MCP in Claude Desktop

Configure the Claude macOS / Windows app to call the Onto MCP server. Standard mcpServers schema in claude_desktop_config.json, then restart Claude.

ℹ

Looking for Claude Code (the CLI)? See MCP in Claude Code — it's a separate product with its own config command.

Install steps

Get an API key

Grab a key from app.buildonto.dev/read/keys. Free tier: 1,000 requests / month.

Open the developer config

In Claude Desktop: Settings → Developer → Edit Config. This opens (or creates) claude_desktop_config.json in your default editor.

Add the Onto server

jsonclaude_desktop_config.json

{
  "mcpServers": {
    "onto": {
      "command": "npx",
      "args": ["-y", "@ontosdk/mcp@latest"],
      "env": {
        "ONTO_API_KEY": "onto_sk_live_your_key_here"
      }
    }
  }
}

Fully quit and restart Claude

On macOS, the menu-bar close button only hides the window — Cmd+Q is required to fully quit and pick up MCP config changes. On Windows, close from the system tray, not just the window.

Config file paths

OS	Path
macOS	~/Library/Application Support/Claude/claude_desktop_config.json
Windows	%APPDATA%\Claude\claude_desktop_config.json

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The file may not exist before you open Settings → Developer → Edit Config for the first time. Claude creates it on demand. If you'd rather create it manually, the parent directory exists from Claude's first launch.

Claude Desktop vs Claude Code

Two different Anthropic products, same vendor. Claude Desktop is the macOS / Windows desktop app (chat UI); Claude Code is the CLI / IDE extension. Both speak MCP but they have different config mechanisms:

Product	How to add an MCP server
Claude Desktop	Edit claude_desktop_config.json directly
Claude Code	Run claude mcp add or edit ~/.claude.json

Verify

After restart, look for the tools icon (slider/hammer) in the input area at the bottom of the Claude window. Clicking it should list all six Onto tools: read_url, score_url, read_and_score, batch, map_site, and extract_data.

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Try it: "Use Onto to read https://stripe.com/pricing and tell me what tiers they offer."