deps: rmcp 3.x / MCP 2026-07-28 — the HTTP session gate is built on what becomes the legacy path

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glass is on rmcp 2.2.0, whose ProtocolVersion::LATEST is 2025-11-25. rmcp
3.x is out and implements MCP 2026-07-28. That revision is mostly a rewrite of
the plumbing rather than a feature release, and one of its changes lands directly
on code glass owns.

What changes underneath

2026-07-28 removes protocol-level sessions: no Mcp-Session-Id header, no
initialize / notifications/initialized handshake (each request carries its
protocol version and client capabilities in _meta), a new mandatory
server/discover RPC, subscriptions/listen in place of the HTTP GET stream and
resources/subscribe, and no SSE resumability (Last-Event-ID). Every result
gains a required resultType discriminator.

crates/glass-mcp/src/serve/session_gate.rs — the single-live-session gate with
last-client-wins takeover — is built entirely on rmcp's LocalSessionManager,
SessionId and the Mcp-Session-Id header. In rmcp 3.x that path is renamed to
what it now is: with_stateful_mode() becomes with_legacy_session_mode(), and
requests negotiating 2026-07-28 are always stateless. The gate does not break —
the legacy path stays — but it stops being the main road, and a client that
speaks only 2026-07-28 gets no session id for the gate to key on. The takeover
semantics that make serve --http reconnect work need a different key (or need to
move out of the transport layer).

Note this is a transport-level concern only. glass's implicit session — one app
under control, tools targeting it without a handle argument — is server-side state
and is unaffected. Worth recording that the new spec explicitly steers servers the
other way (server-minted handles passed as ordinary tool arguments); glass's choice
is deliberate and costs an argument on every call to reverse.

Migration cost (from the rmcp 3.0.0 migration guide)

• call_tool and friends return response enums instead of single result types;
  existing results convert with .into().
• The unified Meta type splits into MetaObject / RequestMetaObject /
  NotificationMetaObject.
• Protocol union enums become #[non_exhaustive] — exhaustive matches need a
  wildcard arm.
• StreamableHttpService requires a ServerHandler rather than a generic
  Service<RoleServer>.
• Snapshot tests must expect "resultType": "complete".
• rmcp 3.x declares rust-version = "1.88". No blocker here — glass is on
  nightly-2026-06-01.

What the upgrade unlocks

• Cache hints (ttlMs / cacheScope) on tools/list. glass's tool list is static
  per build, so a long TTL with cacheScope: "private" is an honest claim.
• Multi-round-trip requests, the replacement shape for elicitation: a handler can
  return InputRequiredResult to ask the user something mid-call. Plausible fits
  are the macOS accessibility grant during onboarding and a confirmation before
  teardown kills an adopted window — both currently handled by returning text that
  asks the agent to ask. Client support is the open question and the wire shape
  just changed, so this is watch-not-build for now.

Non-actions worth recording

• Roots, Sampling and Logging are all deprecated in 2026-07-28. glass uses none
  of them, so there is nothing to unwind.
• glass already returns bad tool input as a tool-execution error rather than a
  protocol error, which is what 2025-11-25 asked for.
• tools/list ordering is already deterministic: rmcp's ToolRouter stores routes
  in a HashMap, but list_all() sorts by name before returning, and the
  generated list_tools calls it. The 2026-07-28 SHOULD for deterministic order
  is satisfied as-is.

No urgency implied — 2025-11-25 remains negotiable and rmcp 2.2 already knows
2026-07-28 as a version constant. This is here so the session-gate coupling is
recorded before someone bumps the dependency and meets it by surprise.