perf(mobile): Metro runs the React Compiler, with a coverage ratchet

[juliusmarminge]

Mobile did not run the React Compiler at all, so its ~1243 manual useMemo/useCallback/memo calls were the only memoization. Unlike web, where oxc in our own Vite config runs the compiler, mobile goes through Metro and babel-preset-expo. So this PR first proves the compiler really runs in that pipeline, then adds a ratchet that tracks which files it compiles.

What changed

• Compiler on: experiments.reactCompiler: true in apps/mobile/app.config.ts. Expo CLI turns this into transform.reactCompiler, Metro's transformer turns that into the supportsReactCompiler caller flag, and babel-preset-expo then adds babel-plugin-react-compiler. Release bundles use panicThreshold: "NONE", so a bailout is silent: the function ships unmemoized. babel-plugin-react-compiler and @babel/core are now explicit mobile devDependencies, so the test resolves the same copies Metro does.
• apps/mobile/src/reactCompilerCoverage.test.ts:
  • Loads metro.config.js, runs its real babelTransformerPath (@expo/metro-config's transformer) on a component, and checks that react/compiler-runtime appears with reactCompiler: "true" and does not appear with "false".
  • Checks that app.config.ts sets the experiment.
  • Compiles every non-test .ts/.tsx file under apps/mobile/src with babel.config.js's presets, using Metro's iOS release caller flags. The only addition is a logger on the preset's own react-compiler option, which collects CompileError/CompileSkip events. The plugin options are therefore exactly the ones Metro uses.
  • Ratchet: KNOWN_BAILOUTS lists every file that bails today, with its reason category. The test fails if a file outside the list bails, and also if a listed file now compiles, so the list can only shrink. It also fails if a listed file no longer exists. I checked both failure directions by hand: adding a try/finally to a compiling screen fails it, and listing a file that compiles fails it.
• Lint: the existing t3code/no-manual-memo now also rejects "use no memo" and "use no forget", and takes an allowMemoHooks option. Mobile runs it with allowMemoHooks: true, so the directives are errors there (0 hits today) while useMemo/useCallback stay legal (about 1243 hits). Dropping the option is the follow-up, once KNOWN_BAILOUTS is empty.
• docs/internals/mobile-development.md: a short section explaining where the compiler is switched on (nothing in babel.config.js), why bailouts are silent, and how the ratchet works.

Coverage today

773 of 861 source files compile. 88 files are allowlisted (files can have more than one category):

Category	Files
try without catch	30
ref read during render	24
try/finally	15
manual memo not preserved	13
throw inside try	8
mutates a frozen value	4
conditional inside try	4
unsupported syntax (import(), ??=, etc.)	4

By directory: features/threads 21, features/pull-requests 13, features/settings 9, state 6, features/files 6, features/home 5, components 5, components/ui 3, native 3, features/review 3, features/layout 3, features/usage 2, features/voice-input 2, and 1 each in cloud, devices, terminal, projects, diffs, connection and lib.

Stacked on top: #20969 (usage), #20970 (Connect onboarding) and #20971 (device preview). Each takes files off the list and drops their manual memos, ending at 84 allowlisted files.

Verifying a built bundle

cd apps/mobile
APP_VARIANT=production npx expo export --platform ios --no-bytecode --output-dir /some/dir
grep -o memo_cache_sentinel /some/dir/_expo/static/js/ios/index-*.js | wc -l

I ran this on Linux. The production iOS bundle has 585 memo_cache_sentinel occurrences with the experiment on, and 2 with it off (those 2 come from a prebuilt dependency). Grep for memo_cache_sentinel, not react/compiler-runtime: Metro rewrites import specifiers to numeric module ids, so the string react/compiler-runtime does not survive bundling (0 hits either way). In unbundled transform output, such as the test's Metro transformer check, react/compiler-runtime is the right marker.

Checks run

• vp test run src/reactCompilerCoverage.test.ts in apps/mobile (864 tests, about 40s)
• vp test run oxlint-plugin-t3code/rules/no-manual-memo.test.ts
• mobile typecheck
• vp lint on the changed files

Measured on an Android emulator; see Measurement below.

Measurement

Scenario stream-reply from the render harness in #21001 at 8d41c049c5: an open thread streams a Claude Haiku 5.5 turn while the composer is idle, with a 12 s window mid-stream. Device: Android emulator, Pixel 9 API 36, on nucbox-1, with Metro and an isolated seeded server on cups. Base main 13aa882c22, then this branch at 7f7d6390d6. Median of 3.

