# Internals

Canonical URL: https://vercel-mcp-reference.vercel.app/internals/
Markdown: https://vercel-mcp-reference.vercel.app/internals.md
Audience: engineer, architect. MCP spec version: 2026-07-28. Last reviewed: 2026-08-26. Status: stable.

How MCP actually works underneath the patterns: the roles, the capability
primitives, the wire transports, and the message flow, plus the part this repo
exists for: what changes when the server is a Vercel Function instead of a
resident process. As of the 2026-07-28 revision the protocol itself is
stateless, so that fit is now native rather than negotiated. Start with the
overview, then dip into the specific mechanism you need. If you only read one
page beyond the overview, read
[serverless sessions](https://vercel-mcp-reference.vercel.app/internals/serverless-sessions/); it is where cross-call state
lives now that the protocol no longer defines sessions at all.

## Pages

- [MCP internals overview](https://vercel-mcp-reference.vercel.app/internals/overview/) - host, client, and server roles; the
  stateless per-request model (no handshake, no session, every request carries
  its own version and capabilities); why each client-server pairing stays
  isolated; and the security implications that follow when every server is a
  remote HTTP endpoint.
- [Capability primitives](https://vercel-mcp-reference.vercel.app/internals/primitives/) - the building blocks a server exposes
  (tools, resources, prompts) and the auxiliary ones, including how a server
  gets model inference or user input under 2026-07-28: an `input_required`
  result the client answers by retrying (MRTR), with roots, sampling, and
  logging documented as deprecated. Plus who is in control of each and the
  full method reference.
- [Transports](https://vercel-mcp-reference.vercel.app/internals/transports/) - Streamable HTTP in depth (one POST per
  message, per-request SSE streams, the mirrored `Mcp-Method`/`Mcp-Name`
  headers, `subscriptions/listen`, the Origin-check 403), stdio for local
  development, and the earlier shapes as history: the session-and-resumability
  era of Streamable HTTP and the legacy HTTP+SSE transport with its
  Redis-on-Vercel caveat.
- [Serverless sessions](https://vercel-mcp-reference.vercel.app/internals/serverless-sessions/) - the flagship page: the
  protocol dropped sessions (SEP-2567) and adopted the shape functions always
  forced, so this page covers where cross-call state can still live
  (server-minted handles, an external store, or nowhere), Fluid compute
  instance reuse, and what `maxDuration` means for long tool calls.
- [The 2026-07-28 stateless revision](https://vercel-mcp-reference.vercel.app/internals/spec-2026-07-28/) - the
  revision-history record: what the now-current published revision changed
  relative to 2025-11-25, why serverless is its happy path, and what the
  pinned handler and SDK client each emit on the wire.
- [Annotated message trace](https://vercel-mcp-reference.vercel.app/internals/message-trace/) - a complete JSON-RPC exchange
  walked line by line under 2026-07-28, from optional discovery through a tool
  call and the `input_required` retry loop, including the HTTP layer a
  deployed server actually sees.
- [Tasks (extension)](https://vercel-mcp-reference.vercel.app/internals/tasks/) - the official `io.modelcontextprotocol/tasks`
  extension (moved out of core in 2026-07-28) for long-running work behind a
  pollable task handle, and how it relates to the
  [async-jobs](https://vercel-mcp-reference.vercel.app/patterns/async-jobs/) backing store.

## Where to look now

- [Patterns](https://vercel-mcp-reference.vercel.app/patterns/) - how these mechanisms compose into
  deployable shapes on Vercel.
- [Security](https://vercel-mcp-reference.vercel.app/security/) - the authorization and identity rules that
  ride on top of the transport and lifecycle described here.
- [Deployment](https://vercel-mcp-reference.vercel.app/deployment/) - the Vercel project mechanics
  (`vercel.json`, environments, Deployment Protection) that every internals
  page assumes.