diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index cc99bbb0..b8a991b6 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -179,7 +179,7 @@ function createClient(
 		Object.assign(headers, copilotHeaders);
 	}
 
-	if (sessionId && model.provider === "openai" && model.baseUrl.includes("api.openai.com")) {
+	if (sessionId) {
 		headers.session_id = sessionId;
 		headers["x-client-request-id"] = sessionId;
 	}
diff --git a/packages/ai/test/openai-responses-copilot-provider.test.ts b/packages/ai/test/openai-responses-copilot-provider.test.ts
index df1ccb8a..ef55603b 100644
--- a/packages/ai/test/openai-responses-copilot-provider.test.ts
+++ b/packages/ai/test/openai-responses-copilot-provider.test.ts
@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { getModel } from "../src/models.js";
 import { streamOpenAIResponses } from "../src/providers/openai-responses.js";
+import type { Model } from "../src/types.js";
 
 type CapturedHeaders = Headers | string[][] | Record<string, string | readonly string[]> | undefined;
 
@@ -22,6 +23,7 @@ function getHeader(headers: CapturedHeaders, name: string): string | null {
 
 async function captureOpenAIResponseHeaders(
 	options: Parameters<typeof streamOpenAIResponses>[2],
+	model: Model<"openai-responses"> = getModel("openai", "gpt-5.4"),
 ): Promise<{ sessionId: string | null; clientRequestId: string | null }> {
 	const captured = { sessionId: null as string | null, clientRequestId: null as string | null };
 	vi.spyOn(globalThis, "fetch").mockImplementation(async (_input, init) => {
@@ -34,7 +36,7 @@ async function captureOpenAIResponseHeaders(
 	});
 
 	const stream = streamOpenAIResponses(
-		getModel("openai", "gpt-5.4"),
+		model,
 		{
 			systemPrompt: "sys",
 			messages: [{ role: "user", content: "hi", timestamp: Date.now() }],
@@ -95,6 +97,17 @@ describe("openai-responses provider defaults", () => {
 		expect(captured).toEqual({ sessionId: "session-123", clientRequestId: "session-123" });
 	});
 
+	it("sets cache-affinity headers for proxy OpenAI Responses requests with a sessionId", async () => {
+		const proxyModel: Model<"openai-responses"> = {
+			...getModel("openai", "gpt-5.4"),
+			provider: "opencode",
+			baseUrl: "https://proxy.example.com/v1",
+		};
+		const captured = await captureOpenAIResponseHeaders({ sessionId: "session-123" }, proxyModel);
+
+		expect(captured).toEqual({ sessionId: "session-123", clientRequestId: "session-123" });
+	});
+
 	it("lets explicit headers override the default OpenAI cache-affinity headers", async () => {
 		const captured = await captureOpenAIResponseHeaders({
 			sessionId: "session-123",