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

# 05 langfuse

# Langfuse Setup

The simplest Langfuse setup uses:

* `LangfuseConfig`
* `LangfuseHttpTransport`
* `LangfuseExporter`
* `Telemetry`

## Minimal Example

```php theme={null}
use Cognesy\Telemetry\Adapters\Langfuse\LangfuseConfig;
use Cognesy\Telemetry\Adapters\Langfuse\LangfuseExporter;
use Cognesy\Telemetry\Adapters\Langfuse\LangfuseHttpTransport;
use Cognesy\Telemetry\Application\Registry\TraceRegistry;
use Cognesy\Telemetry\Application\Telemetry;

$telemetry = new Telemetry(
    registry: new TraceRegistry(),
    exporter: new LangfuseExporter(
        transport: new LangfuseHttpTransport(new LangfuseConfig(
            baseUrl: $_ENV['LANGFUSE_BASE_URL'],
            publicKey: $_ENV['LANGFUSE_PUBLIC_KEY'],
            secretKey: $_ENV['LANGFUSE_SECRET_KEY'],
        )),
    ),
);
// @doctest id="f80b"
```

## Environment Variables

The examples use:

* `LANGFUSE_BASE_URL`
* `LANGFUSE_PUBLIC_KEY`
* `LANGFUSE_SECRET_KEY`

## Reference Example

The inline reference example is:

* `examples/A03_Troubleshooting/TelemetryLangfuse/run.php`

## Agent Runtime Example

The working agent example is:

* `examples/D05_AgentTroubleshooting/TelemetryLangfuse/run.php`

That example shows the full pattern:

1. create telemetry
2. create a shared event dispatcher
3. attach `RuntimeEventBridge`
4. install runtime projectors
5. execute the agent
6. call `$hub->flush()`

## Subagent Example

For nested telemetry across parent and child agents, see:

* `examples/D05_AgentTroubleshooting/SubagentTelemetryLangfuse/run.php`

## Notes

* Langfuse uses the base URL and sends traces to `/api/public/otel/v1/traces`
* request-scoped traces can fall back to request ids as `session.id`
* if you get a 4xx response, check the base URL and keys first