Matthew White contributed to the substrait-io/substrait-java repository by extending the AggregationPhase handling to improve correctness and interoperability between Java and Scala components. He introduced an UNSPECIFIED value to the AggregationPhase enum in Expression.java, addressing gaps in aggregation semantics and enabling more expressive workflows. By mapping this new enum value to the Final aggregate mode in ToAggregateFunction.scala, he unified the behavior across language boundaries and ensured consistent interpretation of aggregation phases. His work involved API modification, enum definition, and pattern matching, resulting in a breaking change that deepened the integration between POJO enums and Scala mappings within the codebase.