A rundown of 19 Claude Code habits that are now outdated or costly, most of them sourced from Anthropic's own docs and recent product changes. The big themes: persona prompts don't help (drop them for location, done-criteria and self-check instructions), context is cheaper to manage than you think if you configure connectors, auto-compact and CLAUDE.md correctly, and several "savings" moves like switching to a cheaper model mid-conversation or leaning on sub-agents actually cost more. It closes with a set of little-used slash commands: /doctor, /insights, /btw and /branch.
- A study of 162 personas across ~2,500 prompts found persona openings performed no better than asking directly, and the Claude Code team recently cut its own system prompt by roughly 80%; better to spend those words on where to look, what "done" looks like, and a self-check line.
- Write instructions positively — "write it as smooth flowing text paragraphs" rather than "do not return markdown" — because negative instructions conflict with the actual task.
- Connectors are added to new routines by default and tool definitions load up front unless you set tool access to "load tools when needed," which people have measured as costing thousands of tokens per session; conversely the old advice to delete rarely-used MCP servers is obsolete since tool search means only ~120 tokens of tool names load, with schemas pulled on demand.
- Sub-agents cost roughly seven times more tokens than standard sessions because each teammate runs its own uncached context, so they suit broad parallel searching but not context-heavy delegation — fork the conversation instead when full context is needed.
- CLAUDE.md files are read once at session start and held in memory, so mid-session edits are ignored until you clear, restart or compact; shorter files work better (Anthropic's guidance points to ~300–350 words, and Claude Code's own system prompt is about 50 instructions), and /doctor can propose trims.
- Anthropic's long-context benchmark on Opus 4.6 scored 93% retrieval at 256K tokens but dropped to 76% at 1 million tokens, meaning roughly one in four buried rules fails to be retrieved — bigger context is not more memory.
- Switching to a cheaper model mid-conversation can cost more than staying put, because each model has its own prompt cache and switching forces you to rebuild it in both directions; the same uncached-context trap applies to enabling fast mode mid-conversation.
- Anthropic calls verification the single most impactful practice, with four escalating levels: an in-prompt check, /goal, stop hooks that block the turn, and an adversarial review agent (which must be told to flag only correctness or stated requirements, or it will invent problems).
- Claude Code keeps only 30 days of resumable conversations by default — set cleanupPeriodDays in settings.json (0 wipes everything, it doesn't mean unlimited), though auto memory files are exempt from the sweep.
The persona prompt myth, tested
0:00–0:50
It kills the single most common prompt-writing habit with a study plus Anthropic's own 80% system prompt cut, and gives the three-part replacement.
They tested 162 different personas across 2 and 1/2 thousand prompts, and the prompts with a persona did no better than just asking the question directly.
Why the cheaper model costs you more
15:38–16:27
A counterintuitive cost mechanic — per-model prompt caches — that reverses the obvious optimisation most users make.
If you're 100,000 tokens into a conversation with Opus and you want to answer a question that's fairly easy to answer, it would actually be more expensive to switch to Haiku at that point than to have Opus answer.
If Claude keeps doing something you don't want despite having a rule against it in your CLAUDE.md, the file is probably too long and the rule is getting lost.
The single most impactful tip in this guide is verification... if there's no check, you're basically acting as the verification loop for Claude.
A bigger context window isn't really more memory. It's basically then going to become harder to actually retrieve the right information.