Anthropic made Claude Opus 5.5, released on Tuesday, cheaper than its predecessor, cutting the price from $5 to $4 per million input tokens and from $25 to $20 per million output tokens. The 1 million-token context window and 128,000-token maximum output are unchanged.

On paper, that makes upgrading an easy decision. In practice, it may not be as simple as changing the model ID.

Anthropic’s migration guide flags four breaking changes that can cause requests built for Opus 5 to return 400 errors after switching to Opus 5.5. Several other changes won’t trigger an error but could still change how an existing agent behaves.

Anthropic’s migration guide flags four breaking changes that can cause requests built for Opus 5 to return 400 errors after switching to Opus 5.5.

The first change involves thinking controls. Opus 5.5 returns a 400 error when a request sets thinking to disabled or uses enabled with budget_tokens, leaving effort as the way to control how much reasoning the model does. Agents that previously switched thinking off for simple steps to save time and tokens will need to assign those steps a lower effort level instead. Because thinking is now always on, responses begin with thinking blocks, so code that assumes the first content block is text will also need to change.

5, so requests that omit the parameter will quietly run at a lower setting. Anthropic recommends setting effort explicitly and re-running effort evaluations, since the right level for each step may have shifted along with cost and latency.

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