While others have been focusing on details of the deal, I’m worried about the overall mechanism. This agreement is a treaty under public international law between a state (the UK) & the EU (a non-state international institution), not (apart from narrow exceptions) conferring

private rights or obligations on persons like you or me, nor to be directly invoked in the domestic legal systems of the Parties. Neither the UK nor the EU has a right of action for breach by the other. This flimsy new regime is meant to replace a robust existing one that’s been

in place governing many aspects of life in the UK for 48 years. The new regime can be abrogated by a mere written notification by diplomatic channels. See Article FINPROV.8 on page 405. It’s a weak, fragile thing.
But how does the new regime start? What happens as 2021 begins?

In certain areas, the EU has what the treaties call exclusive competences. One is trade & international agreements (under certain circumstances).
There are three main types of agreement: customs unions, association agreements & the like, & partnership & cooperation agreements.

In some circumstances trade negotiations with a trade partner have been concluded but have not been either signed or ratified yet. This means that no part of the agreement is in place yet. That is the situation the EU & UK will be in as 2021 begins. The European Parliament’s

consent procedure applies to the ratification of arrangements for the withdrawal from the EU. It’s Rule of Procedure 88 & a simple majority is required. If that includes this deal, then assuming for the sake of argument that the UK Parliament votes for the agreement next week,

at New Year there is no binding treaty because the MEPs have not, or not yet, consented. How can the agreement, which has not been ratified, & so is not in force, be treated as if it were in force before the MEPs have consented? An essential formality isn't in place.

There will be an interregnum from 31 Dec until the formalities are concluded. So effectively there is no deal from 31 Dec anyway, isn't there, because retrospective legislation is not permitted, is it? I hope not: it’s generally regarded with horror. If an Article exists that

allows for retrospective effect, what is it? If there isn’t one, then the UK Gov & European Council must have a card up their sleeves. I suspect they have an agreement under Article 50(3) of the consolidated Treaty on the EU. It says: “3. The Treaties shall cease to apply to

the State in question from the date of entry into force of the withdrawal agreement or failing that, 2 yrs after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.”

So I think the negotiators must have a backup agreement to extend the period: neither side can contemplate the UK crashing out without a deal. President Macron showed us what that'd be like. I may have missed something but if so, please will someone explain what I've missed?