decorator-based mcp server framework with automatic function wrapping
FastMCP provides a high-level decorator API (@mcp.tool(), @mcp.resource(), @mcp.prompt()) that automatically wraps Python functions into MCP protocol handlers. The framework uses Python type annotations to inject context (e.g., via @mcp.use_context), automatically serializes return values into MCP result types, and generates JSON-RPC 2.0 compliant messages without requiring manual handler construction. This eliminates boilerplate compared to the low-level Server API which requires explicit handler registration and result type construction.
Unique: Uses Python decorators and type annotations to eliminate manual MCP protocol construction, automatically generating JSON-RPC handlers and Pydantic-validated schemas from function signatures without requiring developers to understand the underlying MCP specification
vs alternatives: Faster to prototype than raw MCP Server API because decorators handle serialization and validation automatically, but less flexible than low-level APIs for custom protocol behavior
low-level handler-based mcp server with explicit protocol control
The Server class (src/mcp/server/lowlevel/server.py) provides a constructor-based API where developers register handler functions via parameters like on_list_tools=..., on_call_tool=..., on_read_resource=... This approach gives full control over JSON-RPC message construction, session lifecycle, and protocol negotiation. Handlers receive raw MCP request objects and must explicitly construct result types, enabling fine-grained control over error handling, streaming responses, and capability negotiation.
Unique: Provides constructor-based handler registration with explicit control over JSON-RPC message construction and session lifecycle, enabling custom protocol behavior without abstraction layers that hide implementation details
vs alternatives: More flexible than FastMCP for advanced use cases (streaming, custom auth, complex session logic), but requires more boilerplate and protocol knowledge
progress notifications and streaming response support
The SDK supports progress notifications and streaming responses, allowing tools to report progress during long-running operations and stream partial results back to clients. Tools can emit ProgressNotification messages during execution, and clients can subscribe to these notifications to display progress to users. Streaming responses allow tools to return large results incrementally without buffering the entire response in memory.
Unique: Enables tools to emit progress notifications and stream partial results during execution, allowing clients to display real-time progress without waiting for the entire operation to complete
vs alternatives: More responsive than request/response-only APIs because clients receive progress updates and partial results incrementally; better for long-running operations than blocking calls
capability negotiation and protocol version compatibility
The SDK implements MCP capability negotiation during the initialize handshake, allowing servers and clients to advertise their supported features and agree on a common protocol version. Servers declare which capabilities they support (tools, resources, prompts, sampling, etc.), and clients can query these capabilities to determine which features are available. This enables forward/backward compatibility — older clients can work with newer servers by only using supported features.
Unique: Implements capability negotiation during the initialize handshake to enable forward/backward compatibility, allowing clients and servers with different feature sets to interoperate gracefully
vs alternatives: More flexible than fixed protocol versions because capabilities are negotiated dynamically; enables gradual feature adoption without breaking older clients
experimental task system for complex multi-step operations
The SDK includes an experimental task system that allows servers to define complex, multi-step operations that clients can execute. Tasks are similar to tools but support more complex workflows with state management, branching, and progress tracking. This is an early-stage feature designed for future MCP extensions but is available for experimentation.
Unique: Provides an experimental task system for complex multi-step operations with state management, enabling more sophisticated workflows than the standard tool model
vs alternatives: More expressive than tools for complex workflows, but less stable and less widely supported by MCP clients
content type abstraction for structured output and rich formatting
The SDK supports multiple content types (text, image, PDF, etc.) for tool results and resources, allowing servers to return richly formatted responses. Content types are abstracted behind a unified interface, enabling clients to handle different content types appropriately (render images, display PDFs, etc.). This enables tools to return structured, formatted output that LLMs and clients can interpret correctly.
Unique: Abstracts multiple content types (text, image, PDF, etc.) behind a unified interface, enabling tools to return richly formatted results that clients can render appropriately
vs alternatives: More flexible than text-only responses because tools can return structured, formatted output; enables richer user experiences than plain text results
multi-transport abstraction layer with uniform read/write stream interface
The SDK abstracts transport mechanisms (STDIO, SSE, StreamableHTTP) behind a uniform (read_stream, write_stream) interface that carries SessionMessage objects. This allows server and client code to be transport-agnostic — the same handler logic works over STDIO for local development, SSE for browser clients, or StreamableHTTP for production deployments. The transport layer handles serialization/deserialization of JSON-RPC messages and manages connection lifecycle independently of application logic.
Unique: Implements a uniform (read_stream, write_stream) abstraction that decouples application logic from transport implementation, allowing the same server code to run over STDIO, SSE, or StreamableHTTP without modification
vs alternatives: More flexible than transport-specific implementations because application code never depends on transport details; enables seamless migration from local STDIO development to distributed HTTP deployments
pydantic-based json-rpc type system with discriminated unions for automatic message routing
The protocol layer (src/mcp/types.py) defines all MCP messages using Pydantic discriminated unions keyed on the 'method' field. This enables automatic validation and routing of incoming JSON-RPC messages to the correct handler without manual type checking. The type system provides compile-time safety (via type hints) and runtime validation (via Pydantic), ensuring malformed messages are rejected before reaching application handlers. All protocol messages (requests, responses, notifications) are strongly typed.
Unique: Uses Pydantic discriminated unions keyed on the 'method' field to automatically route and validate JSON-RPC messages without manual type checking, providing compile-time and runtime type safety for the entire MCP protocol
vs alternatives: More robust than manual JSON parsing because Pydantic validates all fields and types automatically; stronger guarantees than untyped JSON-RPC implementations
+6 more capabilities