The three in-flight server→client channels during a tool call: progress (ctx.report_progress → the caller's progress_callback=), logging (notifications/message → the client's logging_callback=), and cancellation (abandoning the client's awaiting scope interrupts the server handler). One countdown(steps) tool emits a progress notification and a log line per step; the client asserts both streams arrive in order, then cancels a long call mid-flight by cancelling the enclosing anyio.CancelScope from inside the progress callback (event-driven, no sleep).

Run it

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

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

What to look at

client.py main — opens with async with Client(target, mode=mode, logging_callback=on_log). The story owns that construction; the harness only picks the target and era. logging_callback is constructor-only on Client (no setter after connect), so the callback and the logs list it fills are closed over right above the Client(...) call.
server.py — ctx.report_progress(i, steps, msg) is a silent no-op when the caller passed no progress_callback; the SDK reads the token from the request's _meta for you. The log notification is sent via the raw session.send_notification(...) because the ctx.log() / ctx.info() shorthands are deprecated (SEP-2577) with no non-deprecated replacement yet. related_request_id= keeps the log on this request's response stream — over streamable HTTP an unrelated notification would ride the standalone GET stream instead.
server.py — ctx.request_context.session / ctx.request_context.request_id is the interim 2-hop path; a later release will shorten these.
server.py — the except anyio.get_cancelled_exc_class(): raise block is where a real handler would release resources before re-raising. Never swallow the cancellation exception.
client.py — cancellation is just cancelling the anyio scope around await client.call_tool(...); the SDK sends notifications/cancelled for you on stateful transports. There is no client.cancel(request_id) API.
server_lowlevel.py — the same wire contract built by hand against ServerRequestContext.session directly.

Caveats

Logging is deprecated in the 2026-07-28 protocol (SEP-2577); functional through the deprecation window. Migration: write to stderr or emit OpenTelemetry instead of notifications/message. It is shown here because servers still need to support 2025-era clients during that window. Progress and cancellation are not deprecated. TODO(maxisbey): revisit before beta.
When a request is cancelled the server currently replies with ErrorData(code=0, message="Request cancelled"); the spec says it should not reply at all. The client never observes it (its awaiting task is already cancelled), so this story does not assert on the reply.

Spec

Progress, cancellation, logging

README.md

streaming

Run it

What to look at

Caveats

Spec

See also