stdio

specification/2026-07-28/basic/transports/stdio First recorded · 159 lines, first recorded

# stdio ## Sending Messages ## Receiving Messages ## Request Metadata ## Cancellation ## Shutdown ## Unexpected Termination ## Backward Compatibility

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# stdio

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In the **stdio** transport, the client launches the MCP server as a subprocess.
The two ends communicate over the subprocess's standard streams:

* The server reads JSON-RPC messages from `stdin` and writes JSON-RPC messages to
  `stdout`.
* Each message is a single JSON-RPC request, notification, or response.
* Messages are delimited by newlines, and **MUST NOT** contain embedded newlines.
* The server **MAY** write UTF-8 strings to `stderr` for any logging purposes
  including informational, debug, and error messages.
* The client **MAY** capture, forward, or ignore the server's `stderr` output and
  **SHOULD NOT** assume `stderr` output indicates error conditions.
* The server **MUST NOT** write anything to its `stdout` that is not a valid MCP
  message.
* The client **MUST NOT** write anything to the server's `stdin` that is not a
  valid MCP message.

Standard streams are the canonical channel, but nothing in this binding
depends on them except the process lifecycle. The wire format (one
newline-delimited JSON-RPC message per line over a reliable bidirectional
byte stream) works unchanged over Unix domain sockets, TCP connections, or
any similar channel.
[Custom transports](/specification/2026-07-28/basic/transports#custom-transports)
built on such streams **SHOULD** reuse this framing and the message rules on
this page; only the subprocess-specific aspects (launch, `stderr`, shutdown
by closing the stream, process restart) need channel-specific equivalents.

## Sending Messages

The client sends messages by writing JSON-RPC *requests* and *notifications*
to the server's `stdin`, one message per line. The client **MUST NOT** write
JSON-RPC *responses*.

## Receiving Messages

The client reads server messages from `stdout`, one message per line. All
messages share this single channel; there are no per-request streams.

The server writes three kinds of messages:

1. *Responses* to client requests, correlated by JSON-RPC `id`.
2. *Notifications* that relate to an in-flight request, such as
   `notifications/progress` and `notifications/message`.
3. *Notifications* delivered for an active
   [`subscriptions/listen`][subscriptions-listen] request. Clients **MUST**
   correlate these using the `io.modelcontextprotocol/subscriptionId` field
   in `_meta`; see
   [`SubscriptionsListenRequest`][subscriptions-listen-request].

The server **MUST NOT** write JSON-RPC *requests* to `stdout`.
Server-to-client interactions are carried in
[`InputRequiredResult`][mrtr-input-required] replies; see
[Multi Round-Trip Requests][mrtr].

[mrtr]: /specification/2026-07-28/basic/patterns/mrtr

[mrtr-input-required]: /specification/2026-07-28/basic/patterns/mrtr#inputrequiredresult

[subscriptions-listen]: /specification/2026-07-28/basic/patterns/subscriptions

[subscriptions-listen-request]: /specification/2026-07-28/schema#subscriptionslistenrequest

## Request Metadata

All request metadata for the stdio transport is carried inline in the
JSON-RPC message body. The protocol version, per-request capabilities, and
optional client identity live in
[`_meta.io.modelcontextprotocol/*`][meta-fields];
the method name and arguments live where JSON-RPC puts them. There is no
header layer.

[meta-fields]: /specification/2026-07-28/basic/index#meta

## Cancellation

To cancel an in-flight request, the client **MUST** send a
`notifications/cancelled` notification referencing the request's ID. Because
stdio is a single shared bidirectional channel, there is no per-request stream
to close. Servers **SHOULD** stop work on a cancelled request as soon as
practical and **MUST NOT** send any further messages for it. See
[Cancellation][cancellation] for the full rules.

[cancellation]: /specification/2026-07-28/basic/patterns/cancellation

## Shutdown

The client **SHOULD** initiate shutdown by:

1. Closing the input stream to the child process (the server).
2. Waiting for the server to exit.
3. If the server does not exit within a reasonable time, forcibly terminating
   the process using the mechanism appropriate for the operating system.

On POSIX systems, forced termination typically escalates from
[`SIGTERM`][sigterm]
to `SIGKILL`. On Windows, where POSIX signals are not available, clients can
use [`TerminateProcess`][terminateprocess]
or [Job Objects][job-objects].

Servers **SHOULD** exit promptly when their standard input is closed or reads
return end-of-file. This is the primary graceful-shutdown signal and the only
portable one, so honoring it reduces the need for forced termination.

The server **MAY** initiate shutdown by closing its output stream to the
client and exiting.

## Unexpected Termination

If the server process exits unexpectedly, the client **SHOULD** restart it.
Because the protocol is stateless, any in-flight requests are simply lost and
the client can retry them against the fresh process. Active
[`subscriptions/listen`][subscriptions-listen] streams must also be
re-established after restart.

[sigterm]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html

[terminateprocess]: https://learn.microsoft.com/windows/win32/api/processthreadsapi/nf-processthreadsapi-terminateprocess

[job-objects]: https://learn.microsoft.com/windows/win32/procthread/job-objects

## Backward Compatibility

A client that supports both modern (per-request-metadata) MCP versions and a
legacy version that requires an `initialize` handshake **SHOULD** probe with
[`server/discover`][server-discover] before sending any other request,
setting its preferred modern version in `_meta`. The probe has three
possible outcomes:

* The server returns a `DiscoverResult`: the server is modern. Select a
  mutually supported version from `supportedVersions` and continue.
* The server returns a recognized modern JSON-RPC error such as
  [`UnsupportedProtocolVersionError`][unsupported-version]: the server is
  modern but does not support the requested version. Use one of the versions
  in its advertised `supported` list. Do **not** fall back to `initialize`.
* The server returns any other error, or does not respond within a
  reasonable timeout: the server is legacy. Fall back to the `initialize`
  handshake.

The fallback **MUST NOT** be keyed to one specific error code: legacy servers
respond to unknown pre-`initialize` requests with implementation-defined
errors (commonly `-32601` or `-32602`) or not at all.

A client that only supports modern versions does not need to probe, but
probing is still **RECOMMENDED**: some legacy servers do not validate that a
request arrives after `initialize` and would process an era-ambiguous method
(such as `tools/call`) under legacy semantics. Probing yields a
deterministic failure instead.

See [Versioning: Backward Compatibility][lifecycle-compat] for the era model
and a compatibility matrix for implementors.

[server-discover]: /specification/2026-07-28/schema#discoverrequest

[unsupported-version]: /specification/2026-07-28/schema#unsupportedprotocolversionerror

[lifecycle-compat]: /specification/2026-07-28/basic/versioning#backward-compatibility-with-initialization-based-versions