mcp.zig

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Supported Protocol Version ​

Official Documentation

This library implements Model Context Protocol (MCP) version 2025-11-25. For the official MCP changelog and full specification, please visit modelcontextprotocol.io.

Key Changes in Protocol 2025-11-25 ​

The following changes were introduced in the MCP specification revision 2025-11-25:

Major changes ​

  • Enhance authorization server discovery with support for OpenID Connect Discovery 1.0.
  • Allow servers to expose icons as additional metadata for tools, resources, resource templates, and prompts.
  • Enhance authorization flows with incremental scope consent via WWW-Authenticate.
  • Provide guidance on tool names.
  • Update ElicitResult and EnumSchema to use a more standards-based approach and support titled, untitled, single-select, and multi-select enums.
  • Added support for URL mode elicitation.
  • Add tool calling support to sampling via tools and toolChoice parameters.
  • Add support for OAuth Client ID Metadata Documents as a recommended client registration mechanism.
  • Add experimental support for tasks to enable tracking durable requests with polling and deferred result retrieval.

Minor changes ​

  • Clarify that servers using stdio transport may use stderr for all types of logging, not just error messages.
  • Add optional description field to Implementation interface to align with MCP registry server.json format and provide human-readable context during initialization.
  • Clarify that servers must respond with HTTP 403 Forbidden for invalid Origin headers in Streamable HTTP transport.
  • Updated the Security Best Practices guidance.
  • Clarify that input validation errors should be returned as Tool Execution Errors rather than Protocol Errors to enable model self-correction.
  • Support polling SSE streams by allowing servers to disconnect at will.
  • Clarify GET streams support polling, resumption always via GET regardless of stream origin, event IDs should encode stream identity, disconnection includes server-initiated closure.
  • Align OAuth 2.0 Protected Resource Metadata discovery with RFC 9728, making WWW-Authenticate header optional with fallback to .well-known endpoint.
  • Add support for default values in all primitive types (string, number, enum) for elicitation schemas.
  • Establish JSON Schema 2020-12 as the default dialect for MCP schema definitions.

Other schema changes ​

  • Decouple request payloads from RPC method definitions into standalone parameter schemas.

Governance and process updates ​

  • Formalize Model Context Protocol governance structure.
  • Establish shared communication practices and guidelines for the MCP community.
  • Formalize Working Groups and Interest Groups in MCP governance.
  • Establish SDK tiering system with clear requirements for feature support and maintenance commitments.