Native library reference. Extended CLI examples use jacs-compat. This source reference can include unreleased changes; see the package guide for installation and MCP profiles for available tools.

Create and Verify an Attestation

An attestation signs claims about a subject. Verification authenticates the issuer's statement; the truth of a claim and permission to act require separate evidence and application policy.

The Python example below uses an ephemeral pq2025 agent. It requires the native attestation feature; source builds enable it with the attestation Cargo feature. See Python installation.

Create a claim about a signed document

import hashlib
from jacs.client import JacsClient

client = JacsClient.ephemeral()
signed = client.sign_message({"tool": "calculator", "answer": 42})
content_hash = hashlib.sha256(signed.raw.encode("utf-8")).hexdigest()

attestation = client.create_attestation(
    subject={
        "type": "artifact",
        "id": signed.document_id,
        "digests": {"sha256": content_hash},
    },
    claims=[{
        "name": "purpose",
        "value": "calculation example",
        "assuranceLevel": "self-asserted",
    }],
)
result = client.verify_attestation(attestation.raw)
assert result["valid"]
assert result["crypto"]["signatureValid"]
assert result["crypto"]["hashValid"]

The subject digest here covers the exact UTF-8 signed-document JSON. Preserve those bytes when exchanging the subject. A subject reference alone does not make the referenced artifact available to a verifier.

Local and full verification

verify_attestation() extracts the document's id:version and checks the copy already in this agent's document store. It is not an arbitrary inbound JSON verification API. Full verification additionally examines evidence and derivation:

full = client.verify_attestation(attestation.raw, full=True)
print(full["valid"], full.get("evidence"), full.get("chain"))

Inspect the complete report and errors before accepting it. Packages before 0.16.0 check the signature with the loaded agent's key, so they cannot fully verify another agent's attestation. Since 0.16.0, verification resolves the signer's exact signed key: local verification uses own, enrolled or cached keys and stays offline; full verification also follows the opt-in key discovery settings.

Full verification does not fetch URI-only evidence. Non-embedded evidence whose bytes are unavailable reports digestValid=false; an HTTPS URL and a claimed digest do not form a passing evidence check by themselves. Built-in A2A and Email evidence adapters only record and compare digests, not protocol signatures or signer trust. See custom evidence adapters.

Claims such as reviewed_by=human, a confidence score or an assurance label are assertions until supported by independently verified reviewer evidence. Never use a self-selected label as a substitute for that evidence.

DSSE export

envelope = client.export_attestation_dsse(attestation.raw)
print(envelope["payloadType"], len(envelope["signatures"]))

This produces a DSSE-shaped export. Acceptance by in-toto, SLSA or Sigstore consumers depends on their verifier, algorithms and policy; export alone does not demonstrate interoperability or authorization.

Continue with framework attestations, sign versus attest, or the attestation error catalog.