Learn English – Why does this sentence have wrong subject-verb agreement

Both sentences are correct. It is sometimes very effective to use all in a cleft structure if you want to focus on one particular thing (meaning the only thing,everything, nothing more). All is a pronoun in these sentences. All (that) I +verb is actually an object in the front position and the real subject is delayed . We find it after be. The traditional agreement (a singular noun agrees with a singular and a plural noun with a plural verb form ) is valid. But when all is a direct object we can use either a single or a plural verb after the clause. We can consider what "is left" as an entity.The traditional agreement is used more often.

All (that) I want for Christmas is a new coat. A new coat is all (that) I want for Christmas.

All (that) he had were buckets of water for his severely skinny herd. Buckets of water were all he had.

All I need is names and addresses. All I need are names and addresses.

Every other use requires a singular verb.

All (that) I have is yours.