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.