diff --git a/contextual_orchestrator/orchestrator.py b/contextual_orchestrator/orchestrator.py
index 7953b968c..d9cc60276 100644
--- a/contextual_orchestrator/orchestrator.py
+++ b/contextual_orchestrator/orchestrator.py
@@ -4352,6 +4352,14 @@ def send_synthesis(
                 self._record_failure(final_agent.id)
             if final_agent.group_name and not _is_request_too_large_error(exc):
                 self._group_router.observe_failure(final_agent.id)
+            if (
+                virtual_model
+                and response_format_requested
+                and not isinstance(exc, ProviderRequestTooLargeError)
+            ):
+                raise NoViableAgentError(
+                    retry_after_seconds=max(1, math.ceil(self.circuit_reset_seconds))
+                ) from None
             raise
 
         def provider_output(response: Mapping[str, Any]) -> str:
diff --git a/tests/test_openai_passthrough.py b/tests/test_openai_passthrough.py
index e91bc31f8..01451df02 100644
--- a/tests/test_openai_passthrough.py
+++ b/tests/test_openai_passthrough.py
@@ -779,6 +779,58 @@ def proxy_send_once(self, agent, endpoint, payload):
     assert body["error_message"] == "request body exceeds every eligible provider limit"
 
 
+def test_http_virtual_structured_synthesis_failure_returns_retryable_503() -> None:
+    """A final provider failure is availability, not an internal server bug."""
+
+    class FailingSynthesisClient(ModelClient):
+        def proxy_send_once(self, agent, endpoint, payload):
+            del agent, endpoint, payload
+            raise urllib.error.URLError("synthetic provider outage")
+
+        proxy_send = proxy_send_once
+
+    orchestrator = TaskOrchestrator(
+        [ModelAgent("worker_agent", "worker-model", tags=("response_format",))],
+        client=FailingSynthesisClient(),  # type: ignore[arg-type]
+    )
+    orchestrator.conduct = lambda *args, **kwargs: {  # type: ignore[method-assign]
+        "mode": "conduct",
+        "answer": "evidence",
+        "trace": [{
+            "id": 0,
+            "role": "worker",
+            "agent_id": "worker_agent",
+            "subtask": "Evidence",
+            "access": [],
+            "output": "evidence",
+        }],
+        "verification": {"accepted": True, "reason": "test", "verifier_output": ""},
+    }
+    token = "passthrough_token"
+    server = build_server(
+        orchestrator, port=0, security=SecurityConfig(auth_token=token)
+    )
+    threading.Thread(target=server.serve_forever, daemon=True).start()
+    try:
+        status, body = _post(
+            f"http://127.0.0.1:{server.server_address[1]}/v1/chat/completions",
+            {
+                "model": TaskOrchestrator.AUTO_MODEL,
+                "messages": [{"role": "user", "content": "return JSON"}],
+                "response_format": {"type": "json_object"},
+            },
+            token,
+        )
+    finally:
+        server.shutdown()
+        server.server_close()
+
+    assert status == 503
+    assert body["error"]["code"] == "no_viable_agent"
+    assert body["error"]["detail"]["retry_after_seconds"] == 30
+    assert "synthetic provider outage" not in json.dumps(body)
+
+
 def test_http_chat_completions_accepts_response_format_and_passes_through() -> None:
     server, port, token = _serve()
     url = f"http://127.0.0.1:{port}/v1/chat/completions"
diff --git a/tests/test_passthrough_provider_failover.py b/tests/test_passthrough_provider_failover.py
index 6b33d52a8..5aee05c98 100644
--- a/tests/test_passthrough_provider_failover.py
+++ b/tests/test_passthrough_provider_failover.py
@@ -19,6 +19,7 @@
 )
 from contextual_orchestrator.orchestrator import (
     ModelClient,
+    NoViableAgentError,
     ProviderRequestTooLargeError,
 )
 
@@ -506,7 +507,7 @@ def test_structured_synthesis_records_non_413_failure_on_actual_provider() -> No
         "verification": {"accepted": True, "reason": "test", "verifier_output": ""},
     }
 
-    with pytest.raises(RuntimeError, match="fallback unavailable"):
+    with pytest.raises(NoViableAgentError) as exc_info:
         orchestrator.proxy_completion(
             {
                 "model": TaskOrchestrator.AUTO_MODEL,
@@ -516,6 +517,7 @@ def test_structured_synthesis_records_non_413_failure_on_actual_provider() -> No
             single_agent=False,
         )
 
+    assert "fallback unavailable" not in str(exc_info.value)
     assert orchestrator._circuit["fallback_agent"]["failures"] == 1.0
     assert "primary_agent" not in orchestrator._circuit