Language ${language} is not supported! Please choose from ${

Error message

Language ${language} is not supported! Please choose from ${Object.values(Language)}

What it means

CharacterTextSplitter.getSeparatorsForLanguage maps a Language enum value to markdown-aware separator strings. If the language isn't in the switch's cases, the default branch throws listing all supported languages.

Source

Thrown at packages/rag/src/document/transformers/character.ts:641

        ];
      case Language.PROTO:
        return [
          '\nsyntax ',
          '\npackage ',
          '\nimport ',
          '\nmessage ',
          '\nservice ',
          '\nenum ',
          '\nrpc ',
          '\n\n',
          '\n',
          ' ',
          '',
        ];
      case Language.RST:
        return ['\n=+\n', '\n-+\n', '\n\\*+\n', '\n\\.\\. ', '\n\n', '\n', ' ', ''];
      default:
        throw new Error(`Language ${language} is not supported! Please choose from ${Object.values(Language)}`);
    }
  }
}

View on GitHub (pinned to 75dd419e61)

Solutions

Use a supported Language enum member (e.g. Language.MARKDOWN, Language.RST) instead of a raw/typo'd string.
Check the list in the thrown message (Object.values(Language)) and pick from it.
If you need a language that isn't supported yet, pass plain separators/defaults or fall back to CharacterTextSplitter without markdown-awareness.

Example fix

// before
new CharacterTextSplitter({ separator: '
', language: 'md' as Language });
// after
import { Language } from '@mastra/rag';
new CharacterTextSplitter({ separator: '
', language: Language.MARKDOWN });

Defensive patterns

Strategy: validation

Validate before calling

import { Language } from '@mastra/rag';
const lang: Language | undefined = Object.values(Language).find(v => v === config.language);
if (!lang) throw new Error(`Unsupported language: ${config.language}`);

Type guard

const isSupportedLanguage = (v: unknown): v is Language =>
  typeof v === 'string' && (Object.values(Language) as string[]).includes(v);

Try / catch

try {
  return splitter.splitText(text);
} catch (e) {
  if (e instanceof Error && e.message.includes('is not supported')) {
    return fallbackSplitter.splitText(text);
  }
  throw e;
}

Prevention

Always use Language enum members, never raw strings.
Validate config language values against Object.values(Language) at startup.
After upgrading @mastra/rag, re-check that your chosen language still has a separator table.

When it happens

Trigger: splitText/splitMarkdown with a Language value that has no separator table (e.g. a Language added to the enum but not handled in getSeparatorsForLanguage), or a cast string like 'xyz' as Language passed to the constructor.

Common situations: Passing a raw string from config as Language without validation, upgrading @mastra/rag and using a newly added Language while on a stale splitter path, or typo'd language names.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/7e216bc123372225. Report an issue: GitHub.