Why does a list returned by Stream.toList reject add after a migration?

A terminal collection operation does not automatically promise a mutable result. Review the contract when replacing one stream terminal operation with another. If the next step must append review rows, use stream.collect(Collectors.toCollection(ArrayList::new)), or construct a new ArrayList from the completed result. Keep Stream.toList when a nonmodifiable result is the intended boundary. Avoid depending on the runtime class of a collector implementation that does not promise a particular class. Add a regression test that performs the actual downstream mutation rather than only comparing printed contents. Verify encounter order where the pipeline defines one. The mutable container does not make its elements immutable, and parallel upstream side effects remain a separate correctness issue.