diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 0544a90db3..4013722220 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -44,7 +44,8 @@ import {
 import { CORS_HEADERS } from "../utils/cors.ts";
 import { checkHeapPressureGuard } from "../utils/heapPressure.ts";
 import { normalizeHeaders } from "../utils/headers.ts";
-import { detectFormatFromEndpoint, getTargetFormat } from "../services/provider.ts";
+import { getTargetFormat } from "../services/provider.ts";
+import { resolveChatCoreRequestFormat } from "./chatCore/requestFormat.ts";
 import { injectSystemPrompt } from "../services/systemPrompt.ts";
 import { translateRequest, needsTranslation } from "../translator/index.ts";
 import { FORMATS } from "../translator/formats.ts";
@@ -553,28 +554,6 @@ function buildExecutorClientHeaders(
   return Object.keys(normalized).length > 0 ? normalized : null;
 }
 
-function isCopilotClient(
-  headers: Headers | Record<string, unknown> | null | undefined,
-  userAgent?: string | null
-) {
-  const isMatch = (value: unknown) =>
-    typeof value === "string" && value.toLowerCase().includes("copilot");
-
-  if (isMatch(userAgent)) return true;
-
-  if (headers instanceof Headers) {
-    for (const [key, value] of headers as unknown as Iterable<[string, string]>) {
-      if (isMatch(key) || isMatch(value)) return true;
-    }
-  } else if (headers && typeof headers === "object") {
-    for (const [key, value] of Object.entries(headers)) {
-      if (isMatch(key) || isMatch(value)) return true;
-    }
-  }
-
-  return false;
-}
-
 export function extractSystemRoleMessages(payload: Record<string, unknown>): void {
   if (!Array.isArray(payload.messages)) return;
   const messages = payload.messages as Array<{ role?: unknown; content?: unknown }>;
@@ -878,22 +857,17 @@ export async function handleChatCore({
     credentials.connectionId = connectionId;
   }
 
-  const endpointPath = String(clientRawRequest?.endpoint || "");
-  const sourceFormat = detectFormatFromEndpoint(body, endpointPath);
-  const isResponsesEndpoint =
-    /\/responses(?=\/|$)/i.test(endpointPath) || /^responses(?=\/|$)/i.test(endpointPath);
-  const nativeCodexPassthrough = shouldUseNativeCodexPassthrough({
-    provider,
-    sourceFormat,
+  // Endpoint/format resolution extracted to chatCore/requestFormat.ts (#3501); pure derivation
+  // from the inbound request, destructured so every downstream use stays byte-identical.
+  const {
     endpointPath,
-  });
-  const isDroidCLI =
-    userAgent?.toLowerCase().includes("droid") || userAgent?.toLowerCase().includes("codex-cli");
-  const copilotCompatibleReasoning = isCopilotClient(clientRawRequest?.headers, userAgent);
-  const clientResponseFormat =
-    sourceFormat === FORMATS.OPENAI_RESPONSES && !isResponsesEndpoint && !isDroidCLI
-      ? FORMATS.OPENAI
-      : sourceFormat;
+    sourceFormat,
+    isResponsesEndpoint,
+    nativeCodexPassthrough,
+    isDroidCLI,
+    copilotCompatibleReasoning,
+    clientResponseFormat,
+  } = resolveChatCoreRequestFormat({ clientRawRequest, body, provider, userAgent });
 
   // Check for bypass patterns (warmup, skip) - return fake response
   const bypassResponse = handleBypassRequest(body, model, userAgent);
diff --git a/open-sse/handlers/chatCore/requestFormat.ts b/open-sse/handlers/chatCore/requestFormat.ts
new file mode 100644
index 0000000000..63ad12589b
--- /dev/null
+++ b/open-sse/handlers/chatCore/requestFormat.ts
@@ -0,0 +1,82 @@
+/**
+ * chatCore request endpoint/format resolvers (Quality Gate v2 / Fase 9 — chatCore god-file
+ * decomposition, #3501).
+ *
+ * Pure slice of handleChatCore's request-setup phase: derives the wire-format facts of an inbound
+ * request from its endpoint, body, provider, and user-agent — the source format, whether it targets
+ * the Responses endpoint, native-Codex passthrough eligibility, Droid CLI / Copilot detection, and
+ * the effective client response format (an OpenAI Responses shape off a non-/responses, non-Droid
+ * endpoint collapses back to plain OpenAI). Side-effect-free; behaviour is byte-identical to the
+ * previous inline block. Sits alongside resolveChatCoreRequestSetup as the request-setup phase grows.
