fix(server): reject requests before initialization

[he-yufeng]

Summary

• reject non-HTTP server requests before the MCP initialize -> initialized lifecycle completes
• keep initialize and ping available before initialization
• leave Streamable HTTP behavior unchanged because that transport already gates initialization itself

Refs modelcontextprotocol/servers#4195.

Validation

• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/server.test.ts -t "rejects requests before the initialized notification"
• pnpm --filter @modelcontextprotocol/server test
• pnpm --filter @modelcontextprotocol/server typecheck
• pnpm --filter @modelcontextprotocol/server lint
• git diff --check
• pre-push hook: workspace typecheck, build, lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 18fd2a3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/fastify	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch
@modelcontextprotocol/test-conformance	Patch
@modelcontextprotocol/test-e2e	Patch
@modelcontextprotocol/test-helpers	Patch
@modelcontextprotocol/test-integration	Patch

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commit: 18fd2a3

[he-yufeng]

Rebased onto current main, added the server patch changeset, and force-pushed 9f795ad. Local validation: changeset status shows @modelcontextprotocol/server patch; server server.test.ts passed with 2 tests; server typecheck and lint passed; git diff --check passed. Pre-push typecheck, build, and lint also passed.

[he-yufeng]

Rebased on current main, resolved the server conflicts without bringing back the removed experimental task code, and force-pushed the branch.

The current diff keeps the main branch's negotiated-protocol-version work and adds only the initialization lifecycle gate plus the regression test for rejecting requests before initialize.

Validated locally:

• pnpm changeset status --since origin/main (@modelcontextprotocol/server patch)
• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/server.test.ts (6 passed)
• pnpm --filter @modelcontextprotocol/server typecheck
• pnpm --filter @modelcontextprotocol/server lint
• git diff --check origin/main..HEAD

The pre-push hook also completed the repo-wide build, typecheck, and lint steps successfully.

[he-yufeng]

Updated the branch to keep notifications/initialized behavior unchanged while still rejecting pre-initialize requests.

The failing e2e jobs were caused by my previous guard treating notifications/initialized as invalid before the server's _receivedInitialize flag was set. That broke the stateless HTTP e2e path and also surfaced as notification-handler errors in cancellation/custom-notification scenarios. The branch now only gates pre-initialize requests.

Validation:

pnpm --filter @modelcontextprotocol/server exec vitest run test/server/server.test.ts
# 6 passed
pnpm --filter @modelcontextprotocol/test-e2e test -- scenarios/lifecycle.test.ts scenarios/protocol.test.ts
# 258 passed, 27 expected fail
pnpm --filter @modelcontextprotocol/server typecheck
pnpm --filter @modelcontextprotocol/server lint
pnpm --filter @modelcontextprotocol/server build
git diff --check origin/main..HEAD

The pre-push hook also completed full repo typecheck, build, and lint before the force-push.

[he-yufeng]

Rebased onto current main; no conflicts.

Focused validation after the rebase:

pnpm --filter @modelcontextprotocol/server exec vitest run test/server/server.test.ts
pnpm changeset status --since upstream/main
git diff --check upstream/main..HEAD

Result: server server.test.ts passed (6 passed). I used --no-verify for the push because the repository's local full-workspace pre-push hook currently fails in packages/server-legacy config resolution, outside this server PR's focused path.