Runs (renders/commits): main 3163/41, 3163/39, 2047/31. This PR: 1243/33, 1726/40, 1249/33.

	main	this PR	Δ
renders	3163	1249	-61%
commits	39	33	-15%
renders per commit	77	38	-51%
ThreadListV2Row renders	0	0	0

The per-component output of the 7f7d639 run was not kept, so there is no component split for it. The table below comes from the earlier measurement on main in the same setup (main median 2019/31), before the fix:

Component (main)	Renders	Cause (main)	Renders
AnimatedComponent(View	88	AnimatedComponent(View props:style,children	51
IconButton	57	PressabilityDebugView parent-same-props<-View	24
AppSymbolView	40	TextInputHostProvider props:children	22
PressabilityDebugView	24	AppSymbolView parent-same-props<-View	22
AppText	8	HostView props:style,children	20

What it means: with the compiler on, a streaming thread does about half the rendering work per commit, and the median total fell 61%. Commit count also varies with provider pace. The thread shell files are still on KNOWN_BAILOUTS, so most of the saving is in compiled leaf components under them.

The 7x storm was not reproduced. Before the fix in 7f7d639, an earlier session measured this branch at 14695 renders in 179 commits (main: 2019/31). Nearly all of it was the home ThreadListV2Row and its swipe wrappers, rendering about 154 times per window with cause ThreadListV2Row state#19, the snooze gate's useState tick. By the session that measured the fix, the seeded data had aged past the gate. Compiled #20967 without the fix measured 1499/37, 1093/29, 1673/38 with 0 row renders, so neither run with the fix nor without it could trigger the storm again. The fix stops a compiled row from caching a stale clock and re-arming the gate timer every 50 ms. It is the likely cause, but it isn't confirmed on a device.

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[macroscopeapp]

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — This PR enables React Compiler transformation by default for production mobile bundles and adapts existing navigation and snooze behavior to that transformation. It also adds a mobile lint configuration override that permits manual memo hooks, so the default-build and static-analysis changes require human review.

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📝 Walkthrough

The mobile app now enables React Compiler and checks compiler output and diagnostics across mobile source files. Mobile linting rejects compiler directives and permits memo hooks through an option. Navigation layouts use inline components. Thread snooze timing now uses a stored clock and avoids timers for expired gates.

Changes: Mobile React Compiler

Layer / File(s)	Summary
Enable and check mobile compilation apps/mobile/app.config.ts, apps/mobile/package.json, apps/mobile/src/reactCompilerCoverage.test.ts, apps/mobile/src/Stack.tsx, docs/internals/mobile-development.md	Enables React Compiler in Expo and adds tests for transformed output and compiler diagnostics. Documents compiler configuration and coverage expectations. Navigation layouts are supplied through inline components.
Reject compiler directives in mobile source oxlint-plugin-t3code/rules/no-manual-memo.ts, oxlint-plugin-t3code/rules/no-manual-memo.test.ts, vite.config.ts	The lint rule reports "use memo", "use no memo", and "use no forget". Its allowMemoHooks option permits useMemo and useCallback while keeping directives banned. The mobile override enables the rule as an error with that option.

Thread Snooze Timing

Layer / File(s)	Summary
Track snooze-gate time and expiry apps/mobile/src/features/threads/thread-list-v2-items.tsx	The row stores a timestamp associated with the current thread and uses it for snoozability and expiry checks. The timer skips scheduling when the expiry is null or has passed.

Priority: ➖ Normal

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

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant CoverageTest
  participant MetroTransformer
  participant BabelPreset
  CoverageTest->>MetroTransformer: Transform mobile source with Metro caller flags
  MetroTransformer->>BabelPreset: Compile source with React Compiler options
  BabelPreset-->>CoverageTest: Return transformed output and compiler diagnostics

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Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to 7f7d6

No actionable regression was established in the changed snooze timing. Merge after normal checks.

Pre-merge checks: 4

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: enabling the React Compiler in the mobile Metro pipeline and adding coverage tracking.
Description check		The description is detailed and covers the problem, implementation, scope, verification commands, observed results, performance measurements, limitations, and agent attribution. It does not use the ex…

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Explanation

The description is detailed and covers the problem, implementation, scope, verification commands, observed results, performance measurements, limitations, and agent attribution. It does not use the exact template headings and does not provide a separate approval link, but it explains the established capability and the reason for the configuration change.

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Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	4.9 KiB	−23 B (−0.5%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	−23 B (−1.9%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.8 KiB	−41 B (−0.2%)	29.3 KiB	✅
Codex	Live turn messages	2	1	−1 (−50.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 13aa882 · PR result: 7f7d639 · 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: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

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Actionable comments posted: 2

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @apps/mobile/package.json:
- Line 149: Remove the direct babel-plugin-react-compiler dependency from the
mobile package manifest because babel-preset-expo already provides it; only
retain a direct pin if required, and then add a scoped Knip exception for it.

Review comments at @oxlint-plugin-t3code/rules/no-memo-directives.ts:
- Line 16: Update the directive guard in the rule to exclude both null and
undefined before calling MEMO_DIRECTIVES.has, so only a string reaches the Set
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[juliusmarminge]

Update, fix 7f7d639 (re-measured 2026-10-17): same setup (Pixel 9 API 36 on nucbox-1, Metro and an isolated server on cups, base 13aa882, stream-reply, Claude Haiku 5.5, 12 s window, median of 3), harness at 8d41c04 (state slots count only when the hook has an update queue).

	main	#20967 + fix
total renders	3163	1249
commits	39	33
renders per commit	77	38
ThreadListV2Row renders	0	0

Runs (renders / commits): main 3163/41, 3163/39, 2047/31; #20967 + fix 1243/33, 1726/40, 1249/33. Before the fix, but after the earlier session's data had aged, compiled #20967 measured 1499/37, 1093/29, 1673/38 with 0 row renders, so the storm below did not reproduce in this session either way (see the cause below).

