feat(responses): degrade background mode to synchronous execution

OpenAI Responses API supports background: true for async/deferred runs
(returns 202 with response_id + GET /responses/<id> polling). Implementing
the full background contract requires queue/poll infrastructure that
OmniRoute does not have as a forward proxy.

Previously the translator rejected such requests with HTTP 400
"Unsupported Responses API feature: background mode is not supported by
omniroute". This breaks clients that opportunistically set background=true
for long-running tasks (Capy Captain Pro, Codex agents) even when the
underlying execution would complete in a normal HTTP timeout window.

Degrade silently: strip the background flag and run synchronously. The
client receives the full response in one round-trip with status=completed.
Clients that strictly require the async contract still observe a completed
response on the first poll and can adapt.

The existing test that asserted background=true throws is split into two:
- the unsupported-tool-type branch keeps the throw assertion
- the background branch now asserts the flag is stripped and translation
  completes with the expected messages

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

open-sse/translator/request/openai-responses.ts

@@ -77,13 +77,21 @@ export function openaiResponsesToOpenAIRequest(
     }
   }
 
-  if (root.background) {
-    throw unsupportedFeature(
-      "Unsupported Responses API feature: background mode is not supported by omniroute"
-    );
-  }
-
   const result: JsonRecord = { ...root };
+
+  // background: true requests a deferred Responses API run (the upstream
+  // returns 202 with response_id and the client polls GET /responses/<id>).
+  // OmniRoute is a forward proxy that streams responses synchronously —
+  // implementing the queue/poll contract would require persistence and a
+  // separate retrieval surface. Degrade silently: strip the flag and run
+  // synchronously. The client receives the full response in one round-trip
+  // with status=completed. Clients that set background=true opportunistically
+  // (Capy Captain Pro, Codex agents) work unchanged. Clients that strictly
+  // require the async contract still observe a completed response on the
+  // first poll and can adapt.
+  if (result.background !== undefined) {
+    delete result.background;
+  }
   const messages: JsonRecord[] = [];
   result.messages = messages;
 

tests/unit/translator-openai-responses-req.test.ts

@@ -110,7 +110,7 @@ test("Responses -> Chat filters orphan tool outputs and supports role-based mess
   });
 });
 
-test("Responses -> Chat rejects unsupported built-in tools and background mode", () => {
+test("Responses -> Chat rejects unsupported built-in tools", () => {
   assert.throws(
     () =>
       openaiResponsesToOpenAIRequest(
@@ -124,20 +124,26 @@ test("Responses -> Chat rejects unsupported built-in tools and background mode",
       ),
     (error: any) => error.statusCode === 400 && error.errorType === "unsupported_feature"
   );
+});
 
-  assert.throws(
-    () =>
-      openaiResponsesToOpenAIRequest(
-        "gpt-4o",
-        {
-          input: [],
-          background: true,
-        },
-        false,
-        null
-      ),
-    (error: any) => error.statusCode === 400 && error.errorType === "unsupported_feature"
+test("Responses -> Chat strips background flag and degrades to synchronous execution", () => {
+  // Previously this threw 400 unsupported_feature. OmniRoute is a forward proxy
+  // and cannot host the deferred run + poll contract, so background=true is
+  // silently dropped and the request runs synchronously. Clients that set the
+  // flag opportunistically (Capy Captain Pro, Codex agents) work unchanged.
+  const result = openaiResponsesToOpenAIRequest(
+    "gpt-5.5",
+    {
+      input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+      background: true,
+    },
+    true,
+    null
   );
+  const r = result as Record<string, unknown>;
+  assert.equal(r.background, undefined, "background flag must be stripped from output");
+  assert.ok(Array.isArray(r.messages), "translation must complete and produce messages");
+  assert.equal((r.messages as unknown[]).length, 1, "user message must be preserved");
 });
 
 test("Chat -> Responses converts messages, tool calls, tool outputs, tools and pass-through params", () => {