sampling

Deprecated in the 2026-07-28 protocol (SEP-2577); functional through the deprecation window. Migration: call your LLM provider directly from the server instead of requesting completions through the client. TODO(maxisbey): revisit before beta.

A tool that asks the client's LLM for a completion mid-call — the inverted MCP direction. The server holds no model API key; it awaits ctx.session.create_message(...) and the client's sampling_callback answers. Registering the callback is what makes the client advertise the sampling capability — there is no separate flag.

Run it

# stdio (default — the client spawns the server as a subprocess)
uv run python -m stories.sampling.client

# HTTP — the client self-hosts the server on a free port, runs, then tears it down
uv run python -m stories.sampling.client --http --legacy
# same, against the lowlevel-API server variant
uv run python -m stories.sampling.client --http --legacy --server server_lowlevel

What to look at

• client.py main — async with Client(target, mode=mode, sampling_callback=on_sample) as client:. The callback is an ordinary constructor kwarg; registering it is the whole opt-in.
• client.py on_sample — takes (ClientRequestContext, CreateMessageRequestParams) and returns a CreateMessageResult. A real host calls its LLM provider here; the example returns a canned answer so the round-trip is assertable.
• server.py — await ctx.session.create_message(...) inside the tool body: a server→client request that blocks until the callback answers. There is no Context.sample() sugar; reaching ctx.session is the public path.
• server_lowlevel.py — the same call from ServerRequestContext.session, with the CallToolResult built by hand.

Caveats

• Legacy-era only. sampling/createMessage is a server-initiated request with no 2026-07-28 wire carrier, so this story runs with era = "legacy" and the harness pins the handshake path.
• ctx.session.create_message() is @deprecated; the # pyright: ignore[reportDeprecated] is deliberate. The non-deprecated replacement is to call your LLM provider directly from the server (see the banner above) — there is no successor server→client call.
• ctx.session.* is the interim 2-hop path; a later release will shorten it.
• Client has no sampling_capabilities= kwarg, so the sampling.tools sub-capability (tools-in-sampling) is unreachable from the high-level client. Drop to ClientSession if you need it.

Spec

Sampling — client features

See also

legacy_elicitation/, roots/ — sibling stories that exercise the same legacy server→client request shape.