Likely cause (not reproduced live). ThreadListV2Row hook slot 19 is snoozeGateTick, the useState(0) the snooze-gate timer bumps; I confirmed the slot on device with an inspector probe that dumps hook queues. The row read new Date() in render to compute resolveThreadListV2SnoozeGateExpiryMs. That value is non-null while a turn start is still queued: a user message within 2 minutes and no run timestamp after it. Compiled, the result is cached on thread, so its clock freezes. After expiry the timer bumps the tick, and the re-render reuses the stale value. The effect then re-arms with max(0, expiry - now) + 50 ms, which loops every 50 ms until thread changes. At about 78 ms per cycle, one such row gives about 154 renders in 12 s. The fix makes the gate's clock state, set by the timer and reset when thread changes. A gate still closed past its expiry no longer arms a timer. One thing this doesn't explain: a stuck loop should also commit while idle, and the earlier session saw 0 idle commits. So treat this as the cause of slot 19 changing, not a full account of the 179 commits.

Before fix (original measurement, head 02b4813)

Before fix, head 02b4813: the launch crash is fixed. On an Android emulator (Pixel 9 API 36) the app launches with no useMemoCache errors, and Metro logs no errors. I measured it with the device harness from #21001 (driver head f4fb974). The scenario is stream-reply: an open thread streams a Claude Haiku 5.5 turn while the composer is idle, with a 12 s window mid-stream. Median of 3, same base as main (13aa882), same device, server data and Metro setup.

	main	+#20967	Δ
feed	615	461	-25%
composer	425	336	-21%
header	470	120	-74%
other	509	13934	+2638%
total	2019	14695	+628%
commits	31	179	+477%
renders per commit	69	82	+19%

Runs (total renders / commits): main 2010/29, 2019/31, 3125/37; +#20967 9845/127, 14695/179, 15010/182.

Component (excluding host primitives)	main	+#20967
AnimatedComponent(View	88	1309
GestureDetector	0	616
AppText	8	465
NativeDetector	0	308
LeanReanimatedNativeDetector	0	308
PressabilityDebugView	24	170
AndroidAnchoredMenu	12	163
Menu	7	161
AppSymbolView	40	160
ThreadListV2Row	0	154
ThreadSwipeable	0	154
ThreadSwipeableRow	0	154
Swipeable	0	154
ThreadSwipeActions	0	154
RowPressable	0	154

Top causes, main (excluding host primitives):

• AnimatedComponent(View props:style,children: 51
• PressabilityDebugView parent-same-props<-View: 24
• TextInputHostProvider props:children: 22
• AppSymbolView parent-same-props<-View: 22
• HostView props:style,children: 20
• RNHostView props:children: 20
• SlotView props:style,children: 15
• NativeThemeHost props:style,children: 15

Top causes, +#20967 (excluding host primitives):

• AnimatedComponent(View props:style,children: 651
• AppText parent-same-props<-View: 464
• GestureDetector props:children: 462
• AnimatedComponent(View parent-same-props<-View: 318
• AnimatedComponent(View props:children: 181
• PressabilityDebugView parent-same-props<-View: 170
• ThreadListV2Row state#19: 154
• ThreadSwipeable props:children: 154

The compiler made renders go up, not down, and nearly all of the increase is in components that aren't on the thread screen:

• ThreadListV2Row and its swipe wrappers (ThreadSwipeable, ThreadSwipeableRow, Swipeable, RowPressable, GestureDetector) render about 154 times per window here and 0 times on main. The cause is ThreadListV2Row state#19. This is the home thread list, which stays mounted underneath the thread screen in the native stack. On main every row bails out; compiled, every row re-renders on most commits.
• Commits per window went from 31 to 179. On idle screens (8 s, nothing streaming) both home and thread record 0 commits, so the extra commits come from streaming updates reaching the list.
• The thread screen itself barely moved: ThreadRouteScreen, ThreadDetailScreen, ThreadFeed, ThreadComposer and ThreadHeader render 5–7 times per window on both branches. That's expected: those files are on KNOWN_BAILOUTS, so they aren't compiled. Region totals for feed, composer and header drop 21–74%, but most of that is non-thread components the region attribution sweeps in.

I haven't tracked down which hook state#19 is. A likely place to start is the selectors inside ThreadListV2Row (useThreadPr's useAtomValue(…, useCallback(…)), useSwipeRowDormant's useSyncExternalStore). If the compiler changes the identity of a selector or snapshot getter, a hook that used to bail out would now re-render on every store update. Adding src/features/threads/thread-list-v2-items.tsx to KNOWN_BAILOUTS and re-measuring would confirm it in about 10 minutes.

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