LangSmith

LangSmith is LangChain's tracing platform. It accepts OpenTelemetry traces at https://api.smith.langchain.com/otel (EU: https://eu.api.smith.langchain.com/otel), so it is a regular backend entry: it fans out beside the others, the live store and the Settings card know about it, and status lists it. Until 1.26 the plugin drove LangSmith's run-tree HTTP API instead through a separate code path selected by LANGSMITH_TRACING=true; that path is gone (#415) and LANGSMITH_TRACING no longer selects anything. See Migrating below.

Signals: traces only (the OTLP ingest serves traces; the plugin defaults metrics and logs off). Deployment: cloud (self-host is enterprise-only). Cost: free personal tier + paid.

Configuration

As a backends: entry in ~/.hermes/hermes_otel.yaml:

backends:
  - type: langsmith
    api_key_env: LANGSMITH_API_KEY   # or api_key: lsv2_... inline
    project: hermes-agent            # optional: the Langsmith-Project header
    # region: eu                     # optional: the EU host
    # endpoint: https://api.smith.langchain.com   # optional: the plugin appends /otel/v1/traces

Or from the environment, with no file:

export OTEL_LANGSMITH_API_KEY="lsv2_..."   # the opt-in: selects LangSmith in env-var mode
export OTEL_LANGSMITH_PROJECT="hermes-agent"   # optional

LANGSMITH_API_KEY, LANGSMITH_PROJECT and LANGSMITH_ENDPOINT (the SDK's API base) are honoured as fallback values, but on their own they select nothing: like every vendor's SDK variables they need the OTEL_LANGSMITH_* opt-in or a backends: entry (why).

The plugin sends x-api-key: <key> and, when a project is set, Langsmith-Project: <project>; without a project LangSmith files the traces under your default project.

What you'll see

Each Hermes turn is one trace rooted at the agent (or cron) span with llm.*, api.* and tool.* children, carrying the OTel GenAI attributes (gen_ai.usage.* token counts, gen_ai.request.model, the conversation id) that LangSmith's OpenTelemetry ingest maps onto its run fields. See LangSmith's OpenTelemetry documentation for the exact attribute mapping its ingest applies.

The Hermes dashboard's OTel tab has no query adapter for langsmith, so the entry exports only; read the traces in LangSmith's UI.

Migrating from LANGSMITH_TRACING

Before (run-tree client, up to 1.25)	Now (OTLP preset)
LANGSMITH_TRACING=true + LANGSMITH_API_KEY	OTEL_LANGSMITH_API_KEY=... (or a backends: entry with api_key_env: LANGSMITH_API_KEY)
LANGSMITH_PROJECT	still read; or OTEL_LANGSMITH_PROJECT / project: on the entry
LANGSMITH_ENDPOINT (API base)	still read; the plugin appends /otel/v1/traces
LANGSMITH_WORKSPACE_ID	not sent (the OTLP ingest takes the workspace from the key)
LangSmith or fan-out	LangSmith and every other backend
pip install langsmith for uuid7 run ids	not needed; ids are OTel trace and span ids

Troubleshooting

401 from LangSmith

• The key must be a LangSmith API key (lsv2_...). Check the x-api-key header in the Settings tab's backend card (values are masked; the provenance says where it came from).

Traces land in the wrong project

• Set project: on the entry or OTEL_LANGSMITH_PROJECT; without one LangSmith uses the key's default project.

Nothing exported in env-var mode

• LANGSMITH_API_KEY alone is ignored by design; the plugin logs LANGSMITH_API_KEY is set, but env-var mode only exports to LangSmith when one of OTEL_LANGSMITH_API_KEY, OTEL_LANGSMITH_ENDPOINT is also set. Set OTEL_LANGSMITH_API_KEY.