Anthropic made Opus 5.5 cheaper. Then it broke four things your agent depends on.

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.

Thinking is always on

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.

The default effort level has also dropped from high on Opus 5 to medium on Opus 5.5, 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.

No more forced tool calls

Forced tool use no longer works either, as setting tool_choice to any or tool returns a 400 error, including on the token counting endpoint, where cost estimates built on those settings will fail along with the requests they were meant to price. Many agent loops force a call when a step has to query a database, run code, or reach another service, and Anthropic’s replacement is auto-combined with strict tool use or structured outputs, with the prompt stating when the tool applies.

Routing and conversation history

Thinking blocks are now tied to the model and conversation that produced them. On the Claude API, Fable 5.1 and Mythos 5.1 are the only other models that can read Opus 5.5 thinking blocks, so a router or fallback that hands a conversation to any other model will run those turns without the earlier reasoning instead of returning an error.

That adds another layer for teams already watching whether their agent calls are quietly being routed to an older model. Opus 5.5 can read thinking blocks from Opus 5 and earlier Opus, Sonnet, and Haiku models, but not from Fable or Mythos.

Conversations must also stay append-only for those blocks to remain valid. Trimming old messages, changing tool definitions, summarizing earlier context on the client side, or rewriting the system prompt mid-conversation invalidates existing thinking blocks, and for accounts created on or after August 31, 2026, at midnight UTC, replaying a thinking block after one of those edits returns a 400 error by default. Older accounts get no error, but the invalid blocks still reach the model, and Anthropic says future models will enforce the check for all accounts. Integrations that never edit earlier turns need no code change, and Anthropic says Claude Code, claude.ai, Claude Managed Agents, and the Claude Agent SDK already work this way, while agents that compact their own context should follow the company’s preserved thinking documentation.

The fourth change affects computer-use agents on the Claude API and Google Cloud, where Opus 5.5 rejects the computer_20251124 tool and accepts computer use only through the computer_toolset_20260801 toolset. The request itself gets simpler because the beta header goes away and the toolset entry takes no name or display dimensions, but the agent loop needs more work. Each action now arrives as its own tool_use block identified by the block’s name rather than input.action, a single turn can contain several of them, and every result has to echo toolset_name. The older tool still works on Amazon Bedrock, and Anthropic directs developers on other platforms to the computer use tool’s compatibility documentation.

…a router or fallback that hands a conversation to any other model will run those turns without the earlier reasoning instead of returning an error.

Changes that won’t throw errors

The change most likely to go unnoticed doesn’t produce an error at all. On Opus 5, text Claude writes between tool calls comes back as text blocks, but on Opus 5.5 that narration arrives as progress-update thinking blocks, and at the default thinking.display setting of omitted those blocks are empty.

Any agent interface that streams that narration to users will go silent between tool calls until developers set display to updates, a beta option that returns progress updates while keeping reasoning hidden, or to summarized, which returns both, and then render each non-empty thinking block ahead of the tool call it precedes.

Opus 5.5 also ships with broader safety classifiers. It can return a stop_reason of refusal with stop_details categories that now include bio and reasoning_extraction alongside cyber, and Anthropic’s server-side fallback won’t retry requests declined under reasoning_extraction, handing the refusal back to the application instead.

Agents that don’t handle refusals will stop mid-task, a problem developers have already run into with OpenAI’s safety system cutting off API responses.

The change most likely to go unnoticed doesn’t produce an error at all.

Upgrading from older models

Teams coming from Opus 4.8 need to work through the Opus 5 migration first, which covers thinking being on by default and the response-shape changes that follow, before applying the Opus 5.5 changes. Teams on Opus 4.7 or earlier have more ground to cover, and those on models older than Opus 4.7 also face rejected sampling parameters, rejected manual extended thinking, removed prefill, and a newer tokenizer.

Claude Managed Agents users only need to change the model name. Developers working in Claude Code can run /claude-api migrate to apply the model ID swap, parameter changes, prefill replacement, and effort calibration across a codebase before reviewing a checklist of items to verify by hand.

Anthropic recommends testing the migration in a development environment before switching production traffic. Developers maintaining their own integrations will need to test the pieces around the model, too. Tool calls, model handoffs, conversation history, and user-facing progress updates can all behave differently after the switch, because agent failures often originate outside the model itself.

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