perf(web): reuse completed Markdown prefixes while streaming by juliusmarminge · Pull Request #11193 · pingdotgg/t3code

juliusmarminge · 2026-09-11T04:45:15Z

Streaming a response reparses completed code fences on every chunk. Preserve the parsed prefix through a closed top-level fence and reparse the changing suffix, following the completed-block reuse in Legend Markdown.

The existing whole-document transforms, sanitization, source positions and rendering remain in use. Reference definitions, footnotes, CR/CRLF and BOM input fall back to a full parse. Plain prose, completed messages and custom syntax plugins use the existing path. The cache belongs to one streaming renderer.

Five alternating production-browser runs at 1440 × 1000, each with 20 warm-up updates and 120 measured updates:

Client update time	Base, including minimap change	This layer
Median of run medians	21.5 ms	17.4 ms
Median of run p95s	34.0 ms	30.3 ms

The fixture contains 12 completed TypeScript fences of 30 lines plus growing prose. Every paired run improved; the median reduction is 19.1%. Both builds use the same browser process, origin, fixture and probe. These are synchronous client-update measurements, not provider throughput or user telemetry. Raw samples · Probe, comparison script and parity evidence.

82 focused tests and web typecheck pass. The external CommonMark corpus adds 17,426 full-tree comparisons, including source positions, across its 652 examples. Browser verification retained selected text, DOM identity and exactly the same scroll position through 40 updates. Targeted lint reports two warnings in unchanged code.

This affects streaming Markdown in web and Electron's shared web renderer. Mobile and file/diff viewers are unchanged. Local and remote clients use the same rendering path; no transport, provider or contract changes.

Before, desktop:

After, desktop:

Before navigation and streaming, narrow viewport:

https://gh-file-drop-api-prod-mi5fy3sowv63ufte.pinglabs.workers.dev/f/cf9292628b42deda/parser-before-demo.webm

After navigation and streaming, narrow viewport:

https://gh-file-drop-api-prod-mi5fy3sowv63ufte.pinglabs.workers.dev/f/45f147cf61401e0c/parser-after-demo.webm

Recordings show visual states, not wall-clock speed: the recorder compresses dropped-frame intervals. The table uses separate, unrecorded timing runs; 0.8 seconds of recorder startup was trimmed. Only synthetic fixture data is included. Probes and evidence are not committed.

Integration follow-up: rebased on current main; the integrated stack passes 94 focused Markdown/highlighting tests and 159 timeline tests, plus web typecheck. The timing table remains pinned to the original measured builds.

Model: GPT-6. Harness: Codex.

github-actions · 2026-09-11T04:47:11Z

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	57.0 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	8	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.8 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	8	—	21	✅

Baseline: unavailable · PR result: a660ad9 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

Codex decoded thread snapshot: 113.9 KiB
Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

cursor · 2026-09-11T04:50:16Z

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macroscopeapp · 2026-09-11T04:52:40Z

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR introduces stateful AST caching, parser wrapping, position shifting, and automatic fallback decisions in the production streaming Markdown path. Focused parity tests reduce risk, but the new parsing strategy and automatic processing gate are substantial runtime changes.

No code changes detected at a660ad9. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}

coderabbitai · 2026-09-11T05:51:08Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between 3b9cf6e and a660ad9.

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📒 Files selected for processing (4)

apps/web/package.json
apps/web/src/components/ChatMarkdown.tsx
apps/web/src/markdown-incremental.test.tsx
apps/web/src/markdown-incremental.ts

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📝 Walkthrough

Walkthrough

The web app adds an incremental Markdown parser for eligible streaming messages. It caches completed fenced-code prefixes, integrates the parser into ChatMarkdown, and adds tests for parsing equivalence and fallback cases.

Changes

Incremental Markdown streaming

Layer / File(s)	Summary
Incremental parser and plugin `apps/web/src/markdown-incremental.ts`, `apps/web/package.json`	Adds cached-prefix parsing, AST cloning, position shifting, full-parse fallbacks, and the exported incremental plugin. Adds required development dependencies.
Streaming renderer integration `apps/web/src/components/ChatMarkdown.tsx`	Enables incremental parsing during streaming when fenced code is present and no extra remark plugins are configured.
Parser behavior validation `apps/web/src/markdown-incremental.test.tsx`	Tests cached prefixes, streaming positions, line endings, definitions, edits, repeated renders, transformed nodes, and code-fence edge cases.

