FreeAgent API vs QuickBooks Online API + MCP

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.

QuickBooks Online API + MCP

Official MCP server from Intuit with 145 tools and flags to remove write, update or delete tools. Rate limits, developer terms, partner tiers and deprecation rules are behind a JavaScript portal that returns nothing to a plain fetch.

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

QuickBooks Online API + MCP

1. Store the realmId with the tokens. Every URL needs it and the token alone doesn't name the company
2. Persist the new refresh token from every refresh response. The previous one expires 24 hours later
3. Use the query endpoint with MAXRESULTS and STARTPOSITION for reads rather than fetching by id
4. Send a unique requestid query parameter on writes, so a retry returns the first response instead of posting twice
5. On 429, wait for Retry-After before retrying, and don't retry other 4xx errors