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63 lines
2.7 KiB
Markdown
63 lines
2.7 KiB
Markdown
# sampling
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> **Deprecated** in the 2026-07-28 protocol (SEP-2577); functional through the
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> deprecation window. Migration: call your LLM provider directly from the
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> server instead of requesting completions through the client.
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> TODO(maxisbey): revisit before beta.
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A tool that asks the **client's** LLM for a completion mid-call — the inverted
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MCP direction. The server holds no model API key; it awaits
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`ctx.session.create_message(...)` and the client's `sampling_callback` answers.
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Registering the callback is what makes the client advertise the `sampling`
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capability — there is no separate flag.
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## Run it
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```bash
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# stdio (default — the client spawns the server as a subprocess)
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uv run python -m stories.sampling.client
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# HTTP — the client self-hosts the server on a free port, runs, then tears it down
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uv run python -m stories.sampling.client --http --legacy
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# same, against the lowlevel-API server variant
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uv run python -m stories.sampling.client --http --legacy --server server_lowlevel
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```
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## What to look at
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- `client.py` `main` — `async with Client(target, mode=mode,
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sampling_callback=on_sample) as client:`. The callback is an ordinary
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constructor kwarg; registering it is the whole opt-in.
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- `client.py` `on_sample` — takes `(ClientRequestContext,
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CreateMessageRequestParams)` and returns a `CreateMessageResult`. A real
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host calls its LLM provider here; the example returns a canned answer so the
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round-trip is assertable.
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- `server.py` — `await ctx.session.create_message(...)` inside the tool body: a
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server→client request that blocks until the callback answers. There is no
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`Context.sample()` sugar; reaching `ctx.session` is the public path.
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- `server_lowlevel.py` — the same call from `ServerRequestContext.session`,
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with the `CallToolResult` built by hand.
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## Caveats
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- **Legacy-era only.** `sampling/createMessage` is a server-initiated request
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with no 2026-07-28 wire carrier, so this story runs with `era = "legacy"` and
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the harness pins the handshake path.
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- `ctx.session.create_message()` is `@deprecated`; the
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`# pyright: ignore[reportDeprecated]` is deliberate. The non-deprecated
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replacement is to call your LLM provider directly from the server (see the
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banner above) — there is no successor server→client call.
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- `ctx.session.*` is the interim 2-hop path; a later release will shorten it.
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- `Client` has no `sampling_capabilities=` kwarg, so the `sampling.tools`
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sub-capability (tools-in-sampling) is unreachable from the high-level client.
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Drop to `ClientSession` if you need it.
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## Spec
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[Sampling — client features](https://modelcontextprotocol.io/specification/2025-11-25/client/sampling)
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## See also
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`legacy_elicitation/`, `roots/` — sibling stories that exercise the same legacy
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server→client request shape.
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