RFC 0175: v2 transports and embedded protocols — gRPC demoted or generated, the legacy A2A and MCP profiles gone, negotiation authenticated with a minimum-version policy, the interop threat model written

Summary

grpc-transport.md is Stable with six MUSTs, a proto that calls itself abridged and defers its own messages to a v1.x that never came, no client in the suite, and zero advertisers — Axiom 1's exact failure. supportedTransports lists one baseline, one real transport nobody serves, and two embedded protocols that have richer first-class blocks; nothing reads its mcp or a2a members. The legacy A2A 0.3 and MCP 2025-06-18 profiles have dated sunsets (2027-03-12, 2027-08-12), one dual-era advertiser each, and removal recorded as "a v2 change." Version negotiation on both protocols is fail-closed only for an already-authenticated request, has no minimum-version policy in normative text, no refresh SLA, no audit event, and no threat model; its only witness is a seam a production host never mounts. v2 demotes gRPC, deletes supportedTransports, removes the legacy profiles at the cut, makes negotiation authenticated with an advertised floor, a refresh SLA, a ceiling on MRTR rounds and a content-free audit event, and writes the threat model both Stable documents cite.

Motivation

• grpc-transport.md:3 Stable; api/grpc/openwop.proto:14–20, 68–72 "abridged … deferred to v1.x … a future v1.x minor expands every Struct"; conformance/src/scenarios/grpc-transport.test.ts:5 "SHAPE-ONLY by design … the suite ships no gRPC client"; grpc-transport.md:202 still says RFC 0094 is "in flight"; extensions.json:662 draft, witnessable-gated on a scenario that dials nothing; no grpc in INTEROP-MATRIX.
• capabilities.schema.json:370 supportedTransports: ["rest","mcp","a2a","grpc"]; profiles.md:86/102 read only rest; a2a-integration.md:3 and mcp-integration.md:3 describe compositions, not transports; a2a carries its own transport list inside its block (G4) and mcp.serverMount.transports is its own enum.
• Legacy windows: a2a-integration.md:333 (2027-03-12; "Removal of the 0.3 code path from the corpus is a v2 change"), mcp-integration.md:251 (2027-08-12; same); RFC 0152 G1 / 0153 G1 measured: one dual-era advertiser each (openwop-app), no reference host advertises versions; the -legacy suffix is baked into both profiles item patterns (capabilities.schema.json:2908, 3400), so removal is a pattern change; mcp-integration.md:316 lists the legacy-profile scenarios the cut deletes.
• Negotiation: a2a-integration.md:341 and mcp-integration.md:260 — "for an authenticated request the default is fail-closed"; the exchange itself is not required to be authenticated; "minimum-version policy" exists only as a mitigation phrase in RFC 0152/0153 R1; the refresh SLA is RFC 0147 R10, unwritten; the audit event is RFC 0152/0153 G7 (authorization.decided as carrier, no dedicated type); host-sample-test-seams.md:954 — the wire toward a peer is "what no black-box request to the host's own API can observe," so both R1 rows are blocked on every host.
• mcp-integration.md:283 "the host MUST bound the number of rounds (host policy)" — a MUST with no number (G9). RFC 0100 UQ4 — auth-required in the persisted A2ATaskState enum for the reverse direction, never emitted forward.
• SECURITY/threat-model-interop.md does not exist; a2a-integration.md:509 and mcp-integration.md:331 (G8) cite it; the G7/G8/G9 rows live only in the spec docs — the RFC registers stop at G7 and G5.

Proposal

§A. gRPC

§A.1 grpc-transport.md moves to spec/v2/ext/grpc-transport/ with header witness: unwitnessable (the suite ships no client) and adoption: none; its six MUSTs become SHOULDs of the extension; api/grpc/openwop.proto is retired from the canonical API set (api/v2/ carries OpenAPI and AsyncAPI only) and kept under ext/grpc-transport/openwop.proto as a non-normative sketch. If a Phase 3 contributor generates the proto from the C.2 declaration file and lands a suite client, the extension re-enters core/ by a v2.x additive RFC — the door is named, not left ajar. The capabilities.grpc block is removed from the closed root (an unwitnessable family may not be advertised, RFC 0169 §A.1); grpc-transport.md:202's "in flight" sentence is corrected now (editorial).

§B. supportedTransports is deleted

§B.1 REST is the wire; there is no transport advertisement. A2A and MCP are compositions advertised by their own facets (a2a.versions[], a2a.preferredVersion, a2a.minimumVersion, a2a.profiles[] without -legacy; mcp.revisions[], mcp.preferredVersion, mcp.minimumRevision, mcp.features[], mcp.serverMount.transports[]). RFC 0172 §B axes 11–12 are these facets.

