FreeAgent API vs Pennylane

FreeAgent API

Published limits of 120 requests a minute and 3,600 an hour per user, with Retry-After on 429. No OAuth scopes, so a token can do anything the authorising user can.

Pennylane

The v2 API has per-resource read and write scopes, an OpenAPI definition on every reference page, llms.txt and a breaking-change process that ran over 24 weeks. Access needs a paid Essentiel plan or partner approval, creation endpoints are not idempotent, and the API agreement lets Pennylane change or revoke the API without notice.

FreeAgent API

1. Store the new refresh token from every refresh response. The old one stops working
2. Create the invoice, then PUT /v2/invoices/:id/transitions/mark_as_sent. A draft isn't visible to the customer
3. Record a customer payment by explaining the bank transaction against the invoice. There's no invoice payments endpoint
4. Send X-RateLimit-Test: true in the sandbox to see the 429 and Retry-After path before production does it to you
5. Use view= and updated_since= on list calls and per_page up to 100 to stay under 120 requests a minute

Pennylane

1. Call GET https://app.pennylane.com/api/external/v2/me first to confirm the token, user and company context
2. Stay under 25 requests per 5 seconds per token and wait for the retry-after value on a 429
3. Do not retry a create after a timeout without checking first. Only supplier invoice imports are deduplicated
4. Send filter and sort again with every cursor. The cursor stores position only
5. Send amounts as strings such as "120.00", and ask for :readonly scopes when the task only reads
6. Use the MCP server for reads only. It cannot create, edit or delete, and firm connections need company_id