Add ToolAnnotations support in FastMCP and lowlevel servers (#482)

src/mcp/server/fastmcp/server.py

@@ -41,6 +41,7 @@ from mcp.types import (
     GetPromptResult,
     ImageContent,
     TextContent,
+    ToolAnnotations,
 )
 from mcp.types import Prompt as MCPPrompt
 from mcp.types import PromptArgument as MCPPromptArgument
@@ -176,6 +177,7 @@ class FastMCP:
                 name=info.name,
                 description=info.description,
                 inputSchema=info.parameters,
+                annotations=info.annotations,
             )
             for info in tools
         ]
@@ -244,6 +246,7 @@ class FastMCP:
         fn: AnyFunction,
         name: str | None = None,
         description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> None:
         """Add a tool to the server.
 
@@ -254,11 +257,17 @@ class FastMCP:
             fn: The function to register as a tool
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            annotations: Optional ToolAnnotations providing additional tool information
         """
-        self._tool_manager.add_tool(fn, name=name, description=description)
+        self._tool_manager.add_tool(
+            fn, name=name, description=description, annotations=annotations
+        )
 
     def tool(
-        self, name: str | None = None, description: str | None = None
+        self,
+        name: str | None = None,
+        description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Callable[[AnyFunction], AnyFunction]:
         """Decorator to register a tool.
 
@@ -269,6 +278,7 @@ class FastMCP:
         Args:
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            annotations: Optional ToolAnnotations providing additional tool information
 
         Example:
             @server.tool()
@@ -293,7 +303,9 @@ class FastMCP:
             )
 
         def decorator(fn: AnyFunction) -> AnyFunction:
-            self.add_tool(fn, name=name, description=description)
+            self.add_tool(
+                fn, name=name, description=description, annotations=annotations
+            )
             return fn
 
         return decorator

src/mcp/server/fastmcp/tools/base.py

@@ -8,6 +8,7 @@ from pydantic import BaseModel, Field
 
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.utilities.func_metadata import FuncMetadata, func_metadata
+from mcp.types import ToolAnnotations
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
@@ -30,6 +31,9 @@ class Tool(BaseModel):
     context_kwarg: str | None = Field(
         None, description="Name of the kwarg that should receive context"
     )
+    annotations: ToolAnnotations | None = Field(
+        None, description="Optional annotations for the tool"
+    )
 
     @classmethod
     def from_function(
@@ -38,9 +42,10 @@ class Tool(BaseModel):
         name: str | None = None,
         description: str | None = None,
         context_kwarg: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Tool:
         """Create a Tool from a function."""
-        from mcp.server.fastmcp import Context
+        from mcp.server.fastmcp.server import Context
 
         func_name = name or fn.__name__
 
@@ -73,6 +78,7 @@ class Tool(BaseModel):
             fn_metadata=func_arg_metadata,
             is_async=is_async,
             context_kwarg=context_kwarg,
+            annotations=annotations,
         )
 
     async def run(

src/mcp/server/fastmcp/tools/tool_manager.py

@@ -7,6 +7,7 @@ from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.tools.base import Tool
 from mcp.server.fastmcp.utilities.logging import get_logger
 from mcp.shared.context import LifespanContextT
+from mcp.types import ToolAnnotations
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
@@ -35,9 +36,12 @@ class ToolManager:
         fn: Callable[..., Any],
         name: str | None = None,
         description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Tool:
         """Add a tool to the server."""
-        tool = Tool.from_function(fn, name=name, description=description)
+        tool = Tool.from_function(
+            fn, name=name, description=description, annotations=annotations
+        )
         existing = self._tools.get(tool.name)
         if existing:
             if self.warn_on_duplicate_tools: