Analyzing a Single MLflow Trace

Trace Structure

A trace captures the full execution of an AI/ML application as a tree of spans. Each span represents one operation (LLM call, tool invocation, retrieval step, etc.) and records its inputs, outputs, timing, and status. Traces also carry assessments — feedback from humans or LLM judges about quality.

It is recommended to read references/trace-structure.md before analyzing a trace — it covers the complete data model, all fields and types, analysis guidance, and OpenTelemetry compatibility notes.

Handling CLI Output

Traces can be 100KB+ for complex agent executions. Always redirect output to a file — do not pipe mlflow traces get directly to jq, head, or other commands, as piping can silently produce no output.

# Fetch full trace to a file (traces get always outputs JSON, no --output flag needed)
mlflow traces get --trace-id <ID> > /tmp/trace.json

# Then process the file
jq '.info.state' /tmp/trace.json
jq '.data.spans | length' /tmp/trace.json

Prefer fetching the full trace and parsing the JSON directly rather than using --extract-fields. The --extract-fields flag has limited support for nested span data (e.g., span inputs/outputs may return empty objects). Fetch the complete trace once and parse it as needed.

JSON Structure

The trace JSON has two top-level keys: info (metadata, assessments) and data (spans).

{
  "info": { "trace_id", "state", "request_time", "assessments", ... },
  "data": { "spans": [ { "span_id", "name", "status", "attributes", ... } ] }
}

Key paths (verified against actual CLI output):

What	jq path
Trace state	.info.state
All spans	.data.spans
Root span	.data.spans[] | select(.parent_span_id == null)
Span status code	.data.spans[].status.code (values: STATUS_CODE_OK, STATUS_CODE_ERROR, STATUS_CODE_UNSET)
Span status message	.data.spans[].status.message
Span inputs	.data.spans[].attributes["mlflow.spanInputs"]
Span outputs	.data.spans[].attributes["mlflow.spanOutputs"]
Assessments	.info.assessments
Assessment name	.info.assessments[].assessment_name
Feedback value	.info.assessments[].feedback.value
Feedback error	.info.assessments[].feedback.error
Assessment rationale	.info.assessments[].rationale

Important: Span inputs and outputs are stored as serialized JSON strings inside attributes, not as top-level span fields. Traces from third-party OpenTelemetry clients may use different attribute names (e.g., GenAI Semantic Conventions, OpenInference, or custom keys) — check the raw attributes dict to find the equivalent fields.

If paths don't match (structure may vary by MLflow version), discover them:

# Top-level keys
jq 'keys' /tmp/trace.json

# Span keys
jq '.data.spans[0] | keys' /tmp/trace.json

# Status structure
jq '.data.spans[0].status' /tmp/trace.json

Quick Health Check

After fetching a trace to a file, run this to get a summary:

jq '{
  state: .info.state,
  span_count: (.data.spans | length),
  error_spans: [.data.spans[] | select(.status.code == "STATUS_CODE_ERROR") | .name],
  assessment_errors: [.info.assessments[] | select(.feedback.error) | .assessment_name]
}' /tmp/trace.json

Analysis Insights

• state: OK does not mean correct output. It only means no unhandled exception. Check assessments for quality signals, and if none exist, analyze the trace's inputs, outputs, and intermediate span data directly for issues.
• Always consult the rationale when interpreting assessment values. The value alone can be misleading — for example, a user_frustration assessment with value: "no" could mean "no frustration detected" or "the frustration check did not pass" (i.e., frustration is present), depending on how the scorer was configured. The .rationale field (a top-level assessment field, not nested under .feedback) explains what the value means in context and often describes the issue in plain language before you need to examine any spans.
• Assessments tell you what went wrong; spans tell you where. If assessments exist, use feedback/expectations to form a hypothesis, then confirm it in the span tree. If no assessments exist, examine span inputs/outputs to identify where the execution diverged from expected behavior.
• Assessment errors are not trace errors. If an assessment has an error field, it means the scorer or judge that evaluated the trace failed — not that the trace itself has a problem. The trace may be perfectly fine; the assessment's value is just unreliable. This can happen when a scorer crashes (e.g., timed out, returned unparseable output) or when a scorer was applied to a trace type it wasn't designed for (e.g., a retrieval relevance scorer applied to a trace with no retrieval steps). The latter is a scorer configuration issue, not a trace issue.
• Span timing reveals performance issues. Gaps between parent and child spans indicate overhead; repeated span names suggest retries; compare individual span durations to find bottlenecks.
• Token usage explains latency and cost. Look for token usage in trace metadata (e.g., mlflow.trace.tokenUsage) or span attributes (e.g., mlflow.chat.tokenUsage). Not all clients set these — check the raw attributes dict for equivalent fields. Spikes in input tokens may indicate prompt injection or overly large context.

Codebase Correlation

MLflow Tracing captures inputs, outputs, and metadata from different parts of an application's call stack. By correlating trace contents with the source code, issues can be root-caused more precisely than from the trace alone.

• Span names map to functions. Span names typically match the function decorated with @mlflow.trace or wrapped in mlflow.start_span(). For autologged spans (LangChain, OpenAI, etc.), names follow framework conventions instead (e.g., ChatOpenAI, RetrievalQA).
• The span tree mirrors the call stack. If span A is the parent of span B, then function A called function B.
• Span inputs/outputs correspond to function parameters/return values. Comparing them against the code logic reveals whether the function behaved as designed or produced an unexpected result.
• The trace shows what happened; the code shows why. A retriever returning irrelevant results might trace back to a faulty similarity threshold. Incorrect span inputs might reveal wrong model parameters or missing environment variables set in code.

Example: Investigating a Wrong Answer

A user reports that their customer support agent gave an incorrect answer for the query "What is our refund policy?" There are no assessments on the trace.

1. Fetch the trace and check high-level signals.

The trace has state: OK — no crash occurred. No assessments are present, so examine the trace's inputs and outputs directly. The response_preview says "Our shipping policy states that orders are delivered within 3-5 business days..." — this answers a different question than what was asked.

2. Examine spans to locate the problem.

The span tree shows:

customer_support_agent (AGENT) — OK
├── plan_action (LLM) — OK
│   outputs: {"tool_call": "search_knowledge_base", "args": {"query": "refund policy"}}
├── search_knowledge_base (TOOL) — OK
│   inputs: {"query": "refund policy"}
│   outputs: [{"doc": "Shipping takes 3-5 business days...", "score": 0.82}]
├── generate_response (LLM) — OK
│   inputs: {"messages": [..., {"role": "user", "content": "Context: Shipping takes 3-5 business days..."}]}
│   outputs: {"content": "Our shipping policy states..."}

The agent correctly decided to search for "refund policy," but the search_knowledge_base tool returned a shipping document. The LLM then faithfully answered using the wrong context. The problem is in the tool's retrieval, not the agent's reasoning or the LLM's generation.

3. Correlate with the codebase.

The span search_knowledge_base maps to a function in the application code. Investigating reveals the vector index was built from only the shipping FAQ — the refund policy documents were never indexed.

4. Recommendations.

• Re-index the knowledge base to include refund policy documents.
• Add a retrieval relevance scorer to detect when retrieved context doesn't match the query topic.
• Consider adding expectation assessments with correct answers for common queries to enable regression testing.