Fluent logging results in no-op logging with Logback 1.3.0-alpha5, due to what is seemingly an unnecessary override in ch/qos/logback/classic/Logger.java class that points to a no-op implementation for the LoggingEventBuilder:

@Override
public LoggingEventBuilder makeLoggingEventBuilder(org.slf4j.event.Level level) {
    return new LogbackLoggingEventBuilder(this, level);
}

The LogbackLoggingEventBuilder class is all no-op methods with TODO markers, which was added in a commit last August with commit message "ongoing work".

Based on my very limited understanding, SLF4J 2.0 provides a default implementation for this method, and Logback does not need to override it.

By switching back to Logback 1.3.0-alpha4, fluent logging works as expected.