Yes, from Claude Code and Claude Desktop, with a RareMargin API key. On claude.ai and the Claude mobile apps, custom connectors sign in with OAuth, which RareMargin’s MCP server doesn’t offer yet.

Not inside the ChatGPT app yet: its apps sign in with OAuth, and RareMargin’s MCP server takes an API key. OpenAI’s Codex connects to RareMargin today, reading the key from an environment variable.

It gets the same 17 tools as RareMargin’s built-in assistant: 15 read the books, and 2 draft a journal entry or an accrual that waits for a person to approve it.

So it can answer questions like these from your own books:

• What is our runway, and what changed in burn since last month?
• Which customers owe us money, and which invoices are overdue?
• Which bank lines are still unmatched, and what do they probably match?
• Did any control fail, and what is the evidence?
• Which recurring vendors haven’t billed this month? Draft accruals for them.

An admin creates one in RareMargin under Settings, API & MCP, and picks its role: a Viewer key only reads, and an Accountant key can also draft entries and accruals for approval.

The full key, which starts with ck_, is shown once, so store it in a password or secret manager. Name the key after the assistant, such as “Claude Code”. An Admin key has full access, which an assistant doesn’t need.

Run one command with your API key, and Claude Code lists RareMargin under /mcp. Add --scope user to use it in every project.

claude mcp add --transport http raremargin https://raremargin.com/mcp \
  --header "Authorization: Bearer $RAREMARGIN_API_KEY"

Claude Desktop reaches RareMargin through mcp-remote, a small bridge that runs on Node.js. Add it to claude_desktop_config.json, and keep the API key in a separate headers file.

The file is ~/Library/Application Support/Claude/claude_desktop_config.json on a Mac and %APPDATA%\Claude\claude_desktop_config.json on Windows. Add:

{
  "mcpServers": {
    "raremargin": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://raremargin.com/mcp",
        "--header-file",
        "/path/to/raremargin-headers.txt"
      ]
    }
  }
}

Put this one line in the headers file, then restart Claude Desktop:

Authorization: Bearer YOUR_API_KEY

Add RareMargin to Cursor’s mcp.json, in ~/.cursor for every project or in a project’s .cursor folder, with the API key in a header.

{
  "mcpServers": {
    "raremargin": {
      "url": "https://raremargin.com/mcp",
      "headers": {
        "Authorization": "Bearer ${env:RAREMARGIN_API_KEY}"
      }
    }
  }
}

Cursor fills in ${env:RAREMARGIN_API_KEY} from your environment, so the key stays out of the file.

Add RareMargin to ~/.codex/config.toml, and Codex sends the API key from an environment variable as a Bearer token.

[mcp_servers.raremargin]
url = "https://raremargin.com/mcp"
bearer_token_env_var = "RAREMARGIN_API_KEY"

An assistant can do only what its key’s role allows, and RareMargin’s two write tools create approval requests, never postings, so a person decides what reaches the ledger.

API keys can’t approve anything. Every proposal made over MCP waits for a person, whatever approval rules the company has set, and records that it came over MCP. Revoking the key in Settings cuts the assistant off at once.