How do I export a single trace or session?

In the UI

• Trace: open the trace and click Download JSON in the header. The file (trace-<traceId>.json) contains the trace, its observations, and its scores.
• Session: open the session and click the download icon (Download session as JSON). The file (session-<sessionId>.json) contains the session's traces and comments.

Very large inputs or outputs can be left out of the trace download. To export many traces or observations at once as CSV, JSON, or JSONL, use the export button on the table instead. See Export data from the UI.

Via the API

Fetch all observations of one trace or session with the Observations API v2:

curl -u "$LANGFUSE_PUBLIC_KEY:$LANGFUSE_SECRET_KEY" \
  "https://cloud.langfuse.com/api/public/v2/observations?traceId=<traceId>&fields=core,basic,io,usage&limit=1000"

Use sessionId=<sessionId> instead of traceId for a session, and follow the meta.cursor value to fetch further pages. In the Python SDK: langfuse.api.observations.get_many(trace_id="<traceId>", fields="core,basic,io,usage").