stdio

Publisher

Model Context Protocol (modelcontextprotocol.io)

URL

https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/stdio.md

Published

2026-07-28

Updated

unknown

Accessed

2026-09-16

HTTP status

200

License

not stated on the retrieved page; no-redistribution; short excerpts and locators only

Source notes citing this source

• On stdio, a client that supports both modern (2026-07-28 and later) and legacy initialize-based MCP versions SHOULD probe with server/discover before any other request; a DiscoverResult or a recognized modern JSON-RPC error means the server is modern, while any other error or no response within a reasonable timeout means legacy and the client falls back to initialize, and the fallback MUST NOT be keyed to one specific error code. in context
• On stdio in MCP revision 2026-07-28 the client MUST cancel an in-flight request by sending a notifications/cancelled notification that references the request ID, because there is no per-request stream to close. in context
• In the stdio transport the client launches the MCP server as a subprocess, and messages are newline-delimited on stdin and stdout and MUST NOT contain embedded newlines. in context
• On stdio in MCP revision 2026-07-28 all request metadata is carried inline in the JSON-RPC message body; there is no header layer. in context
• On stdio in MCP revision 2026-07-28 the server MUST NOT write JSON-RPC requests to stdout and the client MUST NOT write JSON-RPC responses to stdin; server-to-client interactions are carried in InputRequiredResult replies. in context
• On stdio in MCP revision 2026-07-28, if the server process exits unexpectedly the client SHOULD restart it; in-flight requests are lost and can be retried against the fresh process. in context
• On stdio in MCP revision 2026-07-28 the client SHOULD initiate shutdown by closing the server's input stream, waiting for the server to exit, and forcibly terminating the process if it does not exit within a reasonable time; servers SHOULD exit promptly when stdin is closed or reads return end-of-file. in context
• On stdio in MCP revision 2026-07-28 all server messages share the single stdout channel; there are no per-request streams. in context
• On stdio the server MAY write UTF-8 strings to stderr for any logging purpose, and the client MAY capture, forward or ignore that output and SHOULD NOT assume stderr output indicates an error condition. in context
• On stdio the server MUST NOT write anything to stdout that is not a valid MCP message, and the client MUST NOT write anything to the server's stdin that is not a valid MCP message. in context
• On stdio in MCP revision 2026-07-28 clients MUST correlate notifications delivered for an active subscriptions/listen request using the io.modelcontextprotocol/subscriptionId field in _meta. in context

Cite this source record

APA

WarmTransfer. (2026, July 28). stdio. WarmTransfer. https://warmtransfer.net/knowledge/sources/mcp-spec-2026-07-28-stdio

BibTeX

@misc{warmtransfer-mcp-spec-2026-07-28-stdio,
  title  = {stdio},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sources/mcp-spec-2026-07-28-stdio},
  note   = {Model Context Protocol (modelcontextprotocol.io), accessed 2026-09-16}
}