trace-audit

Chrome DevTools Trace Audit

Analyze a Chrome DevTools Performance trace and produce a comprehensive performance audit report.

Usage

/trace-audit <path-to-trace.json>

The argument is the absolute path to a Chrome DevTools trace JSON file (exported from the Performance panel).

Workflow

Follow these steps in order. Use parallel tool calls wherever noted.

Step 1 — Validate the trace file

Read the first 100 lines of the file using the Read tool. Confirm it is a valid Chrome DevTools trace by checking for:

• A top-level traceEvents array, or a bare JSON array starting with [
• Event objects with name, cat, ph, ts fields
• Presence of __metadata or TracingStartedInBrowser events

If validation fails, tell the user this doesn't appear to be a Chrome DevTools trace and stop.

Step 2 — Extract metadata

Use Grep on the trace file to extract (run these in parallel):

1. Site URL — grep for TracingStartedInBrowser or navigationStart or FrameCommittedInBrowser and look for a URL in the args
2. Process names — grep for process_name or thread_name to identify renderer, browser, GPU processes
3. Trace time range — grep for the first and last "ts": values to compute trace duration

Step 3 — Run detection passes

Refer to detection-heuristics.md for the full set of patterns and thresholds. Run all detection categories in parallel using Grep. For each category:

• Use the specified grep pattern on the trace file
• Collect matching lines with surrounding context where helpful (-C 1 or -C 2)
• Count matches and extract durations/values from the matched JSON

The detection categories are:

1. Long Tasks (RunTask with dur > 50000)
2. Layout Thrashing (InvalidateLayout → Layout pairs)
3. Forced Reflows (Layout events with stackTrace)
4. rAF Ticker Loops (RequestAnimationFrame frequency)
5. Style Recalc Storms (UpdateLayoutTree with dur > 5000)
6. Paint Storms (Paint events with dur > 3000)
7. GC Pressure (MajorGC / V8.GC_MARK_COMPACTOR)
8. CLS (LayoutShift cumulative score)
9. INP (EventTiming max duration)
10. Network Errors (ResourceReceiveResponse with statusCode >= 400)
11. Redundant Fetches (same URL fetched multiple times)
12. Script Eval (EvaluateScript / CompileScript with dur > 50000)
13. Long Animation Frames (LoAF / LongAnimationFrame)

Step 4 — Aggregate findings

For each detection category:

• Compute total count of flagged events
• Extract the worst offender (max duration or highest score)
• Classify severity: Critical (red) or Warning (yellow) based on the thresholds in detection-heuristics.md
• Skip categories with zero findings

Step 5 — Identify timeline hotspots

Group flagged events by timestamp into time windows (e.g., 500ms buckets). Identify windows where multiple issue categories overlap — these are hotspot ranges that represent the most problematic sections of the trace.

Step 6 — Generate report

Output the report using the structure defined in report-format.md. The report should be:

• Actionable — every issue links to a concrete fix
• Scannable — use tables, severity badges, and clear headings
• Complete — cover all categories, even if just to say "no issues found"