Pennylane vs QuickBooks Online API + MCP

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.

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.

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

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