Is "details" singular?

Solution 1:

Grammar

• "Details" is plural. "Detail" is singular.

• In sentences like "here [to be] [something]", [to be] is the verb and [something] is the subject. The words there and here are never subjects.

• The verb has to agree with its subject. A singular subject takes a singular verb and a plural subject takes a plural verb.

Therefore, "Here are the details you requested" is the correct one.

Usage

As noted by Colin Fine and Kosmonaut in their comments below and by Piet Delport in his answer, "here is [plural]" is commonly used in casual English. Maybe it is more used than the grammatical form where the subject agrees with the verb (to be confirmed). Refer to Piet Deport's answer for a possible explanation.