fix(sse): preserve Responses combo payloads (#8310)

changelog.d/fixes/8310-combo-responses-payload.md

@@ -0,0 +1 @@
+- **fix(sse):** Combo middleware preserves OpenAI Responses request bodies and maps combo system overrides to `instructions`, preventing Responses-native fallbacks from receiving an unsupported `messages` parameter. ([#8310](https://github.com/diegosouzapw/OmniRoute/pull/8310)) — thanks @ridho9

open-sse/services/comboAgentMiddleware.ts

@@ -180,16 +180,24 @@ export function applyComboAgentMiddleware(
 ): { body: Record<string, unknown>; pinnedModel: string | null } {
   if (!comboConfig) return { body, pinnedModel: null };
 
-  let messages: Message[] = Array.isArray(body.messages) ? [...body.messages] : [];
+  const hasMessages = Array.isArray(body.messages);
+  const isResponsesRequest =
+    Object.prototype.hasOwnProperty.call(body, "input") ||
+    Object.prototype.hasOwnProperty.call(body, "instructions");
+  const systemMessage =
+    typeof comboConfig.system_message === "string" && comboConfig.system_message.trim()
+      ? comboConfig.system_message
+      : null;
+  let messages: Message[] = hasMessages ? [...(body.messages as Message[])] : [];
   let pinnedModel: string | null = null;
 
   // Context cache pinning is handled server-side in combo.ts via
   // session_model_history. No client-side <omniModel> tag extraction needed.
   pinnedModel = null;
 
-  // 2. System message override
-  if (comboConfig.system_message && comboConfig.system_message.trim()) {
-    messages = applySystemMessageOverride(messages, comboConfig.system_message);
+  // 2. System message override. Responses API uses top-level instructions instead of messages.
+  if (systemMessage && !isResponsesRequest) {
+    messages = applySystemMessageOverride(messages, systemMessage);
   }
 
   // 3. Tool filter
@@ -206,7 +214,8 @@ export function applyComboAgentMiddleware(
   return {
     body: {
       ...body,
-      messages,
+      ...(isResponsesRequest && systemMessage ? { instructions: systemMessage } : {}),
+      ...(hasMessages ? { messages } : {}),
       ...(filteredTools !== body.tools && { tools: filteredTools }),
     },
     pinnedModel,

tests/unit/services-branch-hardening.test.ts

@@ -184,6 +184,32 @@ test("combo agent middleware covers system override, tool filtering, tag strippi
   assert.equal(passthrough.body, body);
 });
 
+test("combo agent middleware preserves Responses API input and instructions", () => {
+  const body = {
+    model: "combo/default",
+    input: "Reply with exactly: pong",
+  };
+
+  const unchanged = comboAgentMiddleware.applyComboAgentMiddleware(
+    body,
+    { context_cache_protection: true },
+    "openai/gpt-4o"
+  );
+  assert.deepEqual(unchanged.body, body);
+  assert.equal(Object.hasOwn(unchanged.body, "messages"), false);
+
+  const overridden = comboAgentMiddleware.applyComboAgentMiddleware(
+    { ...body, instructions: "client instruction" },
+    { system_message: "combo instruction" },
+    "openai/gpt-4o"
+  );
+  assert.deepEqual(overridden.body, {
+    ...body,
+    instructions: "combo instruction",
+  });
+  assert.equal(Object.hasOwn(overridden.body, "messages"), false);
+});
+
 test("rate limit semaphore covers immediate acquire, timeout, cooldown drain and reset", async () => {
   const release = await rateLimitSemaphore.acquire("model-a", { maxConcurrency: 1 });
   assert.deepEqual(rateLimitSemaphore.getStats()["model-a"], {