diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 339b2de12..d23f366fb 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed GitHub Copilot OpenAI-compatible requests being rejected when the session's native OpenAI service tier was set to `priority`.
+
 ## [16.4.3] - 2026-07-11
 
 ### Fixed
diff --git a/packages/ai/src/types.ts b/packages/ai/src/types.ts
index 5b568de29..0f7830633 100644
--- a/packages/ai/src/types.ts
+++ b/packages/ai/src/types.ts
@@ -141,13 +141,17 @@ function isOpenAIServiceTierApi(api: Api | undefined): boolean {
 	return api === "openai-completions" || api === "openai-responses" || api === "openai-codex-responses";
 }
 
-function hasDedicatedServiceTierControl(provider: Provider | undefined): boolean {
-	return provider === "fireworks";
+function excludesInferredOpenAIServiceTier(provider: Provider | undefined): boolean {
+	// Fireworks has its own priority-only control. GitHub Copilot proxies OpenAI
+	// models but rejects OpenAI's `service_tier` request field.
+	return provider === "fireworks" || provider === "github-copilot";
 }
 
 function isOpenAIServiceTierModel(model: ServiceTierModel): boolean {
 	return (
-		!hasDedicatedServiceTierControl(model.provider) && isOpenAIServiceTierApi(model.api) && isOpenAIModelId(model.id)
+		!excludesInferredOpenAIServiceTier(model.provider) &&
+		isOpenAIServiceTierApi(model.api) &&
+		isOpenAIModelId(model.id)
 	);
 }
 
@@ -159,7 +163,8 @@ function isOpenAIServiceTierModel(model: ServiceTierModel): boolean {
  * `openai/`); Claude on Bedrock/Vertex (api `anthropic-messages`) is the
  * anthropic family even though its provider is `amazon-bedrock`/`google-vertex`.
  * Custom OpenAI-compatible relays that serve OpenAI model ids are OpenAI family
- * too unless that provider owns a separate tier control such as Fireworks.
+ * too unless the provider owns a separate tier control (Fireworks) or rejects
+ * OpenAI's service-tier field (GitHub Copilot).
  */
 export function serviceTierFamily(model: ServiceTierModel): ServiceTierFamily | undefined {
 	const provider = model.provider;
diff --git a/packages/ai/test/github-copilot-openai-base-url.test.ts b/packages/ai/test/github-copilot-openai-base-url.test.ts
index 0b65672df..76cffdaae 100644
--- a/packages/ai/test/github-copilot-openai-base-url.test.ts
+++ b/packages/ai/test/github-copilot-openai-base-url.test.ts
@@ -30,6 +30,13 @@ function getRequestHeader(
 	return new Headers(init?.headers).get(headerName);
 }
 
+async function getRequestBody(input: string | URL | Request, init?: RequestInit): Promise<Record<string, unknown>> {
+	if (input instanceof Request) {
+		return (await input.clone().json()) as Record<string, unknown>;
+	}
+	return JSON.parse(String(init?.body)) as Record<string, unknown>;
+}
+
 function createUnauthorizedResponse(): Response {
 	return new Response(JSON.stringify({ error: { message: "Unauthorized" } }), {
 		status: 401,
@@ -79,6 +86,29 @@ describe("GitHub Copilot OpenAI transport base URL", () => {
 		expect(requestedUrls[0]).toBe("https://api.githubcopilot.com/responses");
 	});
 
+	it("omits OpenAI priority service tier while native OpenAI keeps it", async () => {
+		const requestedBodies: Record<string, unknown>[] = [];
+		const fetchMock = vi.fn(async (input: string | URL | Request, init?: RequestInit) => {
+			requestedBodies.push(await getRequestBody(input, init));
+			return createUnauthorizedResponse();
+		});
+		const requestOptions = {
+			apiKey: testToken,
+			fetch: fetchMock as unknown as typeof fetch,
+			serviceTier: "priority" as const,
+		};
+
+		const copilotModel = getBundledModel("github-copilot", "gpt-5.4") as Model<"openai-responses">;
+		await streamOpenAIResponses(copilotModel, testContext, requestOptions).result();
+
+		const openAIModel = getBundledModel("openai", "gpt-5-mini") as Model<"openai-responses">;
+		await streamOpenAIResponses(openAIModel, testContext, requestOptions).result();
+
+		expect(requestedBodies).toHaveLength(2);
+		expect(requestedBodies[0]?.service_tier).toBeUndefined();
+		expect(requestedBodies[1]?.service_tier).toBe("priority");
+	});
+
 	it("routes structured enterprise credentials to the enterprise chat completions host", async () => {
 		const requestedUrls: string[] = [];
 		const requestedAuthHeaders: Array<string | null> = [];