Claude Skill MCP Checker: Deterministic Audit of Breaking 2026-07-28 Changes
With MCP's 2026-07-28 revision arriving, the protocol's biggest breaking change ever — stateless transport, no more Mcp-Session-Id, OAuth 2.1 — a lot of public servers are quietly broken. Rather than rely on LLM-based guesswork, one developer built a deterministic checker that produces the same output for the same input and cites the exact spec page for every finding.
How the Checker Works
The core is a pure rule engine with 7 rules, exposed in two ways:
- Web demo — paste an endpoint, get a graded report. No install, nothing stored.
- Claude Skill — a
SKILL.mdplus a bundled, dependency-free checker. In Claude Code it handles diagnosis and migration, but you can unzip it and run the checker with any agent (Codex, Cursor).
The repo (MIT, ~86 tests) is on GitHub. There's even a README section called “A rule that was wrong,” with tests pinning both mistakes so they can't recur.
What the Author Learned
One rule was confidently wrong: it told users to upgrade the SDK to ^2 — a version that doesn't exist (it stops at 1.30.0). After deleting it with the note “no v2 exists,” it turned out v2 does exist as a package rename. The lesson: “this doesn't exist” and “you looked in the wrong place” produce identical evidence — a rename looks exactly like an absence. Deterministic isn't the same as correct.
That failure mode shows up everywhere agents write code: confident, plausible, internally consistent claims that just aren't true. The fix isn't to “trust less” — it's to make claims checkable, then actually check them.
Who It's For
Anyone running MCP servers or migrating to the 2026-07-28 spec, plus developers building agentic tools that need their output verified against spec.
📖 Read the full source: r/ClaudeAI
👀 See Also
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