How do I add the BNBCalc MCP server to Claude Code, Claude Desktop, Cursor or VS Code?

What do I need before I start?

Two things: an MCP key from BNBCalc, and an AI app that can send that key in a header with each request. Claude Code, Claude Desktop, Cursor and VS Code all can, so pick whichever you already use.

1. In BNBCalc, open Settings → MCP and click Subscribe, then enter your payment details at checkout. There's no monthly fee, and nothing's charged until you make paid calls.
2. Click Create New Key. The key starts with mcp_, and BNBCalc only shows it in full once, so make sure to copy it somewhere safe before you leave the page.
3. Under Connect a client, pick your app. BNBCalc fills your new key into the setup snippet, so the quickest route is to copy it from there.

Every snippet below uses the same three pieces: the server address https://atlas.bnbcalc.com/mcp, the header name x-bnbcalc-mcp-key, and your key where it says YOUR_MCP_KEY. It has to be your MCP key, because a BNBCalc API key won't work here, and you can use the same key in more than one app. If you ever regenerate it, update every app that uses it.

How do I add the BNBCalc MCP server to Claude Code?

Run this in your terminal, with your key in place of YOUR_MCP_KEY:

claude mcp add --transport http bnbcalc --scope user \
  https://atlas.bnbcalc.com/mcp \
  --header "x-bnbcalc-mcp-key: YOUR_MCP_KEY"

--scope user makes BNBCalc available in every project. Leave it out and Claude Code adds the server to the current project only, which is its default. To check it worked, run claude mcp list, or type /mcp inside a session, and look for bnbcalc with a connected status.

How do I add the BNBCalc MCP server to Claude Desktop?

Claude Desktop can't send a custom header on its own, so its config runs a small bridge called mcp-remote through npx. That means you'll need Node.js 18 or newer installed first.

Open claude_desktop_config.json, which lives in ~/Library/Application Support/Claude/ on a Mac and in %APPDATA%\Claude\ on Windows, and add this. If the file already lists other servers, put the bnbcalc block inside the existing mcpServers section instead:

{
  "mcpServers": {
    "bnbcalc": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://atlas.bnbcalc.com/mcp",
        "--header",
        "x-bnbcalc-mcp-key:${BNBCALC_MCP_KEY}"
      ],
      "env": {
        "BNBCALC_MCP_KEY": "YOUR_MCP_KEY"
      }
    }
  }
}

Leave out the space after the colon in x-bnbcalc-mcp-key:${BNBCALC_MCP_KEY}, since a space there breaks on Windows. Then don't forget to quit Claude Desktop completely and open it again, since it only picks up new servers on a fresh start.

How do I add the BNBCalc MCP server to Cursor?

Add this to ~/.cursor/mcp.json to use BNBCalc in every project, or to .cursor/mcp.json inside one project:

{
  "mcpServers": {
    "bnbcalc": {
      "url": "https://atlas.bnbcalc.com/mcp",
      "headers": {
        "x-bnbcalc-mcp-key": "YOUR_MCP_KEY"
      }
    }
  }
}

Save the file and restart Cursor. You'll find bnbcalc in Cursor's MCP settings with a switch to turn it on, and if it won't connect, the MCP Logs output panel says why.

How do I add the BNBCalc MCP server to VS Code?

Open your user MCP configuration with the MCP: Open User Configuration command and add this:

{
  "servers": {
    "bnbcalc": {
      "type": "http",
      "url": "https://atlas.bnbcalc.com/mcp",
      "headers": {
        "x-bnbcalc-mcp-key": "YOUR_MCP_KEY"
      }
    }
  }
}

The same entry works in a project's .vscode/mcp.json, but since that file sits inside your project, keep it out of version control so your key isn't committed with your code. Run MCP: List Servers to start BNBCalc and check that its tools appear in Copilot's agent mode.

Can I use other MCP apps?

Yes, as long as the app can reach a remote server over streamable HTTP and send a custom header. The BNBCalc server only takes POST requests and doesn't keep sessions, so an app that insists on the older SSE connection won't work. Cline, for example, needs "type": "streamableHttp" in its config, because without it Cline falls back to SSE.

How do I know it's working?

Ask your assistant something free first, such as "What's my BNBCalc MCP usage this month?" or "Calculate the DSCR on $36,000 of operating income and a $400,000 loan at 7% over 30 years." If BNBCalc answers, you're connected, and nothing's been charged.