stacklok / toolhive / 30358418301

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Commit Message

Design MRTR for vMCP and land the typed input_required seam (#6074)

* Design MRTR for the vMCP Modern path

The 2026-07-28 revision removes server-initiated requests outright and
replaces them with client-polled Multi Round-Trip Requests (SEP-2322).
vMCP has no MRTR implementation: elicitation and sampling forward only
over Legacy sessions, and the Modern egress shim declares empty
clientCapabilities, so a compliant Modern backend that needs input
answers -32021 and the call surfaces as an opaque protocol error. With
the Modern dispatch kill-switch being removed (#5959/#6033), that gap
becomes the served behavior.

Add the design the closed-as-not-planned #5759 asked for, as its own
document so it composes with the in-flight edits to doc 10 (#6050,
#6051). Decisions, grounded in the spec text read one day before the
revision's finalization and in RFC-0083:

- Modern-client/Modern-backend rounds are a stateless pass-through:
  per-request capability mirroring, verbatim inputRequests/requestState
  relay, deterministic re-routing of the retry. vMCP adds no state, so
  no durable store is triggered.
- Legacy-client/Modern-backend rounds bridge in-request: vMCP fulfills
  the backend's inputRequests through the existing
  ElicitationRequester/SamplingRequester seams and retries the backend
  call itself, bounded, on one pod — the same bridge go-sdk's own
  serverMultiRoundTripMiddleware performs for its handlers.
- Modern-client/Legacy-backend stays deliberately unbridged, per the
  costed rejection recorded in doc 10; the Tasks extension is the
  sanctioned stateful path.
- Only composite-workflow suspend/resume needs durable state, and it
  takes RFC-0083 D5 verbatim: an opaque >=128-bit handle as the
  client-visible requestState, WorkflowStatus.Owner bound to plaintext
  (iss, sub) via session/binding.Format, owner-checked resume, TTL,
  audit redaction, Redis store behind a core.Config seam.
- Sampling gets no feature work: SEP-2577 deprecates it, a... (continued)

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