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Title: draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison by maxisbey · Pull Request #2322 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison by maxisbey · Pull Request #2322 · modelcontextprotocol/python-sdk

X Title: draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison by maxisbey · Pull Request #2322 · modelcontextprotocol/python-sdk

Description: Draft implementation of Multi Round-Trip Requests (SEP-2322) for the Python SDK. Two commits: lowlevel plumbing, then the handler-shape comparison deck. Counterpart to typescript-sdk#1701 — same weather-lookup tool throughout, so the diff between option files is the argument. Unlike the TS demos, the lowlevel plumbing here is real (not smuggled through JSON text blocks); every option round-trips IncompleteResult through the actual wire protocol. Commit 1: types + lowlevel + client retry loop Where Shape Types src/mcp/types/_types.py IncompleteResult (discriminated by result_type), InputRequest/InputResponse unions, input_responses+request_state folded into RequestParams Server src/mcp/server/lowlevel/server.py on_call_tool return widened to CallToolResult | IncompleteResult | CreateTaskResult Shared src/mcp/shared/session.py send_request accepts TypeAdapter via overload — enables union result parsing Session src/mcp/client/session.py call_tool_mrtr() returns the union; call_tool() stays narrow, raises clearly on IncompleteResult Client src/mcp/client/client.py call_tool() drives the retry loop internally — dispatches embedded input requests to elicitation_callback/sampling_callback/list_roots_callback, retries with collected responses + echoed request_state. max_mrtr_rounds=8 bound. 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. Commit 2: handler-shape comparison SDK primitives in src/mcp/server/experimental/mrtr.py: MrtrCtx.once(key, fn) — idempotency guard tracked in request_state (Option F) ToolBuilder — incomplete_step(...).end_step(...).build(); end_step runs exactly once regardless of round count (Option G) input_response(params, key) — sugar for the guard-first pattern sse_retry_shim() + dispatch_by_version() — comparison artifacts for A/D Option examples in examples/servers/mrtr-options/: Author writes SDK does Hidden re-entry Old client gets E MRTR-native only Nothing No Result w/ default, or error A MRTR-native only Retry-loop over SSE Yes, safe Full elicitation B await elicit() Exception → IncompleteResult Yes, unsafe Full elicitation C One handler, if version branch Version accessor No Full elicitation D Two handlers Picks by version No Full elicitation F MRTR-native + ctx.once wraps once() guard in request_state No (same as E) G Step functions + .build() Step-tracking in request_state No (same as E) Testing 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: naive handler fires twice for one tool call, F and G fire once. tests/client/test_client.py has 8 new E2E tests covering the retry loop (single-round elicitation, multi-round with request_state accumulation, sampling/roots dispatch, round-limit, missing-callback error paths). Not in scope Persistent/Tasks workflow — ServerTaskContext already does input_required; MRTR integration is a separate PR mrtrOnly client flag — trivial to add, not demoed requestState HMAC signing — called out in code comments; demos use plain base64-JSON High-level MCPServer integration (@server.tool decorator shape) — lowlevel-first, this PR stops at Server Exploratory — not intended to merge as-is. Open questions: which of F/G (or both) to ship as SDK primitives, whether to keep call_tool_mrtr as public or fold the union into call_tool once SEP finalises, whether sse_retry_shim belongs in the SDK at all vs docs-only. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update

Open Graph Description: Draft implementation of Multi Round-Trip Requests (SEP-2322) for the Python SDK. Two commits: lowlevel plumbing, then the handler-shape comparison deck. Counterpart to typescript-sdk#1701 — same we...

X Description: Draft implementation of Multi Round-Trip Requests (SEP-2322) for the Python SDK. Two commits: lowlevel plumbing, then the handler-shape comparison deck. Counterpart to typescript-sdk#1701 — same we...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2322

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