Mjucius Cozi

Cozi MCP Server

An unofficial Model Context Protocol server that lets AI assistants like Claude read and update your Cozi Family Organizer lists and calendar.

Each user runs their own instance against their own Cozi account. Your credentials are stored in your MCP client's secure config (Claude Desktop's OS keychain or your local environment) and never leave your machine — the author of this server has no access to your data.

Install

1. MCPB (recommended for Claude Desktop)

Download the latest .mcpb from the Releases page and double-click to install in Claude Desktop. You'll be prompted for your Cozi username and password — they're stored securely in your OS keychain.

This path requires no Node, npm, or Python install on your machine.

2. npx (any other MCP client)

Add this to your Claude Desktop claude_desktop_config.json (or any other MCP client config file):

{
  "mcpServers": {
    "cozi": {
      "command": "npx",
      "args": ["-y", "@mjucius/cozi-mcp"],
      "env": {
        "COZI_USERNAME": "[email protected]",
        "COZI_PASSWORD": "your-password",
        "COZI_READ_ONLY": "true"
      }
    }
  }
}

Requires Node 20+. The package will be downloaded on first run.

Set COZI_READ_ONLY=true to expose only read operations. In read-only mode, the server registers family_members, get_lists, get_list_items, get_calendar, search_calendar, and today; tools that create, update, or delete Cozi data are hidden from MCP clients. Omit the variable, or set it to any value other than 1, true, yes, or on, for the default read-write tool surface.

Troubleshooting

Tools fail with "Authentication failed" even after I corrected my password

COZI_USERNAME and COZI_PASSWORD are read once, when the server process starts. Editing them in your MCP client's settings updates the stored config but does not reach a server that is already running — it keeps presenting the old credentials until it is respawned.

Fully quit and relaunch your MCP client (on macOS, Cmd-Q rather than closing the window). Reloading, reconnecting, or toggling the extension off and on is usually not enough.

How do I tell which version is actually running?

The running server advertises its version in the MCP handshake; your client displays it (Claude Desktop: Settings > Extensions). That is independent of what is in your working tree — installing an MCPB installs whatever is inside the .mcpb file, which is only as current as the last npm run bundle:mcpb. Build the bundle immediately before installing it, or install from a GitHub release.

Repeated failures start returning "Too many failed login attempts"

Five consecutive failed authentications for a username trigger an exponential backoff, capped at 15 minutes; the message states the remaining wait. The counter lives in process memory, so restarting the client clears it.

Trust and Security

Cozi has no OAuth — username/password authentication is the only way the API supports. This server handles that fact honestly:

• Per-user, by architecture. Each user runs their own instance against their own Cozi account. There is no shared backend, no proxy, no multi-tenant database. The author of this server never sees anyone's credentials or data.
• Credentials live only in your MCP client's secure config. Claude Desktop stores them in your OS keychain. The npx path reads them from environment variables you set yourself. Nothing is logged, written to disk by this server, or sent anywhere except https://rest.cozi.com.
• API surface is constrained. This server only contacts rest.cozi.com for the same endpoints the Cozi web app uses (auth, lists, calendar, family members). The full request/response code lives in src/cozi/ — about 500 lines of TypeScript you can audit yourself.
• Open source, MIT licensed. Pin a specific version (@mjucius/[email protected]) if you want a stable target, or fork the repo and run your own build if you want zero supply-chain trust.

Security & trust model

This is a single-user server by design. The Cozi credential holder is the principal — there is no separate per-caller authentication gate, because each user runs their own instance against their own Cozi account. Concretely:

• stdio (npx / MCPB) trusts the local user. Whoever can launch the process and read the configured COZI_USERNAME / COZI_PASSWORD (or the OS keychain entry) is treated as the account owner. The trust boundary is your machine and its user account.
• No multi-tenancy. Each user configures their own instance with their own Cozi account. There is no shared backend, no tenant isolation to breach, and no per-caller authentication beyond possession of valid Cozi credentials — which is the same access model as the Cozi web app itself.

Two defensive measures narrow the blast radius of that model:

• Time-bounded credential cache. Authenticated clients are cached only for a bounded lifetime, so a rotated or revoked Cozi password stops working rather than being honored indefinitely from a stale cached session.
• Failed-login rate limiting. Repeated failed authentication attempts are rate-limited to blunt credential-guessing against the Cozi endpoint.

Tools

The server exposes 15 tools by default, or 6 read-only tools when COZI_READ_ONLY=true (or the MCPB read-only setting) is enabled. Returns are slim dicts with null/empty fields omitted.

Family

• family_members() → [{id, name, color?}] — call this first to get attendee IDs for appointments.

Lists

• get_lists(list_type?) → [{id, title, type, item_count, completed_count}] — list_type is optional, 'shopping' or 'todo'.
• get_list_items(list_id, include_completed=false) → [{id, text, status, position?, header?}] — header: true marks a section heading (e.g. "PRODUCE"), not an item.
• create_list(name, list_type) → {id, title, type}.
• rename_list(list_id, title) → {id, title, type} — the items stay as they are.
• delete_list(list_id) → boolean.

create_list, rename_list and delete_list are hidden in read-only mode.

