The field name of the "message" key should be msg.

• Constant values are defined for slog/log
  
• Field values are defined for the JSONHandler.Handle() implementation

Each JSON log record contains the logging level of the log statement as a string. Different handlers provide that string in uppercase or lowercase. Documentation for slog.Level says that its String() and MarshalJSON() methods will return uppercase but UnmarshalJSON() will parse in a case-insensitive manner.