Quickstart

The fastest way to understand AXON is to watch it say no.

1. Install

   cargo install axon-lang

   One binary, axon. It needs a C compiler on the host (the cryptographic and tokenizer kernels are C23); on Windows, MSVC satisfies that.

2. Write a program that carries regulated data

   type PatientRecord compliance [HIPAA, GDPR] { ssn: String }
   shield PHIShield { scan: [pii_leak] on_breach: halt severity: critical
                      compliance: [HIPAA, GDPR] }
   axonendpoint Api { method: POST path: "/p" body: PatientRecord
                      execute: F output: FlowEnvelope<PatientRecord> shield: PHIShield
                      backend: anthropic compliance: [HIPAA, GDPR] }
   flow F(ssn: String) -> PatientRecord {
     step R { ask: "summarize" output: PatientRecord } }

   Save it as app.axon. Four declarations: a type that carries two regulatory classes, a shield that covers them, an endpoint that crosses a trust boundary, and a flow that does the cognitive work.

3. Check it

   axon check   app.axon   # compile-time compliance verification
   axon dossier app.axon   # regulatory posture, as JSON
   axon audit   app.axon --framework all   # per-framework gap analysis

   axon check exits 0. The endpoint carries PatientRecord, whose classes are {HIPAA, GDPR}, and PHIShield covers both.

4. Now break it

   Delete shield: PHIShield from the endpoint and check again:

   X app.axon  1 error(s)
     error [line 4]: axon-T957 axonendpoint 'Api' carries regulated data
     (kappa = {GDPR, HIPAA}) across a trust boundary but declares no `shield:`.
     Regulated boundaries require a shield whose `compliance:` covers the type's
     kappa — the ESK coverage rule. […] Declaring the classes on the
     endpoint's own `compliance:` does NOT cover them: that list is a label,
     the shield is the control that acts on a breach.

   axon check exits 1.

Why that matters

That failure is a type error, not a lint warning. Nothing downstream builds.

And the rule is a real set difference over the regulatory classes carried by the boundary's body: and output: types — so a shield that covers some of them fails too, naming exactly the ones it misses. It is not a presence check on a label.

The last sentence of the diagnostic is the load-bearing one. Declaring compliance: [HIPAA, GDPR] on the endpoint itself does not satisfy the rule. That list is a label; the shield is the control that acts on a breach. A system that accepted the label would let you certify yourself.

The typo cannot save you either

The regulatory vocabulary is closed (axon-T1214). compliance: [HIPPA] does not compile — it is refused with the suggestion HIPAA. That closure is what makes the coverage rule sound: without it, a symmetric typo on both the type and the shield would satisfy the set difference while protecting nothing.

The same set-difference rule guards channel declarations (axon-T1215: a channel whose payload carries a regulated class needs a covering shield before publish may extrude it), and is re-checked at runtime publish for IR that never met the checker.

Where to go next

Compile-time compliance

The rule in full, and precisely what it does and does not prove.

Installation options

Building from source, the server binary, and the feature flags.