Priority: ➖ Normal

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

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant ChatMarkdown
  participant IncrementalMarkdownPlugin
  participant MarkdownParser
  ChatMarkdown->>IncrementalMarkdownPlugin: enable for eligible streaming text
  IncrementalMarkdownPlugin->>MarkdownParser: parse current Markdown document
  MarkdownParser-->>ChatMarkdown: return Markdown AST for rendering

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to a660a

No merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 3 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly and concisely describes the main change: reusing completed Markdown prefixes during streaming for web performance.
Description check	✅ Passed	The description explains what changed, why it changed, scope, performance results, testing, and UI verification. It includes screenshots and videos for the rendering changes. It does not use the templ…

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 3 files. (1 skipped: 1 unsupported.)

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@maria-rcks

## What's Changed * fix(pr): update labels and reviewers without redundant reloads by @maria-rcks in pingdotgg/t3code#11117 * fix(chat): fold question answers into tool activity by @maria-rcks in pingdotgg/t3code#11014 * fix(usage): flag unpriced model activity instead of showing $0.00 by @maria-rcks in pingdotgg/t3code#11021 * fix(server): let Claude launch args override the derived permission mode by @maria-rcks in pingdotgg/t3code#11026 * fix(editors): accept root paths and Windows servers in Zed remote links by @maria-rcks in pingdotgg/t3code#11044 * fix(web): center pull request unavailable states by @maria-rcks in pingdotgg/t3code#11110 * fix(web): remove sidebar pull request link icon by @maria-rcks in pingdotgg/t3code#11179 * fix(ui): color linked pr counts by aggregate status by @maria-rcks in pingdotgg/t3code#11180 * fix(preview): render website favicons for browser tool activity by @maria-rcks in pingdotgg/t3code#11032 * fix(web): simplify pull request summary sections by @maria-rcks in pingdotgg/t3code#10612 * fix(web): preserve drafts when compacting context by @maria-rcks in pingdotgg/t3code#11103 * fix(server): queue messages during context compaction by @maria-rcks in pingdotgg/t3code#11107 * perf(web): format minimap previews only when opened by @juliusmarminge in pingdotgg/t3code#11181 * perf(web): reuse completed Markdown prefixes while streaming by @juliusmarminge in pingdotgg/t3code#11193 * perf(web): resume syntax highlighting from completed lines by @juliusmarminge in pingdotgg/t3code#11196 * perf(web): preserve completed code-line DOM while streaming by @juliusmarminge in pingdotgg/t3code#11198 * perf(web): huge-thread switch no longer blanks the chat pane by @juliusmarminge in pingdotgg/t3code#11169 **Full Changelog**: pingdotgg/t3code@v0.0.41-nightly.20260911.1520...v0.0.41-nightly.20260911.1533 Upstream release: https://github.com/pingdotgg/t3code/releases/tag/v0.0.41-nightly.20260911.1533

…gg#11193)

juliusmarminge added this pull request to stack #11194 September 11, 2026 04:45

github-actions Bot added vouch:trusted PR author is trusted by repo permissions or the VOUCHED list. size:L 100-499 changed lines (additions + deletions). labels Sep 11, 2026

juliusmarminge marked this pull request as ready for review September 11, 2026 04:50

macroscopeapp Bot reviewed Sep 11, 2026

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Comment thread apps/web/src/markdown-incremental.ts

github-actions Bot added the 📱 Native Change Changes the native fingerprint; merging blocks production OTAs until a new store build ships. label Sep 11, 2026

juliusmarminge added 3 commits September 10, 2026 22:42

perf(web): reuse completed Markdown prefixes while streaming

1a8ac63

fix(web): keep Markdown recovery parses out of the stream cache

acacd58

chore(web): keep the Markdown parser cache helper private

a660ad9

juliusmarminge force-pushed the legend-perf/incremental-markdown branch from a862924 to a660ad9 Compare September 11, 2026 05:43

github-actions Bot removed the 📱 Native Change Changes the native fingerprint; merging blocks production OTAs until a new store build ships. label Sep 11, 2026

juliusmarminge mentioned this pull request Sep 11, 2026

perf(web): format minimap previews only when opened #11181

Merged

juliusmarminge merged commit a9dabbf into main Sep 11, 2026
25 checks passed

juliusmarminge deleted the legend-perf/incremental-markdown branch September 11, 2026 06:50

This was referenced Sep 11, 2026

[Bug]: Heavy CPU Usage on linux #4074

Closed

perf(web): coalesce streaming Markdown renders #4349

Closed

Track performance audit fixes #9661

Open

AIdoesmyjob pushed a commit to AIdoesmyjob/t3code that referenced this pull request Sep 11, 2026

perf(web): reuse completed Markdown prefixes while streaming (pingdot…

a226ac4

…gg#11193)

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perf(web): reuse completed Markdown prefixes while streaming#11193
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