Caveat Lang MCP server

CAVEAT Language

A decision ledger for agents.

Caveat is an embeddable decision runtime with a small rules language. It tracks evidence through computations, preserves the grounds of recorded decisions, and supports reconsideration under authored policies when evidence changes.

The published preview is 0.1.0-rc.19. See the rc.19 release record for what it changes.

npm publication verified 2026-10-12T14:57:45.221Z: exact caveat-lang@0.1.0-rc.19,

tested Linux artifact SHA256 d7cae7dc653a4c53de8d3f8e5b945cc324a14e931adcff004b805577d080672f.

At verification, latest names rc.19 and next remains rc.18; the owner moved next to rc.19 at about 15:01 UTC. Published by publish-npm.yml from tag v0.1.0-rc.19 with an npm provenance attestation.

Publication record.

When a tool result is corrected or a memory turns out to be stale, an application needs to reconsider its decision without losing the reasons for the original. Caveat is a programming language that keeps evidence and caveats with computed values, freezes a decision's grounds, and records why it was reopened.

Try the published preview, 0.1.0-rc.19, in an empty directory with Node 20 or later. No Rust installation is needed:

npm init -y
npm install caveat-lang@0.1.0-rc.19
npx --no-install caveat-lang init
npx --no-install caveat-lang test umbrella.scenarios.json
npx --no-install caveat-lang explain umbrella.cav events.jsonl

The two scenarios pass. The explanation shows umbrella@1 = 70 reopened by sky, still based on rainchance@1 with its forecastis_old caveat. New knowledge changed the decision's status; its original grounds remain visible. Getting started walks through those files, and the language in brief covers the syntax.

The rc.19 verified publication record identifies the exact tested npm artifact. rc.19 is the 1.0 feature-freeze candidate: it adds no language, schema, operation, field, CLI flag or export. It fixes restore of a session that reopened a decision because of a caveat and then took that caveat back; saves written by rc.18 in that state restore in rc.19.

rc.18 added unqualify, which takes back a caveat that qualify added, on every held occurrence of the evidence. The take-back is recorded, not erased, and decisions keep their basis. A save of a program that uses unqualify does not restore in rc.17 or earlier; other saves do. View 0.2 now lists a binding's evidence in the order it was first observed, and explain names the event and payload that observed each ground.

rc.17 introduced View 0.2, source digests in saves and the integration starter (caveat-lang/starter); the digest identifies the source and does not authenticate a save. rc.16 introduced departure of eligible retired history, host-drained archives and collection of unreachable withdrawal groups; see the history integration guide. rc.15 introduced integer clocks, history windows and caveat test checking each program first. rc.14 corrected the package's MCP server name to io.github.WSattazahn/caveat-lang. rc.13 introduced interface JSON and TypeScript declarations, stricter restore checks and refused invalid id_text operations. Restoring a save still does not authenticate its history. A refused operation is not completed; hosts must inspect its outcome. The package has no npm runtime dependencies.

With rc.19 installed, use npx --no-install caveat-lang doctor to check it and npx --no-install caveat-lang demo agent to see observation, assessment, correction and revision in one run. Introduced in rc.7, the unambiguous caveat-lang command remains preferred; caveat is supported shorthand. See CLI names.

Shortened. The full README is on GitHub.