HTMLRewriter: carry the input body's Content-Type onto the transformed Response

[robobun]

Problem

• HTMLRewriter.transform(new Response(Bun.file("index.html"))) returns a Response with no Content-Type. Bun.serve sends application/octet-stream and browsers download the page. A typed Blob, FormData and a string body lose their type too.
• Cause: RewriterPipe::init (src/runtime/api/html_rewriter.rs) copied only materialized headers. A Response has none until its .headers getter runs, and that getter adds a body Blob's type. A string body's text/plain is never a header: Bun.serve derives it from the body. The output body is new, so the type is gone.
• new Response(body, otherResponse), new Response(body, request) and new Request(input, response) copy headers through such a fast path too and missed the Blob type.

Fix

• HeadersRef::for_body builds the headers the getter would add. The getter, Response::clone_headers, the new Response::clone_init and the three fast paths all use it, so every copy reports what the source's .headers reports. A deleted Content-Type stays deleted.
• RewriterPipe::init takes clone_init before it consumes the body, and adds text/plain;charset=utf-8 for a string body with no Content-Type header, as StaticRoute::from_js already does. The transform(string | ArrayBuffer) overloads skip this.
• Verified: test/js/workerd/html-rewriter.test.js (6 of 11 new cases fail on stock bun) and test/js/web/fetch/response.test.ts (fast paths). Self-reviewed: 4 concerns raised, 4 addressed. Other suites in Notes.

Background

• A Response keeps headers in Init.headers: Option<HeadersRef>, a refcounted C++ FetchHeaders. new Response(body) allocates none. The .headers getter does, on first access.
• A string body never gets a header. Bun.serve reads was_string off the body and sends text/plain.
• transform(response) builds a new Response from the input's status and headers with lol-html's output as its body, and consumes the input body.

Notes

• Reading input.headers once before transform() hid the bug for Blob bodies, which is why it went unnoticed.
• In-process, transform(new Response("...")).headers.get("content-type") is now text/plain;charset=utf-8 while the input's own getter still reports null. Cloudflare Workers and the Fetch spec report text/plain;charset=UTF-8 for both. Making new Response("...").headers itself report it is the broader change tracked in Response with text body should have Content-Type of text/plain #8530 / Request doesn't set content-type for string body #17085 / fetch: send text/plain Content-Type for string request bodies #33677 and is not part of this PR. Once a body-level content-type accessor lands there, the string rule in RewriterPipe::init (and the same one in StaticRoute::from_js) folds into HeadersRef::for_body.
• When the output is still streaming as Bun.serve writes the headers (a file larger than one read), stock bun sends no Content-Type at all instead of application/octet-stream. The header fixes both paths.
• An untyped Blob, an ArrayBuffer body and a ReadableStream body imply no type and get no header, same as before.
• Response.json(data, otherResponse) with a typed-Blob otherResponse now keeps that Response's type instead of application/json. That is the same result as when otherResponse.headers had been read first, and what the spec's "set Content-Type unless present" step gives.
• test/js/web/fetch/response.test.ts has a print size snapshot of its own file size, updated for the added test.
• The Bun.serve test case covers a buffered file, a streamed (chunked) file, a typed Blob, a string, and an explicit header.
• Other suites run: test/js/workerd/html-rewriter-{end-error,leak}, test/js/web/html/html-rewriter-doctype, test/js/web/fetch/{headers,fetch_headers,body,body-clone,blob,request-cyclic-reference}, test/js/web/request/request, test/js/node/http/node-fetch, test/js/bun/http/{bun-serve-headers,bun-serve-file,serve-stream-body-error}.
• Repro on bun 1.4.2:

const rewrite = r => new HTMLRewriter().on("p", { element(e) { e.setInnerContent("bye"); } }).transform(r);

rewrite(new Response(Bun.file("index.html"))).headers.get("content-type"); // null
const input = new Response(Bun.file("index.html"));
input.headers;
rewrite(input).headers.get("content-type"); // "text/html;charset=utf-8"

// Bun.serve({ fetch: () => rewrite(new Response(Bun.file("index.html"))) })
//   -> Content-Type: application/octet-stream
// Bun.serve({ fetch: () => rewrite(new Response("<p>hi</p>")) })
//   -> Content-Type: application/octet-stream (direct: text/plain;charset=utf-8)

new Response("x", new Response(new Blob(["x"], { type: "text/html" }))).headers.get("content-type"); // null

• Related: Response: keep the body-derived Content-Type when the body is read before the headers #42015 keeps the body-derived Content-Type on Init from construction on, so it also survives a body that was read or streamed before .headers. It covers the typed-body half of this PR (with the same clone_init / clone_headers entry points) but not the string text/plain rule or the Bun.serve rewriter tests. Whichever lands first, the other rebases down to its remainder. webcore: fix Content-Type lost when Request/Response body is read before headers #32913 is the older per-consumer take on the same ordering bug and leaves HTMLRewriter.transform alone.

