mastra-ai/mastra · error · MastraError

AGENT_GENEREATE_SCORER_NOT_FOUND

AGENT_GENEREATE_SCORER_NOT_FOUND

Error message

Mastra not found when fetching scorer. Make sure to fetch agent from mastra.getAgent()

What it means

A MastraError (id AGENT_GENEREATE_SCORER_NOT_FOUND) thrown when resolving scorer overrides that reference scorers by string name: the agent has no Mastra instance (`this.#mastra` is undefined), so it cannot look up the named scorer in the registry. This happens when the Agent was instantiated standalone rather than fetched/registered via `mastra.getAgent()`.

Source

Thrown at packages/core/src/agent/agent.ts:6573

  }

  /**
   * Resolves scorer name references to actual scorer instances from Mastra.
   * @internal
   */
  private resolveOverrideScorerReferences(
    overrideScorers:
      | MastraScorers
      | Record<string, { scorer: MastraScorer['name']; sampling?: ScoringSamplingConfig; filter?: ScoringFilter }>,
  ) {
    const result: Record<string, { scorer: MastraScorer; sampling?: ScoringSamplingConfig; filter?: ScoringFilter }> =
      {};
    for (const [id, scorerObject] of Object.entries(overrideScorers)) {
      // If the scorer is a string (scorer name), we need to get the scorer from the mastra instance
      if (typeof scorerObject.scorer === 'string') {
        try {
          if (!this.#mastra) {
            throw new MastraError({
              id: 'AGENT_GENEREATE_SCORER_NOT_FOUND',
              domain: ErrorDomain.AGENT,
              category: ErrorCategory.USER,
              text: `Mastra not found when fetching scorer. Make sure to fetch agent from mastra.getAgent()`,
            });
          }

          const scorer = this.#mastra.getScorerById(scorerObject.scorer);
          result[id] = { scorer, sampling: scorerObject.sampling, filter: scorerObject.filter };
        } catch (error) {
          this.logger.warn('Failed to get scorer', { agent: this.name, scorer: scorerObject.scorer, error });
        }
      } else {
        result[id] = scorerObject;
      }
    }

    // Only throw if scorers were provided but none could be resolved

View on GitHub (pinned to 75dd419e61)

Solutions

1. Fetch the agent through the Mastra instance: `const agent = mastra.getAgent('myAgent')` instead of using the raw `new Agent(...)` instance
2. Register the agent on `new Mastra({ agents: { myAgent } })` before using it with scorers
3. Alternatively pass the actual scorer object (not a string name) in `scorers` so no registry lookup is needed

Example fix

// before
const agent = new Agent({ name: 'a', model, instructions });
await agent.generate(prompt, { scorers: { quality: { scorer: 'qualityScorer' } } });
// after
const agent = mastra.getAgent('a');
await agent.generate(prompt, { scorers: { quality: { scorer: 'qualityScorer' } } });

function canUseNamedScorers(agent, scorers) {
  const usesStringScorer = Object.values(scorers ?? {}).some(
    (s) => typeof s?.scorer === 'string'
  );
  const hasMastra = typeof agent.__getMastra === 'function' ? !!agent.__getMastra() : true;
  return !usesStringScorer || hasMastra; // otherwise pass scorer objects directly
}

function isMastraAgent(agent) {
  return typeof agent === 'object' && agent !== null && '__getMastra' in agent;
}

try {
  return await agent.generate(prompt, { scorers });
} catch (e) {
  if (e?.id === 'AGENT_GENEREATE_SCORER_NOT_FOUND' && /Mastra not found/.test(e.message)) {
    logger.warn('agent not attached to Mastra; skipping scoring');
    return runWithoutScoring();
  }
  throw e;
}

When it happens

Trigger: Calling `generate`/`stream` (or the scorer resolution path) with `options.scorers` containing `{ scorer: 'scorerName' }` string references while the agent instance has no `#mastra` reference — i.e., the agent was constructed directly (`new Agent({...})`) and used without registration on a Mastra instance.

Common situations: Tests or scripts that construct agents directly and then pass named scorers; exporting an agent from a module and using it in another app without adding it to `new Mastra({ agents: {...} })`; scorers defined but registered only on the Mastra instance, not the agent.

Related errors

• AGENT_LIST_SUSPENDED_RUNS_NO_STORAGE
• sendNotificationSignal requires a notifications storage doma
• AGENT_DURABLE_METHOD_NOT_AVAILABLE
• sendStateSignal requires Mastra memory
• Mode ${mode.id} requires a defaultModelId when no backing ag

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/73a80cb803ea1c15. Report an issue: GitHub.