WARNING: Formatting of files failed with the following error

Error message

WARNING: Formatting of files failed with the following error: ${error.message}

What it means

After running package migrations, `ng update` attempts to format all touched files (prettier). If formatFiles throws, the migration itself is still complete and successful; this warning reports the formatting failure with the underlying error message.

Source

Thrown at packages/angular/cli/src/commands/update/utilities/migration.ts:237

    switch (files.size) {
      case 0:
        modifiedFilesText = 'No changes made';
        break;
      case 1:
        modifiedFilesText = '1 file modified';
        break;
      default:
        modifiedFilesText = `${files.size} files modified`;
        break;
    }

    if (files.size) {
      try {
        await formatFiles(process.cwd(), files);
      } catch (error) {
        assertIsError(error);

        logger.warn(
          `WARNING: Formatting of files failed with the following error: ${error.message}`,
        );
      }
    }

    logger.info(`  Migration completed (${modifiedFilesText}).`);

    // Commit migration
    if (commit) {
      const commitPrefix = `${packageName} migration - ${migration.name}`;
      const commitMessage = migration.description
        ? `${commitPrefix}\n\n${migration.description}`
        : commitPrefix;
      const committed = commitChanges(logger, commitMessage);
      if (!committed) {
        // Failed to commit, something went wrong. Abort the update.
        return 1;
      }

View on GitHub (pinned to bb72145f9a)

Solutions

Read the inner error.message and fix the specific file it complains about (usually a syntax issue).
Run prettier manually on the affected files to see the full error.
Check/repair your prettier config (.prettierrc, overrides).
Ensure files are writable and not locked; then rerun the migration or format manually.

Example fix

// before (.prettierrc)
{ "parser": "not-a-real-parser" }
// after
{ "printWidth": 100 }

Defensive patterns

Strategy: try-catch

Validate before calling

const filesArr = [...files]; const bad = filesArr.filter(f => !fs.existsSync(f) || !fs.accessSync(f, fs.constants.W_OK)); if (bad.length) throw new Error(`unwritable: ${bad}`);

Type guard

function assertIsError(e: unknown): asserts e is Error { if (!(e instanceof Error)) throw e; }

Try / catch

try { await formatFiles(process.cwd(), files); } catch (error) { assertIsError(error); logger.warn(`WARNING: Formatting of files failed with the following error: ${error.message}`); }

Prevention

Keep prettier config valid and minimal.
Fix syntax errors in generated/migrated files before formatting.
Ensure migrated files are writable and not locked.
Run prettier manually on the workspace regularly to catch issues early.

When it happens

Trigger: `formatFiles(process.cwd(), files)` throws — e.g. a file with syntax prettier cannot parse, unreadable file permissions, or a prettier configuration error in the workspace.

Common situations: Migrations touching generated or malformed code prettier can't parse; a broken .prettierrc; files locked by an editor/AV on Windows; low disk space.

Related errors

AI-assisted analysis of angular/angular-cli@bb72145f9a (2026-08-30). Data as JSON: /api/errors/83b97bcc9677431f. Report an issue: GitHub.