Implement Early Hints to Bun.serve

[rayriffy]

What does this PR do?

Implement to support 103 Early Hints with native Bun.serve. Currently node.js compatible ServerResponse.writeEarlyHints is available to use with Bun but it has performance penalty.

Example of use cases:

Bun.serve({
  async fetch(req, server) {
    server.writeEarlyHints(req, {
      Link: "</app.css>; rel=preload; as=style, </app.js>; rel=modulepreload",
    });

    const html = await renderPage();
    return new Response(html, {
      headers: { "Content-Type": "text/html" },
    });
  },
});

How did you verify your code works?

We do pre-built bun locally and test with our sample code to prove a feature works, together with adding test cases to cover this cases.

should also solves #8690

[claude]

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Walkthrough

The PR adds server.writeEarlyHints(request, headers). It serializes valid headers into HTTP/1.1 103 responses, dispatches them through request contexts, documents the API, and tests HTTP/1.1 and HTTP/3 behavior.

Changes

HTTP Early Hints

Layer / File(s)	Summary
Expose writeEarlyHints packages/bun-types/serve.d.ts, src/runtime/server/server.classes.ts, docs/runtime/http/server.mdx	The public declaration, server prototypes, and runtime documentation expose writeEarlyHints(request, headers).
Serialize and write Early Hints src/runtime/server/server_body.rs, src/runtime/server/AnyRequestContext.rs, src/runtime/server/RequestContext.rs	The runtime validates headers, serializes HTTP/1.1 103 responses, and rejects unavailable, completed, or HTTP/3 requests.
Validate HTTP behavior test/js/bun/http/serve.test.ts, test/js/bun/http/serve-http3.test.ts, test/js/bun/http/fetch-h3.ts	Tests verify response ordering, repeated-write and completed-request results, and the absence of HTTP/3 informational responses.

Possibly related PRs

• oven-sh/bun#33191: Both PRs modify HTTP server response handling in src/runtime/server/server_body.rs, but implement different methods.

Suggested reviewers: alii, jarred-sumner, robobun

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: native Early Hints support for Bun.serve.
Description check	✅ Passed	The description includes both required sections, explains the feature, provides an example, and describes verification and issue coverage.
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.

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[rayriffy]

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[rayriffy]

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/runtime/http/server.mdx`:
- Around line 346-368: Add writeEarlyHints(request: Request, headers:
HeadersInit): boolean to the Server interface in the Reference section, matching
the documented method signature and boolean return contract; leave the
surrounding interface members unchanged.

In `@test/js/bun/http/serve.test.ts`:
- Around line 2230-2294: Add an HTTP/3-focused test alongside the existing
server.writeEarlyHints tests, configuring Bun.serve with HTTP/3 support and
making writeEarlyHints return false for the request. Capture the client response
and assert it contains the final response but no “103 Early Hints” status,
preserving the existing HTTP/1.1 coverage.

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Reviewing files that changed from the base of the PR and between ef32923 and f30aaa0.

📒 Files selected for processing (7)

• docs/runtime/http/server.mdx
• packages/bun-types/serve.d.ts
• src/runtime/server/AnyRequestContext.rs
• src/runtime/server/RequestContext.rs
• src/runtime/server/server.classes.ts
• src/runtime/server/server_body.rs
• test/js/bun/http/serve.test.ts

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/js/bun/http/serve-http3.test.ts`:
- Line 230: Strengthen the HTTP/3 informational-response test around fetchH3 so
it inspects all parsed or raw response header blocks rather than only the final
headers returned by res.headers. Assert that no 103 response block is emitted
and no informational preload Link header appears, while preserving the existing
final-response assertion.

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• test/js/bun/http/serve-http3.test.ts

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/runtime/server/RequestContext.rs`:
- Around line 4483-4501: Update RequestContext::write_informational to also
return false when self.flags.has_written_status() is true, while preserving the
existing response and HTTP/3 guards. Add a regression test that invokes
server.writeEarlyHints after a streaming response has flushed its status line,
then verifies it returns false and produces no HTTP 103 block.

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Reviewing files that changed from the base of the PR and between b58cd46 and 1417bd0.

📒 Files selected for processing (9)

• docs/runtime/http/server.mdx
• packages/bun-types/serve.d.ts
• src/runtime/server/AnyRequestContext.rs
• src/runtime/server/RequestContext.rs
• src/runtime/server/server.classes.ts
• src/runtime/server/server_body.rs
• test/js/bun/http/fetch-h3.ts
• test/js/bun/http/serve-http3.test.ts
• test/js/bun/http/serve.test.ts

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