diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index fdcde3e7fb..5acb4c091d 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -36,6 +36,12 @@ const OPENAI_HEADER_TIMEOUT_DEFAULT = 10_000
 // Custom fetch adapters can opt out when they apply route-specific header timing internally.
 const HEADER_TIMEOUT = Symbol.for("opencode.provider.header-timeout")
 
+export function headerTimeoutForFetch(fetch: unknown, timeout: number | false | undefined) {
+  if (timeout === false) return undefined
+  if (typeof fetch === "function" && fetch[HEADER_TIMEOUT] === false) return undefined
+  return timeout
+}
+
 function wrapSSE(res: Response, ms: number, ctl: AbortController) {
   if (typeof ms !== "number" || ms <= 0) return res
   if (!res.body) return res
@@ -1605,8 +1611,7 @@ export const layer = Layer.effect(
           const fetchFn = customFetch ?? fetch
           const opts = init ?? {}
           const chunkAbortCtl = typeof chunkTimeout === "number" && chunkTimeout > 0 ? new AbortController() : undefined
-          const headerTimeoutMs =
-            headerTimeout === false || customFetch?.[HEADER_TIMEOUT] === false ? undefined : headerTimeout
+          const headerTimeoutMs = headerTimeoutForFetch(customFetch, headerTimeout)
           const headerTimeoutCtl = typeof headerTimeoutMs === "number" ? timeoutController(headerTimeoutMs) : undefined
           const signals: AbortSignal[] = []
 
diff --git a/packages/opencode/test/plugin/codex.test.ts b/packages/opencode/test/plugin/codex.test.ts
index 3169258e60..dd20e1b744 100644
--- a/packages/opencode/test/plugin/codex.test.ts
+++ b/packages/opencode/test/plugin/codex.test.ts
@@ -176,14 +176,24 @@ describe("plugin.codex", () => {
   })
 
   test("can disable websocket HTTP fallback header timeout", async () => {
-    const response = await fetchWithHeaderTimeout(
-      async () => new Response("http"),
-      "https://example.com/v1/responses",
-      undefined,
-      false,
-    )
+    using server = Bun.serve({
+      port: 0,
+      async fetch() {
+        await Bun.sleep(30)
+        return new Response("http")
+      },
+    })
+    const hooks = await CodexAuthPlugin({} as never, { experimentalWebSockets: true })
+    await hooks.config!({ provider: { openai: { options: { headerTimeout: false } } } } as never)
+    const loaded = await hooks.auth!.loader!(async () => ({ type: "api", key: "sk-test" }) as never, {} as never)
+
+    const response = await loaded.fetch!(new URL("/v1/responses", server.url), {
+      method: "POST",
+      body: JSON.stringify({ stream: true }),
+    })
 
     expect(await response.text()).toBe("http")
+    await hooks.dispose?.()
   })
 
   test("deduplicates concurrent Codex token refreshes", async () => {
diff --git a/packages/opencode/test/provider/header-timeout.test.ts b/packages/opencode/test/provider/header-timeout.test.ts
index a3caf4fd8a..ad470cd1d4 100644
--- a/packages/opencode/test/provider/header-timeout.test.ts
+++ b/packages/opencode/test/provider/header-timeout.test.ts
@@ -1,4 +1,4 @@
-import { afterEach, expect } from "bun:test"
+import { afterEach, expect, test } from "bun:test"
 import { createServer, type Server } from "node:http"
 import { streamText } from "ai"
 import { Effect, Layer } from "effect"
@@ -20,6 +20,16 @@ const it = testEffect(
   Layer.mergeAll(Provider.defaultLayer, Env.defaultLayer, Plugin.defaultLayer, CrossSpawnSpawner.defaultLayer),
 )
 
+test("marked custom fetch adapters opt out of outer header timeout", () => {
+  const fetch = Object.assign(() => Promise.resolve(new Response()), {
+    [Symbol.for("opencode.provider.header-timeout")]: false,
+  })
+
+  expect(Provider.headerTimeoutForFetch(fetch, 10_000)).toBeUndefined()
+  expect(Provider.headerTimeoutForFetch(() => Promise.resolve(new Response()), 10_000)).toBe(10_000)
+  expect(Provider.headerTimeoutForFetch(fetch, false)).toBeUndefined()
+})
+
 it.live("headerTimeout does not abort delayed SSE body after headers arrive", () =>
   Effect.gen(function* () {
     const server = yield* Effect.acquireRelease(