.NET / EF Core / AutoMapper causing PostgreSQL "invalid input syntax for type json" error when saving entity

You could use the following function to validate JSON.

private bool IsValidJson(string? json)
{
    if (string.IsNullOrWhiteSpace(json))
        return false;
    var utf8 = Encoding.UTF8.GetBytes(json);
    var utf8Reader = new Utf8JsonReader(utf8);
    if (!JsonDocument.TryParseValue(utf8Reader, out var doc))
        return false;
    doc?.Dispose();
    return true;
}

Then

    .ForMember(dest => dest.RuleJson,
               opt => opt.MapFrom(src => IsValidJson(src.RuleJson) ? src.RuleJson : null))

If you already have a JsonDocument then you should pass that instead, rather than stringifying it, as noted in the documentation.

Note that in your model you should specify the column type for jsonb

    [Column(TypeName = "jsonb")]
    public string? RuleJson { get; set; }  // jsonb column in PostgreSQL