Good test. One edge case worth considering for future coverage: what happens when the Request body has already been consumed (i.e., input.body is a locked/disturbed stream)? Currently input.clone() inside serializeBody would throw. A test documenting whether that throws or gracefully skips caching would be helpful.

Unnecessary .slice() on chunks.

The chunks array was just read from a cloned request's stream in readRequestBodyChunksWithinLimit. These Uint8Array buffers are already owned by this function — nobody else holds a reference to them. The .slice() here creates a defensive copy of each chunk, but it's not needed since the buffers were freshly read from requestClone.body (which itself was cloned from input). The original input request body is untouched.

This is a minor allocation concern, not a bug.

File size check doesn't account for JSON serialization overhead.

The size check uses val.size (raw file bytes), but pushBodyChunk then calls JSON.stringify(...) which adds the key name, {kind: "file", name: ..., type: ..., value: ...} wrapper, and JSON escaping overhead. For a file right at the size limit, the serialized JSON string will be larger than val.size, potentially exceeding the limit without being caught by this pre-check.

In practice this is unlikely to matter (the pushBodyChunk function itself also checks totalBodyBytes against the limit), but the pre-check here is misleadingly precise. Not blocking — just noting the discrepancy.

Non-form Request bodies don't go through pushBodyChunk for size checking.

When a Request body has a non-form content type, the chunks are decoded and pushed directly into bodyChunks (line 315) without going through pushBodyChunk, which means totalBodyBytes is not updated and the per-chunk size limit in pushBodyChunk is never hit. Only the final trailing chunk on line 319 goes through pushBodyChunk.

This is partially mitigated by the streaming size check in readRequestBodyChunksWithinLimit (which enforces the 1 MiB limit during reading), so this won't cause an actual over-limit key. But it means totalBodyBytes is inaccurate for the rest of the function, which could matter if future code depends on it.

Boundary stripping only applies to multipart/form-data, not application/x-www-form-urlencoded.

This is correct behavior (urlencoded doesn't have boundaries), but the conditional reads a bit awkwardly. Consider adding a brief comment explaining the asymmetry, since a reader might wonder why only multipart gets canonicalizedContentType treatment.

Also: when formContentType is "application/x-www-form-urlencoded", canonicalizedContentType stays undefined, so the original content-type header (including any charset=... params) remains in the cache key via collectHeaders. The test on line 525 ("urlencoded Request bodies with different charset headers get separate cache entries") confirms this works correctly. Good.

Good test, but it's testing body passthrough for cache misses only.

This verifies that input (the Request) still has a consumable body after cache key generation on a cache miss. Worth noting: on a cache hit, the input Request is never forwarded to originalFetch, so the body state doesn't matter. And on the stale-while-revalidate path (line 558 in fetch-cache.ts), input is forwarded to originalFetch for the background refetch — so the body also needs to be intact there. This test implicitly covers that since it's the same input reference, but an explicit test for the SWR path with a Request body would be stronger.

Not blocking — the current coverage is adequate for the fix.

Good test for malformed multipart fallback behavior.

This confirms that malformed multipart bodies (where the boundary in the header doesn't match the body) bypass cache entirely (via SkipCacheKeyGenerationError) rather than falling through to raw-byte keying. This directly addresses the concern from the previous review about unstable cache keys from boundary-dependent raw bytes.

Nit: the as unknown as BlobPart[] cast is unnecessary. Uint8Array[] is already assignable to BlobPart[] since Uint8Array extends ArrayBufferView which is a valid BlobPart. This compiles fine without the cast:

Minor: when the Request body has a non-form content type, chunks are pushed directly via pushBodyChunk here (good — the prior review's concern about bypassing pushBodyChunk appears to have been addressed). However, totalBodyBytes is incremented inside pushBodyChunk based on encoder.encode(chunk).byteLength, which re-encodes the already-decoded string. Since readRequestBodyChunksWithinLimit already enforced the 1 MiB limit on the raw bytes, this double-encoding is safe but slightly wasteful for large bodies. Not a real issue — just noting for awareness.

Minor: the val.size pre-check here measures raw file bytes, but the actual data pushed to pushBodyChunk is a JSON string that includes the key name, {kind, name, type, value} wrapper, and JSON escaping. For files near the limit, the serialized JSON will be somewhat larger than val.size. This is effectively a non-issue because pushBodyChunk also enforces the limit on the serialized output, so worst case a slightly-over-limit file gets caught one call later. Just noting the asymmetry.

Worth a brief comment explaining the asymmetry: canonicalizedContentType is only set for multipart/form-data (to strip the boundary) but not for application/x-www-form-urlencoded (which has no boundary to strip). The charset and other params for urlencoded are preserved in the cache key via collectHeaders. The test at line 567 confirms this is correct, but a reader unfamiliar with the code might wonder why only multipart gets special treatment.

Non-blocking — just a readability suggestion.