Developer shares practical guide to writing custom validation rules in Sanity schemas

A developer has published a detailed technical guide on building custom validation rules within Sanity CMS schemas, going beyond the basic examples found in official documentation. The guide covers common production challenges such as async uniqueness checks, cross-field conditionals, and writing clear, editor-friendly error messages. Key techniques include using Rule.custom() with async callbacks to query the Sanity client directly for duplicate-value detection, such as checking whether a barcode already exists in the database. The author highlights the often-overlooked context argument, which provides access to the current document state and a pre-configured client, removing the need for separate client imports. Practical patterns like distinguishing blocking errors from non-blocking warnings are also demonstrated through real schema examples.