MCP Specification Reference. The spec, in plain English.
One page per concept in the Model Context Protocol specification, version 2026-07-28: what it is, why it exists, how it works, and what changed between versions.
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Every page is checked against the official specification and links to the exact section it explains. Each one starts with its status: core, optional, extension or deprecated.
Protocol basics
How MCP is put together
Roles, messages, discovery, capabilities and how the spec is versioned.
How MCP Extensions Work
How MCP extensions work in the 2026-07-28 spec: identifiers, official vs experimental, per-request negotiation, fallback rules and how new ones are approved.
Read the guide →How to Read the MCP Specification (Without Being a Protocol Engineer)
A plain-English map of the MCP specification: where it lives, which version to read, what MUST and SHOULD mean, and the order to read the pages in.
Read the guide →JSON-RPC in MCP: Requests, Responses and Notifications
How MCP uses JSON-RPC 2.0 in the 2026-07-28 spec: request IDs, the new resultType field, error codes, notifications, and what changed from older versions.
Read the guide →MCP Capabilities Explained
What MCP capabilities are, every client and server capability in the 2026-07-28 spec, how they are declared per request, and which ones are now deprecated.
Read the guide →MCP Hosts, Clients and Servers: Who Does What
The three MCP roles as defined in the 2026-07-28 spec: what the host, each client and each server is responsible for, and how the stateless change affects them.
Read the guide →MCP Multi-Round-Trip Requests Explained
How MCP multi-round-trip requests (MRTR) let a server ask for user input mid-call in the stateless 2026-07-28 spec, with JSON examples and security rules.
Read the guide →MCP Server Discovery: How Clients Learn What a Server Offers
How server/discover works in the 2026-07-28 MCP spec: the request and response, when clients call it, caching, and how it replaced the initialize handshake.
Read the guide →MCP Specification Version History and Changelog
Every MCP specification version from 2024-11-05 to 2026-07-28, with the headline changes in each, taken from the official changelogs, in plain English.
Read the guide →MCP Versioning and Deprecation Policy
How MCP versions work (date-based IDs, per-request negotiation) and how its 2026 deprecation policy retires features, with the 12-month window and current list.
Read the guide →The Official MCP SDKs: Languages, Tiers and Versions
The ten official MCP SDKs as of October 2026, their tier (1, 2 or 3), latest release, what each tier promises and how to pick one for the 2026-07-28 spec.
Read the guide →Why MCP Became Stateless in the 2026-07-28 Spec
Why the 2026-07-28 MCP spec removed sessions and the initialize handshake, what stateless means in practice, how to keep state with handles, and what to fix.
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Server features
Tools, resources and prompts
The three things a server offers an AI app, and the rules around them.
How MCP Servers Report Tool Errors
The two ways MCP servers report tool errors, protocol errors and isError results, when to use each, and what changed in the 2025-11-25 and 2026-07-28 specs.
Read the guide →MCP Argument Completion
How MCP argument completion suggests values for prompt and resource template arguments, how completion/complete works in the 2026-07-28 spec, and its limits.
Read the guide →MCP Prompts Explained
What MCP prompts are, how prompts/list and prompts/get work in the 2026-07-28 spec, how they differ from tools and resources, and common mistakes to avoid.
Read the guide →MCP Resource Subscriptions and Change Notifications
How MCP clients get told when resources and lists change, why 2026-07-28 replaced resources/subscribe with subscriptions/listen, and how to migrate.
Read the guide →MCP Resource Templates
How MCP resource templates use RFC 6570 URI templates to expose parameterized data, how clients list and fill them, and what changed in the 2026-07-28 spec.
Read the guide →MCP Resources Explained
What MCP resources are, how resources/list and resources/read work in the 2026-07-28 spec, how they differ from tools, and the changes since 2024.
Read the guide →MCP Tool Annotations: Read-Only, Destructive and Idempotent Hints
What MCP tool annotations mean, the defaults for readOnlyHint, destructiveHint, idempotentHint and openWorldHint, and why clients must not trust them blindly.
Read the guide →MCP Tool Input and Output Schemas
How inputSchema and outputSchema work for MCP tools, which JSON Schema rules apply in the 2026-07-28 spec, and the schema mistakes that break tool calls.
Read the guide →MCP Tool Names and Descriptions: Rules and Good Practice
The MCP spec's rules for tool names (length, characters, uniqueness), how title and description work, and practical tips for descriptions AI models use well.
Read the guide →MCP Tools Explained
What an MCP tool is, how tools/list and tools/call work in the 2026-07-28 spec, what changed since 2024, and the mistakes that trip up new servers.
Read the guide →Structured Tool Output in MCP
How MCP tools return machine-readable results with structuredContent and outputSchema, what changed in the 2026-07-28 spec, and how to stay compatible.
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Utilities and extensions
Elicitation, utilities and extensions
Asking the user, pagination, progress and cancellation, content types, and the Tasks, MCP Apps and Skills extensions.
Cancelling MCP Requests
How to cancel an MCP request in the 2026-07-28 spec, why closing the HTTP stream counts as a cancel, how timeouts fit in, and which race conditions to handle.
Read the guide →MCP Content Types: Text, Images, Audio and Embedded Resources
The five MCP content types a tool can return (text, image, audio, resource links and embedded resources), with JSON examples, annotations and version history.
Read the guide →MCP Elicitation: When a Server Asks the User
What MCP elicitation is, how form and URL modes work in the stateless 2026-07-28 spec, a JSON example, what changed since 2025, and the mistakes to avoid.
Read the guide →MCP Logging: Status in the Current Spec
The MCP logging utility sends server log messages to clients. It is deprecated in 2026-07-28. See how it works now and why stderr and OpenTelemetry replace it.
Read the guide →MCP Progress Notifications
How MCP progress notifications work with a progressToken, what an update contains, what changed in 2026-07-28, and how progress relates to timeouts and Tasks.
Read the guide →MCP Roots: Status and What Replaced Them
MCP roots told servers which folders mattered. They are deprecated in the 2026-07-28 spec. Learn how roots work, when they go and what to use instead.
Read the guide →MCP Sampling: Status and What Replaced It
MCP sampling let servers borrow the client's AI model. It is deprecated in the 2026-07-28 spec. Here is how it works, its removal date and what to use instead.
Read the guide →Pagination in MCP
How MCP pagination works with opaque cursors and nextCursor, which list methods support it, what changed in 2026-07-28, and the client mistakes to avoid.
Read the guide →Skills over MCP
How the Skills over MCP extension lets servers share Agent Skills with skills/list and skills/get, how hosts verify each file, and where client support stands.
Read the guide →The MCP Apps Extension
How the MCP Apps extension lets a server show interactive HTML inside an AI chat, how ui:// resources and sandboxed iframes work, and which clients support it.
Read the guide →The MCP Tasks Extension
How the MCP Tasks extension handles long tool calls with task IDs, polling, mid-task input and cancellation, plus what changed when Tasks left the core spec.
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FAQ
Common questions
What is the latest version of the MCP specification?
Do I need to read the spec to use MCP?
Who maintains the specification?
Where do I start?
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