# Transports: stdio and Streamable HTTP — MCP & Agent-to-Agent Protocols

Source: https://www.geekswithgeeks.com/en/mcp-a2a/m-transports

> Choose how client and server exchange messages.

## Local pipes or remote HTTP

MCP defines how messages travel. With **stdio** the host launches the server as a **child process** and exchanges newline-delimited JSON messages over standard input and output (log to stderr, never stdout, or you corrupt the stream). It is simple and local: the server runs with the user's permissions on their machine. **Streamable HTTP** uses HTTP so servers can run **remotely**, serve many clients, and stream responses (using Server-Sent Events where needed). Remote servers must handle **authentication**, **session management**, rate limits and TLS; the specification recommends **OAuth**-based authorisation for protected remote servers. An older HTTP+SSE transport existed and has been superseded, so check which your SDK version supports.

## stdio versus Streamable HTTP

A comparison to guide the choice.

```text
                 stdio                               Streamable HTTP
where it runs    child process on the user's machine  remote service (or local HTTP)
clients          one (the parent host)                many
auth             inherits the OS user                  OAuth / tokens required for protected data
logging          stderr only (stdout = protocol)       normal server logs
scaling          none needed                           load balancers, sessions, rate limits
best for         local tools, dev, files               shared/team/SaaS tools
```

**Quiz:** Why must a stdio MCP server log to stderr?

- [ ] Logs are forbidden
- [ ] stderr is faster
- [x] stdout carries the protocol messages and logs would corrupt it
- [ ] It encrypts the logs

*Answer:* stdout carries the protocol messages and logs would corrupt it. Any extra text on stdout breaks the JSON message stream.