migration-guide changedmodels/opus-5-5/migration-guide
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3232* **Thinking:** Send no `thinking` field, or send `thinking: {"type": "adaptive"}`, which is equivalent: [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on. `thinking: {"type": "disabled"}` and manual thinking budgets (`thinking: {"type": "enabled", "budget_tokens": N}`) are rejected. See the [before and after for thinking](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-cant-be-disabled).
3333* **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).
3434* **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).
35* **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.
35* **Sampling parameters:** Omit `temperature`, `top_p`, and `top_k`. Use prompting to guide the model's behavior.
3636* **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.
3737* **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).
3838* **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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14971497 Adaptive thinking is steerable through prompting and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort), which replaces the thinking budget as the way to control how much the model reasons. Run an effort sweep on your own evals rather than translating a `budget_tokens` value. The [effort levels table](https://platform.claude.com/docs/en/build-with-claude/effort#effort-levels) describes when to use each level, and [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) covers this model.
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14992. **Sampling parameters removed:** Setting `temperature`, `top_p`, or `top_k` to any non-default value on Claude Opus 4.7 and later models, including Claude Opus 5.5, returns a 400 error. The Python SDK (v1.0 and later) does not define them, and passing them raises a `TypeError`. The safest migration path is to omit these parameters entirely from request payloads. Prompting is the recommended way to guide model behavior on Claude Opus 5.5. If you were using `temperature = 0` for determinism, note that it never guaranteed identical outputs on prior models.
14992. **Sampling parameters removed:** Omit `temperature`, `top_p`, and `top_k`. Prompting is the recommended way to guide model behavior on Claude Opus 5.5. If you were using `temperature = 0` for determinism, note that it never guaranteed identical outputs on prior models.
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150115013. **Thinking content omitted by default:** Thinking blocks still appear in the response stream on Claude Opus 4.7 and later models, but their `thinking` field is empty unless you explicitly opt in. This is a silent change from Claude Opus 4.6, where the default was to return summarized thinking text. To restore it, see item 4 of [Handle thinking in every response](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#thinking-in-every-response).
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17851785* Returns the text between tool calls as `text` blocks.
17861786* Defaults to `high` effort.
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1788Manual extended thinking, non-default sampling parameters, and assistant prefill return a 400 error on both models, so nothing changes there. None of the required changes in the sections for Claude Opus 4.8, Claude Opus 4.7, and Claude Opus 4.6 apply to you.
1788Manual extended thinking, sampling parameters, and assistant prefill return a 400 error on both models, so nothing changes there. None of the required changes in the sections for Claude Opus 4.8, Claude Opus 4.7, and Claude Opus 4.6 apply to you.
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17901790### What changed
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