Follow-ups from PR #71 review panel (MCP SDK 2.2.0)

[carldebilly]

Findings from the eight-lens panel on PR #71 that were deliberately not fixed there, each with the
reason. The two largest were split out at the time: #94 (frozen catalog / */list_changed, still open)
and #93 (MCP App resource handlers miss the flowing request, since closed).

Three of the original findings have since moved out of this issue. What remains below is internal
shape with no consumer path.

• Observability — superseded by mcp: no operator-facing diagnostics for the three failures the server absorbs #99, which names two absorb sites this issue did not (the
  withheld command cause discarded in McpToolAdapter.DescribeFailure, and the swallowed roots prime)
  and settles the ILoggerFactory versus Counter<long> question this issue left open.
• The sessionless revision matched by exact string equality — superseded by mcp: four follow-ups from the #71 review panel #103. Same bug,
  renamed symbol: PR feat(mcp)!: migrate to ModelContextProtocol 2.2.0 — requires SDK 2.x, changes IMcpFeedback.SendMessageAsync, and makes the 2026-07-28 tool list invariant #71 moved McpFeedbackService.IsSessionlessRevision to
  McpProtocolRevisions.CarriesSessionlessFields.
• The hidden-required-option disclosure — carved out as mcp: a cold connection receives the hidden option's target, token and route #104 and milestoned 0.12.0. It was the
  one item here a remote client can observe, and it was buried in an issue labelled internal quality.

What remains: readability and hardening, no consumer path

• McpServerHandler gained a session registry (_sessions, _attachLock, attach/detach,
  routing-subscription lifetime, per-session intro fan-out) on top of options building, seven request
  handlers, primitive generation and the compatibility shim. Two reasons to change in one class, and
  session lifetime is the concern most likely to move when Scoped DI services are shared across hosted sessions despite per-session documentation #70 lands its per-session overlay. An
  internal McpSessionRegistry holding the contexts is the shape.
• McpSessionContext.SnapshotGate exposes a raw SemaphoreSlim whose acquire/release protocol
  lives in the handler, while every other member of the type encapsulates its own invariant.
  ValueTask<IDisposable> AcquireSnapshotGateAsync(ct) keeps the primitive private.
• McpSessionContext is registered into its own service overlay by its caller, so an instance can
  exist whose Services cannot resolve it — a state ResolveContext treats as fatal, guarded only by
  a comment. A static McpSessionContext.Create(...) that builds the overlay, constructs and
  self-registers in one step removes the caller obligation.
• McpRequestServerAccessor.Current and .Effective name two views of one slot with unrelated
  adjectives; call sites read servers.Effective beside servers.Current. CurrentRequest /
  CurrentServer says what each returns.

What stays with #70

Soft roots remain process-wide on the reused-options path: host-set state with no request to belong to,
which is the DI-scoping question #70 exists for. So do the shared snapshot cache and compatibility-intro
flag on that path — currently harmless, since the catalog is pre-built and the shim only exists behind
handlers.

A CreateMcpServer(ITransport, …) entry point — context + McpServer.Create + attach/detach, for hosts
that control connection creation — would give custom transports session-grade isolation including soft
roots, and is a strong candidate for closing #70 for that shape. It is a feature, not a fix.

Origin: PR #71 review panel (issue #51), wave 5.