# Liveness probes

> The five probe kinds, what is sent and what counts as an answer.


A probe is the minimal harmless call that proves an interface is alive. The manifest names it per interface under `interfaces[].probe` with `kind` and `expect`; optionally `target` names a probe URL that differs from the endpoint.

| `kind` | What is sent | `expect` |
|---|---|---|
| `sparql-ask` | an ASK query against the endpoint | `boolean` |
| `http-get` | a plain GET | `ok` (2xx) or `response` |
| `http-head` | a HEAD | `ok` or `response` |
| `mcp-initialize` | the initialize handshake, servers of the handshake era | `response` |
| `mcp-discover` | `server/discover` as the first message, servers of the stateless era (revision 2026-07-28) | `ok` or `response` |

`expect = response` means: any HTTP answer proves liveness, even a 400 to a bare GET. Two MCP probes because MCP has two eras; the wrong probe yields an answer that proves nothing.

## States of a check {#zustaende}

Three, never more: **never checked**, **passed**, **failed**, each with instant, probe and the address actually probed. A measurement against a local instance says so. Nothing is shown as passed that was not measured.

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Source: docs/en/reference/probes.md