🔥 flamebearer

A JavaScript CPU trace analysis tool for agents and humans. Designed for performance optimization loops.

Reads Chrome DevTools Performance recordings (.json / .json.gz) and Node .cpuprofile files, and prints a compact, structured text summary — top CPU offenders, hot source lines, long tasks, with a way to drill down further. No HTML, no GUI: Speedscope already nails that.

Usage

npm install -g flamebearer

flamebearer-node bench.js     # profile a script

flamebearer profile.json.gz   # summarize a Chrome DevTools trace
flamebearer CPU.*.cpuprofile  # summarize node --cpu-prof trace (one or more files or a folder)

With no flags, you get a one-page summary per thread — the answer to “what should I look at in this trace”.

Sample output

Sources:
 file:///Users/mourner/projects/supercluster/

=== CPU.20260521.160113.74872.0.001 ===
samples: 4277  busy: 5309.5 ms (99.9%)  idle: 6.2 ms

Heaviest stacks: (% of thread busy)
  83.6%  run → (anonymous) → load
  63.2%    _cluster → withinInto
  10.0%      sqDist
  18.1%    _createTree → finish → sort
  16.4%      sort → sort → sort
  13.6%        sort

Top CPU (self time):
 2511.9 ms  47.3%  withinInto  node_modules/kdbush/index.js:207
  815.5 ms  15.3%  (garbage collector)
  778.6 ms  14.6%  select  node_modules/kdbush/index.js:289
  531.6 ms  10.0%  sqDist  node_modules/kdbush/index.js:358
  316.8 ms   6.0%  _cluster  index.js:304
  109.2 ms   2.1%  swap  node_modules/kdbush/index.js:346
   48.3 ms   0.9%  (anonymous)  bench.js:1
   36.9 ms   0.7%  compileForInternalLoader  node:internal/bootstrap/realm:383
   35.7 ms   0.7%  swapItem  node_modules/kdbush/index.js:335
   29.9 ms   0.6%  latY  index.js:451
   27.1 ms   0.5%  load  index.js:44

Hot lines (self time): (per source line; includes inlined code)
 711.3 ms  13.4%  node_modules/kdbush/index.js:241  withinInto
 710.1 ms  13.4%  node_modules/kdbush/index.js:227  withinInto
 373.0 ms   7.0%  node_modules/kdbush/index.js:315  select
 331.0 ms   6.2%  node_modules/kdbush/index.js:226  withinInto
 311.1 ms   5.9%  node_modules/kdbush/index.js:314  select
 310.6 ms   5.8%  node_modules/kdbush/index.js:360  sqDist
 279.2 ms   5.3%  node_modules/kdbush/index.js:244  withinInto
 109.8 ms   2.1%  node_modules/kdbush/index.js:359  sqDist
 109.8 ms   2.1%  node_modules/kdbush/index.js:361  sqDist
  84.4 ms   1.6%  index.js:379  _cluster

Drilling down

flamebearer trace.json --stacks load             # summary for a specific function
flamebearer trace.json --thread main --top 30    # restrict threads, more rows
flamebearer trace.json --from 1200 --to 1800     # slice a time range (ms)

flamebearer-node bench.js arg1 -- --stacks load  # pass drilldown flags to the Node wrapper

Run flamebearer --help for the full flag list.

Source maps

Minified frames are de-minified automatically when source maps are available (sibling .map files, inline maps, or maps embedded in a Chrome trace). Pass --no-sourcemap to skip it.

Note on v1

This project used to be about generating an HTML flamegraph for Node traces, but then lay dormant since 2018. In 2026, it was revived with a new purpose: to be a useful CLI tool in the age of AI.

Thanks

• Brendan Gregg for creating the flamegraph concept and maintaining the reference implementation.
• David Mark Clements for creating 0x which originally inspired this project.
• Bernard Cornwell for the amazing books this project took its name from.