"all they need _is_ some words" or "all they need _are_ some words"

Which one is the correct form?

• "all they need is some words"
• "all they need are some words"

Solution 1:

Both forms are perfectly fine English. I disagree with the accepted answer by Ronan, in which it is claimed that only the plural form is acceptable.

To begin with, here are examples from published literature where the author used the singular verb is despite the fact that complement (in boldface; some words in your examples) is in the plural.

All we need is examples from ordinary life. (source)
All we need is other words that mean the same. (source)
All we need is fewer pounds. (source)

The plural form is, of course, also found in published literature:

All we need are a few symbols here and there. (source)
All we need are words like 'reason' and 'evidence'. (source)
All we need are the numbers. (source)

Another consideration is that, as pointed out in CGEL (p. 1423), the construction in question is closely related to the pseudo-cleft construction, namely, to

What they need is/are some words.

But in pseudo-clefts, both singular and plural are acceptable (ComGEL, p. 767):

Pseudo-cleft constructions with a fronted object what may have a plural subject complement (cf10.34):

What we need most is books.

But what is ambivalent in number, often interpreted as equivalent to either 'the thing that' or 'the things that', so that we also find a plural verb in concord with the subject what-clause:

What we need most are books.