FastMCP

Registers MCP tools via addTool() method with pluggable schema validation (Zod, ArkType, or Valibot) that automatically validates parameters before execution. FastMCP wraps the raw MCP SDK's tool handler registration, normalizing parameter validation and error handling across multiple validation libraries without requiring developers to write boilerplate protocol compliance code.

Registers static resources and dynamic resource templates via addResource() and addResourceTemplate() methods that map URIs to lazy-loaded content. Resources are identified by fixed URIs (e.g., 'file://config.json'), while templates use URI patterns (e.g., 'file://docs/{name}') with argument substitution. FastMCP handles URI parsing, argument extraction, and content normalization (text, image, audio) automatically.

Automatically converts exceptions and validation errors from tool/resource/prompt handlers into MCP-compliant error responses. FastMCP catches exceptions, formats error messages, and returns them as MCP error objects without requiring developers to manually implement error serialization. Validation errors from schema validators are automatically converted to MCP error responses.

Allows registration of custom HTTP routes alongside MCP protocol endpoints via custom route handlers. FastMCP exposes the underlying HTTP server, enabling developers to add Express-style middleware and custom routes for health checks, metrics, webhooks, or other HTTP endpoints. Custom routes coexist with MCP protocol handlers on the same server instance.

Manages resource roots (filesystem or URI prefixes) that clients can discover and subscribe to changes. FastMCP allows registration of resource roots and emits rootsChanged events when roots are added/removed. Clients can discover available roots and receive notifications of changes, enabling dynamic resource discovery without polling.

Registers MCP prompts via addPrompt() that accept arguments and return templated content with optional auto-completion suggestions. Prompts are identified by name and can include argument schemas for validation. FastMCP normalizes prompt execution, argument binding, and optional completion suggestions into MCP protocol responses.

Abstracts MCP transport mechanisms via start() method that configures either StdioServerTransport (for local stdio-based clients) or HTTP streaming transport (for remote clients). FastMCP handles transport initialization, connection lifecycle, and message framing automatically. Developers specify transport type via configuration; FastMCP manages the underlying transport setup without exposing transport details.

Manages per-client session state via FastMCPSession instances that track authentication context, client capabilities, and request lifecycle. Sessions are created on client connection and destroyed on disconnect. FastMCP automatically creates sessions and provides them to tool/resource/prompt handlers via Context parameter, enabling handlers to access session-specific state (authenticated user, client capabilities, request ID) without manual session lookup.

Integrates authentication via authenticate() function in ServerOptions and OAuth discovery via OAuth Discovery Endpoints. FastMCP calls authenticate() on client connection to validate credentials and attach user context to session. OAuth Discovery Endpoints allow clients to discover available OAuth providers and initiate OAuth flows. FastMCP handles OAuth proxy setup and token exchange automatically.

Enables streaming of large content (text, images, audio) via streamContent() method in Context parameter, with optional progress reporting via reportProgress(). Streaming is implemented using MCP's streaming protocol, allowing clients to receive content incrementally without waiting for full response. Progress reporting provides real-time feedback on long-running operations.

Automatically normalizes content returned from tools, resources, and prompts into MCP-compliant formats via helper functions (imageContent(), audioContent(), textContent()). Handles MIME type detection, encoding (base64 for binary), and protocol compliance. Developers call these helpers instead of manually constructing content objects.

Provides Context object to all tool/resource/prompt handlers containing requestId, sessionId, log() method, and reportProgress() method. Context enables structured logging and request tracing without requiring developers to manually pass context through function signatures. Log messages are associated with request ID for correlation.

Registers system CLI commands as MCP tools via addTool() with automatic argument parsing and execution. FastMCP handles shell command invocation, output capture, and error handling. Developers specify command name, arguments, and optional working directory; FastMCP executes the command and returns stdout/stderr as tool result.