+ */
+
+import { detectFormatFromEndpoint } from "../../services/provider.ts";
+import { shouldUseNativeCodexPassthrough } from "./passthroughHelpers.ts";
+import { FORMATS } from "../../translator/formats.ts";
+
+/** True when the request originates from a Copilot client (matched by user-agent or any header). */
+function isCopilotClient(
+  headers: Headers | Record<string, unknown> | null | undefined,
+  userAgent?: string | null
+) {
+  const isMatch = (value: unknown) =>
+    typeof value === "string" && value.toLowerCase().includes("copilot");
+
+  if (isMatch(userAgent)) return true;
+
+  if (headers instanceof Headers) {
+    for (const [key, value] of headers as unknown as Iterable<[string, string]>) {
+      if (isMatch(key) || isMatch(value)) return true;
+    }
+  } else if (headers && typeof headers === "object") {
+    for (const [key, value] of Object.entries(headers)) {
+      if (isMatch(key) || isMatch(value)) return true;
+    }
+  }
+
+  return false;
+}
+
+/**
+ * Resolve the per-request endpoint/format facts at the top of handleChatCore. Pure: a function of
+ * the inbound endpoint, the (possibly already-mutated) body, the resolved provider, and the
+ * user-agent.
+ */
+export function resolveChatCoreRequestFormat(opts: {
+  clientRawRequest:
+    | { endpoint?: unknown; headers?: Headers | Record<string, unknown> | null }
+    | null
+    | undefined;
+  body: unknown;
+  provider: string | null | undefined;
+  userAgent: string | null | undefined;
+}) {
+  const { clientRawRequest, body, provider, userAgent } = opts;
+  const endpointPath = String(clientRawRequest?.endpoint || "");
+  const sourceFormat = detectFormatFromEndpoint(body, endpointPath);
+  const isResponsesEndpoint =
+    /\/responses(?=\/|$)/i.test(endpointPath) || /^responses(?=\/|$)/i.test(endpointPath);
+  const nativeCodexPassthrough = shouldUseNativeCodexPassthrough({
+    provider,
+    sourceFormat,
+    endpointPath,
+  });
+  const isDroidCLI =
+    userAgent?.toLowerCase().includes("droid") || userAgent?.toLowerCase().includes("codex-cli");
+  const copilotCompatibleReasoning = isCopilotClient(clientRawRequest?.headers, userAgent);
+  const clientResponseFormat =
+    sourceFormat === FORMATS.OPENAI_RESPONSES && !isResponsesEndpoint && !isDroidCLI
+      ? FORMATS.OPENAI
+      : sourceFormat;
+  return {
+    endpointPath,
+    sourceFormat,
+    isResponsesEndpoint,
+    nativeCodexPassthrough,
+    isDroidCLI,
+    copilotCompatibleReasoning,
+    clientResponseFormat,
+  };
+}
+
+export type ChatCoreRequestFormat = ReturnType<typeof resolveChatCoreRequestFormat>;
diff --git a/tests/unit/chatcore-request-format.test.ts b/tests/unit/chatcore-request-format.test.ts
new file mode 100644
index 0000000000..064a2384f2
--- /dev/null
+++ b/tests/unit/chatcore-request-format.test.ts
@@ -0,0 +1,103 @@
+// tests/unit/chatcore-request-format.test.ts
+// Characterization of resolveChatCoreRequestFormat — the endpoint/format resolution slice extracted
+// from the top of handleChatCore (chatCore god-file decomposition, #3501). Locks the endpointPath
+// construction, the /responses detection, the nativeCodexPassthrough + isDroidCLI + copilot wiring,
+// and the clientResponseFormat downgrade (OpenAI Responses shape off a non-/responses, non-Droid
+// endpoint collapses to plain OpenAI).
