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

# Remote eval OpenTelemetry header propagation

> Propagate OpenTelemetry trace context from a remote eval handler by creating spans and attaching Braintrust request context as span attributes.

export const plans_0 = "Any"

export const deployments_0 = "Any"

export const data_plane_version_0 = undefined

export const use_case_0 = "Use case - Trace distributed requests originating from Braintrust remote eval pods by creating OpenTelemetry spans in the remote eval handler and attaching Braintrust request context as span attributes."

<Note>
  **Applies to:**

  * Plan - {plans_0}
  * Deployment - {deployments_0}
  * {data_plane_version_0}
  * {use_case_0}
</Note>

## Summary

Remote eval requests from the Braintrust UI do not include OpenTelemetry trace context headers (for example, `traceparent` or `baggage`). Braintrust does include request context in the request body so results attach to the Playground run. Start an OTel span inside your remote eval `/eval` handler and add the Braintrust `parent` payload, project ID, or your own run identifier as span attributes to correlate traces across your services.

## What is happening

When the Braintrust UI calls a remote eval service, it sends eval context in the request body, including fields such as `parent`, `project_id`, `data`, `parameters`, and `scores`, but does not propagate OTel trace headers. If your eval handler then makes downstream calls, those calls will not share an incoming OTel trace unless you create or propagate a span yourself. This means distributed traces from the eval pod will be disconnected unless you instrument the eval handler.

## Fix or suggestion

### Start an OTel span in the eval handler

Steps:

1. Instrument your remote eval service with OpenTelemetry.
2. In the `/eval` handler, start a span at the start of the request.
3. Add available Braintrust context from the request body as span attributes, such as `parent`, `project_id`, or another stable run identifier your service controls.
4. Make downstream calls within the span so they appear as child spans.

TypeScript (minimal):

```typescript theme={"theme":{"light":"github-light","dark":"github-dark-dimmed"}}
import { context, trace } from "@opentelemetry/api";

const tracer = trace.getTracer("remote-eval");

export async function evalHandler(req, res) {
  const { parent, project_id: projectId } = req.body ?? {};

  const attributes: Record<string, string> = {};
  if (projectId) {
    attributes["braintrust.project_id"] = String(projectId);
  }
  if (parent) {
    attributes["braintrust.parent"] = JSON.stringify(parent);
  }

  const span = tracer.startSpan("remote-eval.request", {
    attributes,
  });

  try {
    await context.with(trace.setSpan(context.active(), span), async () => {
      // perform downstream calls
    });
    res.sendStatus(200);
  } finally {
    span.end();
  }
}
```

Python (minimal):

```python theme={"theme":{"light":"github-light","dark":"github-dark-dimmed"}}
from opentelemetry import trace

tracer = trace.get_tracer(__name__)

def eval_handler(request):
    body = request.get_json() or {}
    parent = body.get("parent")
    project_id = body.get("project_id")

    attributes = {
        "braintrust.project_id": project_id,
    }
    if parent is not None:
        attributes["braintrust.parent"] = str(parent)

    with tracer.start_as_current_span(
        "remote-eval.request",
        attributes=attributes,
    ):
        # perform downstream calls
        pass

    return ("OK", 200)
```

## How to confirm it worked

* Check your tracing backend for a span named like `remote-eval.request` and verify it contains attributes: `braintrust.project_id` and, when present, `braintrust.parent`.
* Verify downstream spans appear as children with the same trace ID (or that trace IDs match between services when propagating `traceparent`).

## Notes

* [OpenTelemetry docs](https://opentelemetry.io/docs/)
* [W3C trace context](https://www.w3.org/TR/trace-context/)
* [Braintrust tracing quickstart](/tracing-quickstart)