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.

Framework Adapter Attestations

Python adapters accept attest=True to produce an attestation about their output. A signed claim records what its issuer asserted; it does not prove a human reviewed the output or that an action is authorized.

Install the corresponding framework extra. These examples require a native build with attestation support.

LangChain and LangGraph

Wrap a LangChain tool instance:

from langchain_core.tools import tool
from jacs.client import JacsClient
from jacs.adapters.langchain import signed_tool

client = JacsClient.ephemeral()

@tool
def lookup_customer(customer_id: str) -> str:
    """Return the customer status."""
    return "active"

attested_lookup = signed_tool(lookup_customer, client=client, attest=True)

For LangGraph, with_jacs_signing([lookup_customer], client=client, attest=True) constructs a ToolNode. jacs_wrap_tool_call(client=client, attest=True) returns a (request, execute) hook for a tool node; it is not a decorator for an ordinary Python function. For LangChain agents use jacs_signing_middleware(client=client, attest=True).

FastAPI

from fastapi import FastAPI
from jacs.client import JacsClient
from jacs.adapters.fastapi import JacsMiddleware

client = JacsClient.quickstart(name="api", domain="api.example.com")
app = FastAPI()
app.add_middleware(JacsMiddleware, client=client, attest=True)

For an individual route, use @jacs_route(client=client, attest=True). These constructors do not accept claims or default_claims. Request verification only checks signed POST JSON bodies; unsigned requests still pass through, including in strict mode. See FastAPI verification limits.

Anthropic tools

from jacs.client import JacsClient
from jacs.adapters.anthropic import signed_tool

client = JacsClient.ephemeral()

@signed_tool(client=client, attest=True)
def search_database(query: str) -> str:
    return "Found 3 results"

attestation_json = search_database("example")

JacsToolHook(client=client, attest=True) is an async post-tool hook accepting an input dictionary containing tool_response. It does not register or wrap a list of tool schemas.

Custom claims and verification

Use JacsClient.create_attestation(subject=..., claims=..., evidence=...) when you need explicit claims and evidence. The lower-level BaseJacsAdapter also accepts default_claims, and its sign_output(data, extra_claims=...) adds per-output claims. Framework wrappers do not expose all of those arguments.

client.verify_attestation(attestation_json) returns valid, crypto, evidence, chain and errors; it does not return a top-level claims list. The attestation must already be available in this agent's document store. Read the attestation tutorial for exact result keys and full-verification limits.

Signing and attestation failures raise by default, even when strict mode is off. The explicit compatibility options allow_unsigned_output=True and allow_plain_signature_fallback=True respectively permit unsigned output or dropping attestation claims. strict=True disables those fallbacks.