Glossary
mpl terms, defined
The vocabulary of contracts, quality measurement, and audit trails — as used across the docs and the codebase.
- mpl (Meaning Protocol Layer)
- A Rust protocol layer for trustworthy AI agent communication. Runs as a sidecar proxy that validates contracts, scores quality, applies policy, and records tamper-evident proofs.
- stype (semantic type)
- A versioned identifier for the meaning of a payload, e.g. org.calendar.Event.v1, backed by a Draft 2020-12 JSON Schema in the registry.
- Envelope
- The wrapper mpl puts around every message. Carries four fields: stype, payload, sem_hash, and provenance.
- payload
- The actual content of a message, validated against the JSON Schema for its stype.
- sem_hash
- A BLAKE3 hash of the canonicalised payload. Makes each record tamper-evident — any change to the payload changes the hash.
- provenance
- A block recording who emitted a message (agent_id) and why (intent). Travels with the payload and is part of the hashed record.
- QoM (Quality of Message)
- A score for how well a message meets quality expectations, composed from up to six metrics.
- QoM profile
- A named, composable set of QoM metrics and thresholds. qom-basic and qom-strict-argcheck ship in the registry; custom profiles live under registry/profiles/.
- schema_fidelity
- A QoM metric: does the payload match the contract structurally?
- instruction_compliance
- A QoM metric: were upstream constraints respected?
- groundedness
- A QoM metric: are claims backed by sources?
- determinism
- A QoM metric: is output stable across runs?
- ontology_adherence
- A QoM metric: are domain rules respected?
- tool_outcome
- A QoM metric: did side effects match the declared intent?
- Registry
- The store of stype schemas and QoM profiles, organised into org.*, data.*, eval.*, and ai.* namespaces. Served by mpl-registry-api.
- Policy
- A glob-pattern rule applied at the proxy boundary — e.g. require a profile or mandatory provenance fields for stypes matching org.finance.*.
- Sidecar proxy
- The deployment shape of mpl. The proxy runs in front of your existing MCP/A2A/HTTP server; agents point at it and it forwards.
- Transparent mode
- Onboarding mode: the proxy observes and logs but never blocks.
- Production / strict mode
- Enforcement mode: invalid payloads and policy denials are rejected at the proxy.
- mplx
- The CLI crate that runs the proxy and manages schemas (mpl proxy, mpl schemas).
- MCP (Model Context Protocol)
- A client–server transport for agent–tool communication. mpl composes on top of it; it does not replace it.
- A2A (Agent-to-Agent)
- A peer-to-peer transport for agents talking directly to other agents. mpl adds contracts, quality, and audit above it.