What is the stdio transport in MCP?

MCP and AI agents glossary · 1 primary source

The MCP transport for local servers: the client starts the server as a subprocess and they exchange newline-delimited JSON-RPC messages over standard input and output.

Anything the server writes to stdout must be a valid MCP message, which is why printing debug text to stdout breaks a local server; logs go to stderr. Local servers need their runtime installed (Node, Python, Docker…), whereas a remote Streamable HTTP server needs only its URL.

Bridges such as mcp-remote run locally over stdio and forward to a remote server, for clients that only support local servers.

Sources

MCP specification: Transports

Related terms

Streamable HTTP transportThe MCP transport for remote servers: one HTTPS endpoint (such as https://example.com/mcp) that receives JSON-RPC messages as POST requests and may stream replies as server-sent events.
MCP serverA program that exposes tools, resources or prompts to AI applications over the Model Context Protocol.

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