OpenAPI Lambda MCP Server

from __future__ import annotations import inspect import warnings from collections.abc import Callable from dataclasses import dataclass from typing import ( TYPE_CHECKING, Annotated, Any, Generic, Literal, TypeAlias, get_type_hints, ) import mcp.types import pydantic_core from mcp.types import ContentBlock, TextContent, ToolAnnotations from mcp.types import Tool as MCPTool from pydantic import Field, PydanticSchemaGenerationError from typing_extensions import TypeVar import fastmcp from fastmcp.server.dependencies import get_context from fastmcp.utilities.components import FastMCPComponent from fastmcp.utilities.json_schema import compress_schema from fastmcp.utilities.logging import get_logger from fastmcp.utilities.types import ( Audio, File, Image, NotSet, NotSetT, find_kwarg_by_type, get_cached_typeadapter, replace_type, ) if TYPE_CHECKING: from fastmcp.tools.tool_transform import ArgTransform, TransformedTool logger = get_logger(__name__) T = TypeVar("T", default=Any) @dataclass class _WrappedResult(Generic[T]): """Generic wrapper for non-object return types.""" result: T class _UnserializableType: pass ToolResultSerializerType: TypeAlias = Callable[[Any], str] def default_serializer(data: Any) -> str: return pydantic_core.to_json(data, fallback=str).decode() class ToolResult: def __init__( self, content: list[ContentBlock] | Any | None = None, structured_content: dict[str, Any] | Any | None = None, ): if content is None and structured_content is None: raise ValueError("Either content or structured_content must be provided") elif content is None: content = structured_content self.content: list[ContentBlock] = _convert_to_content(result=content) if structured_content is not None: try: structured_content = pydantic_core.to_jsonable_python( value=structured_content ) except pydantic_core.PydanticSerializationError as e: logger.error( f"Could not serialize structured content. If this is unexpected, set your tool's output_schema to None to disable automatic serialization: {e}" ) raise if not isinstance(structured_content, dict): raise ValueError( "structured_content must be a dict or None. " f"Got {type(structured_content).__name__}: {structured_content!r}. " "Tools should wrap non-dict values based on their output_schema." ) self.structured_content: dict[str, Any] | None = structured_content def to_mcp_result( self, ) -> list[ContentBlock] | tuple[list[ContentBlock], dict[str, Any]]: if self.structured_content is None: return self.content return self.content, self.structured_content class Tool(FastMCPComponent): """Internal tool registration info.""" parameters: Annotated[ dict[str, Any], Field(description="JSON schema for tool parameters") ] output_schema: Annotated[ dict[str, Any] | None, Field(description="JSON schema for tool output") ] = None annotations: Annotated[ ToolAnnotations | None, Field(description="Additional annotations about the tool"), ] = None serializer: Annotated[ ToolResultSerializerType | None, Field(description="Optional custom serializer for tool results"), ] = None def enable(self) -> None: super().enable() try: context = get_context() context._queue_tool_list_changed() # type: ignore[private-use] except RuntimeError: pass # No context available def disable(self) -> None: super().disable() try: context = get_context() context._queue_tool_list_changed() # type: ignore[private-use] except RuntimeError: pass # No context available def to_mcp_tool( self, *, include_fastmcp_meta: bool | None = None, **overrides: Any, ) -> MCPTool: """Convert the FastMCP tool to an MCP tool.""" title = None if self.title: title = self.title elif self.annotations and self.annotations.title: title = self.annotations.title return MCPTool( name=overrides.get("name", self.name), title=overrides.get("title", title), description=overrides.get("description", self.description), inputSchema=overrides.get("inputSchema", self.parameters), outputSchema=overrides.get("outputSchema", self.output_schema), annotations=overrides.get("annotations", self.annotations), _meta=overrides.get( "_meta", self.get_meta(include_fastmcp_meta=include_fastmcp_meta) ), ) @staticmethod def from_function( fn: Callable[..., Any], name: str | None = None, title: str | None = None, description: str | None = None, tags: set[str] | None = None, annotations: ToolAnnotations | None = None, exclude_args: list[str] | None = None, output_schema: dict[str, Any] | None | NotSetT | Literal[False] = NotSet, serializer: ToolResultSerializerType | None = None, meta: dict[str, Any] | None = None, enabled: bool | None = None, ) -> FunctionTool: """Create a Tool from a function.""" return FunctionTool.from_function( fn=fn, name=name, title=title, description=description, tags=tags, annotations=annotations, exclude_args=exclude_args, output_schema=output_schema, serializer=serializer, meta=meta, enabled=enabled, ) async def run(self, arguments: dict[str, Any]) -> ToolResult: """ Run the tool with arguments. This method is not implemented in the base Tool class and must be implemented by subclasses. `run()` can EITHER return