fix(error-tracking): excuse the Gecko truncated-response fetch failure

[posthog]

Problem

• A Firefox user whose connection drops mid-response opens an error tracking issue. Nothing of ours failed and the app already recovers, so the issue is pure noise.
• shouldReportApiFailure excuses a transport failure by matching the engine's wording against BROWSER_FETCH_FAILURE_MESSAGES. Gecko has two wordings, and only one was on the list.
• The missing one is global to every kea loader, so any surface can file. The replay player retries a failed source three times and files once per attempt.

Changes

• Firefox truncated responses stop reaching posthog.captureException. They keep toasting wherever they did, and client_request_failure still records the rate.
• The mechanism is one added string in BROWSER_FETCH_FAILURE_MESSAGES:

 export const BROWSER_FETCH_FAILURE_MESSAGES: readonly string[] = [
     'Failed to fetch',
     'Load failed',
     'NetworkError when attempting to fetch resource',
+    'Content-Length header of network response exceeds response Body',
 ]

Engine	Connection dropped	Response arrived short
Chromium	Failed to fetch	—
WebKit	Load failed	—
Gecko	NetworkError when attempting to fetch resource.	Content-Length header of network response exceeds response Body.

• The trailing period is left off, as with the existing Gecko entry — the match is includes.
• MODULE_LOAD_FAILURE_MESSAGES still runs first, so a stale chunk after a deploy keeps reporting.

How did you test this code?

• Added one case to the existing parameterized shouldReportApiFailure block in frontend/src/lib/api-error.test.ts. It catches the regression that no existing case did: a Gecko truncated-response TypeError classified as reportable.
• Ran hogli test frontend/src/lib/api-error.test.ts locally; the suite passes.
• No manual browser reproduction. The wording was confirmed against the exception events in the project rather than by inducing a truncated response in Firefox.

Automatic notifications

• Publish to changelog?

Docs update

None.

🤖 Agent context

Autonomy: Fully autonomous

Agent: Claude Code, Opus 5

• No duplicate: gh pr list --state open --search over the relevant keywords found nothing.
• The string was read off the project's own $exception events, not guessed from memory. An earlier guess at Gecko's wording would have been wrong.
• The change deliberately stops at the allowlist. Widening isBrowserNetworkFailure to match on TypeError would have buried real crashes, which the existing comment already warns about.
• Public artifact: the added string is a browser engine's own error text, already public. Nothing from the session is in the diff.
• CodeRabbit CLI was not available in this environment, so the PR opens without a local review pass.
• Skills invoked: writing-simplified-technical-english.

---

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• frontend/src/lib/api-error.test.ts
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---

📝 Walkthrough

Walkthrough

The browser failure classification now recognizes Gecko’s truncated-response TypeError message. Documentation covers incomplete or dropped responses. A test confirms that this failure is not reported.

Priority: ⬇️ Low

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description is complete and standalone. It explains the problem, user impact, implementation, preserved behavior, regression test, test command, lack of manual browser testing, documentation statu…

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[github-actions]

🤖 CI report

✅ Trunk lane — non-backend lane

This PR is assigned to the non-backend lane. It does not run backend Python tests and may merge in parallel with PRs in other lanes.

⚠️ Complexity (TypeScript) — 1 function above the limit (max 14)

Cyclomatic complexity above the limit in changed typescript files (10 for production files, 15 for test files). Warn only: worth simplifying when you next touch these functions.

Function	Location	Complexity	Limit
shouldReportApiFailure	frontend/src/lib/api-error.ts:194	14	10

✅ Duplication (Python) — clean

New Python code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

✅ Duplication (TypeScript) — clean

New TypeScript code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

⚠️ Bundle size — 🔺 +66 B (+0.0%)

Uncompressed size of every built .js bundle, compared against the base branch.

Total: 69.20 MiB · 🔺 +66 B (+0.0%)

No file changed by more than 1000 B.

_{Posted automatically by build-bundle-size-report · uncompressed bytes from dist-report}

✅ Eager graph — within budget

How much code each root ships on the eager path — downloaded and parsed before the surface is interactive. Measured from the esbuild output chunks (post-tree-shake, static imports only); lazy import() / React.lazy chunks are not counted.

