> ## Documentation Index
> Fetch the complete documentation index at: https://docs.tai42.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Transports

> HTTP, SSE, STDIO, and the UDS path.

A transport is how a client reaches a server. You pick one when you start the
server, and clients connect over it. The runtime speaks HTTP by default and can
also serve over SSE, STDIO, or a fast same-machine Unix-domain-socket path.

## Choosing a transport

`tai serve` takes `--transport`. HTTP is the default; the MCP endpoint is then
served under `/mcp`.

```bash theme={null}
tai serve                        # MCP server over HTTP (default)
tai serve --transport sse        # MCP server over Server-Sent Events
tai serve --transport stdio      # MCP server over standard input/output
```

* **HTTP** — the default network transport, for MCP clients over the network.
* **SSE** — a streaming HTTP transport.
* **STDIO** — for a client that launches the server as a subprocess and talks over
  its standard streams.

## The UDS path

For same-machine communication, `--uds <path>` binds the server to a
Unix-domain socket instead of a host and port — a fast local path that avoids the
network stack. `tai42-kit` supplies the client-side UDS transports and a selector,
`get_mcp_transport`, that picks the right transport for a target: a UDS transport
for a socket, or a plain config for a networked server. A mounted external MCP
server can be reached over any of these — the manifest's `mcp` entry sets `url`,
`uds`, or `command` per server.

## Statefulness and workers

The HTTP transports are **stateful per process**: each MCP session is pinned to the
worker that created it. Under multiple workers a session opened on one worker
breaks when another worker handles its next request, so `tai serve` refuses
`--workers` greater than one on any stateful transport (`http`, `streamable-http`,
`sse`) with an error that names the fix.

The fix is `--stateless-http`, which turns on the underlying stateless HTTP mode and
lifts the refusal — for the HTTP transports only. It has no meaning for `sse` or
`stdio` (SSE has no stateless mode and STDIO has no HTTP sessions), so combining it
with either is itself refused.

`tai serve` also refuses `--workers` greater than one without the
[worker bus](/concepts/worker-bus): sibling workers would serve stale config after a
reload with no channel to converge on. So a multi-worker HTTP server needs both
`--stateless-http` and `TAI_BUS_REDIS_URL`.

| transport              | `workers=1`           | `workers>1`                                                             |
| ---------------------- | --------------------- | ----------------------------------------------------------------------- |
| stdio                  | ok                    | refused                                                                 |
| sse                    | ok                    | refused                                                                 |
| http / streamable-http | ok (stateful default) | refused unless `--stateless-http` **and** the bus (`TAI_BUS_REDIS_URL`) |

Embedding the runtime in your own process applies the session-pinning rule the same
way: pass `create_app(manifest_path="manifest.yml", stateless_http=True)` to run
several workers. The bus, though, is a documented limitation when you embed — the
workers-count rule lives in the `tai serve` CLI, and in-process code cannot count
sibling processes, so a `create_app(..., stateless_http=True)` factory run under an
external `--workers` passes no rule yet runs busless-stale. Set `TAI_BUS_REDIS_URL`
yourself in any multi-process embed. See
[Multi-worker and fleet sync](/guides/embed#multi-worker-and-fleet-sync).

<Note>
  Before binding a UDS path, `tai serve` cleans up a stale socket left by an unclean
  shutdown, but refuses to unlink a socket that answers a connect probe (a live
  server) or a path that is not a socket. This keeps one command from clobbering
  another server's socket.
</Note>

See the [deploy guide](/guides/deploy) for choosing a transport in production and
the [CLI reference](/reference/cli/index) for the full `tai serve` options.