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…he Response The Response constructor and the headers getter copy a typed body's Content-Type (URLSearchParams, FormData, a Blob with a type, Bun.file()) into the header list. A handler that then deletes it saw the delete honored in-process but undone on the wire: render_metadata, StaticRoute and FileRoute derived the type from the body a second time whenever the header list had none. Record on Response.Init that the header list owns the Content-Type once it has held one (from the init, copied from the body, or Response.json), and make the three send paths take it from the header list alone in that case. Responses whose headers were never materialized keep the allocation-free send-time derivation, and string or untyped bodies keep their defaults.
Response::get_content_type (formData(), WebAssembly.compileStreaming) now honors headers_own_content_type too, so a Response cannot be typeless on the wire and still typed for formData(). The three Bun.serve send paths ask the Response (body_content_type / headers_own_content_type) instead of taking a bare flag. Adds a set-then-delete row to the serve matrix and a formData()-after-delete test.
WalkthroughChangesResponses now track whether headers own Content-Type ownership handling
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to This change corrects Content-Type deletion behavior across response serving and parsing paths. The remaining merge-readiness risk is limited to aligning the new parameterized tests with the repository test convention. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Status: ready for review. The diff is green in CI; the remaining red is unrelated. Reproduced on bun 1.4.2/1.4.3 with a raw socket against The new cases in CI: build 113035 had one red test, Self-reviewed before opening: 5 concerns raised, 2 addressed in code, 3 answered in the Notes section of the PR body (string-body default type, |
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Updated 12:26 PM PT - Sep 8th, 2026
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I reviewed this PR and didn't find any bugs. Because it changes user-visible wire behavior for Response Content-Type across every Bun.serve send path and the author explicitly leaves an open design question for a maintainer (whether string bodies should materialize text/plain into the header list, which would let the headers_own_content_type flag go), a human look is still worthwhile.
What was reviewed: the new Init.headers_own_content_type flag's lifecycle — set in the constructor when init headers carry a Content-Type or the body's type is copied in, set in get_or_create_headers on lazy materialization, copied by Init::clone, and reset when a donor Response is used as init; all four Content-Type readers (RequestContext::get_content_type, StaticRoute::from_js, FileRoute::from_js, Response::get_content_type for formData()) route through the same body_content_type()/flag check so they agree. Checked that render_metadata still computes the body MimeType for autoset_filename when the header is suppressed (only needs_content_type flips). Test matrix covers deleted/kept/replaced × typed bodies × dynamic/static/HEAD via raw-socket reads with port: 0; the expect(async () => ...).toThrow() shape in the FormData test matches 75 existing uses in the suite.
Extended reasoning...
Overview
This PR fixes Bun.serve (and in-process consumers like Response.formData()) re-deriving a Content-Type from the response body after the handler has explicitly deleted it from the header list. It adds a headers_own_content_type: bool field to Response's Init struct that becomes true once a Content-Type has entered the header list (from init headers, copied from a typed body like Blob/FormData/URLSearchParams/Bun.file(), or via Response.json()), and every Content-Type reader — RequestContext::get_content_type for dynamic handlers, StaticRoute::from_js, FileRoute::from_js, and Response::get_content_type — now consults this flag and suppresses body-derived fallback when the flag is set but the header list has no Content-Type. Rust changes span ~100 lines across Response.rs and three server files; tests add ~250 lines with a 36-case parameterized wire-level matrix plus a formData() rejection test.
Security risks
None identified. This is Content-Type header emission logic, not auth/crypto/permissions. The change makes behavior more faithful to what the handler explicitly did (respecting a delete rather than silently re-adding), which if anything reduces surprise. No untrusted-input parsing is added; fast_has(ContentType) is a scan of an existing common-header vector. No new allocations or unsafe blocks are introduced — the existing init_mut() JsCell escape hatch is reused with its SAFETY encapsulation unchanged.
Level of scrutiny
Moderate-to-high. This is a user-visible behavior change to a Web-standard API (Response) on Bun.serve's hot response path, and the PR description explicitly poses an open design question to maintainers: whether string bodies should materialize text/plain;charset=utf-8 into the header list at construction (the way undici does, referencing #8530), which would make the stateful flag unnecessary. That is a maintainer-level API-shape decision this review cannot make. The author also documents a known asymmetry the flag approach leaves in place (new Response("str") + headers.set + headers.delete still sends text/plain because string defaults never enter the header list) and two overlapping open PRs (#32913, #42015) that touch the same functions. These are exactly the "large-scale design decisions a human should weigh in on" that the approval guidelines carve out.
Other factors
Test coverage is thorough and follows the repo's conventions: raw net socket reads to assert exact wire headers (buffered until close), port: 0, one .toEqual on a combined actual object per case, controls that keep sending a Content-Type alongside the delete cases, clone() and Response-as-init donor cases, and both dynamic fetch handler and routes (StaticRoute/FileRoute) paths for GET and HEAD. The flag lifecycle is handled at every mutation site I found: constructor (both when init headers already have one and when the body type is copied in), lazy get_or_create_headers, Response.json() via the new put_default_content_type helper, Init::clone copies it, and Init cloned from a donor Response resets it (the donor's flag described the donor's body). render_metadata still gets a body-derived MimeType for autoset_filename's Content-Disposition rule — only the needs_content_type write is suppressed. The bug hunter ran to dry_streak with no findings and no candidates ruled out.
