Split ValueError from unexpected errors during logs.

ValueError only needs a warning.
Unexpected errors need a full traceback.

Author: StarsExpress

src/mcp/server/mcpserver/server.py

@@ -1283,6 +1283,7 @@ async def get_prompt(
         """
         if context is None:
             context = Context(mcp_server=self, subscriptions=self._subscriptions)
+
         try:
             prompt = self._prompt_manager.get_prompt(name)
             if not prompt:
@@ -1296,8 +1297,17 @@ async def get_prompt(
                 description=prompt.description,
                 messages=pydantic_core.to_jsonable_python(rendered),
             )
+
         except MCPError:
             raise
+
+        except ValueError as e:
+            # Expected user-input validation failures, like missing required
+            # arguments, don't need a full traceback for unexpected errors.
+            # Instead, log a concise warning without exc_info.
+            logger.warning(f"Error getting prompt {name}: {e}")
+            raise ValueError(str(e)) from e
+
         except Exception as e:
             logger.exception(f"Error getting prompt {name}")
             raise ValueError(str(e)) from e

tests/server/mcpserver/test_server.py

@@ -1,4 +1,5 @@
 import base64
+import logging
 from pathlib import Path
 from types import SimpleNamespace
 from typing import Any
@@ -1527,6 +1528,35 @@ def prompt_fn(name: str) -> str: ...  # pragma: no branch
             with pytest.raises(MCPError, match="Missing required arguments"):
                 await client.get_prompt("prompt_fn")
 
+    async def test_get_prompt_missing_args_logs_warning_without_traceback(
+        self, caplog: pytest.LogCaptureFixture
+    ) -> None:
+        """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()
+
+        @mcp.prompt()
+        def prompt_fn(name: str) -> str: ...  # Pragma: no branch.
+
+        with caplog.at_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")
+
+        server_records = [r for r in caplog.records if r.name == "mcp.server.mcpserver.server"]
+        assert len(server_records) == 1
+
+        # ValueError should have warning log without exc_info.
+        assert server_records[0].levelno == logging.WARNING
+        assert not server_records[0].exc_info
+
 
 async def test_resource_decorator_rfc6570_reserved_expansion():
     # Regression: old regex-based param extraction couldn't see `path`