Getting the most out of Opus 5.5 in Claude and Claude Code

Anthropic published a usage guide for Opus 5.5 in the Claude apps and Claude Code, noting that the model largely fits existing workflows but behaves differently in three ways: it works for longer on its own, it tells you plainly what it did, and it thinks before every reply. The guide covers prompting the model, steering a long run, and checking results.

It opens with three things to try in a first session: hand over the whole task (stating what "done" looks like and when the model should stop and ask), delete "think carefully"-style lines, and — when a long run ends — read what the model needs from you first.

On asking: give the entire task in one message, name the finish line ("the tests pass," "every endpoint is migrated"), and then let it run. Opus 5.5 keeps going on long, multi-part work better than Opus 5 did, with its biggest gains over prior Opus models on multi-step work such as carrying a change through a large repository until tests pass; early testers had it run long coding tasks for hours with little oversight. A clear finish line tells it when it is done. The guide's example Claude Code prompt asks it to migrate payment endpoints, defines done as every endpoint using the new client with the old client deleted and the test suite passing, and instructs it to stop and ask only if a test fails for an unexplainable reason.

On thinking: remove "think carefully," "think step by step," and similar lines from prompts and saved instructions, because Opus 5.5 always thinks before replying and decides how much. In testing in a chat product, removing such a line made replies start sooner with no clear drop in quality. For a simple question, say "Answer directly"; in Claude Code, change the effort setting to adjust how much it thinks. For mid-run changes, type a follow-up while it works — for example, "Also keep the old endpoint names as aliases" — since runs are longer now and restarting costs more.

For design work, list the styles you do not want. Without design direction, Opus 5.5 falls back on a few default styles, and a general instruction like "avoid a generic look" mostly swaps one default for another, whereas naming specific patterns works much better — the guide's example asks for a personal website with placeholder content that avoids a cream or off-white background and italic accent words.