MCP Compatibility Checker: exact-parser checks for tools/list items

[Naim-arg]

Pre-submission Checklist

• This post follows the MCP community guidelines

What would you like to share?

Hi MCP community,

I built MCP Compatibility Checker, a local open-source beta that checks whether individual tool definitions from an MCP tools/list response are accepted by one frozen client parser: MCP Inspector 2.3.0.

What it does

• Accepts a tool array, { "tools": [...] }, or a JSON-RPC response containing result.tools.
• Returns LOADABLE, NOT_LOADABLE, or fail-closed UNKNOWN for each tool.
• Reports an RFC 6901 JSON Pointer and reason when the parser rejects an item.
• Runs locally without accounts, telemetry, persistent input storage, or LLM-generated verdicts.
• Exports portable JSON and Markdown reports.

How it works

The checker wraps the actual per-item parser from MCP Inspector 2.3.0, frozen at commit:

86d5f58ec8c369839b8237455ad8b1c17b15a984

The setup process verifies the target commit, source state, and lockfile hashes before the parser can return an authoritative result.

What has been verified

The repository includes a self-authored 30-case compatibility corpus covering parser acceptance and rejection, unavailable-validator behavior, supported input envelopes, limits, and HTTP boundary cases.

Observed local result:

• 30/30 corpus cases passed
• 0 mismatches
• 0 skipped cases
• 53 additional diagnostic tests passed

Current limits

This beta checks parser acceptance for one client version only.

LOADABLE does not prove:

• universal MCP specification compliance;
• compatibility with other clients or versions;
• UI rendering or argument-form generation;
• successful tools/call execution;
• semantic correctness of the tool.

The current beta has been verified on Windows. macOS and Linux have not yet been independently verified.

This is an independent open-source project and is not an official compatibility guarantee from the MCP or MCP Inspector maintainers.

Feedback I am looking for

I would especially appreciate feedback from MCP server and client developers:

1. Minimal sanitized tools/list examples that behave differently in a real client.
2. Client name and exact version where the behavior was observed.
3. Review of the LOADABLE / NOT_LOADABLE / UNKNOWN result model.
4. Suggestions for the next client and version to support.
5. Independent testing on macOS or Linux.

Please do not submit API keys, credentials, personal data, or confidential production payloads.

Relevant Links

• GitHub repository
• [README and quick start](https://github.com/Naim-arg/mcp-compatibility-checker#readme)
• [Observed 30-case corpus result](https://github.com/Naim-arg/mcp-compatibility-checker/blob/main/corpus-results/corpus-v1-exact-2026-08-23.json)
• [Contributing and submitting test cases](https://github.com/Naim-arg/mcp-compatibility-checker/blob/main/CONTRIBUTING.md)
• [Report an issue](https://github.com/Naim-arg/mcp-compatibility-checker/issues/new)