MergeWitness

Merging two changes can be deceptively reassuring: each branch passes its own tests, the text merge is clean, and the existing suite stays green. Yet the two features can still interfere when they run together. That leaves maintainers with a hard-to-explain production defect and a costly manual investigation. MergeWitness is a developer workflow for making that interaction visible before release. It prepares four reproducible snapshots—base, change A, change B, and their combined version—and runs the same explicit behavior rule on each one. The measured demo uses a synthetic catalog. One change introduces tenant-specific prices; another introduces a product cache. Each works alone. In the combined version, a price cached for Alpha leaks to Beta. Existing tests exit with code 0 in all four snapshots, but the new rule fails: serving Alpha must not change Beta's price. Bob's original focused probe was independently rerun three times in each snapshot. It passed in Base, Change A, and Change B, then failed three times in the combined snapshot, where Beta should receive 100 but observed 90. An independently strengthened derivative probe found the same interaction and also compared Beta against a fresh catalog. IBM Bob authored a nested-Map repair. The repaired disposable candidate passed at commit 17ed2d8ca9997293383ad589119879c9eb59d94e: the ordinary suite exited with code 0, the strengthened frozen probe passed, and checks confirmed both tenant pricing and the cache remained active. The public laboratory lets a judge run the scenario, inspect the exact input sequence and expected versus observed result, and see the verified repair. The local CLI/MCP workflow evaluates trusted repositories using node --test and frozen probes; its public reports record this synthetic case. A passing probe means that no failure was found by that probe; it never certifies a merge as safe. A second synthetic priority/cursor case tests a different interaction: priority ordering and an ID-based page cursor each pass alone, but their clean combination skips an item. Bob authored that probe and two feature checks. Independent measurement found the probe passing 3/3 in Base/A/B and failing 3/3 in Combined, while ordinary tests stayed green. No repair is claimed for this second case.