# MCP Knowledge > Expert guidance for the Model Context Protocol (MCP), the JSON-RPC 2.0 protocol connecting LLM apps to external tools and data. Covers spec versions 2024-11-05 through 2025-11-25 (architecture, transports, Resources, Prompts, Tools, Sampling, Roots, Elicitation, Tasks, lifecycle, OAuth 2.1, utilities) plus the official language SDKs: the Rust SDK `rmcp` (full guide — `ServerHandler`/`ClientHandler`, `#[tool_router]`/`#[tool_handler]`/`#[task_handler]` macros, `StreamableHttpService`, Cargo features, SEP-1319 tasks, `tokio::io::duplex` test harness) and the TypeScript SDK `@modelcontextprotocol/sdk` (incl. the `pkce-challenge` Vite/bundler resolver failure). Use when building MCP servers or clients in any language, working with transports or OAuth, or debugging SDK-specific issues. Always invoke for questions mentioning MCP, modelcontextprotocol, `rmcp`, tools/list, tools/call, sampling/createMessage, elicitation/create, Streamable HTTP, Mcp-Session-Id, `pkce-challenge`, or `@modelcontextprotocol/sdk`. - Skill: `46ki75/mcp-knowledge` (Agent Skill, multi-file: 88 files) - Install (CLI): `npx skillmds@latest add 46ki75/mcp-knowledge` - Raw SKILL.md: https://api.skillmd.com/api/skills/46ki75/mcp-knowledge/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - License: MIT - Author: 46ki75 (https://skillmd.com/u/46ki75) - Updated: 2026-09-10 - Page: https://skillmd.com/skills/46ki75/mcp-knowledge --- # MCP Skill You are an expert in the Model Context Protocol (MCP) — an open, JSON-RPC 2.0 based protocol that standardizes how LLM applications (hosts) connect to external data sources and tools (servers). MCP provides a stateful session protocol focused on context exchange and sampling coordination, enabling composable integrations across the AI ecosystem. ## What MCP Is MCP follows a client-host-server architecture. A **host** (e.g., an IDE or chat app) runs multiple **clients**, each maintaining a 1:1 stateful session with a **server**. Servers expose capabilities — Resources, Prompts, Tools — and can optionally request LLM completions back from the client (Sampling). The protocol uses JSON-RPC 2.0 for all messages and supports pluggable transports. ## Versions There are four public versions: | Version | Status | Key additions | | :------------- | :------------ | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **2024-11-05** | First release | stdio + HTTP/SSE transports; Resources, Prompts, Tools, Sampling, Roots; Pagination, Logging, Cancellation, Ping, Progress, Completion | | **2025-03-26** | Stable | OAuth 2.1 authorization; Streamable HTTP transport; JSON-RPC batching; tool annotations; audio content type; completions capability | | **2025-06-18** | Stable | Elicitation; structured tool output; resource links in tool results; OAuth Resource Server classification; RFC 8707 Resource Indicators; `MCP-Protocol-Version` header; removed batching; `title` field; `_meta` expansion | | **2025-11-25** | Latest stable | Tasks utility; OpenID Connect discovery; tool/resource/prompt icons; incremental scope consent; URL-mode elicitation; tool calling in sampling; OAuth Client ID Metadata Documents | Use **2025-11-25** for new projects. Use **2024-11-05** only when targeting legacy hosts. ## Core Concepts - **Host** — the LLM application; manages client lifecycle and user consent - **Client** — created by the host; holds one stateful session per server - **Server** — exposes Resources, Prompts, or Tools; may request Sampling - **Capability negotiation** — client and server exchange capability objects during `initialize`; only negotiated capabilities may be used during the session - **Resources** — application-driven context: files, DB schemas, live data (URI-addressed) - **Prompts** — user-triggered prompt templates with optional arguments - **Tools** — model-controlled functions that call external systems - **Sampling** — server-initiated LLM completions routed through the client (human in loop) - **Roots** — client-declared filesystem boundaries the server should respect - **Elicitation** — server requests structured input from the user via the client (2025-06-18+) - **Tasks** — durable async state machines for long-running requests (2025-11-25+) ## Reference Files Each reference file is the full specification document for that topic, converted from the official MDX source at `submodules/modelcontextprotocol/docs/specification/`. ### 2024-11-05 | File | Content | | :-------------------------------------- | :---------------------------------------------------------------------------------------- | | `references/2024-11-05/architecture.md` | Client-host-server architecture, design principles, message types, capability negotiation | | `references/2024-11-05/lifecycle.md` | Connection lifecycle: initialization, operation, shutdown; version negotiation | | `references/2024-11-05/transports.md` | stdio transport; HTTP+SSE transport; custom transports | | `references/2024-11-05/resources.md` | Resources: list, read, templates, subscriptions, URI schemes | | `references/2024-11-05/prompts.md` | Prompts: list, get, listChanged; message content types | | `references/2024-11-05/tools.md` | Tools: list, call, listChanged; inputSchema; error handling | | `references/2024-11-05/sampling.md` | Sampling: createMessage, modelPreferences, hints, human-in-the-loop | | `references/2024-11-05/roots.md` | Roots: list, listChanged; file:// URI constraint | | `references/2024-11-05/logging.md` | Logging: setLevel, notifications/message; syslog severity levels | | `references/2024-11-05/pagination.md` | Cursor-based pagination for list operations | | `references/2024-11-05/completion.md` | Argument autocompletion: completion/complete, ref/prompt, ref/resource | | `references/2024-11-05/cancellation.md` | Request cancellation via notifications/cancelled | | `references/2024-11-05/ping.md` | Ping/pong for connection health | | `references/2024-11-05/progress.md` | Progress tracking via progressToken and notifications/progress | ### 2025-03-26 | File | Content | | :--------------------------------------- | :---------------------------------------------------------------------------------------------------- | | `references/2025-03-26/changelog.md` | Delta from 2024-11-05: OAuth 2.1, Streamable HTTP, tool annotations, audio, completions capability | | `references/2025-03-26/architecture.md` | Architecture (unchanged) | | `references/2025-03-26/lifecycle.md` | Lifecycle (unchanged) | | `references/2025-03-26/transports.md` | Streamable HTTP (POST/GET/SSE, session management, resumability, backwards compatibility) | | `references/2025-03-26/authorization.md` | OAuth 2.1: authorization code, PKCE, dynamic client registration, metadata discovery | | `references/2025-03-26/resources.md` | Resources (unchanged) | | `references/2025-03-26/prompts.md` | Prompts (unchanged) | | `references/2025-03-26/tools.md` | Tools: adds annotations (readOnlyHint, destructiveHint, idempotentHint, openWorldHint); audio content | | `references/2025-03-26/sampling.md` | Sampling: adds audio content type | | `references/2025-03-26/roots.md` | Roots (unchanged) | | `references/2025-03-26/logging.md` | Logging (unchanged) | | `references/2025-03-26/pagination.md` | Pagination (unchanged) | | `references/2025-03-26/completion.md` | Completion (unchanged; server must declare completions capability) | | `references/2025-03-26/cancellation.md` | Cancellation (unchanged) | | `references/2025-03-26/ping.md` | Ping (unchanged) | | `references/2025-03-26/progress.md` | Progress: adds optional message field | ### 2025-06-18 | File | Content | | :--------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------- | | `references/2025-06-18/changelog.md` | Delta from 2025-03-26: elicitation, structured output, resource links, OAuth RS, RFC 8707, MCP-Protocol-Version, batching removed | | `references/2025-06-18/architecture.md` | Architecture (unchanged) | | `references/2025-06-18/lifecycle.md` | Lifecycle: initialized notification is now MUST | | `references/2025-06-18/transports.md` | Transports: MCP-Protocol-Version header required; batching removed | | `references/2025-06-18/authorization.md` | Authorization: OAuth Resource Server classification; RFC 8707 resource indicators | | `references/2025-06-18/resources.md` | Resources: adds title field | | `references/2025-06-18/prompts.md` | Prompts: adds title field | | `references/2025-06-18/tools.md` | Tools: structured output (outputSchema, structuredContent); resource links; title field | | `references/2025-06-18/sampling.md` | Sampling (unchanged) | | `references/2025-06-18/roots.md` | Roots (unchanged) | | `references/2025-06-18/elicitation.md` | Elicitation: elicitation/create, requestedSchema, accept/decline/cancel | | `references/2025-06-18/logging.md` | Logging (unchanged) | | `references/2025-06-18/pagination.md` | Pagination (unchanged) | | `references/2025-06-18/completion.md` | Completion: adds context field for dependent completions | | `references/2025-06-18/cancellation.md` | Cancellation (unchanged) | | `references/2025-06-18/ping.md` | Ping (unchanged) | | `references/2025-06-18/progress.md` | Progress (unchanged) | ### 2025-11-25 | File | Content | | :--------------------------------------- | :--------------------------------------------------------------------------------------------------------------------------- | | `references/2025-11-25/changelog.md` | Delta from 2025-06-18: Tasks, OIDC discovery, icons, URL elicitation, tool calling in sampling, Client ID Metadata Documents | | `references/2025-11-25/architecture.md` | Architecture (unchanged) | | `references/2025-11-25/lifecycle.md` | Lifecycle (unchanged) | | `references/2025-11-25/transports.md` | Transports: OIDC discovery; Client ID Metadata Documents; incremental scope consent; SSE polling | | `references/2025-11-25/authorization.md` | Authorization: OIDC discovery; OAuth Client ID Metadata Documents; incremental scope consent | | `references/2025-11-25/resources.md` | Resources: adds icon field | | `references/2025-11-25/prompts.md` | Prompts: adds icon field | | `references/2025-11-25/tools.md` | Tools: adds icon field; tool name guidance; input validation as tool execution errors | | `references/2025-11-25/sampling.md` | Sampling: tool calling in sampling (tools, toolChoice); sampling.tools capability | | `references/2025-11-25/roots.md` | Roots (unchanged) | | `references/2025-11-25/elicitation.md` | Elicitation: URL mode; updated ElicitResult and EnumSchema | | `references/2025-11-25/logging.md` | Logging: stdio servers may use stderr for all log levels | | `references/2025-11-25/pagination.md` | Pagination (unchanged) | | `references/2025-11-25/completion.md` | Completion (unchanged) | | `references/2025-11-25/cancellation.md` | Cancellation (unchanged) | | `references/2025-11-25/ping.md` | Ping (unchanged) | | `references/2025-11-25/progress.md` | Progress (unchanged) | | `references/2025-11-25/tasks.md` | Tasks (experimental): durable state machines, polling, deferred results, tasks/get, tasks/cancel | ## When to Read Which Files | User is asking about... | Read | | :----------------------------------------- | :------------------------------------------------------ | | MCP architecture / design principles | `references/{version}/architecture.md` | | Connection lifecycle, initialization | `references/{version}/lifecycle.md` | | stdio transport | `references/{version}/transports.md` | | HTTP+SSE transport | `references/2024-11-05/transports.md` | | Streamable HTTP transport | `references/2025-03-26/transports.md` (or later) | | OAuth 2.1 authorization | `references/2025-03-26/authorization.md` (or later) | | OAuth Resource Server / RFC 8707 | `references/2025-06-18/authorization.md` (or later) | | OIDC discovery / Client ID Metadata | `references/2025-11-25/authorization.md` | | Resources (list/read/subscribe) | `references/{version}/resources.md` | | Prompts (list/get) | `references/{version}/prompts.md` | | Tools (list/call/annotations) | `references/{version}/tools.md` | | Structured tool output | `references/2025-06-18/tools.md` (or later) | | Resource links in tool results | `references/2025-06-18/tools.md` (or later) | | Icons on tools/resources/prompts | `references/2025-11-25/tools.md` (or resources/prompts) | | Sampling (createMessage, modelPreferences) | `references/{version}/sampling.md` | | Tool calling inside sampling | `references/2025-11-25/sampling.md` | | Roots (filesystem boundaries) | `references/{version}/roots.md` | | Elicitation (server→user input) | `references/2025-06-18/elicitation.md` (or later) | | URL-mode elicitation | `references/2025-11-25/elicitation.md` | | Logging | `references/{version}/logging.md` | | Pagination | `references/{version}/pagination.md` | | Argument autocompletion | `references/{version}/completion.md` | | Cancellation | `references/{version}/cancellation.md` | | Ping / connection health | `references/{version}/ping.md` | | Progress notifications | `references/{version}/progress.md` | | Tasks (durable async) | `references/2025-11-25/tasks.md` | | Migrating between versions | `references/{target-version}/changelog.md` | | Rust SDK (`rmcp`) — anything Rust-specific | `references/rust-sdk/overview.md` (indexes the rest) | | TypeScript SDK build/bundler errors | `references/typescript-sdk/pkce-challenge.md` | ## Language SDK Guides Beyond the spec itself, this skill ships reference material for the official MCP SDKs. The depth differs by language: the Rust SDK has a full user guide; the TypeScript SDK currently only documents one well-known build-time quirk. These materials are language-specific — Rust guidance does not apply to TypeScript and vice versa. | SDK | Start here | | :---------------------------------------- | :----------------------------------------------------------------------------------------- | | Rust — `rmcp` crate (comprehensive guide) | `references/rust-sdk/overview.md`, then drill via `references/rust-sdk/doc-index.md` | | TypeScript — `@modelcontextprotocol/sdk` | `references/typescript-sdk/pkce-challenge.md` (Vite/bundler `pkce-challenge` resolver fix) | ### Rust SDK (`rmcp`) at a glance `references/rust-sdk/overview.md` covers workspace orientation, version / stability notes (Tier 2 conformance — some 2025-11-25 features are still in motion), Cargo feature flags, and the smallest viable server and client. From there, `references/rust-sdk/doc-index.md` indexes every per-feature file: server primitives (tools, prompts, resources, tasks, sampling, elicitation, roots, transports), client features (handler, requests, sampling, elicitation, roots, transports, testing), and the shared Cargo `feature-flags.md`. The canonical local example is `crates/mcp-server/`; the pinned upstream source of truth is `submodules/mcp-rust-sdk/`. ### TypeScript SDK (`@modelcontextprotocol/sdk`) Currently scoped to one entry — the Vite/Rollup/Webpack resolver error for `pkce-challenge` in browser/SSR builds, with a stub-alias workaround. See `references/typescript-sdk/pkce-challenge.md`. New TypeScript-SDK quirks should be added as sibling files under `references/typescript-sdk/`.