fix(fetch): intercept fetch on fetch level by kettanaito · Pull Request #863 · mswjs/interceptors

kettanaito · 2026-10-08T16:18:28Z

⚠️ Do not merge

This isn't worth the trouble. Once nodejs/undici#5981 is merged and published, bumping the Node.js 26 version will resolve this issue.

Changes

Node fetch interceptor is no longer socket-based. Since feat!: TCP/TLS wrap-based interception #770 it rode on the shared NodeHttpRequestSource: undici
dispatched to a wrapped socket, the raw bytes were parsed back into a Request, and the request event fired in
whatever async context undici's connection pool happened to be in. Now globalThis.fetch is patched directly, same
model as web.ts: Request is built from the caller's args, request event fires synchronously-ish in the caller's
frame, passthrough is realFetch(request.clone(), init) with dispatcher and all other init forwarded verbatim.
No shared code paths changed in behavior for other interceptors. forward-events.ts untouched. ClientRequest/Http
interceptors still socket-based. Fetch-only utils (follow-redirect, create-network-error, clone-response property
descriptors) gained spec-alignment fixes that web.ts also benefits from.

Potential impact

Async context is caller-owned. Fixes the Node 26 flake root-cause: undici 8's https protocol probe drains queued
requests in the first request's ALS frame. Any user relying on ALS inside request listeners (msw boundary(),
per-request logging) now works regardless of undici version.
Listeners see the request as constructed, not as sent on the wire. credentials reflects init, connection: upgrade
and other headers undici would reject reach listeners first, no OPTIONS preflight events for http-polyfill fetch
(happy-dom). Mocked Response.type is default. Response listeners see decompressed bodies.
Passthrough clones once more. Body is read twice at most (listener clone + passthrough clone). Memory-leak-style
tests counting clone() calls (msw many-request-handlers*) shift by one. Streaming request bodies on passthrough are
consumed through the clone, not replayed from raw bytes.
Double handling risk. FetchInterceptor + ClientRequestInterceptor in one BatchInterceptor would previously dedupe
at the socket; now a passed-through fetch over an http-based polyfill is guarded by requestContext.run({ initiator
}), but native undici fetch is only seen by the fetch interceptor. Users mixing both see each request once, from
the fetch side only.
Error surface matches undici. Rejections are TypeError('fetch failed', { cause }), abort rejects with the signal
reason, mocked 1xx rejects, HEAD responses are body-less to the caller.
Undici decoupling. No symbol access, no dispatcher snapshotting; survives undici internals changes. Trade-off:
undici's pre-dispatch validation no longer runs before listeners (msw ws.upgrade test needs its expectation
adjusted).

Implications

This does have serious implications on the interception:

Interception now happens before Undici, not within. It now misses any request validation, default headers, redirect handling, etc. That's a huge loss.
Fetch redirects now listen our internal logic instead of relying entirely on Undici.

coderabbitai · 2026-10-08T16:18:39Z

No actionable comments were generated in the recent review. 🎉

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📒 Files selected for processing (1)

src/interceptors/fetch/node.ts

🚧 Files skipped from review as they are similar to previous changes (1)

src/interceptors/fetch/node.ts

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📝 Walkthrough

Walkthrough

The Node fetch interceptor now wraps global fetch directly and handles passthrough and mocked responses. It emits response events and applies redirect handling. Tests cover request context, response behavior, WebSocket upgrade headers, credentials, and redirects.

Changes

Fetch interception

Layer / File(s)	Summary
Direct fetch interception and request lifecycle `src/interceptors/fetch/node.ts`, `test/features/events/response.test.ts`, `test/modules/fetch/features/fetch-async-context.test.ts`, `test/modules/WebSocket/compliance/websocket-fetch-upgrade.test.ts`, `test/modules/fetch/intercept/fetch-intercept.neutral.test.ts`	The Node interceptor patches global `fetch`, dispatches requests through request handling, and handles passthrough requests, context, aborts, and errors. Tests cover response events, async context, WebSocket upgrade headers, and credentials.
Mocked response handling `src/interceptors/fetch/node.ts`, `src/interceptors/fetch/utils/create-network-error.ts`, `src/interceptors/fetch/web.ts`, `src/utils/clone-response.ts`, `test/modules/fetch/response/*`, `test/modules/fetch/compliance/fetch-response-compression.neutral.test.ts`	Mocked responses handle error and informational statuses, decompression, response URLs, response events, and `HEAD` bodies. Tests check response properties and decoded bytes.
Redirect handling `src/interceptors/fetch/utils/follow-redirect.ts`, `src/interceptors/fetch/web.ts`, `test/modules/fetch/compliance/fetch-follow-redirects.neutral.test.ts`, `test/modules/fetch/response/fetch-redirect-response-event.test.ts`	Redirect handling returns responses without a `Location` header unchanged and accounts for streaming bodies and method changes. Tests cover redirect results and response events.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant GlobalFetch
  participant RequestController
  participant RealFetch
  participant ResponseListener
  Caller->>GlobalFetch: fetch request
  GlobalFetch->>RequestController: handleRequest
  alt Passthrough request
    RequestController->>RealFetch: fetch cloned request with init options
    RealFetch-->>GlobalFetch: response
  else Mocked request
    RequestController-->>GlobalFetch: mocked response
  end
  GlobalFetch->>ResponseListener: emit response event when listeners exist
  GlobalFetch-->>Caller: resolve response

Merge Risk: ⚪ Minimal · up to 79c1d

The change moves Node fetch interception to patching global fetch directly. No remaining merge-blocking risk is evident from the supplied review context.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 15 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

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 identifies the primary change: moving fetch interception to the fetch level.
Description check		The description directly explains the fetch interception redesign, its rationale, behavior changes, risks, and affected utilities.

