diff --git a/docs-site/src/content/docs/reference/adapters.md b/docs-site/src/content/docs/reference/adapters.md
index 715555f847..2b82254895 100644
--- a/docs-site/src/content/docs/reference/adapters.md
+++ b/docs-site/src/content/docs/reference/adapters.md
@@ -123,6 +123,12 @@ body and response, with narrow compatibility rewrites for routed gateways.
 `forward` uses configured static headers without relaying caller authorization; `key` uses the
 configured provider key.
 
+The adapter preserves the incoming client's `User-Agent` as a fallback in both auth modes because
+some Responses-compatible providers use the Codex client fingerprint for compatibility behavior.
+An explicitly configured provider `User-Agent` remains authoritative regardless of header casing;
+if the caller sends none, OpenCodex does not invent one. No other caller header is widened by this
+exception.
+
 Adapter selection does not select the upstream transport. Eligible requests can use the
 [upstream WebSocket proxy route](/reference/proxy-formats/#json-and-sse-output); invalid or unsupported
 WebSocket proxy settings fall back to HTTP/SSE. HTTP fetch-based Responses handling uses Bun's
diff --git a/src/adapters/openai-responses/passthrough.ts b/src/adapters/openai-responses/passthrough.ts
index 4cde7b8d3c..7bdfd123b6 100644
--- a/src/adapters/openai-responses/passthrough.ts
+++ b/src/adapters/openai-responses/passthrough.ts
@@ -64,6 +64,16 @@ export const FORWARD_HEADERS = [
   CODEX_RESPONSES_LITE_HEADER,
 ];
 
+/** Preserve the caller fingerprint unless the provider explicitly owns that header. */
+function applyCallerUserAgentFallback(
+  headers: Record<string, string>,
+  incoming: IncomingMeta,
+): void {
+  if (Object.keys(headers).some(name => name.toLowerCase() === "user-agent")) return;
+  const userAgent = incoming.headers.get("user-agent");
+  if (userAgent) headers["User-Agent"] = userAgent;
+}
+
 /** Replace every `input_image` part under a routed-compaction body with a short marker. */
 function stripInputImagesDeep(value: unknown): unknown {
   if (Array.isArray(value)) return value.map(stripInputImagesDeep);
@@ -221,6 +231,10 @@ export function createResponsesPassthroughAdapter(provider: OcxProviderConfig):
         if (provider.apiKey) headers["Authorization"] = `Bearer ${provider.apiKey}`;
         if (provider.headers) Object.assign(headers, provider.headers);
       }
+      // Some Responses-compatible gateways select their Codex compatibility path from the real
+      // client fingerprint. This is a single non-credential fallback, not broader caller-header
+      // forwarding. Static provider headers remain authoritative in either auth mode.
+      applyCallerUserAgentFallback(headers, incoming);
 
       const forward = provider.authMode === "forward";
       let convertedRoutedCustomToolNames: Set<string> | undefined;
diff --git a/structure/transports/responses.md b/structure/transports/responses.md
index 4a6a664cad..639278c8d0 100644
--- a/structure/transports/responses.md
+++ b/structure/transports/responses.md
@@ -11,6 +11,11 @@ Plaintext collaboration restoration treats a null namespace as absent, rejects n
 provider, lets the selected adapter speak the upstream protocol, then bridges adapter events back to
 Responses-compatible streaming output. For an opted-in key-auth provider, a hosted-search continuation stays bound to the API-key selection that served the first leg; the contract is the [hosted-search continuation binding](../runtime.md#hosted-search-continuation-binding).
 
