diff --git a/open-sse/translator/request/openai-responses.ts b/open-sse/translator/request/openai-responses.ts
index 45beee74b0..6d72f1f5f0 100644
--- a/open-sse/translator/request/openai-responses.ts
+++ b/open-sse/translator/request/openai-responses.ts
@@ -77,13 +77,28 @@ 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: log a marker when true was
+  // actually requested (operators can observe clients that should be
+  // reconfigured) and strip the flag. 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 === true) {
+    const providerStr = toString(credentialRecord.provider);
+    const modelStr = toString(model);
+    console.warn(
+      `BACKGROUND_DEGRADE provider=${providerStr || "unknown"} model=${modelStr || "unknown"}`
     );
   }
-
-  const result: JsonRecord = { ...root };
+  if (result.background !== undefined) {
+    delete result.background;
+  }
   const messages: JsonRecord[] = [];
   result.messages = messages;
 
diff --git a/tests/unit/translator-openai-responses-req.test.ts b/tests/unit/translator-openai-responses-req.test.ts
index e66c4c7469..55b8791713 100644
--- a/tests/unit/translator-openai-responses-req.test.ts
+++ b/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 +124,77 @@ 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 warnLog: string[] = [];
+  const originalWarn = console.warn;
+  console.warn = (msg: unknown) => warnLog.push(String(msg));
+  try {
+    const result = openaiResponsesToOpenAIRequest(
+      "gpt-5.5",
+      {
+        input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+        background: true,
+      },
+      true,
+      { provider: "codex" }
+    );
+    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");
+    assert.ok(
+      warnLog.some((m) => m.startsWith("BACKGROUND_DEGRADE provider=codex model=gpt-5.5")),
+      "BACKGROUND_DEGRADE warning log must be emitted when background=true"
+    );
+  } finally {
+    console.warn = originalWarn;
+  }
+});
+
+test("Responses -> Chat passes through when background flag is unset or false (no degrade log)", () => {
+  // Verifies the inverse of the degradation case: when background is absent or
+  // explicitly false, no warning should be emitted and the request body should
+  // not carry a residual background field. Guards against accidental log spam
+  // and confirms the degradation logic is conditional on background === true.
+  const warnLog: string[] = [];
+  const originalWarn = console.warn;
+  console.warn = (msg: unknown) => warnLog.push(String(msg));
+  try {
+    // Case 1: background unset
+    const r1 = openaiResponsesToOpenAIRequest(
+      "gpt-5.5",
+      { input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }] },
+      true,
+      { provider: "codex" }
+    ) as Record<string, unknown>;
+    assert.equal(r1.background, undefined, "background must be absent on output (unset case)");
+
+    // Case 2: background explicitly false
+    const r2 = openaiResponsesToOpenAIRequest(
+      "gpt-5.5",
+      {
+        input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+        background: false,
+      },
+      true,
+      { provider: "codex" }
+    ) as Record<string, unknown>;
+    assert.equal(r2.background, undefined, "background must be stripped on output (false case)");
+
+    assert.equal(
+      warnLog.filter((m) => m.startsWith("BACKGROUND_DEGRADE")).length,
+      0,
+      "BACKGROUND_DEGRADE must NOT be emitted for unset or false values"
+    );
+  } finally {
+    console.warn = originalWarn;
+  }
 });
 
 test("Chat -> Responses converts messages, tool calls, tool outputs, tools and pass-through params", () => {