mastra-ai/mastra · error · Error

No model generation span found in trace

Error message

No model generation span found in trace

What it means

findPrimaryLLMSpan throws this when the root agent_run span has no child span of type MODEL_GENERATION. The scorer pipeline needs the model-generation span to extract system messages and model I/O for scoring.

Source

Thrown at packages/core/src/evals/scoreTraces/utils.ts:247

  const spanIds = new Set(trace.spans.map(span => span.spanId));
  for (const span of trace.spans) {
    if (span.parentSpanId && !spanIds.has(span.parentSpanId)) {
      throw new Error(`Span ${span.spanId} references non-existent parent ${span.parentSpanId}`);
    }
  }
}

/**
 * Find the most recent model span that contains conversation history
 */
function findPrimaryLLMSpan(spanTree: SpanTree, rootAgentSpan: SpanRecord): SpanRecord {
  const directLLMSpans = getChildrenOfType<SpanRecord>(spanTree, rootAgentSpan.spanId, SpanType.MODEL_GENERATION);
  if (directLLMSpans.length > 0) {
    // There should only be one model generation span per agent run which is a direct child of the root agent span
    return directLLMSpans[0]!;
  }

  throw new Error('No model generation span found in trace');
}

/**
 * Extract common trace validation and span tree building logic
 */
function prepareTraceForTransformation(trace: TraceRecord) {
  validateTrace(trace);
  const spanTree = buildSpanTree(trace.spans);

  // Find the root agent run span
  const rootAgentSpan = spanTree.rootSpans.find(span => span.spanType === 'agent_run') as SpanRecord | undefined;

  if (!rootAgentSpan) {
    throw new Error('No root agent_run span found in trace');
  }

  return { spanTree, rootAgentSpan };
}

View on GitHub (pinned to 75dd419e61)

Solutions

1. Ensure the agent run completed a model call and that tracing of MODEL_GENERATION spans is enabled
2. Verify the trace hierarchy: the model generation span must be a direct child of the root agent_run span
3. Re-run the generation to get a fresh, complete trace before scoring

Example fix

// before
const { input, output } = transformTraceToScorerInputAndOutput(failedRunTrace);
// after
if (!findModelSpan(failedRunTrace)) skipScoring(failedRunTrace.traceId);
else transformTraceToScorerInputAndOutput(failedRunTrace);

const tree = buildSpanTree(trace.spans);
const root = tree.rootSpans.find(s => s.spanType === 'agent_run');
if (!root) throw new Error('No agent_run root');
const hasModelSpan = getChildrenOfType(tree, root.spanId, SpanType.MODEL_GENERATION).length > 0;
if (!hasModelSpan) skipScoring(trace.traceId);

function hasModelGenerationSpan(tree: SpanTree, root: SpanRecord): boolean {
  return getChildrenOfType<SpanRecord>(tree, root.spanId, SpanType.MODEL_GENERATION).length > 0;
}

try {
  const { input, output } = transformTraceToScorerInputAndOutput(trace);
} catch (e) {
  if (e instanceof Error && e.message === 'No model generation span found in trace') {
    logger.warn('Skipping scoring: no LLM span in trace', { traceId: trace.traceId });
  } else throw e;
}

When it happens

Trigger: scoreTrace -> primaryLLMSpan on a trace where the agent run never recorded a model generation span as a direct child of the root agent span — e.g. the run failed before the LLM call, an agent configuration without a model, or instrumentation that nested model spans elsewhere.

Common situations: Agent run errored before invoking the LLM; custom workflows where the model call is not under the root agent span; older core versions emitting a different span hierarchy than the scoring code expects.

Related errors

• Trace is null or undefined
• Trace must have a spans array
• Trace has no spans
• Root agent span has no output
• Span ${span.spanId} references non-existent parent ${span.pa

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/c61b68b17f09d388. Report an issue: GitHub.