§C. Legacy profiles gone at the cut

§C.1 a2a-0.3-legacy and mcp-2025-06-18-legacy do not exist in the v2 tree: the profiles[] item patterns lose the (-legacy)? alternative; the legacy code paths (a2a-integration.md §0.3 mapping, mcp-integration.md live-callback bridges) are absent from spec/v2/; the suite's dual-era A2A peer and MCP server lose their legacy halves; the four mcp-server-* legacy scenarios are deleted. The v1 sunset dates (2027-03-12, 2027-08-12) stand for the v1 tree through Phase 5. §C.2 The adopter inventories (RFC 0152 G1, 0153 G1) are closed as measured: one dual-era advertiser (openwop-app), no reference host; a third-party inventory is not obtainable and the RFCs say so. §C.3 The header-less card rule (a2a-integration.md:349) becomes unconditional: a v2 host serves the card of preferredVersion when no A2A-Version header is present.

§D. Negotiation is a protocol

§D.1 A version-negotiation exchange on either embedded protocol MUST be authenticated: the peer identity is the C.3 Subject of the caller (or the host's own outbound identity), and an unauthenticated negotiation MUST NOT lower the version below preferredVersion. §D.2 A host advertises minimumVersion (A2A) / minimumRevision (MCP); a negotiation that would land below the floor MUST fail closed (interop_version_unsupported, RFC 0152 §B's code, registered in C.4's errors.json) whether or not a policy permits explicit downgrade above the floor. §D.3 Every negotiation outcome — including the fail-closed one — emits negotiation.decided (new v2 event type: { protocol: a2a | mcp, peer: <origin digest>, requested, negotiated | null, outcome: accepted | downgraded | refused, reason }, content-free), replacing the authorization.decided carrier RFC 0152/0153 G7 recommended. §D.4 Refresh SLA (RFC 0147 R10): a host MUST re-evaluate its advertised versions[] / revisions[] against the upstream registry within an advertised refreshedAt window not exceeding 90 days; an advertisement older than its window is non-conformant. §D.5 The silent-downgrade invariants (a2a-version-no-silent-downgrade, mcp-version-no-silent-downgrade) are witnessed by the negotiation.decided event on the host's own event log — a normative surface — rather than only by the §22/§23 invoke seams; the seams stay in the C.1 seams profile for the wire-capture leg.

§E. MRTR and the durable-task projection

§E.1 mcp.mrtr.maxRounds (integer, 1–16, advertised) is the ceiling; the host MUST refuse a further input_required round beyond it (mcp_mrtr_rounds_exceeded, registered in errors.json); RFC 0153 G9 closes. The requestState MUSTs (mcp-integration.md:290–291) carry over unchanged. §E.2 RFC 0100 UQ4: auth-required stays in the persisted A2ATaskState enum for the reverse direction; the forward projection never emits it in v2 either — v2 has no auth interrupt kind, and inventing one here would be C.4's business; recorded as decided, not deferred (RFC 0152 G9 closes as "not in v2.0").

§F. The threat model

§F.1 SECURITY/threat-model-interop.md is written with the sibling template (§1–§8): downgrade, card/runtime drift, cross-tenant lookup through a peer, artifact leakage across the boundary, the anonymous end-user actor (RFC 0165 §B.6), and negotiation replay; its §5 invariants are the two silent-downgrade rows plus interop-negotiation-authenticated, interop-minimum-version-enforced, interop-peer-no-authority-escalation (already registered) — entering invariants.yaml with their tests in Phase 3 (RFC 0167 §C rule). RFC 0152 G8 and 0153 G8 close when the file exists.

Migration table

Persisted-data disposition

Compatibility

breaking (v2). This PR changes no v1.x wire shape. The RFC 0152/0153 register additions copy rows that already exist in the spec documents' own gap tables (register drift, RFC 0174 §C); the grpc-transport.md:202 correction is editorial (RFC 0094 is Accepted).

Conformance

v2 scenarios (suite 2.0.0): no-transport-advertisement (unaided: supportedTransports and grpc absent), legacy-profiles-absent (unaided: no -legacy id validates), negotiation-decided-emitted (gated on a2a/mcp: the host's own event log carries negotiation.decided for a peer exchange driven through the seams profile), minimum-version-refused (gated: a peer offering below the floor is refused with interop_version_unsupported and the event says refused), negotiation-authenticated (gated: an unauthenticated exchange cannot lower the version), refresh-sla (unaided: refreshedAt within the window), mrtr-rounds-ceiling (gated on mcp: round maxRounds + 1 refused), threat-model-template (corpus, shared with C.6).

