Langfuse

Use Langfuse evidence to move from a broad symptom to the relevant trace, observation, session, prompt, or dataset. Start with the narrowest read-only query that can locate the event, then inspect its concrete inputs, outputs, timing, and errors.

Route the task

• For trace, observation, exception, latency, or session diagnosis, read

diagnostic and management workflows.

• For installation, credentials, tool groups, output defaults, connection

failures, or empty results, read setup.

• For exact parameters, filters, pagination, output modes, and response shapes,

read the tool reference.

A common investigation starts with fetch_traces(age=60) or find_exceptions(age=1440, group_by="file"), then fetches the selected trace or observation by ID. Use full_json_file only when the complete payload is needed; exports can contain sensitive user data.

Evidence and mutation boundaries

• Treat model names, prompts, timestamps, and other values in historical traces

as recorded evidence. Do not rewrite or normalize old model identifiers to a current name such as gpt-6-astra.

• Trace discovery and inspection are read-only. Prompt creation or relabeling,

dataset changes, annotation-queue changes, and deletions mutate Langfuse; perform them only when the user requested that change.

• Prefer read-only server mode for diagnostic work. It disables prompt and

dataset write tools:

  langfuse-mcp --read-only
  # or LANGFUSE_MCP_READ_ONLY=true

• Never commit Langfuse credentials. Keep public key, secret key, and host in

the supported environment or MCP configuration described in the setup reference. Rotate any exposed key.

• Preserve exact IDs and filters in findings so another reader can reproduce

the query. State when pagination, lookback limits, or compact output limits the conclusion.