First a rant: the instructions for setting up Day One for access by Claude is missing something important: how to actually set up the MCP server 😂

The whole of their instruction as of this blog post is:

In your AI assistant (for example, Claude Desktop or ChatGPT), open its MCP server / extensions settings and add the Day One MCP server.

I would expect a lot better for a tool like a journaling application used by the general public. 

Day One relies on a local command line app to provide the MCP server for whatever local LLM desktop app you’re using. Unfortunately Claude Desktop doesn’t currently have a way to add a command line tool inside of it’s settings interface and you have to edit a configuration file manually.

In Claude Desktop version 2.2553.1 released on 9/17/26 do the following:

1. Go to “Claude→Settings”.
2. Next go to “Developer” and click the “Edit Config” button.

This will open Finder to the location of claude_desktop_config.json

1. Open this file in a text editor.
2. Add the following at the very top of the file to avoid problems:

"mcpServers": { 
    "dayone-cli": { 
      "command": "/usr/local/bin/dayone", 
      "args": ["mcp"] 
    } 
  },

Save the file and restart Claude Desktop. It should now work (Day One does need to be running).