[coderabbitai]

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📒 Files selected for processing (4)

• docs/runtime/html-rewriter.mdx
• src/runtime/api/html_rewriter.rs
• src/runtime/webcore/Response.rs
• test/js/workerd/html-rewriter.test.js

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Walkthrough

The change propagates input response headers through HTMLRewriter.transform(). Missing headers now derive from response bodies, including typed Blob Content-Type values and string-body defaults. Tests cover files, Blobs, data: URLs, FormData, custom headers, deleted headers, and Bun.serve.

Changes

HTMLRewriter response headers

Layer / File(s)	Summary
Response header derivation src/runtime/webcore/Response.rs	clone_headers() now falls back to body-derived headers. get_or_create_headers() creates Content-Type for typed Blob bodies. set_init_headers() was removed.
HTMLRewriter header propagation and coverage src/runtime/api/html_rewriter.rs, test/js/workerd/html-rewriter.test.js, docs/runtime/html-rewriter.mdx	RewriterPipe clones input headers before output initialization. Tests cover header preservation, derived content types, deleted headers, multipart boundaries, and Bun.serve forwarding. The documentation describes the header derivation rules.

Suggested reviewers: dylan-conway, jarred-sumner, alii

Merge Risk: 🔵 Low · up to 5e66b

Transformed responses now preserve or derive Content-Type values across several body types. File-URL fetch inputs may still lack regression protection for this behavior, creating a bounded compatibility risk.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description check	✅ Passed	The description clearly explains the problem, implementation, scope, verification, test coverage, and related work. It uses different headings from the template, but it includes the required informati…
Title check	✅ Passed	The title clearly and concisely describes the primary change: preserving the input body's Content-Type on the transformed Response.

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[robobun]

^{Updated 7:21 AM PT - Sep 8th, 2026}

❌ @robobun, your commit 7cc64a8 has 1 failures in Build #112888 (All Failures):

• test/js/bun/dns/resolve-dns.test.ts - code 1 on 🍎 any aarch64

---

🧪   To try this PR locally:

bunx bun-pr 38570

That installs a local version of the PR into your bun-38570 executable, so you can run:

bun-38570 --bun

[robobun]

Status: reproduced on bun 1.4.2 and on main with new Response(Bun.file("index.html")), a typed Blob, FormData, fetch("data:text/html,...") and a plain string body as inputs: the transform() output had no Content-Type, and Bun.serve sent application/octet-stream (or no type at all when the output was still streaming); the direct, untransformed Responses sent text/html;charset=utf-8 / text/plain;charset=utf-8. Reading input.headers first hid it for the Blob cases. new Response(body, otherResponse) / new Response(body, request) showed the same gap.

Fix is in this PR: HeadersRef::for_body + Response::clone_init used by RewriterPipe::init and the Response/Request init fast paths, plus text/plain;charset=utf-8 for a string input body. Rebased on main on 2026-09-08. The cases in test/js/workerd/html-rewriter.test.js (transform() carries the input Response's Content-Type) fail on the unfixed build (6 of 11) and pass with it; so does the new init case in test/js/web/fetch/response.test.ts.

CI on 7cc64a8 (#112888): every lane green except test/js/bun/dns/resolve-dns.test.ts on darwin aarch64 (getaddrinfo ETIMEOUT on a negative lookup), which this diff does not touch. Overlaps with #42015, see the PR body.

[claude]

LGTM — focused fix that carries a Blob body's Content-Type onto the transformed Response, with a small behavior-preserving refactor of get_or_create_headers.

What was reviewed:

• clone_headers is called before wire_input takes the body (html_rewriter.rs:925 vs :1039), and set_init at :1011 doesn't touch headers, so the copied headers survive.
• create_headers_from_body: on put() failure the local HeadersRef is dropped (C++ deref) — no leak; get_or_create_headers refactor is behavior-preserving except for the documented OOM edge.
• Removed set_init_headers/clone_init_headers have no other callers (grepped src/).
• Tests cover Bun.file, typed Blob, data: URL, FormData boundary, headers-init, deleted-Content-Type, untyped Blob, and a Bun.serve round-trip; all use tempDir/port: 0/await using.

Extended reasoning...

Overview

This PR fixes HTMLRewriter.transform(response) dropping the input's Content-Type when the input Response was constructed without an explicit headers init (e.g. new Response(Bun.file(...)), typed Blob, FormData, fetch('data:...')). Three files touched: src/runtime/api/html_rewriter.rs (move header clone before wire_input, pass into Init), src/runtime/webcore/Response.rs (replace clone_init_headers with clone_headers that also derives from the body Blob; extract create_headers_from_body and reuse it in get_or_create_headers; delete two dead helpers), and test/js/workerd/html-rewriter.test.js (8 new tests).

