Point at a behavior.
Get the cause.

CRUX walks your history and answers one question. Which commit changed this behavior and which lines did the work.

Four moves. One answer.

Point

You name a behavior. A command. A test. A benchmark. Narrow targets come back fast. Wide ones take longer.

Hunt

CRUX walks 412 commits in range. Each gets a probe run in a sealed sandbox. The walk stops when the behavior flips.

Trim

The flipping commit is the start. CRUX cuts the change down. Hunks drop. Inputs shrink. Env vars narrow. The cut stops when the change stops reproducing.

Report

You get the commit and the trimmed cause. The report cites the run. One command recreates the whole finding.

Every run in a sealed world

Your probe runs against a pinned snapshot. Same toolchain. Same inputs. Same seed. No network surprises. No floating versions. When CRUX reports a flip it proves it with a replay.

crux replay

$ crux replay 7f31ac+batch1024+seed42 World 7f31ac+batch1024+seed42 sealed Toolchain python 3.12.2 pinned Input seed 42 batch size 1024 Commits 7f31ac parent 9c02e7 Probe ./encode.mjs sample.bin Pass 1 before 1.02s 128 bytes Pass 2 after 0.84s 128 bytes Result reproduced. Blame stands. $ █

Pick a behavior. Any behavior.

Test pass or fail

The simplest target. A runner command is enough. Exit code tells the story.

Output equality

Same input. New output. CRUX spots the drift and pins the commit.

Latency

A regression is a behavior too. CRUX hunts timing drift across the range.

Memory

Peak usage climbs across a range. CRUX finds the commit that moved it.

API surface

Signatures appear or vanish. CRUX sees the shape change and names it.

Log lines

A line appears in the logs. That is a behavior as well. Same hunt.

What you never get

No build log dumps

CRUX reads behavior and history. It does not read your CI noise.

No confident guesses

When the cause is fuzzy you get a ranked list with evidence for each candidate.

No unrepeatable findings

A finding without a replay command does not ship.

Skip the guess. Get the build.

The CLI ships free forever. Hosted watch mode comes later. Join the list and the first build lands in your inbox.

Point at a behavior.Get the cause.