From 7ecea4c2ff70a649b9c111c8eba378d27d3ed50d Mon Sep 17 00:00:00 2001
From: Matteo Collina <hello@matteocollina.com>
Date: Wed, 3 Jun 2026 09:49:22 +0200
Subject: [PATCH] fix: preserve raw headers in dispatch handler bridge

Signed-off-by: Matteo Collina <hello@matteocollina.com>
---
 lib/handler/unwrap-handler.js               |  6 +++
 test/node-test/global-dispatcher-version.js | 49 +++++++++++++++++++++
 types/dispatcher.d.ts                       |  2 +
 3 files changed, 57 insertions(+)

diff --git a/lib/handler/unwrap-handler.js b/lib/handler/unwrap-handler.js
index e23b9666cf7..2949faea0fe 100644
--- a/lib/handler/unwrap-handler.js
+++ b/lib/handler/unwrap-handler.js
@@ -13,6 +13,9 @@ class UnwrapController {
 
   [kResume] = null
 
+  rawHeaders = null
+  rawTrailers = null
+
   constructor (abort) {
     this.#abort = abort
   }
@@ -72,11 +75,13 @@ module.exports = class UnwrapHandler {
   }
 
   onUpgrade (statusCode, rawHeaders, socket) {
+    this.#controller.rawHeaders = rawHeaders
     this.#handler.onRequestUpgrade?.(this.#controller, statusCode, parseHeaders(rawHeaders), socket)
   }
 
   onHeaders (statusCode, rawHeaders, resume, statusMessage) {
     this.#controller[kResume] = resume
+    this.#controller.rawHeaders = rawHeaders
     this.#handler.onResponseStart?.(this.#controller, statusCode, parseHeaders(rawHeaders), statusMessage)
     return !this.#controller.paused
   }
@@ -87,6 +92,7 @@ module.exports = class UnwrapHandler {
   }
 
   onComplete (rawTrailers) {
+    this.#controller.rawTrailers = rawTrailers
     this.#handler.onResponseEnd?.(this.#controller, parseHeaders(rawTrailers))
   }
 
diff --git a/test/node-test/global-dispatcher-version.js b/test/node-test/global-dispatcher-version.js
index ed4c0a2e18a..b9b594eb3a0 100644
--- a/test/node-test/global-dispatcher-version.js
+++ b/test/node-test/global-dispatcher-version.js
@@ -93,3 +93,52 @@ test('setGlobalDispatcher mirrors the dispatcher under the v1 symbol that Node.j
   assert.strictEqual(payload.mirroredV1, true)
   assert.strictEqual(payload.mirroredV2, true)
 })
+
+test('Node.js global fetch preserves headers and decoding with an undici Agent dispatcher', () => {
+  const script = `
+    const assert = require('node:assert')
+    const { createServer } = require('node:http')
+    const { once } = require('node:events')
+    const { brotliCompressSync } = require('node:zlib')
+    const { Agent } = require('./index.js')
+
+    ;(async () => {
+      const body = Buffer.from('body content')
+      const compressedBody = brotliCompressSync(body)
+      const server = createServer((_request, response) => {
+        response.writeHead(200, {
+          'content-type': 'application/x-ndjson',
+          'content-encoding': 'br',
+          'another-test-header': 'test-value'
+        })
+        response.end(compressedBody)
+      })
+      server.listen(0)
+      await once(server, 'listening')
+
+      const url = 'http://127.0.0.1:' + server.address().port
+      const cases = [
+        ['global dispatcher', {}],
+        ['custom dispatcher', { dispatcher: new Agent() }]
+      ]
+
+      for (const [label, init] of cases) {
+        const response = await fetch(url, init)
+        const responseBody = Buffer.from(await response.arrayBuffer()).toString('utf8')
+
+        assert.strictEqual(response.headers.get('content-type'), 'application/x-ndjson', label)
+        assert.strictEqual(response.headers.get('content-encoding'), 'br', label)
+        assert.strictEqual(response.headers.get('another-test-header'), 'test-value', label)
+        assert.strictEqual(responseBody, 'body content', label)
+      }
+
+      server.close()
+    })().catch((err) => {
+      console.error(err?.cause?.stack || err?.stack || err)
+      process.exit(1)
+    })
+  `
+
+  const result = runNode(script)
+  assert.strictEqual(result.status, 0, result.stderr)
+})
diff --git a/types/dispatcher.d.ts b/types/dispatcher.d.ts
index 33dc6051626..3f2ec3d498b 100644
--- a/types/dispatcher.d.ts
+++ b/types/dispatcher.d.ts
@@ -212,6 +212,8 @@ declare namespace Dispatcher {
     get aborted(): boolean
     get paused(): boolean
     get reason(): Error | null
+    rawHeaders?: Buffer[] | string[] | IncomingHttpHeaders | null
+    rawTrailers?: Buffer[] | string[] | IncomingHttpHeaders | null
     abort(reason: Error): void
     pause(): void
     resume(): void