Revert "Ensured no traceback at dispatcher boundary."

This reverts commit 8c30d12.

Author: StarsExpress

src/mcp/server/mcpserver/server.py

@@ -1303,20 +1303,15 @@ async def get_prompt(
 
         except PromptValidationError as e:
             # Expected user-input validation failures, like missing required
-            # arguments, don't need a full traceback anywhere in request path.
-
-            # Raising `MCPError` here — the same pattern
-            # `_handle_read_resource` uses for `ResourceNotFoundError` above —
-            # lets `handler_exception_to_error_data` at the dispatcher
-            # boundary recognize this as expected too, so neither this layer
-            # nor the dispatcher's catch-all logs a traceback for it.
+            # arguments, don't need a full traceback. Log a concise warning
+            # instead of the exc_info dump reserved for unexpected errors.
 
             # `Prompt.render` also raises a plain `ValueError` when the prompt
             # function itself throws, so this narrower type is caught here
             # instead of `ValueError` to avoid swallowing that traceback too.
 
             logger.warning(f"Error getting prompt {name}: {e}")
-            raise MCPError(code=INVALID_PARAMS, message=str(e), data={"name": name}) from e
+            raise ValueError(str(e)) from e
 
         except Exception as e:
             logger.exception(f"Error getting prompt {name}")

tests/server/mcpserver/test_server.py

@@ -1531,12 +1531,14 @@ def prompt_fn(name: str) -> str: ...  # pragma: no branch
     async def test_get_prompt_missing_args_logs_warning_without_traceback(
         self, caplog: pytest.LogCaptureFixture
     ) -> None:
-        """Regression for issue #3342: a missing-argument PromptValidationError is
-        an expected validation failure. `MCPServer.get_prompt` logs it as a plain
-        warning (no exc_info) and raises it as `MCPError`, so the dispatcher's own
-        catch-all — which would otherwise log a second, separate traceback for any
-        exception it doesn't recognize as expected — treats it as expected too.
-        No traceback anywhere in the real request path.
+        """Regression for issue #3342: missing-argument ValueErrors are an expected
+        validation failure, so `MCPServer.get_prompt`'s own logger should emit
+        a plain warning without exc_info, instead of a full traceback.
+
+        Note: `jsonrpc_dispatcher` has a separate, intentional catch-all that
+        logs a traceback for any handler exception it doesn't recognize as
+        `MCPError`/`ValidationError`. However, that generic safety net is out of
+        scope here and unaffected by this fix PR #3347.
         """
         mcp = MCPServer()
 
@@ -1545,17 +1547,13 @@ def prompt_fn(name: str) -> str: ...  # pragma: no branch.
 
         # In Python 3.14, coverage.py undercounts a branch when `caplog.at_level`
         # wraps `async with Client(...): with pytest.raises(...): await ...` as
-        # a 4th nesting level around a single `await` statement.
-        # 3 levels of nesting is OK. But 4 is not.
-
-        caplog.set_level(logging.WARNING)
+        # a 4th nesting level around a single `await` statement (3 levels of
+        # nesting is OK; 4 is not). So `caplog.set_level` avoids extra `with` layer.
+        caplog.set_level(logging.WARNING, logger="mcp.server.mcpserver.server")
         async with Client(mcp, mode="legacy") as client:
             with pytest.raises(MCPError, match="Missing required arguments"):
                 await client.get_prompt("prompt_fn")
 
-        # No traceback anywhere in request path. Not just this module's logger.
-        assert not any(r.exc_info for r in caplog.records)
-
         server_records = [r for r in caplog.records if r.name == "mcp.server.mcpserver.server"]
         assert len(server_records) == 1