What's new in Claude Opus 5.5

Claude Opus 5.5 is built for long-running agentic coding and knowledge work, priced at $4 / $20 USD per million input / output tokens. Four breaking changes affect code already running on Claude Opus 5: thinking can't be disabled, forced tool use returns an error, thinking blocks are tied to the model and the conversation, and, on the Claude API and Google Cloud, the earlier computer_20251124 computer use tool is not accepted. The first three also apply on Claude Fable 5.1. A further change alters the response shape without failing any request: text between tool calls comes back in thinking blocks whose text is empty at the default display setting. An application that streams that text to its users as progress updates goes quiet between tool calls until it sets a display value that returns the text.

New model

Adaptive thinking is always on, and the effort parameter controls thinking depth; its default on this model is medium. For the context window, output limits, knowledge cutoff, and prices, see the Claude Opus 5.5 model page; for all current models, see the models overview.

Breaking changes

Thinking can't be disabled

On Claude Opus 5, thinking is on by default and thinking: {"type": "disabled"} is accepted at effort high or below. On Claude Opus 5.5, thinking is always on: a request that sets thinking: {"type": "disabled"}, or a manual budget with thinking: {"type": "enabled", "budget_tokens": N}, returns a 400 invalid_request_error. Omit the thinking field, or send thinking: {"type": "adaptive"}, which is equivalent. No beta header is involved.

The error messages are:

"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.

"thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.

The effort parameter is the control for thinking depth, latency, and cost: lower it where you previously disabled thinking; Optimizing for cost and intelligence has measured results for choosing a level. Because every response can begin with one or more thinking blocks (returned with an empty thinking field at the default display: "omitted"), select content blocks by their type field rather than by position, and pass thinking blocks back unmodified in tool-use loops. Code that already runs on Claude Opus 5 with thinking on needs no change. See Thinking and the migration guide's before and after.

Forced tool use is not supported

Claude Opus 5.5 doesn't support forced tool use. tool_choice set to {"type": "any"} or {"type": "tool", "name": "..."} returns a 400 invalid_request_error:

tool_choice: type "tool" and "any" are not supported for this model.

tool_choice: {"type": "auto"} (the default) and {"type": "none"} are supported, and the same validation applies to the token counting endpoint. For schema-valid JSON, keep tool_choice: {"type": "auto"} and set strict: true with strict tool use, or move the schema to structured outputs. To make the model call a tool rather than reply in text, say in the prompt when the tool applies. The migration guide shows the before and after.

Thinking blocks are tied to the model and the conversation

Every thinking block records which model produced it, and each model reads its own blocks and only some other models'. Claude Opus 5.5 reads thinking blocks from Claude Opus 5 and earlier Opus, Sonnet, and Haiku models, but not from Claude Fable or Claude Mythos models. On the Claude API, Claude Fable 5.1 and Claude Mythos 5.1 read thinking blocks from Claude Opus 5.5; no other model does. A conversation that moves from Claude Opus 5 onto Claude Opus 5.5, or from Claude Opus 5.5 up to Claude Fable 5.1 or Claude Mythos 5.1 on the Claude API, keeps its reasoning. One that moves from Claude Opus 5.5 to any model other than those two, or onto it from a Claude Fable or Claude Mythos model, runs the turns after the switch without the previous model's reasoning. When a request carries a block the target model can't read, the API drops it before the model sees it: the request succeeds, and dropped blocks aren't billed. With the thinking-binding-controls-2026-08-01 beta header the drop is reported in a top-level input_transformations array. See Switching models mid-conversation.

The API also checks whether anything before a Claude Opus 5.5 thinking block (the system prompt, the tools, or an earlier message) has changed since the block was produced. As on Claude Fable 5.1, it enforces that check by default for accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API and on cloud platforms. On those accounts, a request that replays a block after such a change returns a 400 error. To drop the affected blocks instead, send the thinking-binding-controls-2026-08-01 beta header and set thinking.block_binding.prefix_mismatch_behavior to "drop_block". On older accounts, setting that field to either value opts the request in. Keep the conversation append-only so the question never arises: change instructions or tools with mid-conversation system messages rather than edits. See Preserved thinking and the migration guide's note on this change.

The computer_20251124 computer use tool is not supported on the Claude API and Google Cloud

Claude Opus 5 accepts computer use both as the computer_toolset_20260801 toolset and, with the computer-use-2025-11-24 beta header, as the earlier computer_20251124 tool. On the Claude API and Google Cloud, Claude Opus 5.5 supports only the toolset: a request that declares a computer_20251124 tool returns a 400 invalid_request_error. The message names the rejected type, then lists the tool types the model does accept (computer_toolset_20260801 among them) after Did you mean one of; it begins:

'claude-opus-5-5' does not support tool types: computer_20251124.

To move an existing integration on the Claude API or Google Cloud, follow Migrate from computer_20251124: drop the beta header, replace the tools entry with {"type": "computer_toolset_20260801"}, and update your agent loop for member tool_use blocks, batch actions, and toolset_name on results. On Amazon Bedrock, the earlier computer_20251124 tool continues to work on Claude Opus 5.5 as it does on Claude Opus 5, so no change is needed there. For other platforms, see the computer use tool's Compatibility section. Integrations that already use the toolset, and the browser use tool, need no change. The migration guide shows the request before and after.

Feature support

Claude Opus 5.5 supports per-message effort (beta), mid-conversation system messages, task budgets, prompt caching with a 512-token minimum cacheable prompt, batch processing, the Files API, PDF support, vision, and server-side and client-side tools. On the Claude API and Google Cloud, computer use requires the computer_toolset_20260801 toolset (see the breaking change). See each feature's page for model availability.

Fast mode

Fast mode (research preview) is available for Claude Opus 5.5 on the Claude API only; it is not available on Amazon Bedrock, Claude Platform on AWS, Google Cloud, or Microsoft Foundry. Set speed: "fast" with the fast-mode-2026-02-01 beta header. See Fast mode for access, supported models, and pricing.

Define tools in a message (beta)

With the inline-tools-2026-09-15 beta header, a tool_addition block in a mid-conversation system message can carry a full tool definition instead of a reference, so you can add a tool, change its schema, or move a server tool to a newer version mid-conversation without editing tools and without losing the prompt cache. This works on every model that supports mid-conversation tool changes, including Claude Opus 5.5. See Define tools in a message.

Compact on demand (beta)

With the compact-2026-09-04 beta header, a request that sends the top-level compaction parameter returns a signed compaction block summarizing the whole conversation, which you then send first in place of the summarized messages. It is available on the models that support compaction, Claude Opus 5.5 included. You choose when to compact, the request can run in the background, and the thinking blocks in the turns you keep can stay valid after the swap (under the conditions in Compaction and preserved thinking), which matters on Claude Opus 5.5 because its thinking blocks are tied to the conversation. See Compaction on demand for platform availability and the full request flow.

Behavior differences

Claude Opus 5.5 differs from Claude Opus 5 in several ways that show up without any code change. Each has guidance in Prompting Claude Opus 5.5:

• The default effort is medium. A request that omits effort runs at medium; on Claude Opus 5 it ran at high. Set effort explicitly and re-run your sweep; see Calibrate effort.
• More thinking per turn at a given effort level. At the same effort setting the model tends to think more per turn than Claude Opus 5, most of all at xhigh and max. Re-run your effort sweep rather than carrying a setting over, and leave room in max_tokens for the thinking. See Calibrate effort.
• Text between tool calls comes back in thinking blocks. The short notes the model writes between tool calls arrive as progress-update thinking blocks rather than text blocks, so at the default display: "omitted" an application that streams them to its users goes quiet between tool calls, with no error. The migration guide has the fix for receiving them, and User-facing progress updates covers how to ask for more of them.
• More safeguard categories. The model runs a biology safety classifier in addition to the cybersecurity one, and requests that push it to reproduce its internal reasoning in the response text can be declined with the reasoning_extraction category. See Refusals and fallback and Safeguard refusals.
• Sharper reading of charts, diagrams, and screenshots. The model reads values off dense charts and layout-dependent visuals much more precisely without tools, so prompt-side vision workarounds built for earlier models may no longer be needed; image tools still add accuracy on the densest inputs. See Tools for complex visual inputs.

If your Claude Opus 5 integration ran with thinking disabled, see Prompts written for thinking disabled alongside the breaking change. For the capability gains in agentic coding and code review, knowledge work, communication, visual inputs, and computer use, see Capabilities relevant to prompting.

Refusals and fallback

Claude Opus 5.5 ships with safety classifiers, and everything in Refusals and fallback applies. A declined request returns HTTP 200 with stop_reason: "refusal" and a stop_details object naming the policy area, so handle refusals and configure fallback: retry on another model with server-side fallback (fallbacks: "default", in beta, retries on the model Anthropic recommends for that category), the SDK middleware, or your own retry.

Pricing

Claude Opus 5.5 costs $4 USD per million input tokens and $20 USD per million output tokens, below Claude Opus 5's $5 and $25, with 5-minute cache writes at $5, 1-hour cache writes at $8, and cache reads at $0.20 per million tokens (0.05x the base input price). Batch processing is half price: $2 and $10. See Pricing for data residency and tool pricing.

Availability

Claude Opus 5.5 is available on:

• Claude API: all customers, as claude-opus-5-5.
• AWS: Claude in Amazon Bedrock, as anthropic.claude-opus-5-5, and Claude Platform on AWS, as claude-opus-5-5.
• Google Cloud: Claude on Google Cloud, as claude-opus-5-5.
• Microsoft Foundry: Claude in Microsoft Foundry, as claude-opus-5-5.

Migrate from Claude Opus 5

Update your model ID:

model = "claude-opus-5"  # Before
model = "claude-opus-5-5"  # After

Then remove any thinking: {"type": "disabled"} or thinking: {"type": "enabled", ...} settings and choose an effort level instead. Replace tool_choice types any and tool with auto plus strict tool use. If you use computer use through computer_20251124 on the Claude API or Google Cloud, move to the toolset. If your interface shows the text between tool calls, also set thinking.display; see Text between tool calls is returned in thinking blocks. See the migration guide for step-by-step instructions from Claude Opus 5 and earlier models, and the full checklist.

Next steps