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+---
+title: React Compiler Error Triage & Coverage Accounting (MetaMask)
+impact: HIGH
+tags: react-compiler, panicThreshold, error-triage, coverage, babel, build
+---
+
+# Skill: React Compiler Error Triage & Coverage Accounting
+
+The React Compiler **fails open**: when it can't compile a component, it silently skips it and ships the unoptimized original. The build stays green, DevTools shows no warning — you just don't get the memoization. Once the compiler is enabled broadly (metamask-mobile#31171 enabled v1.0.0 app-wide), the question stops being "is it on?" and becomes **"what is it actually compiling, and which of its errors are worth fixing?"** This file is the triage playbook. Extension PR metamask-extension#38007 is the reference implementation.
+
+## The `panicThreshold` ladder
+
+`panicThreshold` controls when a compiler diagnostic fails the build instead of silently skipping the file:
+
+| Setting | Build fails on | Use for |
+|---|---|---|
+| `'none'` (default) | never — every failed file is **silently skipped** | production builds, always |
+| `'critical_errors'` | only critical errors (compiler-internal invariant violations) | CI / debug builds, first ratchet target |
+| `'all_errors'` | every diagnostic, including unsupported syntax | CI / debug builds, end-state ratchet |
+
+**The ratchet strategy** (extension roadmap, MetaMask-planning#6552 → #6553): keep production at `'none'` permanently; aim for a *non-production* build that passes at `'critical_errors'`, then at `'all_errors'`. Each ratchet step turns a class of silent skips into a visible, fixable error list. Never enable a non-`'none'` threshold in a release build — one un-compilable file would block the release for an optimization that is optional by design.
+
+## Triage: unsupported syntax vs. legitimate errors
+
+Compiler diagnostics are **not one bucket**. The logger event's `category` field separates them, and the distinction decides whether you act:
+
+- **`category === 'Todo'` → "unsupported."** Syntax or a pattern the compiler *itself* has not implemented yet. There is **no actionable fix on our side** — rewriting working code to appease an unimplemented compiler path is wasted effort and churn. Count these separately, leave the code alone, and re-check after compiler upgrades.
+- **Any other category (e.g. `InvalidReact`, `InvalidJS`) → legitimate, actionable.** A real Rules-of-React violation in our code (mutation during render, conditional hooks, side effects in render). Fixing it both unlocks compilation *and* removes a latent correctness bug.
+
+A healthcheck that doesn't make this split is noise: the `Todo` count swamps the actionable list and the team learns to ignore the output. The extension's verbose run at enablement (metamask-extension#38007) is the canonical illustration — of 7,308 files processed: 253 compiled, **31 actionable errors**, **7,024 unsupported** (`Todo`). Without the split that reads as ~7,000 hopeless errors; with it, the team's backlog is 31 files and the rest is the compiler's to burn down across upgrades. The extension's webpack wrapper makes the split in ~10 lines:
+
+```ts
+// adapted from metamask-extension development/webpack/utils/loaders/reactCompilerLoaderWrapper.ts
+// (mobile equivalent: pass a `logger` in babel-plugin-react-compiler options)
+logger: {
+  logEvent(filename, event) {
+    switch (event.kind) {
+      case 'CompileSuccess': record(filename, 'compiled'); break;
+      case 'CompileSkip':    record(filename, 'skipped'); break;
+      case 'CompileError': {
+        const category = event.detail?.options?.category ?? event.detail?.category;
+        // 'Todo' = not yet supported by the compiler — no actionable fix on our side
+        record(filename, category === 'Todo' ? 'unsupported' : 'error');
+        break;
+      }
+    }
+  },
+}
+```
+
+The extension exposes this as `yarn webpack --reactCompilerVerbose` (per-file ✅/⏭️/🔍/❌ output + summary stats) and `--reactCompilerDebug={all|critical|none}` (maps to `panicThreshold: '<value>_errors'`). On mobile the same taxonomy is available through the Babel plugin's `logger` option or `eslint-plugin-react-compiler` (the lint rule runs the same analysis the compiler does).
+
+## Coverage accounting
+
+Track four buckets — **compiled / skipped / errors / unsupported** — at file and component granularity, with **worst-status-wins per file** (`error > unsupported > skipped > compiled`): a file with five compiled components and one error is an *error file*, otherwise mixed files inflate the compiled count and the number lies to you.
+
+What the buckets tell you:
+
+- **compiled** — your real optimization coverage. "The compiler is enabled" claims nothing; this number does.
+- **errors** — the actionable backlog. Each is a Rules-of-React fix.
+- **unsupported** — the compiler's backlog, not yours. Trend it across compiler upgrades.
+- **skipped** — intentional exclusions: test/story files, `'use no memo'` directives, and **class components** (never compiled — metamask-mobile#30919 counted 53 at full enablement; migration to function components is the only way to move them into the compiled bucket).
+
+## Staged adoption roadmap
+
+The extension's sequence (epic MetaMask-planning#6549) generalizes to any repo:
+
+1. **Lint clean:** update `eslint-plugin-react-hooks` / `eslint-plugin-react-compiler` to latest; fix violations — these are exactly what the compiler will refuse to compile.
+2. **Audit opt-outs:** every `'use no memo'` carries a reason + TODO; the count only goes down. `grep -rn "use no memo" app --include="*.ts*"`.
+3. **Ratchet `critical_errors`:** non-prod build passes; fix what surfaces.
+4. **Ratchet `all_errors`:** remaining actionable errors fixed; what's left is the `Todo` (unsupported) set, which you wait out.
+
+## Verify
+
+- Per component: `Memo ✨` badge in React DevTools (see [js-profile-react.md](js-profile-react.md)).
+- Per repo: the compiled-files count from the logger stats rises (or at least doesn't silently fall) release over release — silent coverage regressions are the failure mode this file exists to catch.
+- After a compiler version bump: re-run the verbose build and diff the `unsupported` list — `Todo`s that became `compiled` are free wins; new `error`s are regressions to triage.
+
+## Don't over-correct
+
+- **Never "fix" a `Todo`.** Rewriting working code around an unimplemented compiler feature is churn with no perf evidence; the next compiler release may compile it as-is.
+- Don't gate releases on compiler errors (`panicThreshold` stays `'none'` in production builds).
+- Don't treat `skipped` as a problem — tests, stories, and deliberate opt-outs belong there. The smell is *unexplained* `'use no memo'` directives, not the bucket itself.
+- A component without `Memo ✨` is not automatically a bug to chase — check the buckets first; it may be `unsupported`.
+
+## Related
+
+- [mm-react-compiler.md](mm-react-compiler.md) — enabling the compiler in this repo (Babel config, Metro cache, ESLint healthcheck)
+- [js-react-compiler.md](js-react-compiler.md) — how the compiler transforms code; Rules-of-React background
+- [mm-selector-cascade.md](mm-selector-cascade.md) — what the compiler **cannot** fix: unstable values crossing file boundaries (selectors, imported hooks)
diff --git a/domains/performance/skills/performance/references/mm-react-compiler.md b/domains/performance/skills/performance/references/mm-react-compiler.md
index e95d70d6..4f987cfb 100644
--- a/domains/performance/skills/performance/references/mm-react-compiler.md
+++ b/domains/performance/skills/performance/references/mm-react-compiler.md
@@ -51,6 +51,8 @@ React Compiler auto-memoizes components, callbacks, and computed values at build
 
