What causes "Not a log level: " + input" in bazelbuild/bazel?

Pass a plain integer within the range printed by the flag's type description (run bazel help / --help).

bazelbuild/bazel · error · OptionsParsingException

Not a log level: " + input

Error message

Not a log level: " + input

What it means

Thrown by the log-level converter (Converters line 404) when the input is not an integer index into its LEVELS table (an ordered list of java.util.logging levels from most severe to finest). The value is an ordinal like 0..N, not a level name and not an arbitrary number; parse failures and out-of-range indices both surface as this OptionsParsingException.

Source

Thrown at src/main/java/com/google/devtools/common/options/Converters.java:404

  public static class LogLevelConverter extends Converter.Contextless<Level> {

    static final ImmutableList<Level> LEVELS =
        ImmutableList.of(
            Level.OFF,
            Level.SEVERE,
            Level.WARNING,
            Level.INFO,
            Level.FINE,
            Level.FINER,
            Level.FINEST);

    @Override
    public Level convert(String input) throws OptionsParsingException {
      try {
        int level = Integer.parseInt(input);
        return LEVELS.get(level);
      } catch (NumberFormatException | ArrayIndexOutOfBoundsException e) {
        throw new OptionsParsingException("Not a log level: " + input, e);
      }
    }

    @Override
    public String getTypeDescription() {
      return "0 <= an integer <= " + (LEVELS.size() - 1);
    }
  }

  /** Checks whether a string is part of a set of strings. */
  public static class StringSetConverter extends Converter.Contextless<String> {

    private final ImmutableSet<String> values;

    public StringSetConverter(String... values) {
      this.values = ImmutableSet.copyOf(values);
    }

View on GitHub (pinned to e6e199d060)

Solutions

Pass a plain integer within the range printed by the flag's type description (run bazel help <flag> / --help).
Lower numbers are more severe; pick 0 for SEVERE-level-only logging and the maximum for FINEST.
Do not use java.util.logging names; convert them to the ordinal first.

Example fix

# before
bazel --my_log_flag=info build //...
# after
bazel --my_log_flag=2 build //...   # 2 maps to INFO in this converter

Defensive patterns

Strategy: validation

Validate before calling

boolean isParsableLogLevel(String s, int maxIndex) {
  if (s == null) return false;
  try { int i = Integer.parseInt(s); return i >= 0 && i <= maxIndex; }
  catch (NumberFormatException e) { return false; }
}

Type guard

boolean isLogLevelFlagValue(String s) {
  return s != null && s.matches("[0-9]+"); // plus range check against the flag's max
}

Prevention

Map level names to ordinals in your tooling instead of passing names.
Read the valid range from the flag's type description after Bazel upgrades.
Validate log-level configs in a preflight check before long builds.

When it happens

Trigger: Passing --verbosity=WARNING, --verbosity=info (level names), --verbosity=-1, or an index past the last level (the type description reports the valid upper bound, e.g. "0 <= an integer <= 5") to a log-level option.

Common situations: Users writing level names because other tools accept them; guessing an index without reading --help for the flag; stale configs from a Bazel version where the table had more/fewer levels.

Related errors

AI-assisted analysis of bazelbuild/bazel@e6e199d060 (2026-08-14). Data as JSON: /api/errors/de2a01d50faef9c7. Report an issue: GitHub.