Search for why second brains fail and you get the same answer: they become graveyards. People capture more than they ever read back, the vault swells, and the habit dies somewhere around note four hundred.
That is a real problem, and it is not the one that matters here. It describes a system with a human reader, where the cost of a stale note is that you scroll past it. Once a model is reading your context instead, staleness stops being clutter and starts being wrong answers.
The model cannot tell what is out of date
You know your old pricing is old. You remember changing it, you would never quote it, and if you saw it in a folder you would ignore it without thinking.
A model has none of that. It reads what it is given, treats a superseded document with exactly the confidence it treats a current one, and produces an answer built on the wrong thing. Worse, the more carefully you wrote that old document, the more authoritative it looks.
This is the same failure shape as a workflow that breaks quietly: nothing errors, output keeps arriving, and the only signal is that it is subtly wrong in a way you have to already know the answer to catch.
Four things that go stale, in the order they bite
Positions you have changed. Pricing, scope, what you will and will not take on. These are the most damaging because they are the ones a model most confidently repeats.
People and roles. Who does what, who to ask, who owns a relationship. Teams move faster than anyone updates a document about them.
Standards. What good looks like drifts as you get better. A quality bar written eighteen months ago quietly holds your output to a lower line than you would accept today.
Anything with a date in it. Last year’s plan, the current quarter’s targets, the roadmap. These do not decay gradually - they flip from useful to actively misleading on a known date, which is what makes them the easiest to handle.
The routine, which is smaller than it sounds
Review on a trigger, not a calendar. A monthly audit is a chore nobody keeps. The things that actually work are event-shaped: when you change a price, when someone leaves, when you lose or win work for a reason worth recording. The trigger is the decision, and the update takes a minute at the moment you already have the context.
Date anything that expires, and say what supersedes it. A document that opens “as of March 2026, superseded by the Q3 note” is safe to leave in place. One that just stops being true is not. This is the single highest-value habit here and it costs one line.
Delete rather than archive. An archive folder your model can still read is not an archive. If it is wrong, it goes - the version history has it if you ever need it back.
Re-ask a question you know the answer to. Once a month, ask your assistant something about your own business where you already know what good looks like. If the answer has drifted, something underneath it has gone stale. It is the cheapest test available and almost nobody runs it.
Why this is the half people skip
Building a second brain is satisfying. Maintaining one is not, which is why most of the writing about them stops at capture and organisation, and why most of them are quietly untrustworthy within a year.
The routine is what turns it from a project into something that compounds - and it is why the Oxford Second Brain Bootcamp ends with how you will keep it rather than with what is in it. You leave with the thing populated from your own material, and with the habit that stops it decaying. No coding required, and nothing hand-waved.