diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 3f96a6c699..d4caf28454 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -10,6 +10,7 @@ import { buildPostCallGuardrailContext } from "./chatCore/postCallGuardrailConte
 import { storeSemanticCacheResponse } from "./chatCore/semanticCacheStore.ts";
 import { buildNonStreamingResponseHeaders } from "./chatCore/nonStreamingResponseHeaders.ts";
 import { maybeConvertJsonBodyToSse } from "./chatCore/jsonBodyToSse.ts";
+import { assembleStreamingResponseHeaders } from "./chatCore/streamingResponseHeaders.ts";
 import { sanitizeChatRequestBody } from "./chatCore/sanitization.ts";
 import {
   getHeaderValueCaseInsensitive,
@@ -3709,20 +3710,13 @@ export async function handleChatCore({
     await onRequestSuccess();
   }
 
-  const responseHeaders: Record<string, string> = {
-    ...buildStreamingResponseHeaders(providerResponse.headers, {
-      provider,
-      model,
-      cacheHit: false,
-      latencyMs: 0,
-      usage: null,
-      costUsd: 0,
-    }),
-    "x-omniroute-request-id": pendingRequestId,
-  };
-  if (compressionResponseMeta) {
-    responseHeaders[OMNIROUTE_RESPONSE_HEADERS.compression] = compressionResponseMeta;
-  }
+  const responseHeaders = assembleStreamingResponseHeaders({
+    providerHeaders: providerResponse.headers,
+    provider,
+    model,
+    pendingRequestId,
+    compressionResponseMeta,
+  });
 
   // Create transform stream with logger for streaming response
   let transformStream;
diff --git a/open-sse/handlers/chatCore/streamingResponseHeaders.ts b/open-sse/handlers/chatCore/streamingResponseHeaders.ts
new file mode 100644
index 0000000000..a978a0a212
--- /dev/null
+++ b/open-sse/handlers/chatCore/streamingResponseHeaders.ts
@@ -0,0 +1,39 @@
+/**
+ * chatCore streaming response headers (Quality Gate v2 / Fase 9 — chatCore god-file decomposition,
+ * #3501).
+ *
+ * Extracted from handleChatCore's streaming success path: assemble the streaming response header map
+ * — the upstream-derived streaming headers (via buildStreamingResponseHeaders, with zeroed
+ * latency/usage/cost since those are not yet known at stream start), the per-request id, and the
+ * optional compression header. Pure builder (returns a fresh map). Behaviour is byte-identical to
+ * the previous inline block.
+ */
+import { OMNIROUTE_RESPONSE_HEADERS } from "@/shared/constants/headers";
+import { buildStreamingResponseHeaders as defaultBuildStreaming } from "./responseHeaders.ts";
+
+export function assembleStreamingResponseHeaders(
+  args: {
+    providerHeaders: Headers;
+    provider: string | null | undefined;
+    model: string | null | undefined;
+    pendingRequestId: string;
+    compressionResponseMeta?: string | null | undefined;
+  },
+  buildStreamingResponseHeaders: typeof defaultBuildStreaming = defaultBuildStreaming
+): Record<string, string> {
+  const responseHeaders: Record<string, string> = {
+    ...buildStreamingResponseHeaders(args.providerHeaders, {
+      provider: args.provider,
+      model: args.model,
+      cacheHit: false,
+      latencyMs: 0,
+      usage: null,
+      costUsd: 0,
+    }),
+    "x-omniroute-request-id": args.pendingRequestId,
+  };
+  if (args.compressionResponseMeta) {
+    responseHeaders[OMNIROUTE_RESPONSE_HEADERS.compression] = args.compressionResponseMeta;
+  }
+  return responseHeaders;
+}
diff --git a/tests/unit/chatcore-streaming-response-headers.test.ts b/tests/unit/chatcore-streaming-response-headers.test.ts
new file mode 100644
index 0000000000..8b714ca4eb
--- /dev/null
+++ b/tests/unit/chatcore-streaming-response-headers.test.ts
@@ -0,0 +1,65 @@
+// Characterization of assembleStreamingResponseHeaders — the streaming response header builder
+// extracted from handleChatCore's streaming success path (chatCore god-file decomposition, #3501).
+// buildStreamingResponseHeaders is injected so the merge of upstream headers + request-id + the
+// optional compression header is observable. Locks: zeroed latency/usage/cost at stream start, the
+// x-omniroute-request-id, and the compression header only when meta is present.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const { assembleStreamingResponseHeaders } = await import(
+  "../../open-sse/handlers/chatCore/streamingResponseHeaders.ts"
+);
+
+function makeBuild() {
+  const calls: Array<{ headers: unknown; meta: Record<string, unknown> }> = [];
+  const build = (headers: unknown, meta: Record<string, unknown>) => {
+    calls.push({ headers, meta });
+    return { "x-upstream": "kept" };
+  };
+  return { build: build as Parameters<typeof assembleStreamingResponseHeaders>[1], calls };
+}
+
+function baseArgs(overrides: Record<string, unknown> = {}) {
+  return {
+    providerHeaders: new Headers({ "content-type": "text/event-stream" }),
+    provider: "openai",
+    model: "gpt-x",
+    pendingRequestId: "preq-1",
+    compressionResponseMeta: undefined,
+    ...overrides,
+  } as Parameters<typeof assembleStreamingResponseHeaders>[0];
+}
+
+test("merges upstream headers and sets x-omniroute-request-id", () => {
+  const { build } = makeBuild();
+  const h = assembleStreamingResponseHeaders(baseArgs(), build);
+  assert.equal(h["x-upstream"], "kept");
+  assert.equal(h["x-omniroute-request-id"], "preq-1");
+});
+
+test("buildStreamingResponseHeaders receives zeroed latency/usage/cost and cacheHit false", () => {
+  const { build, calls } = makeBuild();
+  assembleStreamingResponseHeaders(baseArgs(), build);
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0].meta.cacheHit, false);
+  assert.equal(calls[0].meta.latencyMs, 0);
+  assert.equal(calls[0].meta.usage, null);
+  assert.equal(calls[0].meta.costUsd, 0);
+  assert.equal(calls[0].meta.provider, "openai");
+  assert.equal(calls[0].meta.model, "gpt-x");
+});
+
+test("no compression meta → no compression header", () => {
+  const { build } = makeBuild();
+  const h = assembleStreamingResponseHeaders(baseArgs({ compressionResponseMeta: undefined }), build);
+  assert.ok(!Object.values(h).includes("engine:z"));
+});
+
+test("compression meta present → compression header set", () => {
+  const { build } = makeBuild();
+  const h = assembleStreamingResponseHeaders(
+    baseArgs({ compressionResponseMeta: "engine:z; source=routing" }),
+    build
+  );
+  assert.ok(Object.values(h).includes("engine:z; source=routing"));
+});