- Combinations.tryUse: only write response when non-null and non-blank, preventing NPE/IllegalArgument for effects-only recipes. - CombinationFactory.fromDto: throw WorldLoadException when a == b, since item singletons make a self-combination impossible at runtime. - Tests: +effectsOnlyRecipeAppliesEffectsWithoutProduceOrResponse (CombinationsTest 7→8), +rejectsSelfCombination (CombinationFactoryTest 4→5). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>