Agents (MCP)

What it does

• It operates the API. One tool per operation in the contract, with its schema as is. Every response starts with a readable line, with amounts in pesos, and carries the FCM-Request-Id.
• It helps you build the integration. It searches this documentation, gives you the official request-signing and webhook-verification examples, and brings guided commands to integrate, walk through the sandbox and prepare for production.
• It diagnoses. It checks the signature your code builds and tells you which part is wrong, verifies webhooks, checks your configuration and waits for the result of a transfer for you.

Guarantees

• Sandbox only. It talks to https://empresas-sandbox.fcmpayments.com. It does not start with the production URL or with an fcm_live_… API key.
• Your keys never leave your machine. It runs locally and every request is signed there (RFC 9421). The API key travels only to the sandbox.
• Safe retries. Writes that move money or change state require an idempotency_key: if the connection drops, the agent retries with the same key and the API returns the same response. Nothing is ever paid twice.
• No secrets in responses. The API key and private keys are masked in everything the server returns.

Access

The @grip-labs/rails-mcp package is in early access for integrators with an active sandbox: ask your contact at Grip Rails for it. It is delivered as a file with its SHA-256 fingerprint and installed with the pinned dependency tree (Node 22 or later):

shasum -a 256 grip-labs-rails-mcp-<version>.tgz      # must match the fingerprint we sent you
mkdir -m 700 grip-rails-mcp
tar -xzf grip-labs-rails-mcp-<version>.tgz -C grip-rails-mcp
cd grip-rails-mcp/package
cp npm-shrinkwrap.json package-lock.json            # installs exactly the pinned tree
npm ci --omit=dev --ignore-scripts --no-audit --no-fund
node dist/main.js --help

Create your sandbox in seconds

No need to ask us for credentials: the MCP sets up a new test entity in the sandbox, with its accounts and two CVUs (Argentine virtual account numbers). From the terminal:

node /absolute/path/grip-rails-mcp/package/dist/main.js init --alta

It generates your Ed25519 signing key pair in ~/.grip-rails/ (the private key with 0600 permissions), creates the entity with a request signed with your key, stores the API key and your key id in ~/.grip-rails/config.json (also 0600) and confirms it. At the end it shows the entity, the CVUs and the exact command to connect the MCP, with no variables needed. With --nombre "…" you give the entity a name. If something is interrupted, run it again: it resumes where it left off, with the same keys.

Or from the agent, without a terminal: connect the MCP without credentials (the first command in Connect it) and ask it to “create my sandbox”. Without credentials it starts in setup mode, with the documentation, the examples, rails_check_setup and rails_create_sandbox, which does the same as init --alta; when it finishes, the MCP switches to full mode without restarting.

Connect it

If you created your sandbox with init --alta, the credentials are already in ~/.grip-rails/config.json and no variables are needed. In Claude Code:

claude mcp add grip-rails -- node /absolute/path/grip-rails-mcp/package/dist/main.js

If you already have sandbox credentials, pass them as variables: the fcm_test_… API key, the keyid (ck_…) of your registered public key and the path to your private key, never the pasted PEM:

claude mcp add grip-rails \
  -e GRIP_RAILS_API_KEY=fcm_test_… \
  -e GRIP_RAILS_KEY_ID=ck_… \
  -e GRIP_RAILS_SIGNING_KEY=/path/to/your-private-key.pem \
  -- node /absolute/path/grip-rails-mcp/package/dist/main.js

It stays in your local configuration. Do not use --scope project with the API key: it would end up written in the repository's .mcp.json.

In Cursor (~/.cursor/mcp.json) or Claude Desktop (claude_desktop_config.json). With config.json, leave out the env block:

{
  "mcpServers": {
    "grip-rails": {
      "command": "node",
      "args": ["/absolute/path/grip-rails-mcp/package/dist/main.js"],
      "env": {
        "GRIP_RAILS_API_KEY": "fcm_test_…",
        "GRIP_RAILS_KEY_ID": "ck_…",
        "GRIP_RAILS_SIGNING_KEY": "/path/to/your-private-key.pem"
      }
    }
  }
}

First test

Ask the agent: “run rails_check_setup”. It answers listo with your entity, the clock skew and the API version. Then try a guided command.

Tools

One per API operation, with the names and schemas of the contract:

And eight helper tools:

Without credentials, there is also rails_create_sandbox: see Create your sandbox in seconds.

Guided commands

They are MCP prompts; in Claude Code they show up with /.

Coming soon

• One-line installation from npm.