Worked on the Altinn/altinn-storage repository to enhance API reliability by addressing a critical issue in the InstancesController Post method. Focused on backend development using C#, the work involved sourcing the appId parameter directly from the query string and implementing robust input validation to ensure non-empty values. The validation logic in GetApplicationOrErrorAsync was tightened to throw errors for null or whitespace appId inputs, improving error handling. Additionally, unit tests for instance creation were refined to submit only required data, reducing brittleness and increasing test resilience. These changes contributed to more stable and predictable API instance creation flows.