Report unprocessable submissions as invalid-submission errors (422) (#14)

* Added tests for TCPServer and improved error handling for JsonRpcHandler and FileHandler

* Remove TCPServer tests that depend on the unmerged feature/tcp branch

tests/io/tcp_server_test.py imports lf_toolkit.io.tcp_server, which only
exists on feature/tcp, so test collection failed on this branch. The tests
belong with that branch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: m-messer

lf_toolkit/io/file_server.py

@@ -6,6 +6,7 @@
 
 from .base_server import BaseServer
 from .handler import Handler
+from .handler import InvalidSubmissionError
 
 
 class FileHandler(Handler):
@@ -16,8 +17,13 @@ async def dispatch(self, req: str) -> str:
         try:
             result = await self.handle(command, request)
             response = {"command": command, "result": result}
+        except InvalidSubmissionError as e:
+            response = {
+                "command": command,
+                "error": {"message": str(e), "code": "INVALID_SUBMISSION"},
+            }
         except Exception as e:
-            response = {"command": command, "error": str(e)}
+            response = {"command": command, "error": {"message": str(e)}}
         return ujson.dumps(response)
 
 

lf_toolkit/io/handler.py

@@ -14,6 +14,13 @@
 from ..shared import Command
 from ..shared import Params
 
+# JSON-RPC error code for submissions the user handler cannot process.
+INVALID_SUBMISSION_CODE = 422
+
+
+class InvalidSubmissionError(Exception):
+    """Raised when the user handler cannot process the submitted response."""
+
 
 class Handler(ABC):
 
@@ -40,6 +47,8 @@ async def _call_user_handler(self, req: str, *args, **kwargs):
                 return await handler(*args, **kwargs)
             else:
                 return await anyio.to_thread.run_sync(handler, *args, **kwargs)
+        except ValueError as e:
+            raise InvalidSubmissionError(str(e)) from e
         except Exception as e:
             raise ValueError(f"Error calling user handler for '{req}': {e}")
 

lf_toolkit/io/rpc_handler.py

@@ -5,7 +5,9 @@
 from jsonrpcserver import async_dispatch
 
 from ..shared import Command
+from .handler import INVALID_SUBMISSION_CODE
 from .handler import Handler
+from .handler import InvalidSubmissionError
 
 
 class JsonRpcHandler(Handler):
@@ -31,6 +33,8 @@ async def wrapped(req: dict):
         try:
             result = await handler.handle(name, {"params": req})
             return Success(result)
+        except InvalidSubmissionError as e:
+            return Error(INVALID_SUBMISSION_CODE, str(e))
         except Exception as e:
             # Pass only the message: the exception object is not JSON
             # serializable, so sending it as `data` makes serializing the

tests/io/file_server_test.py

@@ -81,7 +81,7 @@ async def test_dispatch_exception_handling(self, handler):
 
         assert response["command"] == "bad_command"
         assert "error" in response
-        assert response["error"] == "Invalid command"
+        assert response["error"] == {"message": "Invalid command"}
         assert "result" not in response
 
     @pytest.mark.asyncio
@@ -195,7 +195,7 @@ async def test_run_with_handler_error(self, temp_files, tmp_path):
 
         assert response["command"] == "failing_command"
         assert "error" in response
-        assert response["error"] == "Processing failed"
+        assert response["error"] == {"message": "Processing failed"}
 
     @pytest.mark.asyncio
     async def test_run_missing_request_file(self, temp_files):
@@ -273,3 +273,37 @@ async def test_run_with_complex_nested_data(self, temp_files, tmp_path):
 
         assert response["command"] == "eval"
         assert response["result"] == {"processed": True}
+
+
+class TestFileHandlerUserErrors:
+    """Tests for errors raised by registered user handlers"""
+
+    @pytest.mark.asyncio
+    async def test_value_error_returns_invalid_submission(self):
+        def preview_fn(response, params):
+            raise ValueError(f"Failed to parse SymPy expression: {response}")
+
+        handler = FileHandler()
+        handler.register("preview", preview_fn)
+
+        request = ujson.dumps({"command": "preview", "params": {"response": "A/(w*"}})
+        response = ujson.loads(await handler.dispatch(request))
+
+        assert response["error"] == {
+            "message": "Failed to parse SymPy expression: A/(w*",
+            "code": "INVALID_SUBMISSION",
+        }
+
+    @pytest.mark.asyncio
+    async def test_unexpected_error_has_no_code(self):
+        def preview_fn(response, params):
+            raise RuntimeError("boom")
+
+        handler = FileHandler()
+        handler.register("preview", preview_fn)
+
+        request = ujson.dumps({"command": "preview", "params": {"response": "x"}})
+        response = ujson.loads(await handler.dispatch(request))
+
+        assert "code" not in response["error"]
+        assert "boom" in response["error"]["message"]

tests/io/rpc_handler_test.py

@@ -69,3 +69,43 @@ def chat_health_fn() -> ChatHealthResponse:
 
         assert response["result"]["status"] == "OK"
         assert response["result"]["capabilities"]["supportsChat"] is True
+
+
+class TestJsonRpcHandlerErrors:
+
+    @pytest.fixture
+    def handler(self):
+        return JsonRpcHandler()
+
+    @staticmethod
+    def _eval_request(response):
+        return ujson.dumps({
+            "jsonrpc": "2.0",
+            "method": "eval",
+            "params": [{"response": response, "answer": "x", "params": {}}],
+            "id": 1,
+        })
+
+    @pytest.mark.asyncio
+    async def test_value_error_returns_invalid_submission_code(self, handler):
+        def eval_fn(response, answer, params):
+            raise ValueError(f"Failed to parse SymPy expression: {response}")
+
+        handler.register("eval", eval_fn)
+
+        response = ujson.loads(await handler.dispatch(self._eval_request("A/(w*")))
+
+        assert response["error"]["code"] == 422
+        assert response["error"]["message"] == "Failed to parse SymPy expression: A/(w*"
+
+    @pytest.mark.asyncio
+    async def test_unexpected_error_returns_generic_code(self, handler):
+        def eval_fn(response, answer, params):
+            raise RuntimeError("boom")
+
+        handler.register("eval", eval_fn)
+
+        response = ujson.loads(await handler.dispatch(self._eval_request("x+1")))
+
+        assert response["error"]["code"] == 0
+        assert "boom" in response["error"]["message"]