What's new in Claude Sonnet 5.5

Claude Sonnet 5.5 offers the best combination of speed and intelligence. Five breaking changes affect code already running on Claude Sonnet 5:

• Turn off up-front thinking with between_tools.
• Forced tool use returns an error.
• Thinking blocks are tied to the model and the conversation.
• On the Claude API and Google Cloud, the earlier computer_20251124 computer use tool is not accepted.
• The advisor tool rejects Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 as advisors.

One more change alters the response shape without failing any request: text between tool calls comes back in thinking blocks. An application that streams that text to its users goes quiet between tool calls until it sets a display value that returns the text, or turns off up-front thinking with between_tools.

New model

Adaptive thinking is on by default, and the effort parameter controls thinking depth. Its default on the Claude API is high. The tokenizer is the same as Claude Sonnet 5's, so the same text produces the same token counts. For the context window, output limits, knowledge cutoff, and prices, see the Claude Sonnet 5.5 model page.

For all current models, see the models overview.

Breaking changes

Turn off up-front thinking with between_tools

To turn off up-front thinking on Claude Sonnet 5.5, send thinking: {"type": "between_tools"} instead of "disabled". It's the lowest thinking setting on this model. It's available on every platform that offers Claude Sonnet 5.5. It needs no beta header. The short progress updates the model writes between tool calls still come back as thinking blocks with their summary text. Pass those blocks back unchanged with the rest of the assistant turn. A progress-update block you send back gives the model the full note it wrote, not the summary. If your requests don't use tools, the response contains only text, as with disabled on Claude Sonnet 5.

On Claude Sonnet 5.5, a request that sends thinking: {"type": "disabled"} returns a 400 invalid_request_error whose message points to between_tools.

between_tools is accepted at low, medium, and high effort. At xhigh or max effort, a request with between_tools returns a 400 error. To run at xhigh or max, use adaptive thinking: omit the thinking field or send thinking: {"type": "adaptive"}, which is equivalent. With between_tools, effort can't change mid-conversation: a per-message output_config.effort that differs from the level in effect returns a 400 error. To vary effort per turn, use adaptive thinking.

between_tools takes no other field: display, budget_tokens, or block_binding sent with it returns a 400 error. Manual thinking budgets (thinking: {"type": "enabled", "budget_tokens": N}) return a 400 error. See Thinking and the migration guide's before and after.

Forced tool use is not supported

Claude Sonnet 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. The same check applies to the token counting endpoint. For schema-valid tool input, 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. Each model reads its own blocks and only some other models' blocks. Claude Sonnet 5.5 reads thinking blocks from Claude Sonnet 5, Claude Opus 4.8, Claude Haiku 4.5, and earlier models, but not from Claude Opus 5, Claude Opus 5.5, or any Claude Fable or Claude Mythos model. No other model reads Claude Sonnet 5.5 thinking blocks.

So a conversation that moves from Claude Sonnet 5 onto Claude Sonnet 5.5 keeps its reasoning, and one that moves from Claude Sonnet 5.5 to any other model runs the turns after the switch without it. 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 Sonnet 5.5 thinking block has changed since the block was produced: the system prompt, the tools, or an earlier message. It enforces that check by default for accounts created on or after August 31, 2026, 00:00 UTC, on the Claude API, Amazon Bedrock, and Google Cloud. 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. block_binding works only with thinking: {"type": "adaptive"}. With between_tools, keep the history append-only, or strip the thinking blocks from the edited turn on.

Keep the conversation append-only so the check never fails: 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

On the Claude API and Google Cloud, Claude Sonnet 5.5 supports computer use only through the computer_toolset_20260801 toolset. A request that declares the earlier computer_20251124 tool returns a 400 invalid_request_error. On the Claude API, the message names the rejected type, then lists the tool types the model does accept. It begins:

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

On Amazon Bedrock, Claude Sonnet 5.5 accepts the earlier computer_20251124 tool.

To move an existing integration on the Claude API or Google Cloud, follow Migrate from computer_20251124, which shows the request before and after. 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. The toolset is available on the Claude API and Google Cloud. 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.

Some advisor tool pairings are not supported

With the advisor tool (beta), a Claude Sonnet 5.5 executor needs Claude Mythos 5.1, Claude Fable 5.1, Claude Mythos 5, Claude Fable 5, Claude Opus 5.5, or Claude Opus 5 as its advisor, or Claude Sonnet 5.5 itself. Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5 advisors work with a Claude Sonnet 5 executor, but with a Claude Sonnet 5.5 executor they return a 400 invalid_request_error. Every advisor that Claude Sonnet 5.5 accepts returns its advice encrypted, as an advisor_redacted_result block, so your client can't read the advice text. See the advisor tool's Model compatibility and Result variants.

