Touchstone

Local MCP server for Touchstone — record what your agent did into a tamper-evident, externally-anchored log.

This server runs on your machine and holds your Ed25519 signing key. It signs each event locally and appends it to your recorder, so an agent only has to call touchstone_record({ event_type, payload }). The key never leaves this process. Canonicalization (JCS / RFC 8785) is done locally too, so a malicious or compromised server can't trick you into signing a different commitment than you intended.

Zero dependencies — Node 18+ built-ins only. It's a single file: read it before you trust it.

The remote MCP at https://touchstone.cv/mcp can't sign for you (Touchstone never holds your key), so its touchstone_record expects a signature you computed yourself. Run this server when you want frictionless local signing.

Install

# one-off, no install:
npx -y @touchstone-cv/mcp

# or vendor the single file:
curl -O https://touchstone.cv/touchstone-mcp.mjs

# or clone:
git clone https://github.com/Touchstone-CV/touchstone-mcp && cd touchstone-mcp

Configure

Point your MCP client at it over stdio:

{
  "mcpServers": {
    "touchstone": {
      "command": "npx",
      "args": ["-y", "@touchstone-cv/mcp"],
      "env": {
        "TOUCHSTONE_RECORDER": "rec_...",
        "TOUCHSTONE_SUBJECT": "<your-colony-sub>",
        "TOUCHSTONE_API_KEY": "tsk_...",
        "TOUCHSTONE_SIGNING_KEY": "<base64 Ed25519 32-byte seed>"
      }
    }
  }
}

Env var	Required	Meaning
TOUCHSTONE_RECORDER	yes	Your recorder public id (rec_…)
TOUCHSTONE_SUBJECT	to record	Your Colony sub — the recorder's subject
TOUCHSTONE_API_KEY	yes	API key minted on the recorder (tsk_…)
TOUCHSTONE_SIGNING_KEY	to record	base64 Ed25519 32-byte seed — kept by you, never sent
TOUCHSTONE_KEY_FILE	alt	Path to JSON {"seed_b64":"…"} instead of the inline seed
TOUCHSTONE_BASE_URL	no	Defaults to https://touchstone.cv

To get a recorder + key, see touchstone.cv/developers — agents can self-provision one with their own Colony token (OAuth Token Exchange, RFC 8693), no browser required.

Tools

Tool	What it does
touchstone_record	JCS-canonicalizes payload, signs the commitment locally, appends the entry
touchstone_disclose	Create a shareable /d/<token> disclosure link (proxies to the service)
touchstone_verify	Verify a disclosure bundle (proxies to the service)
touchstone_recorder_info	Fetch your recorder's public info / checkpoint state

Only touchstone_record uses your signing key; the rest proxy to the remote service over your API key.

Selective field disclosure

Call touchstone_record({ event_type, payload, selective_disclosure: true }) to commit each payload field separately — the client computes a salted-field Merkle root locally and signs that as payload_hash, storing the per-field salts. Later you can reveal only a subset:

touchstone_disclose({ seqs: [n], reveal: { n: ["field_a", "field_b"] } })

Revealed fields ship with Merkle proofs against payload_hash (which your signature already covers); withheld fields are salt-bound and their values never appear in the disclosure. The root computation matches the server and the verifiers byte-for-byte.

Verifying the log

A disclosure can be checked by anyone, with no trust in Touchstone — in the browser verifier, the standalone verify.php, or the gossip_check.py split-view checker. Those tools are served from the site (and are each a single auditable file); this repo is just the recording client.

License

Apache-2.0.