Created PromptValidationError for user-input validation failures.

Author: StarsExpress

src/mcp/server/mcpserver/prompts/__init__.py

@@ -1,4 +1,4 @@
-from .base import Prompt
+from .base import Prompt, PromptValidationError
 from .manager import PromptManager
 
-__all__ = ["Prompt", "PromptManager"]
+__all__ = ["Prompt", "PromptManager", "PromptValidationError"]

src/mcp/server/mcpserver/prompts/base.py

@@ -22,6 +22,18 @@
     from mcp.server.mcpserver.context import Context
 
 
+class PromptValidationError(ValueError):
+    """Raised when prompt arguments fail validation (e.g. a required argument is missing).
+
+    A `ValueError` subclass so existing callers that catch `ValueError` are
+    unaffected. It exists to distinguish this expected, user-input validation
+    failure from the generic `ValueError` `Prompt.render` also raises when the
+    prompt function itself raises an unexpected exception - callers that log
+    the two differently (see `MCPServer.get_prompt`) can `except` this
+    specifically without downgrading a genuine crash to a quiet warning.
+    """
+
+
 class Message(BaseModel):
     """Base class for all prompt messages.
 
@@ -166,15 +178,15 @@ async def render(
         through unchanged so the multi-round-trip flow reaches the client.
 
         Raises:
-            ValueError: If required arguments are missing, or if rendering fails.
+            PromptValidationError: If required arguments are missing, or if rendering fails.
         """
         # Validate required arguments
         if self.arguments:
             required = {arg.name for arg in self.arguments if arg.required}
             provided = set(arguments or {})
             missing = required - provided
             if missing:
-                raise ValueError(f"Missing required arguments: {missing}")
+                raise PromptValidationError(f"Missing required arguments: {missing}")
 
         try:
             # Add context to arguments if needed

src/mcp/server/mcpserver/server.py

@@ -72,7 +72,7 @@
 from mcp.server.lowlevel.server import lifespan as default_lifespan
 from mcp.server.mcpserver.context import Context
 from mcp.server.mcpserver.exceptions import ResourceError, ResourceNotFoundError
-from mcp.server.mcpserver.prompts import Prompt, PromptManager
+from mcp.server.mcpserver.prompts import Prompt, PromptManager, PromptValidationError
 from mcp.server.mcpserver.resources import (
     DEFAULT_RESOURCE_SECURITY,
     FunctionResource,
@@ -1301,10 +1301,15 @@ async def get_prompt(
         except MCPError:
             raise
 
-        except ValueError as e:
+        except PromptValidationError 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.
+            # 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 ValueError(str(e)) from e
 

tests/server/mcpserver/test_server.py

@@ -1543,7 +1543,7 @@ async def test_get_prompt_missing_args_logs_warning_without_traceback(
         mcp = MCPServer()
 
         @mcp.prompt()
-        def prompt_fn(name: str) -> str: ...  # Pragma: no branch.
+        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:
@@ -1557,6 +1557,29 @@ def prompt_fn(name: str) -> str: ...  # Pragma: no branch.
         assert server_records[0].levelno == logging.WARNING
         assert not server_records[0].exc_info
 
+    async def test_get_prompt_unexpected_error_still_logs_traceback(self, caplog: pytest.LogCaptureFixture) -> None:
+        """A prompt function raising an unexpected (non-validation) exception must
+        still be logged with a full traceback, even though `Prompt.render` also
+        wraps it as a plain `ValueError` — same wire type as the missing-argument
+        case, but not a `PromptValidationError`, so it must not be downgraded."""
+        mcp = MCPServer()
+
+        @mcp.prompt()
+        def prompt_fn() -> str:
+            raise KeyError("boom")
+
+        with caplog.at_level(logging.WARNING, logger="mcp.server.mcpserver.server"):
+            async with Client(mcp, mode="legacy") as client:
+                with pytest.raises(MCPError):
+                    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
+
+        # Unexpected errors should have error log with exc_info.
+        assert server_records[0].levelno == logging.ERROR
+        assert server_records[0].exc_info is not None
+
 
 async def test_resource_decorator_rfc6570_reserved_expansion():
     # Regression: old regex-based param extraction couldn't see `path`