'Agree' can be used intransitively and transitively. According to Merriam Webster, your example is "chiefly British" - which I guess means it does come up but is rare in the US whereas it is standard in British English (and not "bad form" at all, please note that 'agree to the terms' changes the meaning, 'agree on or upon' is the only option here).
Oxford dict:
2.1 with object Reach agreement about (something) after negotiation.
‘if they had agreed a price the deal would have gone through’
no object ‘the commission agreed on a proposal to limit imports’
https://en.oxforddictionaries.com/definition/agree

MW:
transitive verb
2. chiefly British: to settle on by common consent
e.g. … I agreed rental terms with him … —Eric Bennett
https://www.merriam-webster.com/dictionary/agree

(the 'before' in your example does not belong to 'agreed' of course - i.e., it means 'must be agreed upon before...')