Connect an MCP client

Install the portable CLI:

cargo install jacs-cli --locked
jacs mcp

For clients that accept an mcpServers configuration, the verification-only connection is:

{
  "mcpServers": {
    "jacs": { "command": "jacs", "args": ["mcp"] }
  }
}

The host starts a local stdio process. There is no HTTP listener; diagnostics go to stderr. The only default tool is jacs_verify_document. Select the signer's public evidence independently, parse the returned JSON text, and check its valid field. See the verification use case.

Verify a sample document

To create a local test fixture, run in a private directory (skip this when you already have a signed document and independently selected public identity):

mkdir -m 700 .jacs
jacs create --output .jacs/agent.json > public-identity.json
printf '%s' '{"message":"hello"}' | jacs sign --agent .jacs/agent.json --input - > signed.json
jacs verify --public-identity public-identity.json --input signed.json

The CLI prompts for the key password. Its successful verification reports valid: true. To prepare the equivalent MCP request with Python 3:

import json
from pathlib import Path

trusted = json.loads(Path("public-identity.json").read_text())
request = {
    "jsonrpc": "2.0", "id": 1, "method": "tools/call",
    "params": {"name": "jacs_verify_document", "arguments": {
        "document": Path("signed.json").read_text(),
        "public_key": trusted["public_key"],
        "algorithm": trusted["algorithm"],
    }},
}
print(json.dumps(request))

Send this through your MCP client after its normal connection/initialization. document is an exact JSON string; public_key is canonical standard Base64 of the raw public-key bytes, not PEM. Parse the response's text content as JSON and require valid: true. For identity attribution, also compare signer_id to the independently selected public identity's agent_id. Malformed requests can return protocol/tool errors. Do not accept a key merely because it arrived beside an untrusted document.

Enable local signing explicitly

Start the signer with the encrypted fixture vault created above:

jacs mcp --profile local-sign --agent .jacs/agent.json

The terminal prompts for the password. A noninteractive MCP host must supply JACS_PRIVATE_KEY_PASSWORD through its secret-handling mechanism; do not put passwords in tool arguments, command arguments or committed client config. Configure the client to launch this profile with an absolute vault path:

{
  "mcpServers": {
    "jacs": {
      "command": "jacs",
      "args": ["mcp", "--profile", "local-sign", "--agent", "/absolute/path/.jacs/agent.json"]
    }
  }
}

Local signing enables seven portable tools for the selected vault. Creation and encrypted import require an empty vault and refuse to overwrite an identity. Rotation retains the current password unless the host supplies JACS_NEW_PRIVATE_KEY_PASSWORD; re-encryption requires that new password at startup. The host owns approval of that process-level grant. Unix vault permissions are enforced; platforms without the required filesystem custody adapter can still verify public data.

For agreements and text/images, choose the separate extended MCP profiles. For service integration, use the direct libraries.

Call jacs_sign_document with {"document":"{\"message\":\"hello\"}"}. Its parsed text response contains a document object and algorithm; serialize that object to a JSON string for subsequent verification. Check the active tools/list inventory before calling lifecycle tools.

Portable and extended MCP schemas differ: the extended signer accepts content and returns signed_document; its verifier takes raw key bytes as a JSON array. Use the selected server's contract rather than reusing arguments across them. For other protocols, follow the A2A how-to or W3C DID how-to.