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bug-repro-minimizer

Reduce a failing test case, script, or input file to the smallest version that still reproduces a bug. Use when the user has a large reproduction and asks to minimize, bisect, or isolate it, or says things like "it stops failing when I remove half the file". Do not use when there is no reliable reproduction yet - help build one first, then minimize it.

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Bug Repro Minimizer

Shrink the reproduction until every remaining line is load-bearing. The deliverable is a minimal case plus proof that each removed part was irrelevant.

Prerequisite: a reliable oracle

Before cutting anything, define a command that exits 0 for "bug gone" and non-zero for "bug present" — and run it 3 times on the untouched reproduction. If results vary, the bug is flaky; stop and stabilize the oracle first (fixed seed, pinned versions, retries) or minimization will chase noise.

Reduction loop

  1. Cut coarse to fine. Remove the biggest independent chunk first: a whole file, a function, a config section. Halve, don't nibble — deleting one line at a time on a 2,000-line repro wastes hundreds of runs.
  2. Run the oracle after every single cut. Bug still present → keep the cut. Bug gone → restore the chunk, split it in half, try each half.
  3. Then simplify values. Once no structure can be removed: shrink strings to "x", numbers to 0/1, arrays to one element — reverting any change the oracle rejects.
  4. Stop when stuck. When every remaining element has been tried and each one is required, the case is minimal.

Verify the result

  • Run the oracle 3 times on the final case — still failing every time.
  • Restore any one removed piece at random and confirm the case still fails (guards against accidentally minimizing into a different bug — compare the error message/stack, not just the exit code).

Example

start:  1,842-line SQL dump, importer crashes
cut 1:  drop bottom half            → still crashes (keep)
cut 2:  drop remaining top half     → passes (restore, split)
...
final:  3 lines - a CREATE TABLE and one INSERT with a NUL byte in a varchar

The final report: the 3-line case, the oracle command, and the observation that the NUL byte is the trigger — which is the actual bug report.

Rules

  • Never minimize on the user's only copy; work on a duplicate.
  • Log every cut and its oracle result; the log is the proof of minimality.
  • If two different errors appear during reduction, fork: minimize each separately rather than blending them.

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