refactor: merge static headers without an object spread

src/protocol/body.ts

@@ -379,8 +379,10 @@ export function buildResponsesRequest(
     // is a wire contract to depend on.
     accept: quirks.stream ? "text/event-stream" : "application/json",
     authorization: BEARER_CREDENTIAL_SENTINEL,
-    ...quirks.headers.static,
   };
+  for (const [name, value] of Object.entries(quirks.headers.static)) {
+    headers[name] = value;
+  }
   if (quirks.headers.modelHeader !== undefined) {
     headers[quirks.headers.modelHeader] = model;
   }

src/responses.test.ts

@@ -178,6 +178,18 @@ describe("Responses request builder — quirk matrix", () => {
     expect(request.headers["accept"]).toBe(accept);
   });
 
+  test("static headers override defaults and per-request headers override static ones", () => {
+    const adapter = createOpenAIResponsesAdapter(source, {
+      headers: {
+        static: { accept: "application/x-ndjson", "x-model": "static" },
+        modelHeader: "x-model",
+      },
+    });
+    const { headers } = adapter.buildRequest(turns, "m", {});
+    expect(headers["accept"]).toBe("application/x-ndjson");
+    expect(headers["x-model"]).toBe("m");
+  });
+
   test("typed shape (the default) splits assistant text into output_text", () => {
     const adapter = createOpenAIResponsesAdapter(source, {});
     const assistantTurn: ConversationTurn[] = [