Clarify prompt errors and restore message assertions

Author: Kludex

docs/servers/prompts.md

@@ -63,7 +63,7 @@ That is the entire life of a prompt: listed by name, rendered on demand, dropped
     ```
 
     There is no tool-style error result to hand back to a model, because no model is in the loop:
-    the call raises. You can distinguish invalid arguments from internal errors by the error code.
+    the call raises. You can distinguish missing required arguments from internal errors by the error code.
 
 ### Try it
 

src/mcp/server/mcpserver/server.py

@@ -1352,6 +1352,7 @@ async def get_prompt(
         Raises:
             MCPError: If the prompt name is unknown or a required argument is missing,
                 with code `INVALID_PARAMS`, or if the prompt function raises it.
+            ValueError: If rendering fails.
         """
         if context is None:
             context = Context(mcp_server=self, subscriptions=self._subscriptions)

tests/server/mcpserver/prompts/test_manager.py

@@ -1,4 +1,5 @@
 import pytest
+from inline_snapshot import snapshot
 from mcp_types import INVALID_PARAMS, TextContent
 
 from mcp import MCPError
@@ -110,3 +111,4 @@ def fn(name: str) -> str:  # pragma: no cover
         with pytest.raises(MCPError) as exc_info:
             await manager.render_prompt("fn", None, Context())
         assert exc_info.value.code == INVALID_PARAMS
+        assert exc_info.value.message == snapshot("Missing required arguments: {'name'}")

tests/server/mcpserver/test_server.py

@@ -1568,6 +1568,7 @@ async def test_get_unknown_prompt(self):
             with pytest.raises(MCPError) as exc_info:
                 await client.get_prompt("unknown")
             assert exc_info.value.code == INVALID_PARAMS
+            assert exc_info.value.message == snapshot("Unknown prompt: unknown")
 
     async def test_get_prompt_missing_args(self):
         """Test error when required arguments are missing."""
@@ -1580,6 +1581,7 @@ def prompt_fn(name: str) -> str: ...  # pragma: no branch
             with pytest.raises(MCPError) as exc_info:
                 await client.get_prompt("prompt_fn")
             assert exc_info.value.code == INVALID_PARAMS
+            assert exc_info.value.message == snapshot("Missing required arguments: {'name'}")
 
 
 async def test_resource_decorator_rfc6570_reserved_expansion():