Security risks

None. This only affects which Content-Type header is propagated onto a rewriter's output Response. No auth, crypto, permissions, or untrusted-input parsing paths are involved. Blob content-type strings are already sanitized upstream.

Level of scrutiny

Medium. Response.rs is core Web API surface and get_or_create_headers is called from the .headers getter, Response.json(), Response.redirect(), and the constructor. I traced the refactor: when init.headers is None, both old and new code create an empty FetchHeaders and conditionally put the Blob's Content-Type; the only difference is that the new code assigns init.headers after the fallible put() instead of before, which the PR description documents (headers left unset instead of stored empty on OOM). The HeadersRef local's Drop releases the C++ ref if put() fails, so there is no leak on that path. set_init (called after the output Response is built) only writes method/status_code/status_text, so the headers passed via Init { headers, ... } is preserved.

Other factors

• Verified via grep that the removed set_init_headers and clone_init_headers have no remaining callers.
• clone_headers is called at html_rewriter.rs:925 and wire_input at :1039, so the body is still a Blob when the Content-Type is read — the ordering the fix depends on holds.
• Test coverage is thorough: positive cases (Bun.file, typed Blob, data: URL, FormData boundary, Bun.serve round-trip) that fail on the unfixed build, plus negative pins (headers-init case, deleted Content-Type stays deleted, untyped Blob gets no Content-Type). Tests follow harness conventions (tempDir, port: 0, await using, exact-value assertions).
• No CODEOWNERS on these paths. No prior human review comments to address. CI build is running.

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@test/js/workerd/html-rewriter.test.js`:
- Around line 1591-1597: Add a sibling test alongside the existing data-URL case
that calls fetch() with a file: URL, passes the response through rewrite(), and
asserts the preserved Content-Type and rewritten body using the same
contentTypeAndBody expectations.
- Around line 1556-1558: Add the Bun issue 3334 URL to the regression comment
above the affected test, keeping it as the single issue-URL comment required for
regression tests.

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• src/runtime/api/html_rewriter.rs
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• test/js/workerd/html-rewriter.test.js

[claude]

I reviewed this PR and didn't find any bugs. The fix is focused and well-tested, but since it refactors Response::get_or_create_headers (which backs every .headers access, not just HTMLRewriter), a human look would still be worthwhile.

What was reviewed:

• clone_headers ordering vs. wire_input — called before the body is consumed, so create_headers_from_body still sees the Blob.
• HeadersRef lifetime on error paths — the local drops via RAII if put throws; no leak between allocation and move into Init.
• get_or_create_headers refactor is behavior-preserving except the documented OOM edge (headers left unset instead of stored empty).
• Confirmed set_init_headers/clone_init_headers have no remaining callers.

Extended reasoning...

Overview

Three files: src/runtime/webcore/Response.rs (adds clone_headers + create_headers_from_body, refactors get_or_create_headers to use the new helper, removes set_init_headers/clone_init_headers), src/runtime/api/html_rewriter.rs (calls clone_headers before wire_input and passes the result into the output Init instead of setting it afterward), and 8 new test cases in test/js/workerd/html-rewriter.test.js.

Security risks

None identified. No untrusted-input parsing, no auth/crypto surface. The Content-Type value comes from the input Response's own Blob (content_type_slice()), which is already sanitized when the Blob is constructed.

Level of scrutiny

Medium. The HTMLRewriter-side change is small and localized, but the get_or_create_headers refactor is in a shared helper backing the .headers getter for every Response, so a regression there would be broadly visible. I traced the old vs. new logic and they produce the same outcome (empty headers when the body has no CT; headers with the Blob's CT otherwise), with the one intentional difference on the put-throws-OOM path noted in the PR description.

I also checked memory ownership: create_headers_from_body builds a local HeadersRef before storing, so a failing put drops it via RAII; in RewriterPipe::init the headers local is moved into Init with no fallible ops in between (only heap::alloc_nn and struct construction), so no leak window there either. headers.put on a BunString::ascii value is pure C++ FFI with no user-JS re-entry, so the R-2 borrow discipline is preserved.

Other factors

Test coverage is strong (Bun.file, typed Blob, fetch(data:), FormData boundary, headers-init merge, deleted CT stays deleted, untyped Blob stays null, Bun.serve round-trip). The one CI failure (s3.test.ts on Windows with a 502 Bad Gateway) is unrelated infrastructure. There is still a comment-cop bot flag on line 923 of the current commit and two minor CodeRabbit test-coverage nits — nothing correctness-related, but a maintainer may want to accept or dismiss them before merge.

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

#42015 overlaps with this: it replaces clone_init_headers with an Init clone that carries the input body's Content-Type (kept as a pending value until a header list exists), and RewriterPipe::init writes it into the output's header list. That covers the typed-body cases here (Bun.file(), Blob, FormData). The text/plain default for a string-body input is not in #42015; if that PR lands first, this one reduces to that default on top of it.