Substring Check Matches Sibling Word

What goes wrong

An assertion or existence check tests for a word by substring or prefix instead of exact match, so a sibling word that contains it ('unpaid' for 'paid'), a negated phrase ('no error' for 'error'), or a longer name extending it ('mode 10' for 'mode 1') satisfies the check.

Example

The test checks that the status contains 'paid'; the status is 'unpaid' and the test passes.

Not to be confused with

The outputs differ; the matcher collapses them. Not one value in two spellings compared as unequal (Equivalent values compare different), not test data where two candidates happen to coincide (Coincident Alternative Oracle); the production-side masking twin is Pattern Catches Look-Alike Text.

Discovered by

@sks24git GitHub

Field notes · 5

@sks24git2026-11-05

A test meant to pin each pricing promise checked only a prefix of the sentence that holds it. Drop the trailing qualifier and the suite stays green while the page loses the promise.

Pin the whole sentence, not its first half. A prefix check guards only the words you remembered.
Mutate the qualifier you fear losing. A test that stays green pins nothing.

@sks24git2026-09-14beat 3 review rounds

A guard checks that a doc block mentions the name it documents, by substring. One constant's name is a strict prefix of its sibling's, so a block copied from the sibling and left unedited passes.

Substring matching on names fails where names nest. Anchor the match, or the guard blesses the exact copy-paste it was built to catch.

@sks24git2026-09-12

Two controls, two query keys, and the test matched the link by substring. One key contains the other, so pointing both controls at the same key left every assertion green and the page broken.

Match a whole query parameter, not a substring. One key can contain another and hide the swap.
Ask what your test would say if the code were wrong.

@sks24git2026-09-10

A migration test asserted one substring of the index definition. Point the index at a different column and the assertion still matches, so the wrong index would ship green.

Assert the whole shape of an index: columns, order, predicate. A substring proves the clause exists, not that it is yours.

@sks24git2026-09-10beat 3 review rounds

Both pinned needles were prefixes of a job definition, so appending a tolerate-failure key after them still matched. The claim that either edit alone would be caught was false in both directions.

A prefix match cannot see what is appended after it. Pin the block's end, or the edit simply goes below your pattern.