fix: classify backend value errors as bad gateway

Author: OdinBot33

examples/subscription_bridge/server.py

@@ -589,7 +589,10 @@ def run_backend_structured(
                 stderr = (result.stderr or result.stdout or "backend execution failed").strip()
                 raise RuntimeError(stderr)
             raw = output_path.read_text() if output_path.exists() else result.stdout
-            return _normalize_decision_payload(_load_json_output(raw))
+            try:
+                return _normalize_decision_payload(_load_json_output(raw))
+            except ValueError as exc:
+                raise RuntimeError(str(exc)) from exc
 
     if backend == "claude":
         cmd = [
@@ -610,7 +613,10 @@ def run_backend_structured(
         if result.returncode != 0:
             stderr = (result.stderr or result.stdout or "backend execution failed").strip()
             raise RuntimeError(stderr)
-        return _normalize_decision_payload(_load_json_output(result.stdout))
+        try:
+            return _normalize_decision_payload(_load_json_output(result.stdout))
+        except ValueError as exc:
+            raise RuntimeError(str(exc)) from exc
 
     raise ValueError(f"Unsupported backend: {backend}")
 
@@ -620,24 +626,27 @@ def _respond_for_chat_request(
 ) -> dict[str, Any]:
     prompt = build_chat_prompt(payload)
     if _normalize_tools(payload.get("tools")):
-        decision = run_backend_structured(
-            backend=backend,
-            prompt=_build_structured_decision_prompt(prompt, payload),
-            model=model,
-            workdir=workdir,
-            schema=DecisionSchema,
-        )
-        if decision.get("type") == "tool_calls":
+        try:
+            decision = run_backend_structured(
+                backend=backend,
+                prompt=_build_structured_decision_prompt(prompt, payload),
+                model=model,
+                workdir=workdir,
+                schema=DecisionSchema,
+            )
+            if decision.get("type") == "tool_calls":
+                return build_chat_completion_response(
+                    model=model,
+                    request_id=request_id,
+                    tool_calls=decision.get("tool_calls"),
+                )
             return build_chat_completion_response(
                 model=model,
                 request_id=request_id,
-                tool_calls=decision.get("tool_calls"),
+                content=str(decision.get("content") or ""),
             )
-        return build_chat_completion_response(
-            model=model,
-            request_id=request_id,
-            content=str(decision.get("content") or ""),
-        )
+        except ValueError as exc:
+            raise RuntimeError(str(exc)) from exc
 
     text = run_backend(backend=backend, prompt=prompt, model=model, workdir=workdir)
     return build_chat_completion_response(model=model, request_id=request_id, content=text)
@@ -648,24 +657,27 @@ def _respond_for_responses_request(
 ) -> dict[str, Any]:
     prompt = build_responses_prompt(payload)
     if _normalize_tools(payload.get("tools")):
-        decision = run_backend_structured(
-            backend=backend,
-            prompt=_build_structured_decision_prompt(prompt, payload),
-            model=model,
-            workdir=workdir,
-            schema=DecisionSchema,
-        )
-        if decision.get("type") == "tool_calls":
+        try:
+            decision = run_backend_structured(
+                backend=backend,
+                prompt=_build_structured_decision_prompt(prompt, payload),
+                model=model,
+                workdir=workdir,
+                schema=DecisionSchema,
+            )
+            if decision.get("type") == "tool_calls":
+                return build_responses_api_response(
+                    model=model,
+                    request_id=request_id,
+                    tool_calls=decision.get("tool_calls"),
+                )
             return build_responses_api_response(
                 model=model,
                 request_id=request_id,
-                tool_calls=decision.get("tool_calls"),
+                content=str(decision.get("content") or ""),
             )
-        return build_responses_api_response(
-            model=model,
-            request_id=request_id,
-            content=str(decision.get("content") or ""),
-        )
+        except ValueError as exc:
+            raise RuntimeError(str(exc)) from exc
 
     text = run_backend(backend=backend, prompt=prompt, model=model, workdir=workdir)
     return build_responses_api_response(model=model, request_id=request_id, content=text)

tests/examples/test_subscription_bridge.py

@@ -59,6 +59,55 @@ def fake_run_backend(*, backend: str, prompt: str, model: str | None, workdir: P
     )
 
 
+def test_http_server_returns_502_for_backend_value_errors(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+) -> None:
+    def fake_run_backend_structured(
+        *, backend: str, prompt: str, model: str | None, workdir: Path, schema: dict[str, Any]
+    ) -> dict[str, Any]:
+        raise ValueError("backend emitted malformed JSON")
+
+    monkeypatch.setattr(server, "run_backend_structured", fake_run_backend_structured)
+
+    httpd = server.make_server("127.0.0.1", 0, default_backend="codex", workdir=tmp_path)
+    thread = threading.Thread(target=httpd.serve_forever, daemon=True)
+    thread.start()
+    time.sleep(0.05)
+    try:
+        req = urllib.request.Request(
+            f"http://127.0.0.1:{httpd.server_address[1]}/v1/chat/completions",
+            data=json.dumps(
+                {
+                    "messages": [{"role": "user", "content": "Use the weather tool."}],
+                    "tools": [
+                        {
+                            "type": "function",
+                            "function": {
+                                "name": "get_weather",
+                                "description": "Get the weather for a city.",
+                                "parameters": {
+                                    "type": "object",
+                                    "properties": {"city": {"type": "string"}},
+                                },
+                            },
+                        }
+                    ],
+                }
+            ).encode("utf-8"),
+            headers={"Content-Type": "application/json"},
+        )
+        with pytest.raises(urllib.error.HTTPError) as exc_info:
+            urllib.request.urlopen(req)
+    finally:
+        httpd.shutdown()
+        httpd.server_close()
+        thread.join(timeout=2)
+
+    assert exc_info.value.code == 502
+    payload = json.loads(exc_info.value.read().decode("utf-8"))
+    assert payload == {"error": {"message": "backend emitted malformed JSON"}}
+
+
 def test_build_chat_prompt_from_messages_preserves_roles() -> None:
     payload = {
         "messages": [