Once connected in claude.ai, you can ask things like"what's my Monzo balance?","list my recent transactions", or"how much is in my savings pots?"in any chat — web, desktop, or phone. Authenticate once; it follows you everywhere. No laptop, no local server, nothing to keep running.

Read-only by design: no payments, transfers, or pot movements — the tools simply don't exist, so no amount of prompt injection can move money.

Claude (claude.ai) ──OAuth──> This Worker ──OAuth──> Monzo API │ workers-oauth-provider (KV) + McpAgent (Durable Object)

Worth understanding before you deploy — the design isolates users by construction:

- No standing bank credential in the Worker.The only Worker-level secrets are the Monzo OAuthclient_id/client_secret, which can exchange authorization codes but cannot read any account data by themselves.
- Tokens live per-grant, not per-Worker.When a user completes the OAuth dance, their Monzo tokens are stored encrypted insidetheirgrant (completeAuthorization({ props: ...tokens })). Every MCP request is resolved through the caller's bearer token totheirgrant,theirDurable Object,theirMonzo account. There is no request that reaches another user's data.
- Unauthenticated requests get 401.The/mcpendpoint only answers to bearer tokens the Worker itself issued.
- Monzo adds its own gates.Developer-portal OAuth clients only authorize the client owner (plus explicitly added collaborators), and every login requires approval in the Monzo app on the account holder's phone (SCA).
- Scopes are read-onlyand the payment/transfer endpoints are never called.

Residual risks are the usual ones for any connector: protect the deployed secrets, and treat your claude.ai session like the credential it is.
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Fill secretsin.dev.vars(gitignored) for local dev:

Create the KV namespaceand paste the returned id intowrangler.jsonc(replacing<YOUR_OAUTH_KV_NAMESPACE_ID>):

npx wrangler kv namespace create OAUTH_KV

Add the connector in claude.ai→ Settings → Connectors → Add custom connector →https://<your-worker>.workers.dev/mcp→ authorize. You'll be bounced through Monzo's login (magic-link email + approve in the Monzo app), then Claude shows the five tools.

claude mcp add --transport http monzo https://<your-worker>.workers.dev/mcp

- Monzo only exposes thelast 90 daysof transactions unless you re-authenticate immediately after approving access; this connector accepts that rolling window.
- Monzo developer clients aresingle-userunless Monzo approves your app for production — fine for the personal-use case this is built for.
- Secrets live in.dev.vars(local) and Wrangler secrets (prod) — never in git. The.gitignoreenforces this.

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