“Delete your CLAUDE.md every 6 months”: the Boris Cherny advice almost everyone applies the wrong way

🧹 “Delete your CLAUDE.md every 6 months.” That advice comes from the creator of Claude Code. Yet almost everyone is applying it the wrong way.

Boris Cherny’s advice at YC Startup School

At YC Startup School, Boris Cherny gave a piece of advice that sounds like an own goal: every six months, delete your CLAUDE.md, your skills and your hooks. Then watch how the model behaves.

With Opus 5 he went further: try deleting everything, because the model may no longer need the instructions that previous models required.

The advice comes from Boris Cherny’s talk at YC Startup School, July 2026.

Over 80% of the system prompt removed for Opus 5

As proof that this is more than theory: for Opus 5, the Claude Code team removed over 80% of the system prompt. Without recording any regressions in their coding tests.

The reason is that a system prompt is largely made up of patches created to correct the behaviors that previous models got wrong.

When the model improves, many of those patches suddenly become useless. That’s why it’s possible to cut 80% of the instructions in one go.

The figure comes from the post Anthropic published on July 24, 2026: over 80% of Claude Code’s system prompt removed for Opus 5 and Fable 5, with no measurable loss on coding evaluations.

The second half of the advice: reintroduce one rule at a time

But here comes the part almost nobody mentions.

The advice has a second half: after deleting everything, reintroduce ONE instruction at a time, and only when you see the model genuinely struggle.

Without this step you are simply throwing away your notes.

And above all: your CLAUDE.md is NOT a system prompt. It contains more than patches. So don’t delete it blindly.

The test: which CLAUDE.md rules to keep and which to delete

💡 Practical tip: before deleting a rule, put it through a simple test:

“Would a smarter model, reading my repository, be able to figure it out on its own?”

Is it a preference of yours, like style, tone or conventions? → Keep it: the model can’t guess it.

Is it information that can be derived by reading the code, like the project structure, the commands or the location of files? → It can be deleted.

Is it a constraint that MUST be respected, for example a security or business rule? → Keep it: it’s non-negotiable.

Someone who applied this criterion to a CLAUDE.md made up of 79 rules found that only 22% could actually be deleted. The rest was context that was still necessary.

Having fewer instructions is not always better. Having the right ones is.

How often do you clean up your CLAUDE.md? Have you ever tried deleting it entirely to see what happens?