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { resolveChatCoreRequestFormat } from "../../open-sse/handlers/chatCore/requestFormat.ts";
+import { shouldUseNativeCodexPassthrough } from "../../open-sse/handlers/chatCore/passthroughHelpers.ts";
+import { FORMATS } from "../../open-sse/translator/formats.ts";
+
+const base = { body: { messages: [{ role: "user", content: "hi" }] }, provider: "openai", userAgent: "unit-test" };
+
+test("chat/completions endpoint → openai source, not a responses endpoint, no downgrade", () => {
+  const r = resolveChatCoreRequestFormat({
+    ...base,
+    clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Headers() },
+  });
+  assert.equal(r.endpointPath, "/v1/chat/completions");
+  assert.equal(r.sourceFormat, FORMATS.OPENAI);
+  assert.equal(r.isResponsesEndpoint, false);
+  assert.equal(r.clientResponseFormat, FORMATS.OPENAI);
+  assert.equal(r.nativeCodexPassthrough, false); // provider !== codex
+  assert.equal(r.isDroidCLI, false);
+  assert.equal(r.copilotCompatibleReasoning, false);
+});
+
+test("/responses endpoint → openai-responses source + isResponsesEndpoint, kept (no downgrade)", () => {
+  const r = resolveChatCoreRequestFormat({
+    body: { input: "x" },
+    provider: "openai",
+    userAgent: "unit-test",
+    clientRawRequest: { endpoint: "/v1/responses", headers: new Headers() },
+  });
+  assert.equal(r.sourceFormat, FORMATS.OPENAI_RESPONSES);
+  assert.equal(r.isResponsesEndpoint, true);
+  assert.equal(r.clientResponseFormat, FORMATS.OPENAI_RESPONSES);
+});
+
+test("Responses-shaped body on a /chat/completions endpoint downgrades clientResponseFormat to openai", () => {
+  const r = resolveChatCoreRequestFormat({
+    body: { input: "describe" }, // input + no messages → openai-responses via body
+    provider: "openai",
+    userAgent: "unit-test",
+    clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Headers() },
+  });
+  assert.equal(r.sourceFormat, FORMATS.OPENAI_RESPONSES);
+  assert.equal(r.isResponsesEndpoint, false);
+  assert.equal(r.clientResponseFormat, FORMATS.OPENAI); // downgraded
+});
+
+test("Droid CLI suppresses the downgrade (clientResponseFormat stays openai-responses)", () => {
+  const r = resolveChatCoreRequestFormat({
+    body: { input: "describe" },
+    provider: "openai",
+    userAgent: "Droid/1.2 codex-cli",
+    clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Headers() },
+  });
+  assert.equal(r.isDroidCLI, true);
+  assert.equal(r.sourceFormat, FORMATS.OPENAI_RESPONSES);
+  assert.equal(r.clientResponseFormat, FORMATS.OPENAI_RESPONSES); // !isDroidCLI is false → no downgrade
+});
+
+test("copilotCompatibleReasoning detects copilot via header or user-agent", () => {
+  const viaUa = resolveChatCoreRequestFormat({
+    ...base,
+    userAgent: "GitHubCopilotChat/0.1",
+    clientRawRequest: { endpoint: "/v1/chat/completions", headers: new Headers() },
+  });
+  assert.equal(viaUa.copilotCompatibleReasoning, true);
+
+  const viaHeader = resolveChatCoreRequestFormat({
+    ...base,
+    clientRawRequest: {
+      endpoint: "/v1/chat/completions",
+      headers: new Headers({ "x-client": "copilot-vscode" }),
+    },
+  });
+  assert.equal(viaHeader.copilotCompatibleReasoning, true);
+});
+
+test("nativeCodexPassthrough delegates to shouldUseNativeCodexPassthrough (codex + responses)", () => {
+  const r = resolveChatCoreRequestFormat({
+    body: { input: "x" },
+    provider: "codex",
+    userAgent: "unit-test",
+    clientRawRequest: { endpoint: "/v1/responses", headers: new Headers() },
+  });
+  assert.equal(
+    r.nativeCodexPassthrough,
+    shouldUseNativeCodexPassthrough({
+      provider: "codex",
+      sourceFormat: r.sourceFormat,
+      endpointPath: r.endpointPath,
+    })
+  );
+});
+
+test("missing clientRawRequest → empty endpointPath", () => {
+  const r = resolveChatCoreRequestFormat({ ...base, clientRawRequest: null });
+  assert.equal(r.endpointPath, "");
+});