improvement(mothership): stream retry state machine, progressive re-rendering

[icecrasher321]

Summary

• Progressive chat rendering
• Harden copilot stream/replay handling

Type of Change

• Bug fix

Testing

Tested manually

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		Apr 24, 2026 6:47pm

[cursor]

PR Summary

Medium Risk
Medium risk because it changes Copilot SSE replay behavior (new comment frames/keepalives) and refactors progressive message rendering state, which can affect reconnect reliability and chat UI scrolling/rendering under load.

Overview
Copilot stream replay now writes SSE comments: an immediate : accepted acknowledgment plus periodic : keepalive comments during idle replay polling, with replay enqueue helpers updated to track last-write time and stop cleanly on enqueue failures.

Chat UI now progressively stages message rendering via useProgressiveList, including scroll-to-bottom behavior during staging and a memoized lookup for the preceding user message used by MessageActions.

Go stream fetch instrumentation is hardened by ending the fetch span and recording header timing when fetchGo throws before a response is available; tests were added to assert stream loops do not retry on HTTP errors or network failures.

^{Reviewed by Cursor Bugbot for commit 7bba0c5. Configure here.}

[greptile-apps]

Greptile Summary

This PR hardens the copilot stream/replay state machine with three improvements: (1) the replay route now immediately emits an accepted SSE comment and periodic keepalive comments to prevent proxy timeouts, (2) the gap-replay loop correctly advances cursor, currentRequestId, and sawTerminalEvent per envelope (a real bug fix from the prior code that silently dropped these updates), and (3) the chat UI renders messages progressively using a refactored useProgressiveList hook backed by a structured state machine instead of scattered mutable refs.

Confidence Score: 5/5

Safe to merge — all findings are P2 informational notes; no functional regressions identified.

The gap-replay cursor/state fix is a genuine correctness improvement. The two P2 notes are both self-correcting and do not affect user-visible behaviour.

No files require special attention beyond the noted P2 items in route.ts and use-progressive-list.ts.

Important Files Changed

Filename	Overview
apps/sim/app/api/copilot/chat/stream/route.ts	Hardens replay stream: adds accepted SSE comment on open, keepalive comments every 15 s, and fixes the gap-replay loop to correctly advance cursor, currentRequestId, and sawTerminalEvent.
apps/sim/hooks/use-progressive-list.ts	Refactors progressive list hook to a structured state machine with two separate effects; a stale closure in the rAF step can cause one frame of premature caughtUp but the sync effect corrects it immediately.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/mothership-chat.tsx	Integrates useProgressiveList for staged message rendering and replaces per-message O(n) .reverse().find() with a single O(n) useMemo pass.
apps/sim/lib/copilot/request/go/stream.ts	Wraps fetchGo in try/catch so network-level errors properly close the fetch OTel span before re-throwing.
apps/sim/app/api/copilot/chat/stream/route.test.ts	Adds mock for encodeSSEComment and asserts ': accepted\n\n' is the first chunk of every replay response.
apps/sim/lib/copilot/request/go/stream.test.ts	Adds three no-retry tests for 502, 402, and network errors.

_{Reviews (1): Last reviewed commit: "improvement(mothership): stream rety sta..." | Re-trigger Greptile}

[icecrasher321]

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^{Reviewed by Cursor Bugbot for commit 7bba0c5. Configure here.}