mcp — an MCP server and client in Go, standard library only

No third-party dependencies, no code generation, no reflection tricks. go.mod has no require block and it stays that way. The package serves MCP (stdio, unix socket) and consumes it (the Client), sharing one Framer / JSON-RPC / protocol core across both roles — which is what lets one service front another.

mcp/
  jsonrpc.go        JSON-RPC 2.0 wire types, ID normalisation, error codes
  framing.go        line framing (NDJSON) and Content-Length framing
  protocol.go       MCP data types for all three protocol revisions
  server.go         registry, capability derivation, pagination, lifecycle
  session.go        per-connection state machine, read/write loops, outbound calls
  dispatch.go       method handlers, URI template matching
  transport.go      stdio, Unix sockets, systemd activation, accept loop
  client.go         MCP client: Dial, Initialize, ListTools, CallTool, server→client dispatch
  peercred_linux.go SO_PEERCRED
  peercred_other.go fallback
  umask_unix.go     socket creation under a umask
  server_test.go    protocol conformance tests over a real socket
  client_test.go    client↔server round trip over an in-memory pipe
  harden_test.go    limits, quotas, deadlines, injection, cache invalidation
  audit_fixes_test.go regressions: cursor-cache bound, large-id keys, dup Content-Length
  protocol_content_test.go  Content constructors, SessionFromContext
  transport_test.go systemd no-activation paths
  helpers_test.go   test helpers (short socket paths)
  bench_test.go     benchmarks
app/mcpd/           reference daemon (the mcpd binary)
example/stdio/      minimal runnable server (one echo tool over stdio)
docs/               introduction · executive-summary · architecture · userguide · plan/
build/mcp           cross-compile + package + install the daemon

Documentation

• Start here — docs/introduction.md · docs/executive-summary.md
• Deep dive — docs/architecture.md (this README is the reference; the docs are the guided tour)
• Operations — docs/userguide.md
• Examples — example/README.md

Module github.com/netstar-labs/mcp — the root package mcp is the importable library; app/mcpd is the reference daemon.

Usage

srv := mcp.New(mcp.Options{Name: "mcpd", Version: "1.0.0"})

srv.AddTool(mcp.Tool{
    Name:        "sha256",
    InputSchema: json.RawMessage(`{"type":"object","properties":{"input":{"type":"string"}},"required":["input"]}`),
}, func(ctx context.Context, s *mcp.Session, args json.RawMessage) (*mcp.CallToolResult, error) {
    var in struct{ Input string `json:"input"` }
    if err := json.Unmarshal(args, &in); err != nil {
        return mcp.ToolError("invalid arguments: %v", err), nil
    }
    sum := sha256.Sum256([]byte(in.Input))
    return mcp.ToolText("%x", sum), nil
})