) so ACK and chunk listeners go through the latest socket.

Chat recreation → stable ID

Replaced id: agent._pk with id: initialMessagesCacheKey (based on URL + agent namespace + instance name). This identifier is stable across socket recreations, so the AI SDK's Chat is never recreated when _pk changes. The in-flight makeRequest survives and correctly transitions status to "streaming".

Double STREAM_RESUMING → localRequestIdsRef guard

Added localRequestIdsRef.current.has(data.id) check before the fallback path to prevent duplicate ACK/replay.

Edge cases handled

• Double STREAM_RESUMING — localRequestIdsRef guard prevents duplicate ACK/replay
• React strict mode — useRef singleton survives mount/unmount/remount; second reconnectToStream overwrites resolver; first times out harmlessly
• _pk change (async queries) — transport singleton survives; agent ref updated; ACK/listeners use latest socket; stable Chat ID prevents Chat recreation
• WS not open on refresh — PartySocket queues send(); works regardless of readyState
• Connection drops mid-resume — PartySocket EventTarget survives reconnection
• Timeout (no active stream) — resolves null after 5s, resolver cleared
• Agent switches — transport's agent ref updated every render; old resolver orphaned

Testing

• 23 transport unit tests (18 original + 5 new agent-swap tests) covering: resolver lifecycle, ACK, activeRequestIds tracking, timeout, strict mode double-call, chunk reception/completion/filtering, error handling, send failure tolerance, double-RESUMING dedup, agent property update, resolver surviving agent swap, ACK routing to new agent, chunk listener on new agent, full end-to-end agent swap
• All 290 tests pass (23 new + 267 existing)
• All 45 projects typecheck

[changeset-bot]

🦋 Changeset detected

Latest commit: ff7026b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/ai-chat	Patch

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commit: 807365f