Root	Eager (shipped)	Δ vs base	Budget
entry (logged-out pages, app bootstrap) src/index.tsx	1.56 MiB · 22 files	no change	████████░░ 84.6% of 1.84 MiB
logged-out boot: index + App + bootApp (preloaded by every page, including /login) src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts	3.55 MiB · 628 files	🔺 +66 B (+0.0%)	█████████░ 88.2% of 4.03 MiB
authenticated shell (every logged-in page) src/scenes/AuthenticatedShell.tsx	8.39 MiB · 2,772 files	🔺 +66 B (+0.0%)	█████████░ 88.3% of 9.51 MiB

🟢 node_modules/monaco-editor/ stays out of src/index.tsx
🟢 src/lib/components/ActivityLog/describers stays out of src/index.tsx
🟢 [object Object] stays out of src/index.tsx
🟢 [object Object] stays out of src/index.tsx
🟢 node_modules/monaco-editor/ stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/layout/navigation-3000/navigationLogic.tsx stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/scenes/dashboard/dashboardLogic.tsx stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/lib/lemon-ui/LemonMarkdown/ stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/lib/components/RichContentEditor/ stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/lib/components/CodeSnippet/ stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 src/taxonomy/core-filter-definitions-by-group.json stays out of src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts
🟢 node_modules/monaco-editor/ stays out of src/scenes/AuthenticatedShell.tsx
🟢 src/lib/components/ActivityLog/describers stays out of src/scenes/AuthenticatedShell.tsx
🟢 [object Object] stays out of src/scenes/AuthenticatedShell.tsx
🟢 src/scenes/session-recordings/player/sessionRecordingPlayerLogic.ts stays out of src/scenes/AuthenticatedShell.tsx
🟢 [object Object] stays out of src/scenes/AuthenticatedShell.tsx
🟢 [object Object] stays out of src/scenes/AuthenticatedShell.tsx

Largest files eagerly shipped from src/index.tsx

Size	File
126.8 KiB	../node_modules/.pnpm/react-dom@18.3.1_react@18.3.1/node_modules/react-dom/cjs/react-dom.production.min.js
24.6 KiB	../node_modules/.pnpm/buffer@6.0.3/node_modules/buffer/index.js
6.3 KiB	../node_modules/.pnpm/react@18.3.1/node_modules/react/cjs/react.production.min.js
4.5 KiB	../node_modules/.pnpm/@jspm+core@2.1.0/node_modules/@jspm/core/nodelibs/browser/process.js
3.9 KiB	../node_modules/.pnpm/scheduler@0.23.2/node_modules/scheduler/cjs/scheduler.production.min.js
1.4 KiB	../node_modules/.pnpm/base64-js@1.5.1/node_modules/base64-js/index.js
1.3 KiB	src/index.tsx
1.3 KiB	src/RootErrorBoundary.tsx
912 B	../node_modules/.pnpm/ieee754@1.2.1/node_modules/ieee754/index.js
854 B	src/scenes/ChunkLoadErrorBoundary.tsx

Largest files eagerly shipped from src/index.tsx + src/scenes/App.tsx + src/scenes/bootApp.ts

Size	File
301.9 KiB	../node_modules/.pnpm/posthog-js@1.434.4_@types+react@18.3.27_react@18.3.1/node_modules/posthog-js/dist/module.mjs
267.7 KiB	../node_modules/.pnpm/@posthog+icons@0.38.0_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/@posthog/icons/dist/posthog-icons.es.js
126.8 KiB	../node_modules/.pnpm/react-dom@18.3.1_react@18.3.1/node_modules/react-dom/cjs/react-dom.production.min.js
104.6 KiB	src/lib/api.ts
82.6 KiB	src/products.tsx
68.5 KiB	src/lib/lemon-ui/icons/icons.tsx
63.4 KiB	src/lib/utils/eventUsageLogic.ts
38.8 KiB	../node_modules/.pnpm/@dnd-kit+core@6.0.8_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/@dnd-kit/core/dist/core.esm.js
33.9 KiB	../node_modules/.pnpm/kea@4.0.0-pre.6_patch_hash=139b8d1f1304f9d9da452a9a1244c94ea679dbcb85687d8999563146879fb6f5_react@18.3.1/node_modules/kea/lib/index.cjs.js
28.5 KiB	src/scenes/scenes.ts

Largest files eagerly shipped from src/scenes/AuthenticatedShell.tsx

Size	File
301.9 KiB	../node_modules/.pnpm/posthog-js@1.434.4_@types+react@18.3.27_react@18.3.1/node_modules/posthog-js/dist/module.mjs
267.7 KiB	../node_modules/.pnpm/@posthog+icons@0.38.0_react-dom@18.3.1_react@18.3.1__react@18.3.1/node_modules/@posthog/icons/dist/posthog-icons.es.js
265.0 KiB	src/taxonomy/core-filter-definitions-by-group.json
153.8 KiB	../node_modules/.pnpm/re2js@0.4.1/node_modules/re2js/build/index.esm.js
126.8 KiB	../node_modules/.pnpm/react-dom@18.3.1_react@18.3.1/node_modules/react-dom/cjs/react-dom.production.min.js
104.6 KiB	src/lib/api.ts
98.4 KiB	../packages/quill/packages/quill/dist/index.js
93.3 KiB	../node_modules/.pnpm/prosemirror-view@1.40.1/node_modules/prosemirror-view/dist/index.js
90.6 KiB	../node_modules/.pnpm/@tiptap+core@3.20.6_@tiptap+pm@3.20.6/node_modules/@tiptap/core/dist/index.js
82.6 KiB	src/products.tsx

