Most don’t need extra thinking. Renames, format fixes, “explain this line” — Claude answers in a beat and gets it right.
Hard problems are different. A weird bug that doesn’t reproduce locally, an architecture decision between two reasonable options, a refactor where the wrong choice cascades — these are where Claude’s default speed works against you. You want it to slow down and reason.
Two ways to ask for that.
Informal: trigger words in the prompt
The convention that’s taken hold: drop phrases like “think harder”, “think step by step”, or “ultrathink” into your prompt and Claude shifts into extended reasoning before answering.
Ultrathink: I’m seeing inconsistent invoice totals in production but the staging tests pass. Walk through everything that could cause a divergence between the two environments.
You don’t have to be precious about it. It’s a hint, not a setting. Use it when the problem is open-ended and you’d rather wait twenty extra seconds for a better answer.
Official: /effort
Inside any , run /effort to see the levels:
- low — fewer , faster, cheaper. Mechanical edits, format passes.
- medium — the everyday default on most plans.
- high — the everyday default on Team / Enterprise. Most code work lives here.
- xhigh — a step past high, when the problem is genuinely hard but you don’t need the ceiling.
- max — hard debugging, architecture, multi-step reasoning. Slower and more expensive but materially more careful.
You can also set it at launch: claude --effort max.
There’s a keyboard shortcut for the one-off case too — toggles thinking for the next turn only, without changing the session’s effort level.
When to reach for which
- About to do a batch of one-line renames?
/effort low. - Normal feature work? Default (high or medium depending on plan).
- Stuck on a real bug, or making a decision that’s hard to reverse?
/effort maxor “think harder” in the prompt. - Architecture proposal you want a second opinion on?
/effort maxand ask Claude to argue against itself.
The rough rule: match the effort to the cost of getting it wrong. Cheap-to-revert work doesn’t need extra thinking. Hard-to-revert work pays for the extra cycles many times over.
The dial turns down, too
The reverse move is just as useful and almost nobody uses it. When you want a fast answer and don’t need the reasoning:
Give me a quick answer — I just need the gist.
Worth knowing: asking Claude to “walk me through your reasoning” or “double-check every number” is a different thing from asking it to think harder. Current already reason before answering; those phrases mostly add output for you to read rather than care taken. If you want more care, use /effort or “think harder”. If you want to see the reasoning because you’ll check it yourself, ask for it — just know which of the two you’re buying.