Telemetry

yoagent instruments the loop with tracing spans — structured, timed, nested units your observability stack can consume. With no subscriber installed the overhead is near-zero (a cached per-callsite interest check); nothing is exported unless you opt in.

The span tree

agent_loop                (model)
└─ llm_stream             (turn, model, tokens_in, tokens_out, tokens_cached, cost_usd, error)
└─ tool                   (tool, tool_call_id, is_error)
  • llm_stream — one per turn, wrapping the provider call. Token counts are recorded from real usage; cost_usd is recorded when the ModelConfig has pricing configured (CostConfig).
  • tool — one per tool execution, with the tool name and error status; duration comes free with the span.

Local: print spans

#![allow(unused)]
fn main() {
tracing_subscriber::fmt()
    .with_span_events(tracing_subscriber::fmt::format::FmtSpan::CLOSE)
    .init();
}

Run cargo run --example telemetry to see it.

Production: OpenTelemetry

The OTel bridge is application-side — yoagent needs no OTel dependency (that's the point of tracing). Install the tracing-opentelemetry layer and the same spans flow to any OTLP backend — Datadog, Grafana Tempo, Honeycomb, Jaeger:

#![allow(unused)]
fn main() {
// opentelemetry_otlp 0.27+, opentelemetry_sdk 0.27+, tracing-opentelemetry 0.28+
use opentelemetry::trace::TracerProvider as _;
use tracing_subscriber::layer::SubscriberExt;

let exporter = opentelemetry_otlp::SpanExporter::builder()
    .with_tonic()
    .build()?;
let provider = opentelemetry_sdk::trace::SdkTracerProvider::builder()
    .with_batch_exporter(exporter)
    .build();

let subscriber = tracing_subscriber::registry()
    .with(tracing_opentelemetry::layer().with_tracer(provider.tracer("yoagent-app")));
tracing::subscriber::set_global_default(subscriber)?;
}

(The OTel crates rework their builder APIs between releases — if this snippet drifts, the authoritative wiring is the tracing-opentelemetry docs; yoagent only emits standard tracing spans and does not depend on OTel.)

Because these are ordinary tracing spans, an agent call nests inside your app's existing request traces (e.g. an axum handler span) automatically.

What it buys you

  • Cost attribution — dollars per turn/model in your dashboards, from the same CostConfig data as session_cost_usd.
  • Latency diagnosis — "40s request: 8s provider, 30s in one bash tool."
  • Audit — which tools ran, with what outcome, per session.