SEP-2133: Extensions framework for MCP by pja-ant · Pull Request #2133 · modelcontextprotocol/modelcontextprotocol

pja-ant · 2026-01-22T15:14:49Z

This SEP ports the Extensions proposal from issue #1724 to the new PR-based SEP format (per SEP-1850).

Summary

Establishes a framework for extending the Model Context Protocol while maintaining interoperability, clear governance, and appropriate legal structure.

Key Points

Extension identifiers: Reverse domain notation with extension name (e.g., io.modelcontextprotocol/oauth-client-credentials)
Official extensions: Live under github.com/modelcontextprotocol/ext-* repositories
Extensions Track: New SEP type for extension proposals (follows same process as Standards Track)
Capability negotiation: Extensions advertised via extensions field in ClientCapabilities/ServerCapabilities
Legal requirements: Includes trademark, antitrust, licensing, and contributor license grant provisions

Design Consistency

The patterns described are consistent with existing official extensions (ext-apps and ext-auth).

References

Issue: SEP-1724: Extensions #1724
SEP workflow: SEP-1850: Migrate SEP process from issues to pull requests #1850

seps/2133-extensions.md

dsp-ant

We should add the extension page outline to this PR.

seps/2133-extensions.md

dsp-ant · 2026-01-26T18:46:47Z

This was accepted by @modelcontextprotocol/core-maintainers unanimously:

Voting results:

7x yes,
2x yes with changes,
0x no votes.

Once comments are addressed, this can be merged and status changed to final. @pja-ant please ensure that SDK's are implementing the necessary changes.

Co-authored-by: Che Liu <cheliu@google.com>

- Update definition to be more flexible (modular, specialized, experimental) - Add collision prevention guidance for extension identifiers - Define what constitutes a breaking change - Add example links to MAINTAINERS.md files - Add SHOULD for working/interest groups - Add hyperlink to SEP guidelines - Require reference implementation before acceptance - Clarify minor updates don't need SEP review - Update graceful degradation to allow rejecting requests - Clarify backwards compatibility concerns for extensions - Update to RFC 2119 language throughout

- Add 'extensions' field to ClientCapabilities and ServerCapabilities in draft schema - Create extensions.mdx documentation page listing ext-auth and ext-apps - Add extensions navigation to docs.json under Community tab - Add Extension Negotiation section to lifecycle specification - Add example JSON files for extension capabilities - Regenerate schema.json and schema.mdx

The merge-base changed after approval.

pja-ant changed the title ~~SEP: Extensions framework for MCP~~ SEP-2133: Extensions framework for MCP Jan 22, 2026

pja-ant mentioned this pull request Jan 22, 2026

SEP-1761: Add MCP extensions framework #1761

Closed

9 tasks

pja-ant marked this pull request as ready for review January 22, 2026 16:44

localden added SEP in-review SEP proposal ready for review. labels Jan 22, 2026

github-project-automation bot added this to SEP Review Pipeline Jan 22, 2026

localden assigned pcarleton Jan 22, 2026

localden moved this to In Review in SEP Review Pipeline Jan 22, 2026

pwwpche reviewed Jan 23, 2026

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