We agreed the job in a message thread and the paperwork never followed

A price offered, a start date accepted, a thumbs-up: agreements made in message threads are still agreements, and work starting on the back of one usually settles the point. What's missing is not a contract but an administrable one - the thread never states the payment cycle, the variation procedure, the notice addresses or whose terms apply, so every one of those questions gets answered later, from positions, instead of earlier, from agreement.

If a dispute lands first, both sides pay to reconstruct what was agreed from fragments - and whoever kept the better records wins the reconstruction. The cheap fix is to paper the deal while both parties still agree what it was: scope, price, dates and terms in one signed document, even a short one. Where that moment has already passed, get the essentials confirmed in writing now; the thread plus a confirming email beats the thread alone.