Claude Code Best Practices: 3 Lessons from 400,000 Sessions

I used to think Claude Code best practices were a matter of taste. Plan mode or not. Long CLAUDE.md or short. Pick what suits you, move on. Then Anthropic scored roughly 400k sessions from over 235k users against hard evidence of success. Tests passing, commits landing, users confirming they got what they asked for. Taste turned out to be measurable. In this article, I’ll walk through what separated the sessions that worked from the ones that didn’t. The gap had nothing to do with the model. It was behaviour. And the study didn’t define expertise by job title or years of experience. It read three things off the transcript: One thing to note before we get into them: expertise here is task-specific. A senior engineer asking their first Rust question is a beginner at Rust. An accountant who’s never written Python, but who tells Claude exactly which reconciliation rules to enforce and catches the edge case it fumbles at month end, is an expert at that task. All ten of the largest occupation groups landed within seven points of software engineers. The study found that in novice sessions, each prompt set off about five Claude actions and roughly 600 words…