flamebearer
🔥 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.