Debug Your LangWatch Instrumentation

This recipe uses the LangWatch MCP to inspect your production traces and identify instrumentation issues.

Prerequisites

The LangWatch MCP must be installed with a valid API key. See MCP Setup.

Step 1: Fetch Recent Traces

Call search_traces with a recent time range (last 24h or 7d) to get an overview:

• How many traces are there?
• Do they have inputs and outputs populated, or are they <empty>?
• Are there labels and metadata (user_id, thread_id)?

Step 2: Inspect Individual Traces

For traces that look problematic, call get_trace with the trace ID to see the full span hierarchy:

• Empty input/output: The most common issue. Check if autotrack_openai_calls(client) (Python) or experimental_telemetry (TypeScript/Vercel AI) is configured.
• Disconnected spans: Spans that don't connect to a parent trace. Usually means @langwatch.trace() decorator is missing on the entry function.
• Missing labels: No way to filter traces by feature/version. Add labels via langwatch.get_current_trace().update(metadata={"labels": ["feature_name"]}).
• Missing user_id/thread_id: Can't correlate traces to users or conversations. Add via trace metadata.
• Slow spans: Unusually long completion times may indicate API timeouts or inefficient prompts.

Step 3: Read the Integration Docs

Use fetch_langwatch_docs to read the integration guide for the project's framework. Compare the recommended setup with what's in the code.

Step 4: Apply Fixes

For each issue found:

1. Identify the root cause in the code
2. Apply the fix following the framework-specific docs
3. Run the application to generate new traces
4. Re-inspect with search_traces to verify the fix

Step 5: Verify Improvement

After fixes, compare before/after:

• Are inputs/outputs now populated?
• Are spans properly nested?
• Are labels and metadata present?

Common Issues and Fixes

Issue	Cause	Fix
All traces show <empty> input/output	Missing autotrack or telemetry config	Add autotrack_openai_calls(client) or experimental_telemetry: { isEnabled: true }
Spans not connected to traces	Missing @langwatch.trace() on entry function	Add trace decorator to the main function
No labels on traces	Labels not set in trace metadata	Add metadata={"labels": ["feature"]} to trace update
Missing user_id	User ID not passed to trace	Add user_id to trace metadata
Traces from different calls merged	Missing langwatch.setup() or trace context not propagated	Ensure langwatch.setup() called at startup