Unsupported file format: ${fileExtension}. Supported formats

Error message

Unsupported file format: ${fileExtension}. Supported formats: ${supportedExtensions.join(', ')}

What it means

resolveOutputFilePath validates that a file input has an extension matching one of repomix's supported output formats (derived from the default output file paths: xml, md, json, txt). If the extension is not recognized, it throws because it cannot infer the packed-output format from the file.

Source

Thrown at src/mcp/tools/attachPackedOutputTool.ts:81

          await fs.access(outputFilePath);
          const format = getFormatFromFileName(fileName);
          return { filePath: outputFilePath, format };
        } catch {
          // File doesn't exist, continue to next
        }
      }

      throw new Error(
        `No repomix output file found in directory: ${inputPath}. Looking for: ${possibleFiles.join(', ')}`,
      );
    }

    // If it's a file, check if it's a supported format
    const supportedExtensions = Object.values(defaultFilePathMap).map((file) => path.extname(file));
    const fileExtension = path.extname(inputPath).toLowerCase();

    if (!supportedExtensions.includes(fileExtension)) {
      throw new Error(
        `Unsupported file format: ${fileExtension}. Supported formats: ${supportedExtensions.join(', ')}`,
      );
    }

    const format = getFormatFromExtension(fileExtension);
    return { filePath: inputPath, format };
  } catch (error) {
    if (error instanceof Error && error.message.includes('ENOENT')) {
      throw new Error(`File or directory not found for path: ${inputPath}`, { cause: error });
    }
    throw error;
  }
}

/**
 * Get format from file name
 */
function getFormatFromFileName(fileName: string): string {

View on GitHub (pinned to f465ad9093)

Solutions

Rename the file to use a supported extension (.xml, .md, .json, .txt)
Regenerate the output with the format you want: `repomix --style xml` producing repomix-output.xml
Pass a supported-format file path directly to the tool

Example fix

// before
await attachPackedOutput({ path: './out/repomix-output.yaml' });
// after
await $`mv out/repomix-output.yaml out/repomix-output.xml`; // ensure real xml content
await attachPackedOutput({ path: './out/repomix-output.xml' });

Defensive patterns

Strategy: validation

Validate before calling

const ok = /\.(xml|md|json|txt)$/i.test(inputPath);
if (!ok) throw new Error(`Rename ${inputPath} to a .xml/.md/.json/.txt repomix output`);

Try / catch

try {
  await attachPackedOutput({ path: inputPath });
} catch (e) {
  if (String(e.message).startsWith('Unsupported file format')) {
    // rename/copy to a supported extension and retry
  } else throw e;
}

Prevention

Keep repomix outputs at their default filenames
Never rename outputs to non-standard extensions
Store format in the filename extension when scripting

When it happens

Trigger: Calling the attach tool with a file whose extension is not .xml/.md/.json/.txt (e.g. .yaml, .log, .out, or a file with no extension), or an unusual case variant handled by path.extname but not in the map.

Common situations: Renaming repomix-output.xml to something like repomix-output.bak or packing to a custom filename with a non-standard extension; on some systems a file saved without extension after an editor session.

Related errors

AI-assisted analysis of yamadashy/repomix@f465ad9093 (2026-08-29). Data as JSON: /api/errors/9854222d1ed004e5. Report an issue: GitHub.