$ show mark3labs/mcp-go
mark3labs/mcp-go is a production-oriented Go SDK implementing both server and client sides of the Model Context Protocol (MCP). The codebase is a library, not a standalone application: it exposes server.NewMCPServer, client.NewClient, and transport constructors for stdio, SSE, Streamable HTTP, and in-process communication. It supports tools, resources, prompts, tasks, sampling, elicitation, roots, completions, subscriptions, multi-round-trip requests, OAuth, OpenTelemetry tracing, schema validation, and cursor-based pagination. The SDK maintains dual-era protocol support, negotiating between legacy handshake-based versions (2024-11-05 through 2025-11-25) and the modern stateless 2026-07-28 core, with automatic per-request era detection and bridging for server-initiated requests. Architecturally, the repository is organized as modular Go packages: mcp defines protocol types and schema helpers, server implements JSON-RPC dispatch, sessions, middleware, hooks, tasks, and HTTP/SSE/stdio transports, while client provides a high-level client plus a transport abstractions layer. Notable engineering strengths include defensively coded transports, resumable SSE streams with event replay, an OAuth layer hardened against RFC 9728 resource-binding attacks, cached JSON Schema validation, and an extensive test suite including conformance tests, race-condition tests, and e2e HTTP tests. The repository also ships a separate otel submodule for OpenTelemetry integration and a docs site under www/. This is purely an application/infrastructure repository; there are no data pipelines, notebooks, or research artifacts. The primary data flow is JSON-RPC messages exchanged between MCP servers and clients, with SSE used for notifications, streaming, and server-to-client requests. The code is strongly standards-driven and tightly tracked to the evolving MCP specification. While the dual-era protocol support adds significant complexity and maintenance burden, the implementation is well-tested, security-conscious, and contains several genuinely reusable components. A few stale example files and commented-out tests exist, but they do not materially affect the quality of the core SDK.
The LLM identified 5 files worth highlighting for reuse:
A transparent transport decorator wrapping transport.Interface. It conditionally re-exposes optional interfaces such as BidirectionalInterface and HTTPConnection via type assertions, so callers still see the original transport's capabilities. It also supports payload redaction. This is a clean, self-contained reusable pattern for any Go transport stack that needs observability without altering behavior.
A schema cache designed to amortize JSON Schema compilation costs across cold starts. It supports build-time warm-up and disk persistence, making it directly useful for serverless deployments of MCP servers. The cache is isolated from business logic and can be extracted into other projects that need schema compilation caching.
Implements resumable SSE streams for Streamable HTTP with Last-Event-ID support, per-session event recording, and replay-under-lock semantics ensuring events are delivered once and never duplicated. It defines a pluggable EventStore boundary and includes transient heartbeat handling. This is a strong, reusable pattern for any HTTP streaming server that needs connection recovery without data loss.
A compiled JSON Schema validation pipeline for tool arguments, with a two-level cache keyed by tool name and schema digest. It handles nil-safe receivers and invalidates cached schemas when tools are re-registered. The approach of compiling schemas lazily and caching by digest is a reusable pattern for validating structured inputs at scale.
A full subprocess transport for MCP clients over stdin/stdout/stderr. It provides serialized writes, response routing by request ID, a 64KB stderr drain ring buffer, and graceful shutdown escalation from SIGTERM to SIGKILL. This is a self-contained process-management pattern applicable to any CLI-based client-server protocol.