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Inline comments:
In `@test/js/bun/http/bun-serve-headers.test.ts`:
- Around line 470-485: Replace the parameterized for...of test loop in
test/js/bun/http/bun-serve-headers.test.ts (lines 470-485) with describe.each(),
preserving each case’s expected and actual assertions. Apply the same conversion
to the loop in test/js/web/html/FormData.test.ts (lines 888-908); both sites
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src/runtime/server/FileRoute.rssrc/runtime/server/RequestContext.rssrc/runtime/server/StaticRoute.rssrc/runtime/webcore/Response.rstest/js/bun/http/bun-serve-headers.test.tstest/js/web/html/FormData.test.ts
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Problem
Bun.servesends a Content-Type the handler deleted.const r = new Response(new URLSearchParams("q=1")); r.headers.delete("content-type"); return r;has no Content-Type in-process, but the client getsapplication/x-www-form-urlencoded;charset=UTF-8. Same forFormData, a typedBlob,Bun.file(), a deleted init Content-Type, androutes.headersgetter copy the body's type into the header list (Response.rs). The send paths (get_content_typeinRequestContext.rs,StaticRoute::from_js,FileRoute::from_js) andformData()then derive it from the body again when the list has none.Fix
ResponseInitgetsheaders_own_content_type: the header list has held a Content-Type (from the init, copied from the body, orResponse.json()).clone()copies it.new Response(body, otherResponse)recomputes it.Response. With the flag set and no Content-Type in the list, the send paths send none andformData()rejects, as in undici and Deno. The header list is then the single source.Responsewhose headers were never materialized keeps the allocation-free send-time derivation (new Response("str")is unchanged). String and untyped bodies keep their defaults.test/js/bun/http/bun-serve-headers.test.ts(36 new raw-socket cases, 15 fail on 1.4.3),test/js/web/html/FormData.test.ts(1 new, fails on 1.4.3). Other suites in Notes.Background
new Response(body)appends the body's type to the header list at construction, and undici and Deno serialize that list.headersinit it allocates noFetchHeadersand derives the Content-Type at send time. The firstresponse.headersaccess materializes the list and copies the type in.routes: { "/x": response }snapshots the headers inStaticRoute::from_js/FileRoute::from_jswith the same fallback.Notes
new Response(Bun.file(p).stream(), { headers: {...} })has a header list from the start, but its body is a stream with no type. At send timeto_blob_if_possible()turns the lazy file stream back into a fileBlobwhose type comes from the extension, and the wire getstext/javascriptetc. today. A stateless rule drops that. The flag records whether the Content-Type decision actually reached the header list. The stateless rule becomes possible if a maintainer decides that a string body should puttext/plain;charset=utf-8into the header list the way undici does (the questionResponsewith text body should haveContent-Typeoftext/plain#8530 asks). That is a separate, visible change (new Response("x").headers.get("content-type")isnulltoday), so it is not made here. Question for a maintainer: is that materialization wanted? If yes, this flag can go.render_metadatastill computes the body'sMimeTypewhen the flag suppresses the header, because theContent-Dispositionfilename rule (autoset_filename) keys on its category. Only thecontent-typewrite is skipped.get_content_typeis unchanged: header list, elseblob.content_type(), else image sniff, elsetext/plainfor strings, elseapplication/octet-stream. The flag only stops it after step one.formData()/compileStreamingread the same rule, the send paths ask theResponseinstead of taking a bare flag), 3 answered here:new Response("str", { headers: { "content-type": x } })plus delete now sends none, butnew Response("str")plusheaders.setplusheaders.deletestill sendstext/plain, because Bun never records a string body's default type in the header list (see the first note). Only a Content-Type that was in the header list is tracked.new Response(r.body, r)after deleting fromrgets the type again whenr's body was an in-memory typedBlob: Bun turns a blob-backed stream body back into the typedBlobat construction (Body.rs, kept for Content-type - multipart/form-data header is missing #14988), so the newResponsepairs a typed body with a header list that has no Content-Type, and copies it in. In-process and wire agree there. Left as is.Initfield. They do not fix the delete case: after the header list is materialized they fall back to the body's type at send time like main does. Whichever lands second rebases, the fields compose. server: don't send a fallback content-type on detached responses #30997 rewrites the detached-body condition 50 lines below the call changed here, no textual overlap.Blob/ArrayBufferbodies keep the image sniff andapplication/octet-streamfallback. Afetch()-createdResponsewhose own headers are edited is not flagged (new Response(res.body, res)is, it goes through the constructor). The client side (fetch(request)afterrequest.headers.delete("content-type")) is a separate path infetch.rs.fetchhandler already sends no Content-Type for these bodies (it renders metadata from an empty blob, Bun.serve: resolve a file body for HEAD the same way as for GET #41585 and Bun.serve: send Date on bodiless responses, and mirror GET's Content-Type on HEAD #33427 are about that). The test pins GET dynamic, GET static and HEAD static.boolinInit, one extrafast_has(ContentType)(a scan of the common-header vector) innew Response(body, { headers }). No newFetchHeadersallocation anywhere.serve.test.ts(2 pre-existing environment failures, same on stock bun: root port, loopback dev mode),bun-serve-static,bun-serve-file,bun-serve-routes,bun-serve-html,bun-server,web/fetch/body,body-clone,response,wasm-streaming,fetch.test.ts(environment-only failures: ipv6, public internet, root permissions, ASAN timeouts),web/html/FormData.[human-review] gate passed · iteration 2 · 6 files touched
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