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Applies to:
  • Plan -
  • Deployment -

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Summary

Goal: Calculate average cost, LLM calls, or spans per trace in Braintrust Monitor. Features: Custom measures, estimated_cost(), per-trace averages.

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Configuration steps

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Step 1: Use a custom measure

Use a custom measure when you need a trace-level average. The built-in Cost measure computes totals, rather than averages, and using avg(estimated_cost()) on the Monitor page enables you to average matching spans, rather than traces. In the chart’s Measures section, click the </> button next to the measure dropdown to switch from the visual builder to a raw BTQL expression editor.

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Step 2: Divide by distinct traces

You can use count_distinct(root_span_id) as the denominator for per-trace metrics. Average cost per trace: 
sum(estimated_cost()) / count_distinct(root_span_id)
Average LLM calls per trace: 
sum(span_attributes.type = 'llm') / count_distinct(root_span_id)
Average spans per trace: 
count(1) / count_distinct(root_span_id)

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Step 3: Check your filters

These formulas calculate averages over the spans included by the chart’s filters. If you filter to a specific span name, span type, or root spans only, the denominator counts only traces represented in that filtered span set. For example, if you added a filter where span_attributes.name = 'agent' then the measure count_distinct(root_span_id) will only include root spans whose trace contains a span that meets that filter condition. For average cost per span instead, use: 
avg(estimated_cost())