What is System.Text.Json and how does it compare to Newtonsoft.Json?

System.Text.Json is .NET's built-in, high-performance JSON library (added in .NET Core 3.0) designed to be allocation-efficient with Span-based parsing. Newtonsoft.Json (Json.NET) is the long-standing third-party library with a much richer feature set. Comparison: Performance: System.Text.Json is 2-3× faster and uses far less memory. AOT/trimming: System.Text.Json has full source generator support for AOT; Newtonsoft relies on reflection. Features: Newtonsoft has more features (property name remapping case-insensitive by default, reference loops, constructor-based deserialization, LINQ to JSON, JSON path). System.Text.Json is strict by default (case-sensitive, throws on unknown properties). Migration gotchas: null handling, enums, datetime formats, property casing differ. When to use each: new .NET 6+ projects → System.Text.Json. Complex scenarios needing rich JSON manipulation, custom converters, or Newtonsoft-specific behaviors → Newtonsoft. ASP.NET Core uses System.Text.Json by default.