_{Posted automatically by check-eager-graph · sizes are eager output bytes (shipped, post-tree-shake) from the esbuild metafile · part of #32479}

✅ Toolbar bundle — eager 2.36 MiB within budget

What the toolbar ships to customer pages, measured from the esbuild output (minified, post-tree-shake). The eager set is the entry plus everything statically imported from it — fetched before any feature runs; deferred chunks load lazily. The eager guardrail is 5.72 MiB. Each output file must also stay below 10 MB, where CloudFront stops compressing it. The module boundary is enforced separately by check-toolbar-graph.

Metric	Size	Δ vs base	Budget
Eager (shipped) entry + static imports	2.36 MiB · 19 files	no change	████░░░░░░ 41.2% of 5.72 MiB
Deferred (lazy)	2.09 MiB · 44 files	no change	n/a — loads on demand
Loader dist/toolbar.js	1.2 KiB	no change	█░░░░░░░░░ 6.0% of 19.5 KiB

Largest eagerly-shipped chunks

Size	File
782.3 KiB	dist/toolbar/toolbar-app-BE564NTX.css
649.5 KiB	dist/toolbar/chunk-chunk-GZKLML6Z.js
483.6 KiB	dist/toolbar/chunk-chunk-2PKLVDXN.js
138.1 KiB	dist/toolbar/chunk-chunk-2JKMD6OO.js
131.8 KiB	dist/toolbar/chunk-chunk-FDH2IBXT.js
75.1 KiB	dist/toolbar/toolbar-app-ELGH35JS.js
69.0 KiB	dist/toolbar/chunk-chunk-TSAL54PB.js
35.6 KiB	dist/toolbar/chunk-chunk-NKOHS3J4.js
21.0 KiB	dist/toolbar/chunk-chunk-TSQMXSSY.js
6.8 KiB	dist/toolbar/chunk-chunk-DV7IWQNF.js

_{Posted automatically by check-toolbar-size · sizes are toolbar output bytes (shipped, post-tree-shake) from the esbuild metafile}

✅ Dist folder size — 🔺 +1.1 KiB (+0.0%)

Total size of the built frontend/dist folder (all assets), compared against the base branch.

Total: 1510.66 MiB · 🔺 +1.1 KiB (+0.0%)

[posthog]

👋 Visual changes detected for this PR.

Review and approve in PostHog Visual Review

If these changes are unexpected, they may be caused by a flaky test or a broken snapshot on master. Don't approve — rerun the job or wait for a fix.

[trunk-io]

   

Failed Test	Failure Summary	Logs
Scenes-App/SidePanels SidePanelNotebooks smoke-test	The test timed out while waiting for a loading indicator or spinner to disappear.	Logs ↗︎

View Full Report ↗︎ ⋅ Docs

[posthog]

Visual review gate: the 4 diffs are calendar drift, not this PR

This PR changes one string in frontend/src/lib/api-error.ts and one test case. It renders no UI, so the 4 changed snapshots cannot come from it. Triaged rather than approved, per the bot's own advice above.

What changed. All 4 are the same story in two states and two themes:

products-data-modeling-model-health-summary--completed--{dark,light}
products-data-modeling-model-health-summary--running--{dark,light}

Comparing baseline against current, the only visible delta is one line of text:

	Last successful refresh
Baseline (master, 2026-09-14)	2 days ago
This run (2026-09-21)	10 days ago

Root cause. ModelHealthSummary.stories.tsx pins an absolute date and the card renders it as a relative string:

lastSuccessfulSyncAt: '2026-09-12T10:00:00Z',

So the rendered text drifts by exactly the age of the baseline — 8 days here, matching the 8 days between the baseline commit and this run. Re-running CI will reproduce it identically. Dimensions are unchanged (896x130) and diff is ~0.38%, consistent with a few glyphs.

The sibling createdAt="2026-01-10T10:00:00Z" renders "8 months ago", which is why only the refresh line moves day to day.

Not specific to this branch. Three other open PRs on unrelated work show the identical signature (total 6018 / changed 4 / unchanged 6014): #104122, #104123, #104126.

Suggested fix, in its own PR — make the fixture relative so the rendered string is constant, e.g. derive lastSuccessfulSyncAt from dayjs().subtract(2, 'day'), or freeze the clock for the story. That belongs with the data modeling owners rather than in an error-tracking change.

Not approving or finalizing the baseline here: the diff is in another product's component and re-baselining it from this PR would hide the drift rather than fix it.