Is the task specialized?

The city of San Francisco contracted with a builder to do several tasks associated with roadwork and refurbishment. The contractor assigned the work to another contractor. The city sought breach of contract for, among other things, inappropriate assignment in that the service was of a personal nature, which under common law cannot be unilaterally assigned to a third party. The contractor argued that the roadwork in the contract was not of a personal nature and thus could lawfully be assigned by the original contractor to the third-party contractor.