.NET 10 JSON Console Logging: Stop Parsing State.Message

The .NET 10 JSON console logging change is small enough to miss during an upgrade: the formatted message still exists, but a typical record no longer duplicates it at State.Message . A collector, script, or snapshot test that reads only that nested property can start returning null while the application continues logging normally. I treat console JSON as a schema whenever another process parses it. That means a runtime upgrade deserves a contract test, not just a visual check in a terminal. The practical fix is to read the top-level Message , keep State for structured values, and retain a narrow fallback for older records. Why .NET 10 JSON console logging breaks nested-message parsers Before .NET 10, a normal AddJsonConsole record commonly repeated the rendered text: { "Message" : "Order 42 moved to ready." , "State" : { "Message" : "Order 42 moved to ready." , "OrderId" : 42 , "Status" : "ready" , "{OriginalFormat}" : "Order {OrderId} moved to {Status}." } } In .NET 10, the typical shape keeps one rendered message at the top level: { "Message" : "Order 42 moved to ready." , "State" : { "OrderId" : 42 , "Status" : "ready" , "{OriginalFormat}" : "Order {OrderId} moved to {Status}." } } Microsoft documents this as a behavioral breaking change and recommends that parsers use the top-level property. The official compatibility note also gives an essential caveat: State.Message may still appear when its content differs from the top-level value. I therefore do not reject a record merely because both properties exist. This is not a loss of structured logging data. OrderId , Status , and {OriginalFormat} remain useful fields inside State . The part that changed is where a consumer should get the rendered sentence. Prefer the top-level Message and keep State structured A legacy-only extractor is brittle because it assumes the duplicate is the contract: static string ? ReadLegacyOnly ( JsonElement root ) => root . TryGetProperty ( "State" , out var state ) && state . TryGetPro