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Phoenix

Arize Phoenix is an open-source LLM observability platform with a tracing UI built around the OpenInference span convention. It runs as a single container, takes ~5 seconds to start, and pretty-prints JSON previews.

Signals: traces + metrics. Deployment: local (single container) or Arize AX cloud. Cost: OSS (self-host) / commercial (cloud).

Local (Docker)​

A ready-to-use compose file ships with the plugin:

cd ~/.hermes/plugins/hermes_otel
docker compose -f docker-compose/phoenix.yaml up -d

Then:

export OTEL_PHOENIX_ENDPOINT="http://localhost:6006/v1/traces"
export OTEL_PROJECT_NAME="hermes-agent"

Open the UI at http://localhost:6006. Traces show up in the hermes-agent project.

Arize AX Cloud​

Phoenix also runs as a hosted service. Point at your cloud endpoint and attach the ingest key:

export OTEL_PHOENIX_ENDPOINT="https://app.phoenix.arize.com/v1/traces"
# Headers can be passed via config.yaml:
# ~/.hermes/plugins/hermes_otel/config.yaml
backends:
  - type: phoenix
    endpoint: https://app.phoenix.arize.com/v1/traces
    headers:
      api_key: ${PHOENIX_API_KEY}

What you'll see​

Phoenix is built around LLM-specific spans, so the UI understands the plugin's span types natively:

  • session.* / cron spans appear as top-level traces with the turn summary on them (tool count, skills, final status).
  • llm.* spans show the user message in the Input panel and the assistant response in the Output panel (pretty-printed JSON when conversation capture is on).
  • api.* spans carry the token counts (llm.token_count.prompt, llm.token_count.completion, llm.token_count.total), the finish_reason, and the HTTP duration.
  • tool.* spans show the arguments (Input) and the result (Output). Errors map to StatusCode.ERROR so the Phoenix error filter works.

Attribute convention​

Phoenix uses OpenInference. hermes-otel emits that convention on every span (llm.token_count.*, input.value, output.value) alongside the gen_ai.* convention for other backends.

See Attribute conventions for the full dual-convention table.

Metrics​

Phoenix accepts OTLP metrics in addition to traces. The plugin's token/tool/cost metrics flow automatically when Phoenix is selected — no extra config.

Troubleshooting​

"No traces show up in Phoenix"

  • Check the endpoint includes /v1/traces — Phoenix doesn't redirect from the collector root.
  • Confirm the container is listening: curl -I http://localhost:6006 should return 200.
  • Turn on debug logging: export HERMES_OTEL_DEBUG=true, run a Hermes turn, check ~/.hermes/plugins/hermes_otel/debug.log for the OTLP POST response.

"Spans are missing input/output previews"

  • If capture_previews: false or HERMES_OTEL_CAPTURE_PREVIEWS=false is set, previews are intentionally suppressed. Remove the setting, restart Hermes.

"Tokens show as zero"

  • Phoenix keys off llm.token_count.* — the plugin emits these on api.* spans (not llm.*). Check the child api.* span, not the parent.