The Option B footgun was: await elicit() looks like a suspension point but
is actually a re-entry point, so everything above it runs twice. Option H
fixes that by making it a REAL suspension point — the coroutine frame is
held in a ContinuationStore across MRTR rounds, keyed by request_state.
Handler code stays exactly as it was in the SSE era:
async def my_tool(ctx: LinearCtx, location: str) -> str:
audit_log(location) # runs exactly once
units = await ctx.elicit("Which units?", UnitsSchema)
return f"{location}: 22°{units.u}"
The wrapper linear_mrtr(my_tool, store=...) translates this into a standard
MRTR on_call_tool handler. Round 1 starts the coroutine; elicit() sends
IncompleteResult back through the wrapper and parks on a stream. Round 2's
retry wakes it with the answer. The coroutine continues from where it
stopped — no re-entry, no double-execution.
Trade-off: server holds the frame in memory between rounds. Client sees
pure MRTR (no SSE, independent requests), but server is stateful within
a single tool call. Horizontally-scaled deployments need sticky routing on
the request_state token. Same operational shape as Option A's SSE hold,
without the long-lived connection.
SDK pieces (src/mcp/server/experimental/mrtr/linear.py):
- LinearCtx with async elicit(message, PydanticSchema) -> instance
- ContinuationStore — owns the task group, TTL-based frame expiry
- linear_mrtr(handler, store=...) — the wrapper
- ElicitDeclined raised when user declines/cancels
7 E2E tests including the key assertion: side-effects above await fire
exactly once (the test measures audit_log count).
Python-SDK counterpart to typescript-sdk#1701. Seven ways to write the
same weather-lookup tool so the diff between files is the argument.
SDK primitives (src/mcp/server/experimental/mrtr.py):
- MrtrCtx.once() — idempotency guard tracked in request_state (Option F)
- ToolBuilder — structural step decomposition; end_step runs exactly once
regardless of round count (Option G)
- input_response() — sugar for the guard-first pattern
- sse_retry_shim() — Option A comparison artifact (pragma no-cover until
LATEST_PROTOCOL_VERSION bumps past the MRTR gate)
- dispatch_by_version() — Option D comparison artifact
Option examples (examples/servers/mrtr-options/):
- E (degrade-only): the SDK default. MRTR-native; pre-MRTR gets a default
or error. Both quadrant rows collapse here.
- A (SSE shim): SDK emulates retry over SSE. Safe re-entry, hidden loop.
- B (await shim): exception-based. UNSAFE — hidden double-execution above
await. Not a ship target; for contrast.
- C (version branch): explicit if/else in handler body.
- D (dual handler): two functions, SDK picks by version.
- F (ctx.once): idempotency guard, opt-in per side-effect.
- G (ToolBuilder): no above-the-guard zone; end_step structurally
unreachable until all elicitations complete.
The invariant test (tests/experimental/test_mrtr.py) parametrises E/F/G
against the same Client + callback to prove identical wire behaviour —
the server's internal choice doesn't leak. The footgun test measures
audit_log count to prove F and G actually hold the guard (naive handler
fires twice; F and G fire once).
Both F and G depend on request_state integrity. The demos use plain
base64-JSON; a production SDK MUST HMAC-sign the blob.
Lowlevel plumbing for Multi Round-Trip Requests:
Types:
- IncompleteResult with result_type discriminator, input_requests, request_state
- InputRequest/InputResponse unions (elicitation, sampling, roots)
- input_responses + request_state fields on RequestParams
Server (lowlevel):
- on_call_tool return widened to include IncompleteResult
Session:
- send_request accepts TypeAdapter (overload) for union result parsing
- call_tool_mrtr() returns CallToolResult | IncompleteResult
- call_tool() stays narrow, raises on IncompleteResult with migration hint
Client:
- call_tool() drives MRTR retry loop internally — dispatches embedded
input requests to elicitation/sampling/list_roots callbacks, retries
with collected responses + echoed request_state
- max_mrtr_rounds bound (default 8)
The client-side delta from today's code is zero: elicitation_callback is the
same function whether it fires from SSE push or MRTR retry.
Max Isbey