nodejs/node · error · MBErr

Unknown mixin "%s"

Error message

Unknown mixin "%s"

What it means

During config flattening, MB walks the mixin list of a config (and recursively any nested 'mixins'). Each name must exist in the top-level `mixins` map of the mb config file. An undefined reference aborts flattening.

Source

Thrown at deps/v8/tools/mb/mb.py:653

  def FlattenConfig(self, config):
    mixins = self.configs[config]
    vals = self.DefaultVals()

    visited = []
    self.FlattenMixins(mixins, vals, visited)
    return vals

  def DefaultVals(self):
    return {
      'args_file': '',
      'cros_passthrough': False,
      'gn_args': '',
    }

  def FlattenMixins(self, mixins, vals, visited):
    for m in mixins:
      if m not in self.mixins:
        raise MBErr('Unknown mixin "%s"' % m)

      visited.append(m)

      mixin_vals = self.mixins[m]

      if 'cros_passthrough' in mixin_vals:
        vals['cros_passthrough'] = mixin_vals['cros_passthrough']
      if 'args_file' in mixin_vals:
        if vals['args_file']:
          raise MBErr('args_file specified multiple times in mixins '
                      'for %s on %s' %
                      (self.args.builder, self.args.builder_group))
        vals['args_file'] = mixin_vals['args_file']
      if 'gn_args' in mixin_vals:
        if vals['gn_args']:
          vals['gn_args'] += ' ' + mixin_vals['gn_args']
        else:
          vals['gn_args'] = mixin_vals['gn_args']

View on GitHub (pinned to 1b2de5e052)

Solutions

1. Grep the mb config file for the named mixin to confirm it is undefined.
2. Add the missing mixin definition under the top-level mixins map.
3. Correct the typo in the referencing config/mixin.

Example fix

// before
'mixins': ['linux', 'relase']   // typo: 'relase'
// after
'mixins': ['linux', 'release']

# Lint the mb config: every referenced mixin must be defined.
import astpp  # or json if config is JSON
cfg = load_mb_config(config_file)
known = set(cfg['mixins'].keys())
def check(mixins):
    for m in mixins:
        assert m in known, f"unknown mixin {m!r}"
        if 'mixins' in cfg['mixins'][m]:
            check(cfg['mixins'][m]['mixins'])
for name, entry in cfg['configs'].items():
    if isinstance(entry, dict):
        for phase, c in entry.items():
            check(c.get('mixins', []))
    else:
        check(entry.get('mixins', []))

When it happens

Trigger: A config or mixin references 'mixins': ['foo', 'bar'] where 'bar' is not declared under the config file's top-level mixins: block (includes transitively referenced mixins).

Common situations: Typo in a mixin name; mixin renamed/removed without updating all references; a merge that left a dangling reference; copy-paste of a config without its dependency mixins.

Related errors

• args_file specified multiple times in mixins for %s on %s
• Phase %s doesn't exist for %s on %s
• Must not specify a build --phase for %s on %s
• Cannot generate isolate for %s since it is an additional_com
• did not generate any of %s

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/638f9df4d5ec7bbe. Report an issue: GitHub.