+The `openai-responses` adapter preserves the incoming `User-Agent` as a non-credential fallback in
+both key and forward modes. A configured provider header with that name wins case-insensitively;
+when the caller omits it, the adapter invents no client identity. This does not widen the canonical
+forward credential/metadata allowlist or copy any other caller header.
+
 Retired Codex Spark has no model-specific tool or Responses Lite override; general Lite handling and
 namespace scrubbing remain shared compatibility behavior. Codex quota/reset evidence follows the
 [shared/Reserve policy](../providers/openai-tiers.md#public-provider-contract), including suppression of retired model-derived evidence before shared recovery.
diff --git a/tests/codex-integration/codex-metadata-integrity.test.ts b/tests/codex-integration/codex-metadata-integrity.test.ts
index a9dc25ed4a..d6f21f87e8 100644
--- a/tests/codex-integration/codex-metadata-integrity.test.ts
+++ b/tests/codex-integration/codex-metadata-integrity.test.ts
@@ -160,6 +160,78 @@ describe("Codex metadata integrity", () => {
     expect(sync.headers.session_id).toBe("sess-real-2");
     expect(sync.headers["thread-id"]).toBe("thread-real-2");
   });
+
+  test("Responses preserves caller User-Agent as a fallback in key and forward modes", async () => {
+    for (const provider of [
+      {
+        adapter: "openai-responses",
+        baseUrl: "https://gateway.example/v1",
+        authMode: "key",
+        apiKey: "test-key",
+      },
+      {
+        adapter: "openai-responses",
+        baseUrl: "https://chatgpt.com/backend-api/codex",
+        authMode: "forward",
+      },
+    ] satisfies OcxProviderConfig[]) {
+      const request = await createResponsesPassthroughAdapter(provider).buildRequest(minimalParsed(), {
+        headers: new Headers({ "User-Agent": "codex_cli_rs/0.154.0" }),
+      });
+      expect(new Headers(request.headers).get("user-agent")).toBe("codex_cli_rs/0.154.0");
+    }
+  });
+
+  test("configured User-Agent wins case-insensitively and a missing caller value stays absent", async () => {
+    const configured = await createResponsesPassthroughAdapter({
+      adapter: "openai-responses",
+      baseUrl: "https://gateway.example/v1",
+      authMode: "key",
+      headers: { "uSeR-aGeNt": "operator-agent/1" },
+    }).buildRequest(minimalParsed(), {
+      headers: new Headers({ "User-Agent": "caller-agent/1" }),
+    });
+    expect(new Headers(configured.headers).get("user-agent")).toBe("operator-agent/1");
+    expect(Object.keys(configured.headers).filter(name => name.toLowerCase() === "user-agent"))
+      .toHaveLength(1);
+
+    const absent = await createResponsesPassthroughAdapter({
+      adapter: "openai-responses",
+      baseUrl: "https://gateway.example/v1",
+      authMode: "key",
+    }).buildRequest(minimalParsed(), { headers: new Headers() });
+    expect(new Headers(absent.headers).has("user-agent")).toBe(false);
+  });
+
+  test("the preserved User-Agent is the value received by the HTTP upstream", async () => {
+    let resolveObserved!: (value: string | null) => void;
+    const observed = new Promise<string | null>(resolve => { resolveObserved = resolve; });
+    const upstream = Bun.serve({
+      hostname: "127.0.0.1",
+      port: 0,
+      fetch(request) {
+        resolveObserved(request.headers.get("user-agent"));
+        return Response.json({ id: "response-fixture", output: [] });
+      },
+    });
+    try {
+      const built = await createResponsesPassthroughAdapter({
+        adapter: "openai-responses",
+        baseUrl: `http://127.0.0.1:${upstream.port}/v1`,
+        authMode: "key",
+      }).buildRequest(minimalParsed(), {
+        headers: new Headers({ "User-Agent": "codex_cli_rs/receiver-proof" }),
+      });
+      await fetch(built.url, {
+        method: built.method,
+        headers: built.headers,
+        body: built.body,
+      });
+      expect(await observed).toBe("codex_cli_rs/receiver-proof");
+    } finally {
+      upstream.stop(true);
+    }
+  });
 });
 
 describe("Codex request transport metadata", () => {