Otis

Otis MCP

The source code of Otisbase, a free MCP server for client billing, built for consultants and small agencies.

Otis has no app or dashboard. Your AI assistant is the interface: it records clients, work, expenses and payments in Otis, and issues the invoices you approve. Otis stores the records and enforces the rules. It numbers invoices, calculates totals, requires your exact total before it issues an invoice, and never rewrites an issued invoice. Otis runs no AI model; every result comes from the records and the rules in this repository.

This repository is the complete service that runs at mcp.otisbase.com: the MCP server, email sign-in and the storage layer. It runs on Cloudflare Workers. The otisbase.com website is maintained separately.

Use the hosted service

Add this URL as a remote MCP server in your assistant:

https://mcp.otisbase.com/mcp

Sign-in uses OAuth with an email code, so there is no API key. How to install has a guide for Claude, ChatGPT, Claude Code, Codex, Cursor, Muse, Grok Bot and Mistral Vibe. The hosted service is free. Records are stored in the EU.

• MCP docs: the endpoint, the rules and every tool.
• Otis Plugins: the plugin for Claude and Codex: a menu and nine workflow skills.
• Skills: what each skill does.

What Otis records

Area	Records
Business	Business details, tax settings and invoice numbering, used by its owner
Clients	Clients and their billing details
Work	Billable time, services, reported AI usage and expenses
Invoices	Drafts, issued invoices with immutable contents, discounts and tax
Payments	Full and partial payments, with references that prevent duplicates
Export	A complete export of the business records

How it works

Part	Implementation
Runtime	One Cloudflare Worker (src/index.ts)
MCP server	@modelcontextprotocol/server over Streamable HTTP at /mcp (src/mcp.ts)
Sign-in	@cloudflare/workers-oauth-provider with an email code (src/auth.ts)
Business records	SQLite in one Durable Object per business (src/business-store.ts)
Accounts and sign-ins	Durable Objects (src/account-store.ts, src/sign-in-store.ts)
OAuth state	Workers KV
Email	Cloudflare Email Sending, for sign-in codes and security notices
Data location	Durable Objects in the EU jurisdiction (src/regions.ts)
Sign-in pages	Server-rendered HTML with its own small stylesheet (src/html.ts, src/shell.ts)

Every business operation checks the caller's current membership. Amounts are integers in minor units. Writes take a request ID, so a retried call does not record the same fact twice.

Repository layout

Path	Contents
src/	Worker, MCP tools, sign-in, storage, billing rules and public docs
test/	Integration tests in Cloudflare's local runtime
scripts/	Live endpoint checks, sign-in check and the plugin generator
spec/	The product specification: scope, billing rules, service and design

Read the spec before you change behavior. Start with spec/01-README.md. The spec wins over the decision log.

Develop

You need Node 24 or newer.

npm ci
npm run check
npm test

Tests run in Cloudflare's local runtime. Email delivery is simulated in tests.

To run the Worker locally:

npm run dev -- --local --ip 127.0.0.1 --port 8788

Then open http://127.0.0.1:8788/health. The local runtime uses local storage only.

npm run plugins:build generates the Claude and Codex plugins into plugins/ from src/workflows.ts. They are published to otisbase/otis-plugins.

Self-host

You can run your own copy on a Cloudflare account. Edit wrangler.jsonc before you deploy:

1. Set account_id to your Cloudflare account.
2. Create a KV namespace for OAuth state and put its ID in kv_namespaces. For EU storage, create it in the EU jurisdiction.
3. Replace the two routes with your own domains.
4. Set the vars:
   • ORIGIN: the URL of the MCP host.
   • SITE_ORIGIN: the URL of your website. Website pages opened on the MCP host redirect there, and the sign-in pages link to its Terms and Privacy Policy.
   • EMAIL_FROM: the sender for sign-in codes. The domain must be set up for Cloudflare Email Sending.
   • FEEDBACK_TO: the address that receives feedback sent through the feedback_send tool.
   • DO_JURISDICTION: eu keeps Durable Objects in the EU. Remove it to let Cloudflare choose.
   • SIGN_IN_ENABLED: true to allow sign-in.
5. Set ratelimits.namespace_id to an ID that is unique in your account.

Then deploy:

npx wrangler login
npm run deploy

The sign-in pages and emails name Otisbase. Change them in src/html.ts, src/auth.ts and src/notify.ts, and publish your own terms and privacy policy, before you offer your copy to other people.

Contribute

Bug reports, fixes and ideas are welcome. See CONTRIBUTING.md. To report a security problem, see SECURITY.md. You can also send feedback from your assistant: ask it to send feedback to Otis.

License

The code is released under the MIT License.

These items are not covered by the MIT License:

• The Otisbase and Otis names and logos.
• The Geist fonts in src/fonts/, under the SIL Open Font License.

The copy and check icons in src/html.ts come from Phosphor Icons (MIT).