a list of ContentBlocks, or a tuple of (list of ContentBlocks, dict of structured output). """ raise NotImplementedError("Subclasses must implement run()") @classmethod def from_tool( cls, tool: Tool, *, name: str | None = None, title: str | None | NotSetT = NotSet, description: str | None | NotSetT = NotSet, tags: set[str] | None = None, annotations: ToolAnnotations | None | NotSetT = NotSet, output_schema: dict[str, Any] | None | NotSetT | Literal[False] = NotSet, serializer: ToolResultSerializerType | None = None, meta: dict[str, Any] | None | NotSetT = NotSet, transform_args: dict[str, ArgTransform] | None = None, enabled: bool | None = None, transform_fn: Callable[..., Any] | None = None, ) -> TransformedTool: from fastmcp.tools.tool_transform import TransformedTool return TransformedTool.from_tool( tool=tool, transform_fn=transform_fn, name=name, title=title, transform_args=transform_args, description=description, tags=tags, annotations=annotations, output_schema=output_schema, serializer=serializer, meta=meta, enabled=enabled, ) class FunctionTool(Tool): fn: Callable[..., Any] @classmethod def from_function( cls, fn: Callable[..., Any], name: str | None = None, title: str | None = None, description: str | None = None, tags: set[str] | None = None, annotations: ToolAnnotations | None = None, exclude_args: list[str] | None = None, output_schema: dict[str, Any] | None | NotSetT | Literal[False] = NotSet, serializer: ToolResultSerializerType | None = None, meta: dict[str, Any] | None = None, enabled: bool | None = None, ) -> FunctionTool: """Create a Tool from a function.""" parsed_fn = ParsedFunction.from_function(fn, exclude_args=exclude_args) if name is None and parsed_fn.name == "<lambda>": raise ValueError("You must provide a name for lambda functions") if isinstance(output_schema, NotSetT): final_output_schema = parsed_fn.output_schema elif output_schema is False: # Handle False as deprecated synonym for None (deprecated in 2.11.4) if fastmcp.settings.deprecation_warnings: warnings.warn( "Passing output_schema=False is deprecated. Use output_schema=None instead.", DeprecationWarning, stacklevel=2, ) final_output_schema = None else: # At this point output_schema is not NotSetT and not False, so it must be dict | None final_output_schema = output_schema # Note: explicit schemas (dict) are used as-is without auto-wrapping # Validate that explicit schemas are object type for structured content if final_output_schema is not None and isinstance(final_output_schema, dict): if final_output_schema.get("type") != "object": raise ValueError( f'Output schemas must have "type" set to "object" due to MCP spec limitations. Received: {final_output_schema!r}' ) return cls( fn=parsed_fn.fn, name=name or parsed_fn.name, title=title, description=description or parsed_fn.description, parameters=parsed_fn.input_schema, output_schema=final_output_schema, annotations=annotations, tags=tags or set(), serializer=serializer, meta=meta, enabled=enabled if enabled is not None else True, ) async def run(self, arguments: dict[str, Any]) -> ToolResult: """Run the tool with arguments.""" from fastmcp.server.context import Context arguments = arguments.copy() context_kwarg = find_kwarg_by_type(self.fn, kwarg_type=Context) if context_kwarg and context_kwarg not in arguments: arguments[context_kwarg] = get_context() type_adapter = get_cached_typeadapter(self.fn) result = type_adapter.validate_python(arguments) if inspect.isawaitable(result): result = await result if isinstance(result, ToolResult): return result unstructured_result = _convert_to_content(result, serializer=self.serializer) if self.output_schema is None: # Do not produce a structured output for MCP Content Types if isinstance(result, ContentBlock | Audio | Image | File) or ( isinstance(result, list | tuple) and any(isinstance(item, ContentBlock) for item in result) ): return ToolResult(content=unstructured_result) # Otherwise, try to serialize the result as a dict try: structured_content = pydantic_core.to_jsonable_python(result) if isinstance(structured_content, dict): return ToolResult( content=unstructured_result, structured_content=structured_content, ) except pydantic_core.PydanticSerializationError: pass return ToolResult(content=unstructured_result) wrap_result = self.output_schema.get("x-fastmcp-wrap-result") return ToolResult( content=unstructured_result, structured_content={"result": result} if wrap_result else result, ) @dataclass class ParsedFunction: fn: Callable[..., Any] name: str description: str | None input_schema: dict[str, Any] output_schema: dict[str, Any] | None @classmethod def from_function( cls, fn: Callable[..., Any], exclude_args: list[str] | None = None, validate: bool = True, wrap_non_object_output_schema: bool = True, ) -> ParsedFunction: from fastmcp.server.context import Context if validate: sig = inspect.signature(fn) # Reject functions with *args or **kwargs for param in sig.parameters.values(): if param.kind == inspect.Parameter.VAR_POSITIONAL: raise ValueError("Functions with *args are not supported as tools") if param.kind == inspect.Parameter.VAR_KEYWORD: raise ValueError( "Functions with **kwargs are not supported as tools" ) # Reject exclude_args that don't exist in the function or don't have a default value if exclude_args: for arg_name in exclude_args: if arg_name not in sig.parameters: raise ValueError( f"Parameter '{arg_name}' in exclude_args does not exist in function." ) param = sig.parameters[arg_name] if param.default == inspect.Parameter.empty: raise ValueError( f"Parameter '{arg_name}' in exclude_args must have a default value." ) # collect name and doc before we potentially modify the function fn_name = getattr(fn, "__name__", None) or fn.