Glossary

The terms this book and the tools’ output use, in the sense they use them.

Agreement check. The project’s calibration: on every CI run the official conformance suite drives the reference server and host, the sessions are recorded, and the validator’s verdicts on them are compared with the suite’s. A disagreement nobody has explained fails the build.

Capture. Recording a session as a trace. mcp-trace-capture does it for any implementation; the reference server and host can also record their own sessions.

Capability gate. The capability a clause depends on, such as server.tools. A gated clause applies only to a session that negotiated that capability; in a session that did not, it is not applicable.

Check. One mechanical test in the validator, with an ID such as base.request-id-unique. A clause is judged by one or more checks. Every check is shown to fail on at least one committed violation trace.

Clause (or requirement). One normative sentence of the specification — a MUST, MUST NOT, SHOULD or SHOULD NOT — entered in the registry under a stable ID such as BASE-003. The book’s reference pages list every one. A MAY permits rather than binds, so no trace can break one and the built-in registries enter none: requirements --level may is accepted, for a custom --registry that has some, and lists nothing otherwise.

Corpus. The committed traces the validator is tested against: conforming sessions, one violating trace per check, and recordings of real sessions, each with a golden report. See the corpus chapter.

Event. One line of a trace: a protocol message, an HTTP exchange’s metadata, or a transport opening or closing. See the trace format.

Excluded. A clause no recorded trace can judge — it is about an implementation’s internals, its user interface, or something that leaves nothing on the wire. The registry gives the reason for each.

Finding. One concrete violation, addressed to a clause and, where possible, to the event (seq) that broke it.

Golden report. The committed expected report for a corpus trace. A change in the validator’s output on any corpus trace is a test failure until the golden is regenerated and the difference reviewed.

Level. A clause’s RFC 2119 strength. A broken MUST or MUST NOT is a fail; an unfollowed SHOULD or SHOULD NOT is a warn, which fails the run only under --strict.

Not applicable. The outcome of a capability-gated clause in a session that never negotiated the capability.

Not observed. The outcome of a clause whose subject never appeared in the session — nothing was paginated, no error was returned. It is not a pass: the session had no opportunity to break it.

Registry. The specification as data: every clause of a revision with its ID, level, the party it binds, its verbatim quote and either its checks or its exclusion. The registry set holds one registry per revision.

Revision. A dated version of the MCP specification: 2026-07-28 (current) or 2025-11-25. The validator judges the revision a session declares, unless --revision names one.

Reference server, reference host. mcp-everything-server and mcp-reference-host: an MCP server and client built on the official Rust SDK, which pass the official suite and are used to calibrate the validator.

seq. An event’s position in its trace, assigned by the recorder when it observed the event. It is the only ordering the validator uses.

Session. One conversation between a client and a server, from opening to closing. The validator judges each trace as one session.

Trace. A recorded session in the trace format: JSON Lines, one event per line.

Verdict. A report’s conclusion: pass, pass with warnings, fail, or unsupported (the build cannot judge a clause its registry names — a mismatched installation). A run over several traces that could not judge one of them — unreadable, malformed, of a revision this build does not know, or judging nothing — reads incomplete, whatever the others say. The exit status carries it.