API Update: New Comment Log for Supplier Invoices

In September 2026, we are launching a dedicated and structured log for comments on supplier invoices in the Fortnox API.

The new solution replaces the previous Comments field with an append-only log, where each entry receives a timestamp, the author's name, and a unique UUID. The previous Comments field will be deprecated.

CommentLog Included in Invoice Responses

Every response returning a full supplier invoice object — such as GET, POST, and PUT (for creating, cancelling, bookkeeping, etc.) — now includes the CommentLog array nested alongside the legacy Comments field.

For full details on payload structures, please refer to our API specification.

Backward Compatibility & Field Separation

Legacy Field Preservation: Any text historically stored in the legacy Comments field will continue to appear in that field and can still be updated using PUT as usual.
Strict Field Separation: The legacy single-comment is preserved in its original Comments field but is not surfaced in the new CommentLog array. Legacy comments and the new structured comment log live entirely independently of each other.

Fortnox UI Behavior & Critical Impact on Integrations

To ensure a smooth transition, it is vital to understand how the Fortnox web application interacts with these fields and how it affects your integration:

Fortnox UI Behavior: Within the Fortnox web interface, all new comments added to supplier invoices will exclusively be saved to the new CommentLog. However, the Fortnox UI will still display legacy comments from the old Comments field to preserve historical context for the user.
The Risk of Missing Comments: Because the Fortnox UI only writes to the new log, integrations that have not been updated to read the CommentLog array will fail to display any new comments written by users inside Fortnox.
Hybrid Workflows: This is especially critical for customers who work in both the Fortnox web interface and your integrated system. If your integration is not updated, comments added in Fortnox will be missing on your side, potentially leading to communication gaps and double-work.

Do you need to adjust anything? Yes. To ensure your users get a consistent and complete view of all invoice communication, your integration needs to be updated to read from the new CommentLog array. We highly recommend that you implement the update during the second week of October.