 On opted-in paths you can gradually drop hand-written `useMemo`/`useCallback`/`React.memo` once the compiler is verified working — but do it deliberately and re-measure. Off opted-in paths, manual memoization still matters.
 
+**Exception — effect dependencies.** Keep any `useMemo`/`useCallback` whose output is used as a `useEffect` dependency, here or in a consumer: the compiler's memoization is not guaranteed to match the manual strategy, and a mismatch causes over/under-firing of effects or infinite loops — a correctness change, not a perf tweak. Official guidance is to leave existing manual memoization in place and only omit it in *new* code ([reactwg/react-compiler#16](https://github.com/reactwg/react-compiler/discussions/16)).
+
 ## What breaks compilation (it will skip the component)
 
 - Mutating props or state during render.
@@ -68,4 +70,5 @@ Fix the ESLint `react-compiler` warnings on a path before/after opting it in.
 ## Related
 
 - [js-react-compiler.md](js-react-compiler.md) — upstream reference on how the compiler transforms code
+- [mm-react-compiler-error-triage.md](mm-react-compiler-error-triage.md) — triaging compiler errors (`Todo`/unsupported vs actionable), `panicThreshold` ratcheting, and measuring real coverage
 - [mm-selector-memoization.md](mm-selector-memoization.md) — fix data-layer re-renders the compiler can't