Fabric Graph - Query Set - REST API - Create GraphQuerySet

Thanks for sharing the details , I’ve run into something very similar while working with the Fabric Graph APIs. Based on what you’ve described, your observations are on point.

The behavior where an “empty” Query Set can be created successfully, but fails when a definition object is included (with only a generic "Unknown" error), usually indicates that the endpoint is not fully implemented or still evolving, particularly around the definition payload.

A few things to keep in mind:

• The GraphQuerySet APIs (especially definition-related operations) are still relatively new and somewhat immature compared to Semantic Models or Reports.
• The "Unknown" error is unfortunately common when the backend validation for the definition object fails silently.
• In many cases, even slight deviations from the expected schema (including missing required nested properties or unsupported fields) result in this generic failure.
• Unlike other Fabric item APIs, the error messaging and validation here are not yet as robust, which makes troubleshooting difficult.

What I recommend trying next:

• Compare your payload strictly against the example in the official docs and ensure no extra properties are included.
• Start with the simplest possible definition (minimal valid structure) and then incrementally add components.
• Validate whether the same definition works when created through the UI (if available) , this helps confirm if it’s an API limitation vs payload issue.
• Capture the requestId/activityId from the response and, if possible, check Fabric logs (or raise a support ticket) , this is often the only way to get more meaningful diagnostic information.

Reality check:
Based on current behavior, it does look like the definition part of this API is not yet fully stable/production-ready, even though documentation exists.

If this is blocking your scenario, you may want to:

• Create the Query Set first (empty via API), and
• Apply/edit the definition through UI or alternative supported flows (if available),
  until the API matures further.

Hope this helps , you’re not alone in seeing this behavior.