Feature support

Claude Sonnet 5.5 supports per-message effort (beta), mid-conversation system messages, mid-conversation tool changes (beta), prompt caching with a 512-token minimum cacheable prompt, batch processing, the Files API, PDF support, vision, and server-side and client-side tools. Per-message effort, mid-conversation system messages, and mid-conversation tool changes aren't available on Claude Sonnet 5, whose minimum cacheable prompt is 1,024 tokens. 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.

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 that summarizes the whole conversation. You then send that block first, in place of the summarized messages. You choose when to compact, and the thinking blocks in the turns you keep can stay valid after the swap, under the conditions in Compaction and preserved thinking. That matters on Claude Sonnet 5.5 because its thinking blocks are tied to the conversation. See Compaction on demand for platform availability and the full request flow.

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. 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. See Define tools in a message.

Thinking blocks stay with the account that produced them

Thinking blocks that Claude Sonnet 5.5 produces work only in the account that produced them, or in an account linked to it. When another account sends one of these blocks, the API drops the block before the model sees it, and the request succeeds. On the Claude API and Google Cloud, with the thinking-binding-controls-2026-08-01 beta header, the response lists each dropped block in input_transformations with reason: "organization_binding_mismatch". Blocks from earlier models aren't affected. See Preserved thinking.

Behavior differences

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

• Effort levels are recalibrated. An effort level doesn't produce the same amount of thinking as it did on Claude Sonnet 5. Re-run your effort sweep rather than carrying a setting over. Start at high unless your workload is agentic or latency-sensitive. For agentic coding and multistep tool use, start at medium for well-specified tasks and move to high for harder or longer ones. For chat and other latency-sensitive work, start at medium or low.
• Text between tool calls comes back in thinking blocks. Between tool calls, notes longer than a sentence or two come back as progress-update thinking blocks. Shorter remarks stay text. At the default display: "omitted", the progress-update blocks' text is empty, so an application that streams those notes to its users goes quiet between tool calls, with no error. If you turn off up-front thinking with between_tools, the text comes back. The migration guide shows how to receive it.
• Safeguard categories. The model's safeguards can decline a request in five stop_details categories. "cyber" means the request could enable cyber harm. "bio" means it could enable biological harm. "frontier_llm" means it could assist the development of competing AI models. "reasoning_extraction" means it asks the model to reproduce its internal reasoning in the response text. "general_harms" means it falls under another usage-policy area. See Refusals, fallback, and billing.

Refusals, fallback, and billing

Everything in Refusals and fallback applies to Claude Sonnet 5.5. A declined request returns HTTP 200 with stop_reason: "refusal" and a stop_details object naming the policy area. Handle refusals and configure fallback. Server-side fallback (fallbacks: "default", in beta, on the Claude API) retries "cyber" and "frontier_llm" declines on Claude Sonnet 5. It doesn't retry "bio", "reasoning_extraction", or "general_harms" declines. You can also use the SDK middleware or your own retry. Whether a refusal that arrives before any output is billed depends on its refusal category, and it counts against your rate limits either way. See How refusals are billed.

Pricing

Claude Sonnet 5.5 has the same prices as Claude Sonnet 5, including prompt caching and batch processing rates. See Pricing for the full list, data residency, and tool pricing.

Availability

Claude Sonnet 5.5 is available on:

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

Migrate from Claude Sonnet 5

Update your model ID:

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

Then check six things:

1. If your code turns thinking off with disabled, send between_tools instead, at high effort or below.
2. Replace tool_choice types any and tool with auto plus strict tool use.
3. Keep conversations append-only. A request that replays a Claude Sonnet 5.5 thinking block after an edit to earlier history can return a 400 error. See Thinking blocks are tied to the model and the conversation.
4. If you use computer use through computer_20251124 on the Claude API or Google Cloud, move to the toolset.
5. If you use the advisor tool with a Claude Opus 4.8, Claude Opus 4.7, or Claude Sonnet 5 advisor, switch to an advisor that Claude Sonnet 5.5 accepts.
6. If your interface shows the text between tool calls, set thinking.display when you use adaptive thinking. With between_tools, the text comes back without it. See Text between tool calls is returned in thinking blocks.

The migration guide has step-by-step instructions from Claude Sonnet 5 and earlier models, and the full checklist.

Next steps