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The moment your tunnel opens, HeyMCP performs the MCP handshake against your server itself — as a real client, over the real routing — and reports what it found:
Or, when it isn’t ok:
This exists because agent clients fail silently. Claude.ai in particular tells you nothing at all when it can’t use your server. The probe is the thing that speaks up first.

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What it does

1

initialize

Sends initialize with Accept: application/json, text/event-stream, asking for protocol revision 2025-11-25 — the newest the official SDKs ship, rather than the newest that exists, because a server handed a version it has never heard of is entitled to refuse outright.
2

notifications/initialized

The lifecycle notification, in order, because some servers refuse tools/list before it.
3

tools/list

Collects your tools and their input schemas.
4

The checks

Runs all eleven rules against what it found. See the checks reference.
Every request after initialize carries the negotiated protocol version, not a hardcoded one. Sending a hardcoded version is how a client ends up telling a server it speaks a revision that server just declined — the source of the Bad Request: Unsupported protocol version failure real clients hit.

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It goes in the front door

The probe reaches your server through the same edge, routing, path policy and event tee that a real agent hits — not by dialling your local port directly. So a misconfiguration that would break Claude.ai breaks the probe first, which is the entire point. A probe that took a shortcut would happily pass on a setup no real client can use.

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When your server refuses SSE

If your server rejects Accept: text/event-stream with a 406 or 415, the probe doesn’t give up. It names the problem — transport/accept-sse — and retries without SSE so you still get the rest of the report. A half report with a clear cause beats no report.

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Running it without a tunnel

You don’t need a public URL to get a report:
Handshake plus checks, straight against your local server, nothing published. Good in CI — it exits 2 if anything will break, 1 on warnings only, 0 when clean. See heymcp probe.

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Re-probing after a fix

In the inspector’s Handshake tab, re-run the probe against the live tunnel. Reports are kept as history rather than overwritten, so you can see whether the change you just made actually cleared the finding.

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Protocol versions HeyMCP knows

If your server negotiates something outside this list, you get a mcp/protocol-version warning — either it’s a typo, or your HeyMCP build is older than your server and wants updating.
2026-07-28 is in the list because Claude already sends it. A server that answers with it is ahead of HeyMCP, not broken, and is not flagged.