Items

• add_item(list_id, text, position=0) → {id, text}.
• update_item(list_id, item_id, text?, completed?) → {id, text, status} — pass either or both. Non-atomic when both are passed: the text is updated first, then the status.
• remove_items(list_id, item_ids) → boolean.

All item tools are hidden in read-only mode.

Calendar

• today() → {date, weekday, time, timezone} — the current local time on your machine. Every tool result also ends with this as a separate line, so the assistant resolves "tomorrow" against your clock rather than its own (often UTC). Set the standard TZ env var (e.g. TZ=America/New_York) if the server runs somewhere other than your own computer.
• get_calendar(year, month) → [{id, subject, day, all_day, start?, end?, end_day?, attendees?, location?, notes?, reminders?, kind?, read_only?, source?, recurring?, recurrence?, series_start?, series_end?, skipped_days?}]. day is the day the entry happens; end_day appears only on multi-day events, which Cozi lists in every month they overlap (so day may fall outside the month you asked for). A recurring series is listed once per occurrence in the month, on the days Cozi itself expands it to: each entry has the series id, its own day (and end_day when the occurrence spans days), series_start, and recurrence (Cozi's wording of the rule). Occurrences deleted in the Cozi app are left out and named in skipped_days; a series with no occurrence left in a month is not listed for that month. kind marks non-appointments such as birthdays; read_only items (holidays, subscribed calendars, named by source) cannot be changed. reminders are minutes before the start.
• search_calendar(query) → {appointments: [...same fields, last_day?, next_days?], more?} — Cozi's own search across months. A recurring series reports its latest past occurrence and the next ten. Cozi searches from a few years back onward, and a brand-new appointment can take a few seconds to show up.
• create_appointment(subject, start, end, attendees?, all_day=false, notes='', location?, repeat?, reminders?, notify?) — start and end are ISO datetimes (e.g. '2026-06-15T10:00:00'). For all-day events (all_day=true) a bare date (e.g. '2026-06-15') is also accepted and end may equal start; a bare date on a timed event is an error. Put end on a later date for a multi-day event; the result reports the span as end_day. An end before start is an error. repeat makes it a series: {frequency: 'daily'|'weekly'|'monthly'|'yearly', interval?, weekdays?, monthly_by?: 'day_of_month'|'weekday', until?} (no repeats forever). are minutes before the start, e.g. .

Holidays, subscribed calendars and birthdays are refused by update_appointment and delete_appointment. notify=true on any calendar write sends Cozi's change notification to the attendees who have notifications on (like the Cozi app's "notify" option); it is off by default.

create_appointment, update_appointment, and delete_appointment are hidden in read-only mode.

Workflow tip

When creating or updating appointments with specific attendees, call family_members() first and use those id values in the attendees arg. Calendar tools are scoped to a (year, month) page — pass the same year/month back when updating or deleting an appointment from that page.

Migration from v1 (Python)

v2.0 is a Node/TypeScript rewrite of the previous Python implementation, distributed as MCPB and npx. The runtime changed AND the tool surface was consolidated — if you have prompts written against v1, update them as follows:

The legacy v1 Python source is preserved at git tag v1.0.0 for reference.

Development

Requires Node 20+ (see .nvmrc).

nvm use
npm install
npm test               # vitest, mocked at the CoziClient boundary; no credentials
npm run test:live      # live conformance suite against real Cozi (creds.env); required before a release
npm run typecheck
npm run build          # tsup → dist/
npm run dev            # local stdio dev with COZI_USERNAME / COZI_PASSWORD env vars
npm run bundle:mcpb    # produces cozi-mcp.mcpb at repo root

The repo layout:

cozi_mcp/
├── src/
│   ├── server.ts              # MCP server factory
│   ├── bin.ts                 # npx + MCPB stdio entry point
│   ├── instructions.ts
│   ├── cozi/                  # Inlined Cozi HTTP client (no separate npm package)
│   └── tools/                 # 13 MCP tools
├── tests/                     # vitest, mocks CoziClient at the boundary
├── manifest.json              # MCPB manifest (Claude Desktop)
└── package.json

The Cozi HTTP client is inlined under src/cozi/ rather than published as a separate npm package — it's small, only useful for this MCP server, and avoids the supply-chain surface area of a separate dependency. If you'd prefer the Python equivalent for your own projects, see py-cozi-client.

Acknowledgments

• The ?apikey=coziwc|v…_production requirement on the Cozi auth endpoint was reverse-engineered from the live my.cozi.com web bundle by Wetzel402/py-cozi PR #3. Without that discovery, every login attempt from a server environment fails with a misleading 401 regardless of credential validity.
• Built on the Model Context Protocol and its TypeScript SDK by Anthropic.

Trademark and affiliation

Cozi and the Cozi logo are trademarks of Cozi Group Inc. This project is unofficial and not affiliated with, endorsed by, or sponsored by Cozi Group Inc. Use of the Cozi API is at your own risk and subject to Cozi's Terms of Service.

License

MIT — see LICENSE.

Contributing

PRs welcome. Please run npm test and npm run typecheck before submitting.