__class__.__name__ fn_doc = inspect.getdoc(fn) # if the fn is a callable class, we need to get the __call__ method from here out if not inspect.isroutine(fn): fn = fn.__call__ # if the fn is a staticmethod, we need to work with the underlying function if isinstance(fn, staticmethod): fn = fn.__func__ prune_params: list[str] = [] context_kwarg = find_kwarg_by_type(fn, kwarg_type=Context) if context_kwarg: prune_params.append(context_kwarg) if exclude_args: prune_params.extend(exclude_args) input_type_adapter = get_cached_typeadapter(fn) input_schema = input_type_adapter.json_schema() input_schema = compress_schema( input_schema, prune_params=prune_params, prune_titles=True ) output_schema = None # Get the return annotation from the signature sig = inspect.signature(fn) output_type = sig.return_annotation # If the annotation is a string (from __future__ annotations), resolve it if isinstance(output_type, str): try: # Use get_type_hints to resolve the return type # include_extras=True preserves Annotated metadata type_hints = get_type_hints(fn, include_extras=True) output_type = type_hints.get("return", output_type) except Exception: # If resolution fails, keep the string annotation pass if output_type not in (inspect._empty, None, Any, ...): # there are a variety of types that we don't want to attempt to # serialize because they are either used by FastMCP internally, # or are MCP content types that explicitly don't form structured # content. By replacing them with an explicitly unserializable type, # we ensure that no output schema is automatically generated. clean_output_type = replace_type( output_type, { t: _UnserializableType for t in ( Image, Audio, File, ToolResult, mcp.types.TextContent, mcp.types.ImageContent, mcp.types.AudioContent, mcp.types.ResourceLink, mcp.types.EmbeddedResource, ) }, ) try: type_adapter = get_cached_typeadapter(clean_output_type) base_schema = type_adapter.json_schema(mode="serialization") # Generate schema for wrapped type if it's non-object # because MCP requires that output schemas are objects if ( wrap_non_object_output_schema and base_schema.get("type") != "object" ): # Use the wrapped result schema directly wrapped_type = _WrappedResult[clean_output_type] wrapped_adapter = get_cached_typeadapter(wrapped_type) output_schema = wrapped_adapter.json_schema(mode="serialization") output_schema["x-fastmcp-wrap-result"] = True else: output_schema = base_schema output_schema = compress_schema(output_schema, prune_titles=True) except PydanticSchemaGenerationError as e: if "_UnserializableType" not in str(e): logger.debug(f"Unable to generate schema for type {output_type!r}") return cls( fn=fn, name=fn_name, description=fn_doc, input_schema=input_schema, output_schema=output_schema or None, ) def _serialize_with_fallback( result: Any, serializer: ToolResultSerializerType | None = None ) -> str: if serializer is not None: try: return serializer(result) except Exception as e: logger.warning( "Error serializing tool result: %s", e, exc_info=True, ) return default_serializer(result) def _convert_to_single_content_block( item: Any, serializer: ToolResultSerializerType | None = None, ) -> ContentBlock: if isinstance(item, ContentBlock): return item if isinstance(item, Image): return item.to_image_content() if isinstance(item, Audio): return item.to_audio_content() if isinstance(item, File): return item.to_resource_content() if isinstance(item, str): return TextContent(type="text", text=item) return TextContent(type="text", text=_serialize_with_fallback(item, serializer)) def _convert_to_content( result: Any, serializer: ToolResultSerializerType | None = None, ) -> list[ContentBlock]: """Convert a result to a sequence of content objects.""" if result is None: return [] if not isinstance(result, (list | tuple)): return [_convert_to_single_content_block(result, serializer)] # If all items are ContentBlocks, return them as is if all(isinstance(item, ContentBlock) for item in result): return result # If any item is a ContentBlock, convert non-ContentBlock items to TextContent # without aggregating them if any(isinstance(item, ContentBlock) for item in result): return [ _convert_to_single_content_block(item, serializer) if not isinstance(item, ContentBlock) else item for item in result ] # If none of the items are ContentBlocks, aggregate all items into a single TextContent return [TextContent(type="text", text=_serialize_with_fallback(result, serializer))]