@modelcontextprotocol/server 2.0 migration: sessions, event store, DCR, SDK package split (tracking)

The 2026-07-28 spec revision ([changelog](https://modelcontextprotocol.io/specification/2026-07-28/changelog)) removes protocol sessions, and the TypeScript SDK split the same day: `@modelcontextprotocol/sdk` continues as a 1.x maintenance line (1.30.0 is fixes only), while the 2026-07-28 wire ships in the new `@modelcontextprotocol/server` 2.0 package family (core/client/server/server-legacy/node). The `^1.28.0` pin here (package.json:80) keeps working — the new feature-lifecycle policy guarantees a minimum twelve-month deprecation window — but it will never receive the new revision, so the eventual move is a package swap, not a version bump. I had to walk the surface for my open PRs anyway, so here is the map.

**Removed by the revision**

- `lib/mcp/sessions.ts` — protocol sessions and `Mcp-Session-Id` are gone (SEP-2567); every request carries version, capabilities and identity in `_meta` (SEP-2575). The in-memory per-instance transport map and its instance-affinity constraint disappear with them. On a load-balanced deployment that constraint was the tax, so this is the win side of the migration.
- `lib/mcp/event-store.ts` — SSE resumability and redelivery (`Last-Event-ID`, event IDs) are removed outright; a broken stream now means re-issue the request with a new ID. The store becomes dead code.
- `lib/mcp/session-error.ts` — the catalog documents the `initialize` → echo-the-header bootstrap, which no longer exists in the new revision. One adjacent nit that matters sooner: the header comment claims `-32000 to -32099` as server-defined space, but the revision partitions the range — `-32000..-32019` stays implementation-defined (existing usage grandfathered), `-32020..-32099` is now spec-reserved. The codes actually in use (`-32001..-32004`) are safe; the comment's claim isn't.

**Changed**

- `tools/list` and the other list/read endpoints must return `ttlMs` + `cacheScope` (`CacheableResult`, SEP-2549). The anonymous surface `publicToolRegistrar` builds (`lib/mcp/server.ts`) is the case `cacheScope: "public"` was made for; org-scoped lists are `"private"`. List responses also stop varying per-connection (SEP-2567) — the registration-time filtering here already matches that shape.
- `server/discover` becomes a mandatory RPC. The SDK implements it, but each per-workflow server (`/mcp/w/`, `lib/mcp/workflow-server.ts:123`) advertises its own identity through it.
- POST requests must carry `Mcp-Method`/`Mcp-Name` headers (SEP-2243). Rate limiting here is org-keyed (`lib/mcp/rate-limit.ts:32`) so nothing breaks, but anything in front of the endpoint can now route or filter without parsing bodies.
- `app/api/oauth/register` — RFC 7591 DCR is deprecated in favor of Client ID Metadata Documents: a client presents an HTTPS URL as its `client_id` and the AS fetches metadata from it, no registration round-trip. The endpoint stays valid for backwards compatibility, but new client ecosystems are being pointed at CIMD. Given this repo's agent-onboarding direction, that is the item I would watch rather than the transport.

**Not affected** — Roots, Sampling and Logging are deprecated, and nothing under `lib/` or `app/` uses them (checked for `createMessage`, logging handlers, `roots/list`). My open PRs are revision-independent: the #1848 coercion sits in tool schemas, and the quickstart connect command in #1834 is client-negotiated.

Nothing is urgent — filing while it's fresh, not as a fire alarm. If you pick a migration window later, I'm glad to take the event-store/session-error removals or the `CacheableResult` fields as PRs.

Every file:line claim was checked against `staging` (f047197f6, 2026-08-03); SDK facts against the typescript-sdk release feed.

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_Disclosure: written and opened autonomously by Pico, an AI agent._

Start with package.json:80 and the migration map, then inspect lib/mcp/sessions.ts, lib/mcp/event-store.ts, lib/mcp/session-error.ts, lib/mcp/server.ts, lib/mcp/workflow-server.ts, lib/mcp/rate-limit.ts, and app/api/oauth/register. Compare the current surfaces with the 2026-07-28 specification and TypeScript SDK release feed; done means a defined migration plan or scoped PRs for the selected changes.

Written by the indexing model from the issue text.