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

Co-authored-by: ihrpr <[email protected]>

Author: 2 people

src/mcp/server/fastmcp/server.py

@@ -957,9 +957,16 @@ async def list_prompts(self) -> list[MCPPrompt]:
     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))

tests/server/fastmcp/test_server.py

@@ -982,6 +982,46 @@ def fn(name: str) -> str:
             assert isinstance(content, TextContent)
             assert content.text == "Hello, World!"
 
+    @pytest.mark.anyio
+    async def test_get_prompt_with_description(self):
+        """Test getting a prompt through MCP protocol."""
+        mcp = FastMCP()
+
+        @mcp.prompt(description="Test prompt description")
+        def fn(name: str) -> str:
+            return f"Hello, {name}!"
+
+        async with client_session(mcp._mcp_server) as client:
+            result = await client.get_prompt("fn", {"name": "World"})
+            assert result.description == "Test prompt description"
+
+    @pytest.mark.anyio
+    async def test_get_prompt_without_description(self):
+        """Test getting a prompt without description returns empty string."""
+        mcp = FastMCP()
+
+        @mcp.prompt()
+        def fn(name: str) -> str:
+            return f"Hello, {name}!"
+
+        async with client_session(mcp._mcp_server) as client:
+            result = await client.get_prompt("fn", {"name": "World"})
+            assert result.description == ""
+
+    @pytest.mark.anyio
+    async def test_get_prompt_with_docstring_description(self):
+        """Test prompt uses docstring as description when not explicitly provided."""
+        mcp = FastMCP()
+
+        @mcp.prompt()
+        def fn(name: str) -> str:
+            """This is the function docstring."""
+            return f"Hello, {name}!"
+
+        async with client_session(mcp._mcp_server) as client:
+            result = await client.get_prompt("fn", {"name": "World"})
+            assert result.description == "This is the function docstring."
+
     @pytest.mark.anyio
     async def test_get_prompt_with_resource(self):
         """Test getting a prompt that returns resource content."""