diff --git a/packages/ai/src/api/openai-codex-responses.ts b/packages/ai/src/api/openai-codex-responses.ts
index 9c1831a02..a6bbc2ce7 100644
--- a/packages/ai/src/api/openai-codex-responses.ts
+++ b/packages/ai/src/api/openai-codex-responses.ts
@@ -1643,7 +1643,11 @@ function buildBaseCodexHeaders(
 	accountId: string,
 	token: string,
 ): Headers {
-	const headers = new Headers(initHeaders);
+	// Defaults first so model and caller headers can override them, matching the other providers.
+	const headers = new Headers({ originator: "pi", "User-Agent": getPiUserAgent() });
+	for (const [key, value] of Object.entries(initHeaders || {})) {
+		headers.set(key, value);
+	}
 	for (const [key, value] of Object.entries(additionalHeaders || {})) {
 		if (value === null) {
 			headers.delete(key);
@@ -1653,8 +1657,6 @@ function buildBaseCodexHeaders(
 	}
 	headers.set("Authorization", `Bearer ${token}`);
 	headers.set("chatgpt-account-id", accountId);
-	headers.set("originator", "pi");
-	headers.set("User-Agent", getPiUserAgent());
 	return headers;
 }
 
diff --git a/packages/ai/test/openai-codex-stream.test.ts b/packages/ai/test/openai-codex-stream.test.ts
index 9aabc09c3..9283a95d6 100644
--- a/packages/ai/test/openai-codex-stream.test.ts
+++ b/packages/ai/test/openai-codex-stream.test.ts
@@ -797,6 +797,55 @@ describe("openai-codex streaming", () => {
 		expect(capturedHeaders?.get("x-client-request-id")).toBe("x".repeat(64));
 	});
 
+	it("lets model and caller headers override originator and User-Agent", async () => {
+		const token = mockToken();
+		let capturedHeaders: Headers | undefined;
+		const encoder = new TextEncoder();
+		vi.stubGlobal(
+			"fetch",
+			vi.fn(async (_input: string | URL, init?: RequestInit) => {
+				capturedHeaders = init?.headers instanceof Headers ? init.headers : undefined;
+				return new Response(
+					new ReadableStream<Uint8Array>({
+						start(controller) {
+							controller.enqueue(encoder.encode(buildSSEPayload({ status: "completed" })));
+							controller.close();
+						},
+					}),
+					{ status: 200, headers: { "content-type": "text/event-stream" } },
+				);
+			}),
+		);
+
+		const model: Model<"openai-codex-responses"> = {
+			id: "gpt-5.1-codex",
+			name: "GPT-5.1 Codex",
+			api: "openai-codex-responses",
+			provider: "openai-codex",
+			baseUrl: "https://chatgpt.com/backend-api",
+			reasoning: true,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 400000,
+			maxTokens: 128000,
+			headers: { originator: "my-app" },
+		};
+		const context = normalizeContext({
+			systemPrompt: "You are a helpful assistant.",
+			messages: [{ role: "user", content: "Say hello", timestamp: Date.now() }],
+		});
+
+		await streamOpenAICodexResponses(model, context, {
+			apiKey: token,
+			transport: "sse",
+			headers: { "user-agent": "my-app/1.0", Authorization: "Bearer ignored" },
+		}).result();
+
+		expect(capturedHeaders?.get("originator")).toBe("my-app");
+		expect(capturedHeaders?.get("User-Agent")).toBe("my-app/1.0");
+		expect(capturedHeaders?.get("Authorization")).toBe(`Bearer ${token}`);
+	});
+
 	it("preserves gpt-5.5 xhigh reasoning effort from simple options", async () => {
 		const tempDir = mkdtempSync(join(tmpdir(), "pi-codex-stream-"));
 		process.env.PI_CODING_AGENT_DIR = tempDir;