---
url: /mcp.zig/guide/protocol-version.md
description: >-
  MCP protocol version support including 2026-07-28, 2025-11-25, and earlier
  versions with stateless per-request model.
---

# Supported Protocol Version

::: info Official Documentation
This library implements **Model Context Protocol (MCP) version 2026-07-28**.
For the official MCP changelog and full specification, please visit [modelcontextprotocol.io](https://modelcontextprotocol.io/).
:::

## Key Changes in Protocol 2026-07-28

The following breaking changes were introduced in the MCP specification revision 2026-07-28:

## Breaking Changes

* **Stateless Protocol**: No more `initialize`/`notifications/initialized` handshake. Every request carries `io.modelcontextprotocol/protocolVersion`, `io.modelcontextprotocol/clientCapabilities`, and optionally `io.modelcontextprotocol/clientInfo` in `_meta`.
* **`server/discover`**: Now a mandatory RPC that servers MUST implement (replaces initialization). Returns `supportedVersions`, `capabilities`, `serverInfo`.
* **MRTR (Multi Round-Trip Requests)**: Servers use `InputRequiredResult` with `resultType: "input_required"` and `inputRequests` field instead of server-initiated JSON-RPC requests (sampling, elicitation, roots). Client retries original request with `inputResponses`.
* **`subscriptions/listen`**: Replaces `resources/subscribe`/`resources/unsubscribe` and HTTP GET endpoint. Uses long-lived POST-response streams with notification filters.
* **No protocol-level sessions** on Streamable HTTP. `Mcp-Session-Id` removed. Servers are stateless per-request.
* **All results carry `resultType`**: `"complete"` or `"input_required"`.
* **Deprecated features**: Roots, Sampling, Logging (still functional but new implementations should not adopt). HTTP+SSE transport deprecated. `ping`, `logging/setLevel`, `notifications/roots/list_changed` removed.
* **New HTTP headers**: `MCP-Protocol-Version`, `Mcp-Method`, `Mcp-Name`, `Mcp-Param-*` (from tool `x-mcp-header` annotations).
* **Caching**: `ttlMs` and `cacheScope` required on `server/discover`, `tools/list`, `prompts/list`, `resources/list`, `resources/templates/list`, `resources/read` results.
* **Error codes restructured**: `-32020` (HeaderMismatch), `-32021` (MissingRequiredClientCapability), `-32022` (UnsupportedProtocolVersion).
* **Streamable HTTP**: GET endpoint removed, SSE resumability via `Last-Event-ID` removed.
* **OAuth 2.1** authorization framework with Protected Resource Metadata (RFC 9728).
* **`_meta` key naming rules** with reverse DNS prefix convention.
* **`icons` property** added to implementations, tools, prompts, resources.
* **Tool names**: 1-128 chars, case-sensitive, alphanumeric + underscore/hyphen/dot.
* **`outputSchema`** for structured tool results with `structuredContent` field.
* **Completion**: `completion/complete` method for argument autocompletion.
* **Caching utility** with TTL-based freshness and cache scope.

## Backward Compatibility

This library maintains backward compatibility with older protocol versions:

| Version    | Status        |
| ---------- | ------------- |
| 2026-07-28 | Supported  |
| 2025-11-25 | Compatible |
| 2025-06-18 | Compatible |
| 2025-03-26 | Compatible |
| 2024-11-05 | Compatible |