How to Write JSON Logs in ASP.NET Core

Plain-text logs are nice to read, and hard for a log tool to search. AddJsonConsole writes one JSON object per entry, with each value in a field of its own. Keep plain text in Development, and use JSON everywhere else.

What You Need

The .NET 10 SDK, and a small project: dotnet new web --no-https -n WebDemo. It's built in.

Step 1: Turn It On Outside Development

Replace Program.cs:

Program.cs

var builder = WebApplication.CreateBuilder(args);

// One JSON object per log entry outside Development, for log tools to read.
if (!builder.Environment.IsDevelopment())
{
    builder.Logging.AddJsonConsole(options =>
    {
        options.UseUtcTimestamp = true;
        options.TimestampFormat = "yyyy-MM-ddTHH:mm:ss.fffZ";
    });
}

var app = builder.Build();

app.MapGet("/orders/{id:int}", (int id, ILogger<Program> logger) =>
{
    logger.LogInformation("Looking up order {OrderId}", id);
    return id;
});

app.Run();

• IsDevelopment leaves your own terminal readable.
• UseUtcTimestamp keeps every server on one clock.
• TimestampFormat adds a Timestamp field, which isn't there by default.

Step 2: Compare the Two

With no launch profile and no ASPNETCORE_ENVIRONMENT, the app runs as Production:

Terminal 1

dotnet run --no-launch-profile --urls http://localhost:5300

Send a request to http://localhost:5300/orders/42, and the log says:

Output, as Production

{"Timestamp":"2026-11-06T06:24:41.702Z","EventId":0,"LogLevel":"Information","Category":"Program","Message":"Looking up order 42","State":{"OrderId":42,"{OriginalFormat}":"Looking up order {OrderId}"}}

As Development, in plain text:

Output, as Development

info: Program[0]
      Looking up order 42

Look at State: the value from {OrderId} has a field of its own, as a number. So a log tool can find State.OrderId = 42, which is what you want from structured logging. That only works if you log with named placeholders. With $"…{id}", there's nothing to separate.

Do It in Configuration Instead

Microsoft's documentation shows the same through appsettings.json:

appsettings.json

"Logging": {
  "Console": {
    "FormatterName": "json",
    "FormatterOptions": {
      "UseUtcTimestamp": true,
      "TimestampFormat": "yyyy-MM-ddTHH:mm:ss.fffZ"
    }
  }
}

That applies in every environment, so Development has to switch back, with "FormatterName": "simple" in appsettings.Development.json. When I did that, the timestamp options carried over into the plain logs, and the timestamp stuck to the log level. Setting it in code, once, is simpler.

Keep Secrets Out

JSON makes logs easier to search, including for whoever reads them later. So:

• Never log a connection string. How to Log the Database Settings When an ASP.NET Core App Starts logs what you need without it.
• Keep EnableSensitiveDataLogging off, so that EF Core logs @p0='?' and not the values.
• Don't log people's names and emails. I searched a production run's log for a customer's email, and for each password: none were there.

Common Mistakes

Mistake	What happens	Do this instead
Interpolating with $"…" in a log call	No fields, and an analyzer warning	Use named placeholders
JSON in Development too	Your terminal's hard to read	Check the environment
Assuming Timestamp is there	It isn't, without TimestampFormat	Set it

Troubleshooting

Symptom	Likely cause	Fix
Plain text in production	The environment is Development	Check it
' in messages	The JSON writer escapes ' by default	Valid JSON: tools read it correctly
CA1848	The analyzer wants LoggerMessage	The fix
2026-11-06T07:50:17.723Zinfo:	A timestamp format with no space, in plain logs	The fix

Conclusion

builder.Logging.AddJsonConsole outside Development gives log tools a field for each value, and leaves your own terminal readable.

Additional Resources

• Console log formatting — the JSON formatter and its options.
• Logging in .NET — categories, levels and placeholders.

Related

• How to Log with the LoggerMessage Source Generator in .NET C#
• How to Set Different Log Levels per Environment in appsettings.json
• How to Log the Database Settings When an ASP.NET Core App Starts