diff --git a/src/proxy-capture/runtime.test.ts b/src/proxy-capture/runtime.test.ts
index 8024cd3320f..d56d599caab 100644
--- a/src/proxy-capture/runtime.test.ts
+++ b/src/proxy-capture/runtime.test.ts
@@ -78,6 +78,42 @@ describe("debug proxy runtime", () => {
     expect(sessionEvents.some((event) => event.kind === "response")).toBe(true);
   });
 
+  it("normalizes symbol-bearing request headers before calling patched fetch targets", async () => {
+    fetchTarget.fetch = async (_input: RequestInfo | URL, init?: RequestInit) => {
+      const headers = new Headers(init?.headers);
+      expect(headers.get("content-type")).toBe("application/json");
+      expect(headers.get("x-hidden")).toBe("yes");
+      return new Response("{}", { status: 200 });
+    };
+    const headers = { "content-type": "application/json" } as Record<string, string> & {
+      [key: symbol]: unknown;
+    };
+    Object.defineProperty(headers, "x-hidden", {
+      value: "yes",
+      enumerable: false,
+    });
+    Object.defineProperty(headers, Symbol("sensitiveHeaders"), {
+      value: new Set(["content-type"]),
+      enumerable: false,
+    });
+
+    initializeDebugProxyCapture("test", settings, deps);
+    await fetchTarget.fetch("https://api.example.com/messages", {
+      method: "POST",
+      headers,
+      body: "{}",
+    });
+    await new Promise((resolve) => setImmediate(resolve));
+    finalizeDebugProxyCapture(settings, deps);
+
+    const request = events.find((event) => event.kind === "request");
+    expect(JSON.parse(String(request?.headersJson))).toMatchObject({
+      "content-type": "application/json",
+      "x-hidden": "yes",
+    });
+    expect(Object.getOwnPropertySymbols(headers)).toHaveLength(1);
+  });
+
   it("redacts sensitive request and response headers before persistence", async () => {
     initializeDebugProxyCapture("test", settings, deps);
     captureHttpExchange(
diff --git a/src/proxy-capture/runtime.ts b/src/proxy-capture/runtime.ts
index 4ae326a8118..11b50b46bb1 100644
--- a/src/proxy-capture/runtime.ts
+++ b/src/proxy-capture/runtime.ts
@@ -1,5 +1,6 @@
 import { randomUUID } from "node:crypto";
 import { URL } from "node:url";
+import { normalizeRequestInitHeadersForFetch } from "../infra/fetch-headers.js";
 import { resolveDebugProxySettings, type DebugProxySettings } from "./env.js";
 import {
   closeDebugProxyCaptureStore,
@@ -174,8 +175,9 @@ function installDebugProxyGlobalFetchPatch(
   fetchTarget[DEBUG_PROXY_FETCH_PATCH_KEY] = { originalFetch };
   fetchTarget.fetch = (async (input: RequestInfo | URL, init?: RequestInit) => {
     const url = resolveUrlString(input);
+    const normalizedInit = normalizeRequestInitHeadersForFetch(init);
     try {
-      const response = await originalFetch(input, init);
+      const response = await originalFetch(input, normalizedInit);
       if (url && /^https?:/i.test(url)) {
         captureHttpExchange(
           {
@@ -184,17 +186,18 @@ function installDebugProxyGlobalFetchPatch(
               (typeof Request !== "undefined" && input instanceof Request
                 ? input.method
                 : undefined) ??
-              init?.method ??
+              normalizedInit?.method ??
               "GET",
             requestHeaders:
               (typeof Request !== "undefined" && input instanceof Request
                 ? input.headers
-                : undefined) ?? (init?.headers as Headers | Record<string, string> | undefined),
+                : undefined) ??
+              (normalizedInit?.headers as Headers | Record<string, string> | undefined),
             requestBody:
               (typeof Request !== "undefined" && input instanceof Request
                 ? (input as Request & { body?: BodyInit | null }).body
                 : undefined) ??
-              (init as (RequestInit & { body?: BodyInit | null }) | undefined)?.body ??
+              (normalizedInit as (RequestInit & { body?: BodyInit | null }) | undefined)?.body ??
               null,
             response,
             transport: "http",
@@ -225,7 +228,7 @@ function installDebugProxyGlobalFetchPatch(
             (typeof Request !== "undefined" && input instanceof Request
               ? input.method
               : undefined) ??
-            init?.method ??
+            normalizedInit?.method ??
             "GET",
           host: parsed.host,
           path: `${parsed.pathname}${parsed.search}`,