public LogBuilder atTrace() {
    return atLevel(Level.TRACE);    // correct
}
public LogBuilder atDebug() {
    return atLevel(Level.DEBUG);    // correct
}
public LogBuilder atInfo() {
    return atLevel(Level.INFO);     // correct
}
public LogBuilder atWarn() {
    return atLevel(Level.WARN);     // correct
}
public LogBuilder atError() {
    return atLevel(Level.ERROR);    // correct
}
@Override
public LogBuilder atFatal() {
    return atLevel(Level.TRACE);    // BUG: should be Level.FATAL
}

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

// Requires log4j-to-slf4j bridge with SLF4J/Logback backend
// Logger level set to INFO (standard production configuration)
public class AtFatalLogLoss {
    private static final Logger logger = LogManager.getLogger(AtFatalLogLoss.class);

    public static void main(String[] args) {
        // Traditional API: works correctly
        logger.fatal("TRADITIONAL: System crash detected");
        // Output: logged at ERROR level in SLF4J ✓

        // Fluent API: message silently lost
        logger.atFatal().log("FLUENT: System crash detected");
        // Output: NOTHING — logged at TRACE, filtered out ✗
    }
}

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.read.ListAppender;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.junit.jupiter.api.Test;
import org.slf4j.LoggerFactory;

import static org.junit.jupiter.api.Assertions.*;

class SLF4JAtFatalLogLossTest {

    @Test
    void atFatal_message_is_silently_lost_in_production_config() {
        LoggerContext logbackContext = (LoggerContext) LoggerFactory.getILoggerFactory();
        ch.qos.logback.classic.Logger logbackLogger = logbackContext.getLogger("test.AtFatal");
        logbackLogger.setLevel(ch.qos.logback.classic.Level.INFO);

        ListAppender<ILoggingEvent> appender = new ListAppender<>();
        appender.start();
        logbackLogger.addAppender(appender);

        Logger log4jLogger = LogManager.getLogger("test.AtFatal");

        // Traditional API: message logged correctly as ERROR
        log4jLogger.fatal("traditional fatal");
        assertEquals(1, appender.list.size());
        assertEquals(ch.qos.logback.classic.Level.ERROR, appender.list.get(0).getLevel());

        appender.list.clear();

        // Fluent API: message SILENTLY LOST
        log4jLogger.atFatal().log("fluent fatal");
        assertEquals(1, appender.list.size(),
            "atFatal().log() should produce a log event, but the message is silently lost");
    }

    @Test
    void atFatal_logs_at_trace_level_instead_of_error() {
        LoggerContext logbackContext = (LoggerContext) LoggerFactory.getILoggerFactory();
        ch.qos.logback.classic.Logger logbackLogger = logbackContext.getLogger("test.AtFatalLevel");
        logbackLogger.setLevel(ch.qos.logback.classic.Level.TRACE);

        ListAppender<ILoggingEvent> appender = new ListAppender<>();
        appender.start();
        logbackLogger.addAppender(appender);

        Logger log4jLogger = LogManager.getLogger("test.AtFatalLevel");

        log4jLogger.atFatal().log("this should be ERROR level");

        assertEquals(1, appender.list.size());
        // FAILS: actual level is TRACE, not ERROR
        assertEquals(ch.qos.logback.classic.Level.ERROR, appender.list.get(0).getLevel(),
            "atFatal() should map to SLF4J ERROR, but actual level is: " + appender.list.get(0).getLevel());
    }
}