Project " " has a non-string reporter in Karma config. This…

Error message

Project "${projectName}" has a non-string reporter in Karma config. This cannot be automatically mapped to Vitest.

What it means

Warning from extractReporters in karma-processor.ts. The migration expects karma.conf.js `reporters` entries to be plain strings; if an entry is not a string (e.g. an array-style multi-reporter entry, an object, or a function returned by a custom framework), it cannot be mapped and is skipped with this warning.

Solutions

Flatten the reporters array to plain strings in karma.conf.js before migrating so entries can be mapped.
Replace object/function reporter registrations with supported string reporter names.
Remove the non-string entries if the reporters are no longer needed and re-run the migration.
After migration, manually add equivalent Vitest reporters/plugins in the generated config.

Example fix

// before (karma.conf.js)
reporters: ['progress', ['junit', { outputDir: 'reports' }]],
// after
reporters: ['progress', 'junit'], // junit options moved to junitReporter config

Defensive patterns

Strategy: validation

Validate before calling

const reporters = karmaConfig?.reporters ?? [];
if (reporters.some(r => typeof r !== 'string')) {
  console.warn('reporters contains non-string entries; flatten before migrating');
}

Type guard

function allStringReporters(reporters: unknown): reporters is string[] {
  return Array.isArray(reporters) && reporters.every(r => typeof r === 'string');
}

Prevention

Keep karma.conf.js reporters as a flat array of plain strings.
Move multi-reporter options into the plugin's own config section rather than nested arrays.
Lint karma.conf.js with a schema check before running the migration.

When it happens

Trigger: Running the migrate-karma-to-vitest migration when a karma.conf.js reporters array contains a non-string element — e.g. nested arrays as produced by karma's multi-reporter form, or objects/functions assigned by reporter plugins.

Common situations: Hand-written karma.conf.js using non-standard reporter values; configs assembled programmatically; Karma plugins that register reporters as objects in the reporters array.

Related errors

AI-assisted analysis of angular/angular-cli@bb72145f9a (2026-08-30). Data as JSON: /api/errors/24216b928b881ae6. Report an issue: GitHub.

Appendix: source

Thrown at packages/schematics/angular/migrations/migrate-karma-to-vitest/karma-processor.ts:45

          mappedReporters.push('default');
        } else if (r === 'kjhtml') {
          context.logger.warn(
            `Project "${projectName}" uses the "kjhtml" reporter. ` +
              `This has not been automatically mapped. ` +
              `For an interactive test UI in Vitest, consider setting the "ui" option to true in your test target options ` +
              `and installing "@vitest/ui".`,
          );
        } else if (SUPPORTED_REPORTERS.has(r)) {
          mappedReporters.push(r);
        } else {
          context.logger.warn(
            `Project "${projectName}" uses a custom Karma reporter "${r}". ` +
              `This reporter cannot be automatically mapped to Vitest. ` +
              `Please check the Vitest documentation for equivalent reporters.`,
          );
        }
      } else {
        context.logger.warn(
          `Project "${projectName}" has a non-string reporter in Karma config. ` +
            `This cannot be automatically mapped to Vitest.`,
        );
      }
    }
    if (mappedReporters.length > 0) {
      options['reporters'] = [...new Set(mappedReporters)];
    }
  }
}

function extractCoverageSettings(
  analysis: KarmaConfigAnalysis,
  options: Record<string, json.JsonValue | undefined>,
  projectName: string,
  context: SchematicContext,
): void {
  const coverageReporter = analysis.settings.get('coverageReporter');

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