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pkg-pr-new · 2026-10-08T16:19:52Z

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commit: 79c1d75

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/interceptors/fetch/node.ts:
- Around line 87-95: Move the fetchInit snapshot in the fetch interceptor to
immediately after constructing request, before request handling can yield.
Preserve its existing fields so implementation-specific options such as
dispatcher are forwarded, and keep realFetch using that snapshot.

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📥 Commits

Reviewing files that changed from the base of the PR and between 49d7901 and 17377e5.

📒 Files selected for processing (15)

src/interceptors/fetch/node.ts
src/interceptors/fetch/utils/create-network-error.ts
src/interceptors/fetch/utils/follow-redirect.ts
src/interceptors/fetch/web.ts
src/utils/clone-response.ts
test/features/events/response.test.ts
test/modules/WebSocket/compliance/websocket-fetch-upgrade.test.ts
test/modules/fetch/compliance/fetch-follow-redirects.neutral.test.ts
test/modules/fetch/compliance/fetch-response-compression.neutral.test.ts
test/modules/fetch/features/fetch-async-context.test.ts
test/modules/fetch/intercept/fetch-intercept.neutral.test.ts
test/modules/fetch/response/fetch-head-request.test.ts
test/modules/fetch/response/fetch-informational-response.test.ts
test/modules/fetch/response/fetch-redirect-response-event.test.ts
test/modules/fetch/response/fetch-response-init.neutral.test.ts

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mswjs/interceptors#863 was closed unmerged. msw's maintainer expects nodejs/undici#5982, released in undici 8.12.3, to fix the post-abort reconnect leak once our Node bundles it.

### Motivation Supersedes #39655. Dependabot's version of this bump fails typecheck and several unit tests. ### Description msw is only used by tests. - `request.clone()` now keeps the declared body type, where msw 2 returned a plain `Request` whose `json()` was `any`. The three SQL handlers that destructure the body now declare it with `http.post<never, ExtendedRequest>`. - `HttpResponse` is now generic, so two pending handlers resolve a plain `Response`. - `onUnhandledRequest` is renamed to `onUnhandledFrame`. msw 3 ignores the old name at runtime, so keeping it would let unhandled requests reach the real network instead of erroring. `vitest.setup.ts` uses the new name. - msw 3 answers a few ticks later, so `AlterClusterModal`'s "closes the modal on success" test now waits for `onClose` instead of checking right after the click. - **Test server:** msw 3 mocks Node's `fetch` at the socket level (undici). When a test aborts an in-flight request, undici opens a replacement socket it never writes to. msw passes that idle socket through to the real network, and the next fetch can reuse it and fail with `fetch failed`. That made a few tests flaky under CI load. - `src/api/mocks/server.ts` now builds the server with msw's `defineNetwork` and the interceptor that patches `fetch` directly, so mocked requests never touch undici. - This uses msw's experimental API, with a TODO to return to `setupServer` once mswjs/interceptors#863 (the upstream fix, still a draft) ships. - With faster responses, one Object Explorer spinner test now holds its request open, as its sibling tests already do. msw 3 requires Node 22.12 or later. CI and the Volta pin run Node 26, and #39683 updates `engines.node`. ### Verification Existing tests, plus five consecutive full-suite runs on Node 26 (CI's Node version) with no failures. 🤖 Generated with [Claude Code](https://claude.com/claude-code)

fix(fetch): intercept fetch on fetch level

17377e5

leedqin mentioned this pull request Oct 8, 2026

console: bump msw to 3 MaterializeInc/materialize#39684

Merged

kettanaito marked this pull request as ready for review October 9, 2026 11:15

coderabbitai Bot reviewed Oct 9, 2026

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Comment thread src/interceptors/fetch/node.ts Resolved

kettanaito added the wontfix This will not be worked on label Oct 9, 2026

fix(fetch): snapshot init before passthrough

79c1d75

kettanaito marked this pull request as draft October 12, 2026 17:08

kettanaito closed this Oct 26, 2026

jwh3times mentioned this pull request Nov 6, 2026

Migrate frontend tests to MSW 3 jwh3times/GuardianTracker#469

Closed

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fix(fetch): intercept fetch on fetch level - #863

fix(fetch): intercept fetch on fetch level#863
kettanaito wants to merge 2 commits into
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fix/undici-missing-dispatcher

kettanaito commented Oct 8, 2026 •

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kettanaito commented Oct 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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⚠️ Do not merge

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Potential impact

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