Transports

Overview

protomcp supports two transports, both implemented by the official MCP Go SDK. The transport determines how your MCP client connects to protomcp. Your server code is unaffected — it always communicates with protomcp via the same unix socket protocol.

Set the transport with --transport:

pmcp dev server.py --transport http --port 8080

---

stdio (default)

Flag: --transport stdio

Reads MCP requests from stdin, writes responses to stdout. The MCP client spawns protomcp as a child process and communicates over its standard I/O.

Use when: Your MCP client supports subprocess spawning (Claude Desktop, Cursor, most local clients).

Config:

{
  "mcpServers": {
    "mytools": {
      "command": "pmcp",
      "args": ["dev", "/path/to/server.py"]
    }
  }
}

No --host or --port needed. stdio is the simplest and most compatible option.

---

Streamable HTTP

Flag: --transport http

Runs an HTTP server using the official SDK’s StreamableHTTPHandler. Supports session management, multiple concurrent clients, and SSE-based streaming responses. This is the modern MCP HTTP transport that replaces the older SSE transport.

Use when:

• Your MCP client connects over HTTP instead of spawning a subprocess
• You need to handle multiple concurrent MCP clients
• You’re running protomcp on a remote server
• Load balancers or proxies are in the path

pmcp run server.py --transport http --host 0.0.0.0 --port 8080

MCP client config:

{
  "mcpServers": {
    "mytools": {
      "url": "http://myserver:8080/mcp"
    }
  }
}

---

Compatibility matrix

Transport	MCP clients	Hot reload	Multiple clients	Session management
stdio	Claude Desktop, Cursor, most local	Yes	No (one client)	Single session
http	Modern HTTP clients	Yes	Yes	Per-session via Mcp-Session-Id

---

Host and port flags

--host and --port only apply to the HTTP transport:

Flag	Default	Description
--host	localhost	Address to bind
--port	8080	Port to bind

For local-only access, use the default localhost. For external access:

pmcp run server.py --transport http --host 0.0.0.0 --port 8080