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Mirror of https://github.com/modelcontextprotocol/experimental-ext-variants — Status: Experimental. This repository provides a multi-language reference implementation of the variants proposal for the Model Context Protocol (MCP), as described in SEP-2053.
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fix: merge Extensions in unionCapabilities (#9) 
## Summary

`unionCapabilities` merges `Experimental` map entries from inner variant
servers but skips the `Extensions` field entirely. This means any
extension registered via `ServerCapabilities.AddExtension()` on an inner
server (e.g. `io.modelcontextprotocol/skills`) is silently dropped from
the front proxy's `InitializeResult`.

- Add the same first-wins merge logic for `Extensions` that already
exists for `Experimental`
- Bump `go-sdk` to v1.4.1 which introduced the `Extensions` field on
`ServerCapabilities`
- Add unit test for `unionCapabilities` extensions merging
- Add integration test verifying extensions survive the variant path
end-to-end

## Test plan

- [x] `TestUnionCapabilities_MergesExtensions` — verifies first-wins
merge semantics for extensions
- [x] `TestIntegration_ExtensionsAdvertised` — verifies extensions
registered on inner servers appear in the client's `InitializeResult`
- [x] All existing tests pass (27/27)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2026-05-29 09:17:04 +01:00
.github/workflows fix: merge Extensions in unionCapabilities (#9) 2026-05-29 09:17:04 +01:00
conformance Merge pull request #1 from modelcontextprotocol/chore/scaffolding 2026-01-27 20:10:28 +00:00
go/sdk fix: merge Extensions in unionCapabilities (#9) 2026-05-29 09:17:04 +01:00
python/sdk Merge pull request #1 from modelcontextprotocol/chore/scaffolding 2026-01-27 20:10:28 +00:00
typescript/sdk Merge pull request #1 from modelcontextprotocol/chore/scaffolding 2026-01-27 20:10:28 +00:00
.gitignore Merge pull request #1 from modelcontextprotocol/chore/scaffolding 2026-01-27 20:10:28 +00:00
LICENSE Initial commit 2026-01-26 10:54:20 +00:00
README.md Add example servers for SEP-2053 variant patterns (#4) 2026-03-01 12:42:43 +05:30

README.md

MCP Variants (Experimental Extension)

Status: Experimental. This work is for prototyping and feedback only, and is not an accepted or official MCP extension.

This repository provides a multi-language reference implementation of the variants proposal for the Model Context Protocol (MCP), as described in SEP-2053.

Implementations

Language Directory Package Status
Go go/sdk/ github.com/modelcontextprotocol/experimental-ext-variants/go/sdk Planned
Python python/sdk/ mcp-ext-variants Planned
TypeScript typescript/sdk/ @ext-modelcontextprotocol/variants Planned

Examples (Go)

The Go implementation includes runnable example servers under go/sdk/examples/server/:

model-optimized/

Same tools, different descriptions per LLM family.

https://github.com/user-attachments/assets/5bb60cd2-291f-4940-abfe-f53852d35470

github/

Different tool sets per variant with custom ranking.

https://github.com/user-attachments/assets/d770ac55-e988-4d89-bdf8-bcb3cf1e2e53

research/

Context budget management via description verbosity.

trading/

API versioning, lifecycle statuses, and deprecation info.

variants-stdio/

Minimal single-variant setup.

CI/CD

This monorepo uses path-based CI workflows to efficiently test only what changes:

How It Works

  1. Language-specific workflows (python.yml, go.yml, typescript.yml)

    • Only trigger when their language directory or workflow file changes
    • Run all tests, linting, and checks for that language

  2. Status check workflow (status-check.yml)

    • Runs on every PR to verify required checks passed
    • Determines what needs to pass based on which files changed
    • This is the only required check in branch protection

Examples

  • Change python/sdk/file.py → Only Python CI runs → PR requires Python checks to pass
  • Change both Go and TypeScript files → Both CIs run → PR requires both to pass
  • Change only README.md → No language CIs run → PR can merge immediately

Forcing All Checks

To run all language checks regardless of changed files:

  • In a PR: Comment /test all (only works for repo owners/members/collaborators)
  • Manually: Use GitHub Actions UI or CLI to trigger individual workflows

Adding New Required Checks

  1. Add your check to the appropriate language workflow (e.g., python.yml):

    python-security-scan:
  name: "Security Scan"
  runs-on: ubuntu-latest
  steps:
    - name: Run security checks
      run: # your commands here

  2. Update the status check in .github/workflows/status-check.yml:

    const requiredChecks = {
  python: [
    'Python CI / Linting',
    'Python CI / Unit Tests (3.10)',
    // ... existing checks ...
    'Python CI / Security Scan'  // ← Add your new check
  ],

  3. Submit PR - Your new check is now required for all relevant changes!

License

Apache License 2.0 - See LICENSE file for details

Resources