Tier 2 Application: cxxmcp (C++ SDK) #2814

Tier 2 Application: cxxmcp (C++ SDK)

Repository: https://github.com/caomengxuan666/cxxmcp
Maintainer: @caomengxuan666 (sole maintainer)
Language: C++17
Current version: v1.1.1

Summary

cxxmcp is a community-driven C++ SDK implementing the full MCP specification, including both the 2025-11-25 stable and 2026 draft spec versions. It provides both server and client transports (Streamable HTTP, SSE, stdio), OAuth 2.1 authorization, and comprehensive conformance coverage.

Conformance Results

All results from local modelcontextprotocol/conformance --suite all. cxxmcp supports both spec versions:

Known Exception

The single server failure (http-header-validation / ServerRejectsMissingMethodHeader) is an upstream issue in the TypeScript SDK conformance client — it does not send the required Mcp-Method header per SEP-2243. The cxxmcp client correctly sends this header. Tracked at:

• typescript-sdk#2176
• conformance#323
• Fix PR: typescript-sdk#2178

Once PR #2178 is merged, cxxmcp can enable strict SEP-2243 validation to reach 110/110 (100%).

Ecosystem Contribution: SEP-2243 Discovery

During conformance testing, cxxmcp discovered a significant issue in the MCP ecosystem: the conformance test suite is self-contradictory. The test ServerRejectsMissingMethodHeader enforces a MUST-level requirement (SEP-2243's Mcp-Method header), but the conformance suite's own reference client — the TypeScript SDK — does not satisfy this requirement. In other words, the test suite tests a MUST requirement that its own reference implementation fails.

This discovery led to three upstream issues:

• typescript-sdk#2176 — Bug report: TypeScript SDK conformance client omits the required Mcp-Method header
• conformance#323 — Discovery: conformance suite is self-contradictory (tests a MUST that its own client violates)
• typescript-sdk#2178 — Fix PR by another contributor, adding the missing header

This is noteworthy because it demonstrates that a community C++ SDK surfaced a gap that affected the entire MCP ecosystem's conformance infrastructure.

Comparison with RMCP (official Rust SDK)

Using identical --suite all runs on the same machine:

Tier-Check Automated Assessment

npx @modelcontextprotocol/conformance tier-check --repo caomengxuan666/cxxmcp --skip-conformance

Evidence Links

• SDK repo: https://github.com/caomengxuan666/cxxmcp
• Conformance examples: https://github.com/caomengxuan666/cxxmcp-examples
• Full run notes: cxxmcp-examples/CONFORMANCE_STATUS.md
• Conformance evidence doc: docs/conformance_evidence.md
• SEP-2243 contribution: typescript-sdk#2176, conformance#323, typescript-sdk#2178

Note on Tier Selection

While cxxmcp's conformance scores meet Tier 1 thresholds (99% server, 100% client), applying for Tier 2 given:

• Sole maintainer — cannot commit to Tier 2's full maintenance SLA without additional contributors
• C++ is a niche language in the MCP ecosystem
• spec_tracking gap due to delayed first release after 2025-11-25 spec

The conformance evidence and tier-check results are available for community review.