test: LegacyTestClient + statelessAcceptance(HTTP) + conformance dispatchV2 target

NEW test/integration/__fixtures__/testClient.ts: LegacyTestClient —
advertises only pre-2026 versions so connect() skips discover probe.

NEW statelessAcceptance.test.ts (HTTP scenarios): Server stateless
dispatch, SubscriptionBackend, handleHttp, StreamableHTTP zero-change
consumer, audit invariants.

conformance: extract everythingServerSetup.ts; add
everythingServerDispatchV2.ts target wired to run-server-conformance.sh.

Author: felixweinberger

.changeset/stateless-2026-06.md

@@ -0,0 +1,25 @@
+---
+'@modelcontextprotocol/core': major
+'@modelcontextprotocol/server': major
+'@modelcontextprotocol/client': major
+---
+
+2026-06 stateless protocol support (SEP-2575, SEP-2567, SEP-2322).
+
+`Server` and `Client` now support the 2026-06 stateless connection model
+alongside the existing pre-2026 model. They remain the same classes (still
+extending `Protocol`); the new behavior is additive.
+
+- `Client.connect()` auto-probes `server/discover` and falls back to the
+  legacy `initialize` handshake.
+- `Server` gained `subscriptions` and `statelessHandlers()`; transports route
+  per-message via the `MCP-Protocol-Version` header / `_meta` key.
+- `handleHttp(server, opts)` is a new Fetch-API entry point: one shared
+  `Server` instance, no `Transport`, no `connect()`.
+- `client.subscribe(filter)` opens a `subscriptions/listen` stream.
+- `Transport` interface gained optional `setStatelessHandlers?` and
+  `sendAndReceive?` for custom transports.
+- Prefer `ctx.mcpReq.{elicitInput, requestSampling, listRoots, log}` inside
+  handlers; works under both protocols (MRTR under 2026-06).
+
+See `docs/migration.md` for the full guide.

test/conformance/package.json

@@ -34,6 +34,8 @@
         "test:conformance:server": "scripts/run-server-conformance.sh --expected-failures ./expected-failures.yaml",
         "test:conformance:server:all": "scripts/run-server-conformance.sh --suite all --expected-failures ./expected-failures.yaml",
         "test:conformance:server:run": "npx tsx ./src/everythingServer.ts",
+        "test:conformance:server:dispatchv2": "PORT=3036 SERVER_ENTRY=./src/everythingServerDispatchV2.ts scripts/run-server-conformance.sh --expected-failures ./expected-failures.yaml",
+        "test:conformance:server:dispatchv2:run": "PORT=3036 npx tsx ./src/everythingServerDispatchV2.ts",
         "test:conformance:all": "pnpm run test:conformance:client:all && pnpm run test:conformance:server:all"
     },
     "devDependencies": {

test/conformance/scripts/run-server-conformance.sh

@@ -6,14 +6,15 @@ set -e
 
 PORT="${PORT:-3000}"
 SERVER_URL="http://localhost:${PORT}/mcp"
+SERVER_ENTRY="${SERVER_ENTRY:-./src/everythingServer.ts}"
 
 # Navigate to the repo root
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 cd "$SCRIPT_DIR/.."
 
 # Start the server in the background
-echo "Starting conformance test server on port ${PORT}..."
-npx tsx ./src/everythingServer.ts &
+echo "Starting conformance test server (${SERVER_ENTRY}) on port ${PORT}..."
+PORT="${PORT}" npx tsx "${SERVER_ENTRY}" &
 SERVER_PID=$!
 
 # Function to cleanup on exit