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Two layers, zero YAML

Observability is not something you declare in agents.yml — there’s no observability: section. It’s controlled entirely by environment variables, and one layer is always on:

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Structured logging

Always active, regardless of any environment variable. Every run emits structured log lines at the engine’s key stages: run started, routing decisions, tool calls started/succeeded/failed, run ended. Use the --log-level flag to control verbosity:
This is a fixed part of the runtime — there’s nothing to turn on.

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Langfuse tracing

Langfuse is an open-source LLM observability platform — traces, prompt/token usage, latency, per-agent breakdowns. Extra self-enables it the moment two environment variables are present:
.env
That’s the entire integration. No YAML, no code, no flag. Both keys must be present, or Langfuse stays off — there’s no partial state.
Using a self-hosted Langfuse instance instead of Langfuse Cloud? Add LANGFUSE_HOST alongside the two keys above — it’s read by the underlying Langfuse SDK the same way.

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What happens if it’s misconfigured

If the two keys are set but the Langfuse client fails to initialize for any reason, Extra logs a warning and the engine keeps running without tracing — a broken Langfuse setup never breaks your agents.
This is the opposite failure mode from runtime hooks, which are fail-closed by default for anything security-critical. Observability failing open is deliberate — losing traces is acceptable; losing service to your customers is not.

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Adding another backend

The engine’s observability layer is a small internal registry of callback providers — logging and Langfuse are the two currently wired in. There’s no plugin mechanism for adding a third-party backend from YAML today; that would require a code change to the engine itself, not a configuration change to your spec.