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HomeDocsClaude Developer Platform 30 Sep 2026
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migration-guide changedmodels/opus-5-5/migration-guide

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329329* Remove any assistant-message prefills; Claude Opus 4.6 already rejects them.
330330* Verify tool call JSON parsing uses a standard JSON parser.
331* Move from `client.beta.messages.create` to `client.messages.create`: adaptive thinking and effort need no beta namespace.
331* Move from `client.beta.messages.create()` (python, typescript, ruby; csharp: `client.Beta.Messages.Create()`; go: `client.Beta.Messages.New()`; java: `client.beta().messages().create()`; php: `$client->beta->messages->create()`; cli: `ant beta:messages create`) to `client.messages.create()` (python, typescript, ruby; csharp: `client.Messages.Create()`; go: `client.Messages.New()`; java: `client.messages().create()`; php: `$client->messages->create()`; cli: `ant messages create`): adaptive thinking and effort need no beta namespace.
332332* Remove the `effort-2025-11-24` beta header (the effort parameter does not require it).
333333* Remove the `fine-grained-tool-streaming-2025-05-14` beta header.
334334* Remove the `interleaved-thinking-2025-05-14` beta header (adaptive thinking enables interleaved thinking automatically).
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15471547The first item is required on Claude Opus 5.5; the rest are recommended.
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15491. **Migrate to adaptive thinking (required):** `thinking: {"type": "enabled", "budget_tokens": N}` returns a 400 error on Claude Opus 4.7 and later models. The before and after is item 1 of the [breaking changes for migrating from Claude Opus 4.6](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#opus-46-breaking-changes). The migration also moves from `client.beta.messages.create` to `client.messages.create`: adaptive thinking and effort do not require the beta SDK namespace or any beta headers.
15491. **Migrate to adaptive thinking (required):** `thinking: {"type": "enabled", "budget_tokens": N}` returns a 400 error on Claude Opus 4.7 and later models. The before and after is item 1 of the [breaking changes for migrating from Claude Opus 4.6](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#opus-46-breaking-changes). The migration also moves from `client.beta.messages.create()` (python, typescript, ruby; csharp: `client.Beta.Messages.Create()`; go: `client.Beta.Messages.New()`; java: `client.beta().messages().create()`; php: `$client->beta->messages->create()`; cli: `ant beta:messages create`) to `client.messages.create()` (python, typescript, ruby; csharp: `client.Messages.Create()`; go: `client.Messages.New()`; java: `client.messages().create()`; php: `$client->messages->create()`; cli: `ant messages create`): adaptive thinking and effort do not require the beta SDK namespace or any beta headers.
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15512. **Remove effort beta header:** The effort parameter does not require a beta header. Remove `betas=["effort-2025-11-24"]` from your requests.
15512. **Remove effort beta header:** The effort parameter does not require a beta header. Remove the `effort-2025-11-24` beta from your requests.
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15533. **Remove fine-grained tool streaming beta header:** Fine-grained tool streaming does not require a beta header. Remove `betas=["fine-grained-tool-streaming-2025-05-14"]` from your requests.
15533. **Remove fine-grained tool streaming beta header:** Fine-grained tool streaming does not require a beta header. Remove the `fine-grained-tool-streaming-2025-05-14` beta from your requests.
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15554. **Remove interleaved thinking beta header:** With adaptive thinking, interleaved thinking is automatic on every model that supports adaptive thinking. Remove `betas=["interleaved-thinking-2025-05-14"]` from your requests.
15554. **Remove interleaved thinking beta header:** With adaptive thinking, interleaved thinking is automatic on every model that supports adaptive thinking. Remove the `interleaved-thinking-2025-05-14` beta from your requests.
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155715575. **Migrate to output\_config.format:** If using structured outputs, update `output_format={...}` to `output_config={"format": {...}}`. The `output_format` parameter is deprecated and will be removed in the future. To use it anyway, add the `structured-outputs-2025-11-13` beta header. Without it, the API returns a 400 error. The Python SDK (v1.0 and later) does not accept `output_format={...}` on `client.beta.messages.create()` or `count_tokens()`. The `output_format=Model` argument of the `parse()` and `stream()` helpers is unchanged.
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