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.md plus 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.