Langfuse Skill

Debug your AI systems through Langfuse observability.

Triggers: langfuse, traces, debug AI, find exceptions, set up langfuse, what went wrong, why is it slow, datasets, evaluation sets

Setup

Step 1: Get credentials from https://cloud.langfuse.com → Settings → API Keys

If self-hosted, use your instance URL for

LANGFUSE_HOST

Step 2: Install MCP (pick one):

# Claude Code (project-scoped, shared via .mcp.json)
claude mcp add \
  --scope project \
  --env LANGFUSE_PUBLIC_KEY=pk-... \
  --env LANGFUSE_SECRET_KEY=sk-... \
  --env LANGFUSE_HOST=https://cloud.langfuse.com \
  langfuse -- uvx --python 3.11 langfuse-mcp

# Codex CLI (user-scoped, stored in ~/.codex/config.toml)
codex mcp add langfuse \
  --env LANGFUSE_PUBLIC_KEY=pk-... \
  --env LANGFUSE_SECRET_KEY=sk-... \
  --env LANGFUSE_HOST=https://cloud.langfuse.com \
  -- uvx --python 3.11 langfuse-mcp

Step 3: Restart CLI, verify with

/mcp

codex mcp list

Step 4: Test:

fetch_traces(age=60)

Read-Only Mode

For safer observability without risk of modifying prompts or datasets, enable read-only mode:

# CLI flag
langfuse-mcp --read-only

# Or environment variable
LANGFUSE_MCP_READ_ONLY=true

This disables write tools:

create_text_prompt

create_chat_prompt

update_prompt_labels

create_dataset

create_dataset_item

delete_dataset_item

For manual

.mcp.json

references/setup.md

Playbooks

"Where are the errors?"

find_exceptions(age=1440, group_by="file")

→ Shows error counts by file. Pick the worst offender.

find_exceptions_in_file(filepath="src/ai/chat.py", age=1440)

→ Lists specific exceptions. Grab a trace_id.

get_exception_details(trace_id="...")

→ Full stacktrace and context.

"What happened in this interaction?"

fetch_traces(age=60, user_id="...")

→ Find the trace. Note the trace_id.

If you don't know the user_id, start with:

fetch_traces(age=60)

fetch_trace(trace_id="...", include_observations=true)

→ See all LLM calls in the trace.

fetch_observation(observation_id="...")

→ Inspect a specific generation's input/output.

"Why is it slow?"

fetch_observations(age=60, type="GENERATION")

→ Find recent LLM calls. Look for high latency.

fetch_observation(observation_id="...")

→ Check token counts, model, timing.

"What's this user experiencing?"

get_user_sessions(user_id="...", age=1440)

→ List their sessions.

get_session_details(session_id="...")

→ See all traces in the session.

"Manage datasets"

list_datasets()

→ See all datasets.

get_dataset(name="evaluation-set-v1")

→ Get dataset details.

list_dataset_items(dataset_name="evaluation-set-v1", page=1, limit=10)

→ Browse items in the dataset.

create_dataset(name="qa-test-cases", description="QA evaluation set")

→ Create a new dataset.

create_dataset_item(
  dataset_name="qa-test-cases",
  input={"question": "What is 2+2?"},
  expected_output={"answer": "4"}
)

→ Add test cases.

create_dataset_item(
  dataset_name="qa-test-cases",
  item_id="item_123",
  input={"question": "What is 3+3?"},
  expected_output={"answer": "6"}
)

→ Upsert: updates existing item by id or creates if missing.

"Manage prompts"

list_prompts()

→ See all prompts with labels.

get_prompt(name="...", label="production")

→ Fetch current production version.

create_text_prompt(name="...", prompt="...", labels=["staging"])

→ Create new version in staging.

update_prompt_labels(name="...", version=N, labels=["production"])

→ Promote to production. (Rollback = re-apply label to older version)

Quick Reference

fetch_traces(age=N)

fetch_trace(trace_id="...", include_observations=true)

fetch_observations(age=N, type="GENERATION")

fetch_observation(observation_id="...")

get_error_count(age=N)

find_exceptions(age=N, group_by="file")

fetch_sessions(age=N)

get_user_sessions(user_id="...", age=N)

list_prompts()

get_prompt(name="...", label="production")

list_datasets()

get_dataset(name="...")

list_dataset_items(dataset_name="...", limit=N)

create_dataset_item(dataset_name="...", item_id="...")

age

Troubleshooting

MCP connection fails

• Verify credentials: check

  LANGFUSE_PUBLIC_KEY

  ,

  LANGFUSE_SECRET_KEY

  ,

  LANGFUSE_HOST

• Restart CLI after adding/updating MCP config
• Test MCP independently:

  fetch_traces(age=60)

  — if this fails, the issue is MCP, not the skill
• See

  references/setup.md

  for detailed troubleshooting

No traces found

• Increase the

  age

  parameter (default lookback may be too short)
• Verify your application is sending traces to the correct Langfuse project
• Check

  LANGFUSE_HOST

  points to the right instance (cloud vs self-hosted)

Permission denied

• Regenerate API keys from Langfuse dashboard
• Ensure keys have the required scopes for the operation
• Write operations require read-write keys (not read-only mode)

References

• references/tool-reference.md

  — Full parameter docs, filter semantics, response schemas

• references/setup.md

  — Manual setup, troubleshooting, advanced configuration