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migration-guide changedmodels/opus-5-5/migration-guide

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3131* **Effort:** Control thinking depth with the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort), the only request parameter that controls it. All five levels (`low`, `medium`, `high`, `xhigh`, `max`) are supported, and the default is `medium`. See [Recommended effort levels for Claude Opus 5.5](https://platform.claude.com/docs/en/build-with-claude/effort#recommended-effort-levels-for-claude-opus-5-5).
3232* **Tool choice:** Use `tool_choice` `{"type": "auto"}` (the default) or `{"type": "none"}`. Forcing a tool call with `{"type": "any"}` or `{"type": "tool", "name": "..."}` is rejected. See the [before and after for tool choice](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#forced-tool-use).
3333* **Sampling parameters:** Omit `temperature`, `top_p`, and `top_k`, or leave them at their defaults: any other value is rejected. Use prompting to guide the model's behavior.
34* **Prefill:** Don't end `messages` with a prefilled assistant turn: it is rejected. Use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) or system prompt instructions instead.
34* **Prefill:** Don't end `messages` with a prefilled assistant turn: it is rejected. Use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) or system prompt instructions instead. On Amazon Bedrock, structured outputs aren't available for Claude Opus 5.5, so use system prompt instructions there.
3535* **Computer use:** On the Claude API and Google Cloud, declare computer use as the `computer_toolset_20260801` toolset; the earlier `computer_20251124` tool is rejected there. See the [computer use breaking change](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#computer-use-toolset).
3636* **Context window:** No context-window beta header is needed. The [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) is the default, and a header sent for older models has no effect.
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284284* Update the model ID to `claude-opus-5-5`.
285285* Remove `thinking: {"type": "disabled"}` and `thinking: {"type": "enabled", ...}`; choose an effort level instead.
286286* Set `effort` explicitly: the default is `medium`, where Claude Opus 5's is `high`.
287* Replace `tool_choice` types `any` and `tool` with `auto` plus strict tool use or structured outputs.
287* Replace `tool_choice` types `any` and `tool` with `auto` plus strict tool use or structured outputs, or with `auto` alone on Amazon Bedrock.
288288* If you use computer use on the Claude API or Google Cloud, declare `computer_toolset_20260801` (no beta header) instead of `computer_20251124` and update your agent loop for the toolset. On Amazon Bedrock, keep `computer_20251124`; check the computer use tool's [Compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#compatibility) section for other platforms.
289289* If a router or fallback can move a conversation from Claude Opus 5.5 to another model, expect that model to run without Claude Opus 5.5's thinking blocks (Claude Fable 5.1 and Claude Mythos 5.1 on the Claude API are the exception and keep them). Claude Opus 5.5 itself reads thinking from Claude Opus 5, from earlier Opus, Sonnet, and Haiku models, and, on the Claude API and Google Cloud, from Claude Sonnet 5.5, but not from Claude Fable or Claude Mythos models.
290290* Read content blocks by `type`, and pass `thinking` blocks back unmodified in tool-use loops.
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568568 
569569#### Forced tool use is not supported
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571`tool_choice` types `any` and `tool` return a 400 error (`tool_choice: type "tool" and "any" are not supported for this model.`), including on the token counting endpoint. Use `auto` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) or [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs), and say in the prompt when the tool applies. Strict tool use accepts a subset of JSON Schema, so check each tool's `input_schema` before you add `strict: true`. Every object in the schema must set `additionalProperties: false`; see [JSON Schema limitations](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#json-schema-limitations). See [Forced tool use is not supported](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#forced-tool-use-is-not-supported).
571`tool_choice` types `any` and `tool` return a 400 error (`tool_choice: type "tool" and "any" are not supported for this model.`), including on the token counting endpoint. Use `auto` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) or [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs), and say in the prompt when the tool applies. Strict tool use accepts a subset of JSON Schema, so check each tool's `input_schema` before you add `strict: true`. Every object in the schema must set `additionalProperties: false`; see [JSON Schema limitations](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#json-schema-limitations). On Amazon Bedrock, structured outputs, which include strict tool use, aren't available for Claude Opus 5.5. There, send `auto` without `strict`, say in the prompt when the tool applies, and validate the tool input in your code. See [Forced tool use is not supported](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#forced-tool-use-is-not-supported).
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573573Before. Claude Opus 5 accepts this request, and Claude Opus 5.5 rejects it with a 400 error:
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15121512 Update your `max_tokens` parameters to give additional headroom, including compaction triggers, and re-test any code path that estimates tokens client-side or assumes a fixed token-to-character ratio. Use the [Token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting) to verify. Prompting interventions, [`task_budget`](https://platform.claude.com/docs/en/build-with-claude/task-budgets), and [`effort`](https://platform.claude.com/docs/en/build-with-claude/effort) can help control costs; these controls may trade off model intelligence.
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15145. **Prefill removal (already in effect on Claude Opus 4.6):** Prefilling assistant messages returns a 400 error on Claude Opus 4.6 and later Opus models, including Claude Opus 5.5, so this is a change only if you come from Claude Opus 4.5 or earlier. Use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs), system prompt instructions, or `output_config.format` instead.
15145. **Prefill removal (already in effect on Claude Opus 4.6):** Prefilling assistant messages returns a 400 error on Claude Opus 4.6 and later Opus models, including Claude Opus 5.5, so this is a change only if you come from Claude Opus 4.5 or earlier. Use [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) or system prompt instructions instead. On Amazon Bedrock, structured outputs aren't available for Claude Opus 5.5, so use system prompt instructions there.
15151515 
15161516### Behavior changes
15171517 
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