> ## Documentation Index
> Fetch the complete documentation index at: https://docs.openlayer.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Session cost

> Learn how to use the session cost test to monitor total LLM cost per session

## Definition

The session cost test monitors **LLM cost aggregated to the session level**. For
each session, Openlayer sums the per-trace cost column across all turns in the
session, then exposes window-level aggregates you can threshold against. No LLM
evaluator is involved — it's a deterministic aggregation.

## Taxonomy

* **Task types**: LLM.
* **Availability**: <Tooltip tip="Continuously evaluate your models and datasets as you iterate on their versions.">development</Tooltip>
  and <Tooltip tip="Monitor a model in production, measure its health, check for drifts and set up alerts.">monitoring</Tooltip>.
* **Evaluation level**: session.
* **Computation**: deterministic aggregation.

## Why it matters

* Cost per trace is useful, but cost per session is what maps to a user's real
  experience — users don't see individual LLM calls, they see conversations.
* Mean/median session cost tracks baseline spend; `totalCost` catches the bill
  for the full window.
* Pair with [Session record count](/tests/catalog/session-record-count) to distinguish
  "long session, predictable cost" from "short session, expensive turn".

## Available measurements

| Measurement            | What it means                                                             |
| ---------------------- | ------------------------------------------------------------------------- |
| `totalCost`            | Sum of cost across **all traces in the window** (global, not per-session) |
| `meanCostPerSession`   | Mean of per-session cost sums across sessions in the window               |
| `medianCostPerSession` | Median of per-session cost sums across sessions in the window             |

## Required columns

* **Session ID**: Groups turns belonging to the same conversation.
* **Cost**: Per-trace cost (usually populated automatically by the Openlayer client
  or via OpenTelemetry).

## Test configuration examples

<CodeGroup>
  ```json Monitoring theme={null}
  [
    {
      "name": "Mean session cost below $0.50",
      "description": "Alert when average session cost exceeds $0.50",
      "type": "performance",
      "subtype": "sessionCost",
      "thresholds": [
        {
          "insightName": "sessionCost",
          "measurement": "meanCostPerSession",
          "operator": "<=",
          "value": 0.5
        }
      ],
      "subpopulationFilters": null,
      "mode": "monitoring",
      "usesProductionData": true,
      "evaluationWindow": 3600,
      "delayWindow": 0
    }
  ]
  ```
</CodeGroup>

## Related

* [Session token count](/tests/catalog/session-token-count) — token-based view of the same underlying usage.
* [Session record count](/tests/catalog/session-record-count) — how many turns contribute to the session cost.
* [Mean cost](/tests/catalog/mean-cost), [Max cost](/tests/catalog/max-cost), [Total cost](/tests/catalog/total-cost) — trace-level cost metrics.