Cut ask-route preflight latency and batch client data-stream appends

[ross0x01]

Summary

Review of t3code's perf: commits against our chat pipeline, porting the patterns that apply. Most of their client-side work (structural row sharing, memoized markdown, debounced drafts, LegendList) already exists here; this PR covers the three gaps.

• Ask route preflight: /api/chat ran ~10 serial awaits before streamText with zero Promise.all. User customization now overlaps the message fetch, project context overlaps the extra-usage config, and the notes fetch starts early instead of adding a serial Convex round-trip right before the model call (mirrors what the Trigger agent route already does). Net: three sequential round-trips removed from time-to-first-token.
• Latency attribution: the chat wide event now carries preflight.duration_ms and stream.first_chunk_ms via a new markFirstChunk on the chat logger, wired to the runner's onModelChunk hook. The ask route previously had no way to tell preflight latency from provider latency.
• Client data-stream batching: every data part (including each terminal output chunk) copied the whole dataStream array and re-ran every consumer effect. useBatchedDataStreamAppend coalesces appends into one state update per 100ms window (throttle-first, order preserved, nothing dropped). Chat resets clear pending parts so a stale chat cannot resurface.

Test plan

• tsc --noEmit clean
• New tests: useBatchedDataStreamAppend, wide-event timing, notes preload
• 83 suites / 1253 tests pass across lib/api, lib/chat, app/hooks, and message components
• Verify preflight and first_chunk_ms fields appear in the chat wide event in PostHog after deploy

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements
  • Chat responses now appear more smoothly during streaming, with incoming content grouped into efficient updates.
  • Faster chat startup through improved parallel loading of conversation context and personalization settings.
  • Preloaded notes are reused when available, reducing unnecessary data retrieval.
  • Stream timing measurements now more accurately reflect response preparation and first-content delivery.
  • Streaming cleanup is more reliable when stopping a response or switching chats.

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Project	Deployment	Actions	Updated
hackerai	Ready	Preview	Sep 3, 2026 1:45pm UTC

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• app/components/chat.tsx

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📝 Walkthrough

Walkthrough

Changes

The PR adds batched client-side stream updates, parallelizes chat preflight reads, reuses preloaded notes, and adds preflight and first-chunk timing to wide events.

Chat stream performance

Layer / File(s)	Summary
Batched client stream updates app/hooks/useBatchedDataStreamAppend.ts, app/hooks/__tests__/useBatchedDataStreamAppend.test.ts, app/components/chat.tsx	Chat now batches ordered stream parts within a configurable window. Stop and chat-change cleanup clears pending parts. Tests cover flushing, ordering, clearing, and unmount cleanup.
Stream timing instrumentation lib/logger.ts, lib/api/chat-logger.ts, lib/__tests__/wide-event-timing.test.ts	Wide events now record preflight duration and optional first-model-chunk duration. The logger exposes markFirstChunk() and preserves the first timestamp.
Chat preflight and note loading lib/api/chat-handler.ts, lib/api/chat-stream-helpers.ts, lib/api/__tests__/chat-stream-helpers-notes.test.ts	Chat preflight database reads now run concurrently. Notes load early and pass into injectNotesIntoMessages, which still fetches notes when no preload is provided. The first model chunk marks the chat logger.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🟡 Moderate · up to 74d18

Chat preflight changes may still allow free non-agent requests to include notes in provider prompts contrary to the intended eligibility policy. This should be resolved before merge.

Sequence Diagram(s)

sequenceDiagram
  participant chat-handler
  participant chat-logger
  participant WideEventBuilder
  chat-handler->>chat-logger: onModelChunk
  chat-logger->>WideEventBuilder: markFirstChunk()
  WideEventBuilder->>WideEventBuilder: record first chunk timestamp
  chat-logger->>WideEventBuilder: setStreamResult()

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 9 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the two main changes: reducing chat-route preflight latency and batching client data-stream appends.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@lib/api/chat-handler.ts`:
- Around line 371-374: Update shouldIncludeNotes in the preloaded-notes flow to
require the same subscription and request-mode eligibility used by notesEnabled,
so free non-agent Ask requests remain excluded even when include_notes is unset.
Preserve customization behavior for eligible requests and add a regression test
covering free Ask note injection.

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 6ba4b49 and d729cef.

📒 Files selected for processing (9)

• app/components/chat.tsx
• app/hooks/__tests__/useBatchedDataStreamAppend.test.ts
• app/hooks/useBatchedDataStreamAppend.ts
• lib/__tests__/wide-event-timing.test.ts
• lib/api/__tests__/chat-stream-helpers-notes.test.ts
• lib/api/chat-handler.ts
• lib/api/chat-logger.ts
• lib/api/chat-stream-helpers.ts
• lib/logger.ts

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