fix(responses): insert developer message before compaction_trigger

injectDeveloperMessage was appending to raw.input after the compaction_trigger
item, causing ChatGPT backend to reject with 'compaction_trigger must be the
final input item' when multi-agent guidance fired on ultra/max effort models.

Now splices the developer message before compaction_trigger when present.

Author: lidge-jun

src/server/responses.ts

@@ -122,7 +122,15 @@ export function injectDeveloperMessage(parsed: OcxParsedRequest, text: string):
   parsed.context.messages.push({ role: "developer", content: text, timestamp: Date.now() });
   const raw = parsed._rawBody as { input?: unknown } | undefined;
   if (raw && Array.isArray(raw.input)) {
-    raw.input.push({ type: "message", role: "developer", content: [{ type: "input_text", text }] });
+    const devItem = { type: "message", role: "developer", content: [{ type: "input_text", text }] };
+    // compaction_trigger must remain the final input item (codex-rs + ChatGPT backend both
+    // validate this). Insert the developer message BEFORE the trigger when present.
+    const last = raw.input[raw.input.length - 1];
+    if (last && typeof last === "object" && (last as { type?: string }).type === "compaction_trigger") {
+      raw.input.splice(raw.input.length - 1, 0, devItem);
+    } else {
+      raw.input.push(devItem);
+    }
   }
 }
 

tests/multi-agent-compat.test.ts

@@ -100,6 +100,21 @@ describe("injectDeveloperMessage", () => {
     expect((parsed._rawBody as { input: unknown }).input).toBe("plain");
     expect(parsed.context.messages.at(-1)!.content).toBe("note");
   });
+
+  test("inserts BEFORE compaction_trigger so it stays the final input item", () => {
+    const parsed = parsedFixture({ reasoning: "max" });
+    const rawBody = parsed._rawBody as { input: unknown[] };
+    rawBody.input = [
+      { type: "message", role: "user", content: "long conversation" },
+      { type: "compaction_trigger" },
+    ];
+    injectDeveloperMessage(parsed, "guidance text");
+    const input = rawBody.input;
+    expect(input).toHaveLength(3);
+    expect((input[1] as { type: string }).type).toBe("message");
+    expect((input[1] as { role: string }).role).toBe("developer");
+    expect((input[2] as { type: string }).type).toBe("compaction_trigger");
+  });
 });
 
 describe("sanitizeEncryptedContentInPlace", () => {