Logger category is silently dropped in JSON log format (IncludeCategory has no effect)

[madmox]

Describe the bug

With Amazon.Lambda.Logging.AspNetCore, the ILogger category name is included in log output in plain-text mode (governed by LambdaLoggerOptions.IncludeCategory), but in JSON log format the category is dropped entirely: the JSON branch of LambdaILogger.Log only forwards the message template and arguments, and JsonLogMessageFormatter has no category concept either. IncludeCategory = true is silently ignored.

The category is the primary key for locating and filtering .NET logs ("which class/middleware emitted this?"). After switching a function from text to JSON log format, CloudWatch Logs Insights queries can no longer filter on it, and log lines can only be traced back to their source by searching for the literal message text.

Regression Issue

• Select this option if this issue appears to be a regression.

(Behavioral regression when migrating a function from text to JSON log format: the category was present in every text log line, and there is no way to opt back into it.)

Expected Behavior

The logger category available in JSON records — ideally as a dedicated property, e.g.:

{
    "timestamp": "2026-07-04T22:43:57.529Z",
    "level": "Information",
    "category": "MyApp.Filters.IdentityValidatorActionFilter",
    "message": "Successfully authenticated user"
}

Current Behavior

{
    "timestamp": "2026-07-04T22:43:57.529Z",
    "level": "Information",
    "message": "Successfully authenticated user"
}

No trace of the category anywhere in the record, regardless of LambdaLoggerOptions.IncludeCategory.

Reproduction Steps

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddAWSLambdaHosting(LambdaEventSource.RestApi);
builder.Logging.ClearProviders();
builder.Logging.AddLambdaLogger();
var app = builder.Build();
app.MapGet("/", (ILogger<Program> logger) =>
{
    logger.LogInformation("Hello from a categorized logger");
    return "ok";
});
app.Run();

Deploy with LoggingConfig.LogFormat: JSON, invoke /, and observe that the record contains no Program/category information, while the same app with text log format produces info: Program[0] Hello from a categorized logger.

Possible Solution

Emit the category as a top-level JSON property ("category"), or at minimum honor IncludeCategory by prefixing the rendered message.

Additional Information/Context

No response

AWS .NET SDK and/or Package version used

Amazon.Lambda.AspNetCoreServer 10.1.1
Amazon.Lambda.AspNetCoreServer.Hosting 2.1.0
Amazon.Lambda.Logging.AspNetCore 5.0.0
Amazon.Lambda.Core 3.1.0

Targeted .NET Platform

.NET 10 (managed dotnet10 runtime)

Operating System and version

Amazon Linux 2023 (Lambda managed runtime)

[madmox]

Related: #2468

[madmox]

Workaround we implemented (unit-tested, not yet validated on a deployed function), in case it helps others: since every named template parameter becomes a top-level JSON property, our custom ILoggerProvider prepends a "[{Category}] " placeholder to the message template and inserts the category name as the first argument. The category then surfaces both in the rendered message and as a queryable Category property in CloudWatch Logs Insights.

Note that this only works reliably combined with a fix for #2468 (arguments must be supplied in placeholder order, otherwise the extra leading argument shifts everything).

A proper fix would presumably be a category field in the JSON envelope emitted by JsonLogMessageFormatter, but the template trick has the advantage of requiring no runtime-side change.

[normj]

Thanks for reporting the issue and I'm investigating the issue.