fix: normalize headers reassigned in an onRequest hook

[greymoth-jp]

resolveFetchOptions returns options.headers as a Headers instance, but an onRequest hook can reassign it to a plain object (a valid HeadersInit). In v2 the request flow reads context.options.headers.get("content-type") immediately after the hook runs, so reassigning a plain object throws TypeError: context.options.headers.get is not a function on any payload request with a JSON-serializable body.

This was already handled on v1 in #524 (and #436 before it), but the v2 rewrite reintroduced it. The normalization now lives inside the JSON-body branch and runs after that first .get() call, so it never gets a chance to run.

Repro:

await $fetch("/api", {
  method: "POST",
  body: { hello: "world" },
  onRequest(ctx) {
    ctx.options.headers = { "x-foo": "bar" }; // throws before this fix
  },
});

The fix mirrors #524: re-normalize options.headers back to a Headers instance right after the onRequest hook, and drop the now-redundant re-creation in the body branch (resolveFetchOptions already returns a fresh Headers, so the later .set() calls can mutate it directly).

Added a regression test covering both a Headers instance and a plain object reassigned in the hook, matching the v1 test. The new test fails without the change (headers.get is not a function) and passes with it. Full suite (29) is green and eslint / prettier / tsc --noEmit are clean.

Summary by CodeRabbit

• Bug Fixes
  • Improved request header handling so headers remain safe to read and update after request hooks modify them.
  • Fixed an issue where reassigned headers could cause inconsistent request behavior when using different header formats.
• Tests
  • Added coverage for header reassignment scenarios to confirm requests still send the expected headers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f825c818-1ef3-4d8e-a926-58f29da14d0f

📥 Commits

Reviewing files that changed from the base of the PR and between 9102908 and 67b26c6.

📒 Files selected for processing (2)

• src/fetch.ts
• test/index.test.ts

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📝 Walkthrough

Walkthrough

In $fetchRaw, headers are now normalized to a Headers instance after onRequest hooks run, guarding against reassignment to other HeadersInit shapes. The redundant headers re-initialization in the JSON body branch is removed. A new test covers both Headers instance and plain-object reassignment scenarios.

Changes

Header normalization after onRequest hooks

Layer / File(s)	Summary
Normalization logic in $fetchRaw src/fetch.ts	After onRequest hooks run, context.options.headers is coerced to a Headers instance if it isn't one already. The redundant context.options.headers re-initialization inside the JSON body branch is removed, relying on the earlier normalization.
onRequest header reassignment tests test/index.test.ts	New test case asserting that reassigning ctx.options.headers to a Headers instance or a plain object in onRequest results in the correct normalized headers on the sent request.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐇 When headers get swapped in the hook's little game,
I wrap them in new Headers(...) all the same.
No matter the shape — object, instance, or more —
normalization ensures they behave as before.
Consistent and tidy, the fetch can proceed! 🎉

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main fix: normalizing headers reassigned in an onRequest hook.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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