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74 lines
3.7 KiB
Markdown
74 lines
3.7 KiB
Markdown
# streaming
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The three in-flight server→client channels during a tool call: **progress**
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(`ctx.report_progress` → the caller's `progress_callback=`), **logging**
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(`notifications/message` → the client's `logging_callback=`), and
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**cancellation** (abandoning the client's awaiting scope interrupts the server
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handler). One `countdown(steps)` tool emits a progress notification and a log
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line per step; the client asserts both streams arrive in order, then cancels a
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long call mid-flight by cancelling the enclosing `anyio.CancelScope` from
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inside the progress callback (event-driven, no `sleep`).
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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.streaming.client
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uv run python -m stories.streaming.client --server server_lowlevel
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# HTTP — the client self-hosts the server on a free port, runs, then tears it
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# down
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uv run python -m stories.streaming.client --http
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# same, against the lowlevel-API server variant
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uv run python -m stories.streaming.client --http --server server_lowlevel
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```
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## What to look at
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- `client.py` `main` — opens with `async with Client(target, mode=mode,
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logging_callback=on_log)`. The story owns that construction; the harness only
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picks the target and era. `logging_callback` is constructor-only on `Client`
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(no setter after connect), so the callback and the `logs` list it fills are
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closed over right above the `Client(...)` call.
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- `server.py` — `ctx.report_progress(i, steps, msg)` is a silent no-op when the
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caller passed no `progress_callback`; the SDK reads the token from the
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request's `_meta` for you. The log notification is sent via the raw
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`session.send_notification(...)` because the `ctx.log()` / `ctx.info()`
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shorthands are deprecated (SEP-2577) with no non-deprecated replacement yet.
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`related_request_id=` keeps the log on this request's response stream — over
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streamable HTTP an unrelated notification would ride the standalone GET
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stream instead.
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- `server.py` — `ctx.request_context.session` / `ctx.request_context.request_id`
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is the interim 2-hop path; a later release will shorten these.
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- `server.py` — the `except anyio.get_cancelled_exc_class(): raise` block is
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where a real handler would release resources before re-raising. **Never
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swallow** the cancellation exception.
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- `client.py` — cancellation is just cancelling the `anyio` scope around
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`await client.call_tool(...)`; the SDK sends `notifications/cancelled` for
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you on stateful transports. There is no `client.cancel(request_id)` API.
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- `server_lowlevel.py` — the same wire contract built by hand against
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`ServerRequestContext.session` directly.
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## Caveats
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- **Logging is deprecated** in the 2026-07-28 protocol (SEP-2577); functional
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through the deprecation window. Migration: write to stderr or emit
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OpenTelemetry instead of `notifications/message`. It is shown here because
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servers still need to support 2025-era clients during that window. Progress
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and cancellation are **not** deprecated. TODO(maxisbey): revisit before beta.
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- When a request is cancelled the server currently replies with
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`ErrorData(code=0, message="Request cancelled")`; the spec says it should not
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reply at all. The client never observes it (its awaiting task is already
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cancelled), so this story does not assert on the reply.
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## Spec
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[Progress](https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/progress),
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[cancellation](https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/cancellation),
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[logging](https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/logging)
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## See also
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`parallel_calls/` (concurrent in-flight calls), `error_handling/` (the
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cancellation error path), `tools/` (the basics this builds on).
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