exploring-llm-traces

What this skill does

# Exploring LLM traces with MCP tools

PostHog captures LLM/AI agent activity as traces. Each trace is a tree of events representing
a single AI interaction — from the top-level agent invocation down to individual LLM API calls.

## Available tools

| Tool                            | Purpose                                                       |
| ------------------------------- | ------------------------------------------------------------- |
| `posthog:query-llm-traces-list` | Search and list traces; can return large multi-trace payloads |
| `posthog:query-llm-trace`       | Get a single trace by ID with full event tree                 |
| `posthog:read-data-schema`      | Discover custom event/person properties before filtering      |
| `posthog:execute-sql`           | Ad-hoc SQL for complex trace analysis                         |

## Event hierarchy

See the [event reference](./references/events-and-properties.md) for the full schema.

```text
$ai_trace (top-level container)
  └── $ai_span (logical groupings, e.g. "RAG retrieval", "tool execution")
        ├── $ai_generation (individual LLM API call)
        └── $ai_embedding (embedding creation)
```

Events are linked via `$ai_parent_id` → parent's `$ai_span_id` or `$ai_trace_id`.

## Workflow: debug a trace or session from a URL

### Step 1 — Classify the URL

First inspect the path. Do not treat every UUID-looking value as a trace ID.

- `/ai-observability/traces/<trace_id>` or legacy `/llm-analytics/traces/<trace_id>` / `/llm-observability/traces/<trace_id>` is a single trace. Fetch it with `posthog:query-llm-trace`.
- `/ai-observability/sessions/<session_id>` or legacy `/llm-analytics/sessions/<session_id>` is an AI session, not a trace. Fetch traces with `posthog:query-llm-traces-list` filtered by event property `$ai_session_id`.

Preserve `date_from` / `date_to` query parameters from the URL when present.
If none are present but the URL has a `timestamp` query parameter, use that timestamp as the anchor and query an absolute window around it, for example `timestamp - 36h` to `timestamp + 36h`.
This handles exact session links whose UI timestamp may be offset from the stored event timestamps while keeping the query bounded.
If the URL has neither explicit dates nor `timestamp`, use a safe default like `{"date_from": "-7d"}`.

For exact trace and session URLs, skip schema discovery for the standard `$ai_*` fields used below. These are AI observability built-ins, not project-specific custom properties.

### Step 2 — Fetch trace data

For a trace URL, call `posthog:query-llm-trace` with:

```json
{
  "traceId": "<trace_id>",
  "dateRange": { "date_from": "-7d" }
}
```

For a session URL, call `posthog:query-llm-traces-list` with:

```json
{
  "dateRange": { "date_from": "<timestamp_minus_36h>", "date_to": "<timestamp_plus_36h>" },
  "filterTestAccounts": false,
  "limit": 20,
  "properties": [{ "type": "event", "key": "$ai_session_id", "value": ["<session_id>"], "operator": "exact" }]
}
```

Use the URL's `date_from` / `date_to` values in the session query if present.
If the URL only has `timestamp`, calculate the absolute date range from that timestamp instead of using a relative range like `-1h`.
Set `filterTestAccounts: false` for an exact URL so the requested trace is not hidden by account filters.

The result contains the event tree with all properties.
The response may be large — when it exceeds the inline limit, Claude Code auto-persists it to a file.

From the result you get:

- Every event with its type (`$ai_span`, `$ai_generation`, etc.)
- Span names (`$ai_span_name`) — these are the tool/step names
- Latency, error flags, models used
- Parent-child relationships via `$ai_parent_id`
- `_posthogUrl` — **always include this in your response** so the user can click through to the UI

### Step 3 — Parse large results with scripts

When the result is persisted to a file (large traces with full `$ai_input`/`$ai_output_choices`),
use the [parsing scripts](./scripts/) to explore it.

**Start with the summary** to get the full picture, then drill into specifics:

```bash
# 1. Overview: metadata, tool calls, final output, errors
python3 scripts/print_summary.py /path/to/persisted-file.json

# 2. Timeline: chronological event list with truncated I/O
python3 scripts/print_timeline.py /path/to/persisted-file.json

# 3. Drill into a specific span's full input/output
SPAN="tool_name" python3 scripts/extract_span.py /path/to/persisted-file.json

# 4. Full conversation with thinking blocks and tool calls
python3 scripts/extract_conversation.py /path/to/persisted-file.json

# 5. Search for a keyword across all properties
SEARCH="keyword" python3 scripts/search_traces.py /path/to/persisted-file.json
```

All scripts support `MAX_LEN=N` env var to control truncation (0 = unlimited).

## Investigation patterns

### "Did the agent use the tool correctly?"

1. Find the `$ai_span` for the tool call (look at `$ai_span_name`)
2. Check `$ai_input_state` — what arguments were passed to the tool?
3. Check `$ai_output_state` — what did the tool return?
4. Check `$ai_is_error` — did the tool call fail?

### "Was the context correct?" / "Were the right files surfaced?"

1. Find the `$ai_generation` event where the LLM made the decision
2. Check `$ai_input` — this is the full message history the LLM saw
3. Look at preceding `$ai_span` events for retrieval/search steps
4. Check their `$ai_output_state` — what content was retrieved and fed to the LLM?

### "Did the subagent work?"

1. In the structural overview, find spans that are children of other spans (via `$ai_parent_id`)
2. The parent span is the orchestrator; child spans are subagent steps
3. Check each child's `$ai_output_state` and `$ai_is_error`
4. If a child span contains `$ai_generation` events, those are the subagent's LLM calls

### "Why did the LLM say X?"

1. Use `search_traces.py` to find where the text appears: `SEARCH="the text" python3 scripts/search_traces.py FILE`
2. This shows which event and property path contains it
3. Check the `$ai_input` of that generation to see what the LLM was told before it said X

## Constructing UI links

The trace tools return `_posthogUrl` — always surface this to the user.

You can also construct links manually:

- **Trace detail**: `https://app.posthog.com/ai-observability/traces/<trace_id>?timestamp=<url_encoded_timestamp>&event=<optional_event_id>`
- **Traces list with filters**: returned in `_posthogUrl` from `query-llm-traces-list`

The `timestamp` query param is **required** — use the `createdAt` of the earliest event in the trace, URL-encoded (e.g. `timestamp=2026-04-01T19%3A39%3A20Z`).

When presenting findings, always include the relevant PostHog URL so the user can verify.

## Finding traces

Use `posthog:query-llm-traces-list` to search and filter traces.

**CRITICAL: Never assume event names, property names, or property values from training data.**
Every project instruments different custom properties. For open-ended searches and custom filters, call
`posthog:read-data-schema` first to discover what properties and values actually exist in the project's
data before constructing filters.

The exception is exact AI observability trace/session URLs: use the built-in `$ai_trace_id` / `$ai_session_id`
fields directly and skip schema discovery.

### Discovering the schema first

Before filtering traces, discover what's available:

1. **Confirm AI events exist** — call `posthog:read-data-schema` with `kind: "events"` and look for `$ai_*` events
2. **Find filterable properties** — call `posthog:read-data-schema` with `kind: "event_properties"` and `event_name: "$ai_generation"` (or another AI event) to see what properties are captured
3. **Get actual values** — call `posthog:read-data-schema` with `kind: "event_property_values"`, `event_name: "$ai_generation"`, and `property_name: "$ai_model"` to see real model names in use

Only then construct the `query-llm-traces-list` call with property filters.

Thi