Adding description field to the FastMCP get_prompt method (#614)

Co-authored-by: ihrpr <inna@anthropic.com>

src/mcp/server/fastmcp/server.py

@@ -957,9 +957,16 @@ class FastMCP:
     async def get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -> GetPromptResult:
         """Get a prompt by name with arguments."""
         try:
-            messages = await self._prompt_manager.render_prompt(name, arguments)
+            prompt = self._prompt_manager.get_prompt(name)
+            if not prompt:
+                raise ValueError(f"Unknown prompt: {name}")
 
-            return GetPromptResult(messages=pydantic_core.to_jsonable_python(messages))
+            messages = await prompt.render(arguments)
+
+            return GetPromptResult(
+                description=prompt.description,
+                messages=pydantic_core.to_jsonable_python(messages),
+            )
         except Exception as e:
             logger.exception(f"Error getting prompt {name}")
             raise ValueError(str(e))