Falsifiability — one row per normative requirement

Adversarial review

1. Demoting gRPC to unwitnessable while a grpc block exists in the schema contradicts RFC 0169 §A.1. Disposition: the block is removed from the v2 root (row C8.3); a demoted extension is documented, not advertised. 2. Deleting supportedTransports removes the one field that let a client learn REST is served. Disposition: REST is the wire by definition (grpc-transport.md:3 already says "REST + SSE remains the REQUIRED wire surface"); a field that can only say rest says nothing. 3. Removing the -legacy pattern before 2027-03-12 / 2027-08-12 breaks the v1 promise. Disposition: it does not — the v1 tree keeps the profiles and the dates through Phase 5; only spec/v2/ lacks them, and a v2 host that still speaks A2A 0.3 does so as a non-advertised private behavior, which is what "removed from the corpus" means. 4. Authenticated negotiation is unwitnessable without the invoke seams. Disposition: the exchange is driven through the C.1 seams profile, but the record (negotiation.decided) lands on the host's own event log, a normative surface; §D.3's leg is witnessable — gated, §D.1/§D.2's peer-capture legs stay honestly seam-gated. 5. A 90-day refresh SLA is arbitrary. Disposition: it is the shortest window both upstreams' own lifecycle policies exceed (A2A/MCP twelve-month deprecation minimums); recorded as G1 for re-measurement at the first refresh. 6. RFC 0100 UQ4 is "decided" by not deciding. Disposition: the decision is that v2.0 has no auth interrupt kind; a future additive RFC may add one; the enum member stays for reverse-direction fidelity; RFC 0152 G9 closes as "not in v2.0", which is a decision with a date. 7. The RFC 0152/0153 registers lack the G7–G9 rows the charter cites. Disposition: copied into the registers in this PR with the spec-doc rows as their sources (RFC 0174 §C register-drift rule); the spec docs' tables are retired into gaps.json in Phase 3 (RFC 0174 §E.3).

Alternatives considered

1. Generate the proto now and keep gRPC in core. Rejected: no client, no advertiser, and the declaration file (C.2) does not exist until Phase 3; the door is named in §A.1. 2. Keep supportedTransports as ["rest"]. Rejected: Axiom 2 — a field with one legal value is an alias for its own absence. 3. Keep the legacy profiles through the v1 dates in spec/v2/. Rejected: the dates are v1's; v2 starts without the code paths, as both RFCs already say. 4. Do nothing. Rejected: two Stable documents cite a threat model that does not exist.

Unresolved questions

1. Whether negotiation.decided should carry the peer origin in clear or as a digest. Recommended: digest (content-free audit, RFC 0128 purpose label rides separately).

Implementation notes (non-normative)

openwop-app advertises both eras of both protocols today and drops the legacy halves in Phase 4; MyndHyve advertises neither A2A nor MCP versions and has no obligation under §D. The suite's fake peer and server (a2a-fake-peer.ts, mcp-fake-server.ts) are the negotiation-decided-emitted drivers.

Acceptance criteria

• [x] Draft → Active: RFC text; rows C8.1–C8.8; the discovery codemod extended with fixtures; RFC 0152/0153 register rows G7–G9 added; grpc-transport.md:202 corrected; deprecation row supported-transports-field; ledger row; adversarial review. (This PR.)
• [ ] Active → Accepted (Phase 3): spec/v2/ext/grpc-transport/; the facets and patterns in schemas/v2/capabilities.schema.json; spec/v2/core/interop.md; negotiation.decided in the v2 event registry; SECURITY/threat-model-interop.md; the eight scenarios in suite 2.0.0; openwop-app passes no-transport-advertisement, legacy-profiles-absent, negotiation-decided-emitted, refresh-sla.

References

• RFC 0167 §A (Axioms 1–2), §C, §E.1 axes 11–12, §E.3; RFC 0094 §H; RFC 0100 UQ4; RFC 0152 §B, G1, G3, G7–G9, R1; RFC 0153 §A/§C, G1, G3, G7–G9, R1; RFC 0147 G11, G12, R10; RFC 0169 §A.1; RFC 0171 §A/§B; RFC 0172 §B; RFC 0165 §B.6.
• spec/v1/grpc-transport.md, api/grpc/openwop.proto; spec/v1/a2a-integration.md §Discovery, §B, §Open spec gaps; spec/v1/mcp-integration.md §B, §C.1, §Open spec gaps; spec/v1/host-sample-test-seams.md §22–§23; spec/v1/capabilities.md:162; schemas/capabilities.schema.json (supportedTransports, a2a, mcp, grpc); conformance/src/scenarios/grpc-transport.test.ts; SECURITY/threat-model-*.md (the template).