> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ai-stats.phaseo.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Anthropic: Claude Opus 4.5 to 4.6

> The concrete API and behavior changes to review when upgrading from Claude Opus 4.5 to Claude Opus 4.6.

# Anthropic: Claude Opus 4.5 to 4.6

Use this guide when you are moving from `anthropic/claude-opus-4.5` to `anthropic/claude-opus-4.6`.

Claude Opus 4.6 is not just a model-ID swap. Anthropic's migration guidance includes request-shape changes, removed beta flags, and stricter behavior around prefills.

## What changed

* assistant prefills are no longer supported on Claude 4.6 models and now return `400`
* structured output moved from `output_format` to `output_config.format`
* adaptive thinking is now handled through `output_config.effort`
* older beta headers should be removed
* Opus 4.6 specifically no longer needs the `interleaved-thinking-2025-05-14` beta header
* Opus 4.6 adds a 1M-token context window in beta and improves long-running coding and agentic tasks

## What to change in your integration

### 1. Remove assistant prefills

If you were forcing JSON or a response prefix by partially pre-seeding the assistant turn, that pattern now breaks on 4.6. Move that control into:

* clearer system instructions
* examples in the user or system prompt
* structured outputs through `output_config.format`

### 2. Migrate structured outputs

If you still send `output_format`, update the request body to use `output_config.format` instead.

### 3. Review thinking configuration

If you were depending on older thinking betas, migrate to the current `effort` setting inside `output_config` and retest latency-sensitive routes.

### 4. Remove stale beta flags

Audit hard-coded beta headers in SDK wrappers, middleware, and gateway presets. Leaving obsolete flags in place creates noisy failures during rollout.

## What to test

### Prompt behavior

* long coding and refactor prompts
* multi-step research or document workflows
* prompts that previously depended on assistant prefills
* long-context tasks if you plan to use the 1M-token beta window

### Contracts and tools

* schema pass rate after moving to `output_config.format`
* tool-call argument quality on multi-step agent tasks
* retry behavior when the model refuses or reformats output
* regression handling in code-review and debugging workflows

### Operations

* median and p95 latency with the new thinking settings
* token consumption on longer runs
* timeout rate on large-context tasks
* success rate versus Opus 4.5 on your production eval set

## Safe rollout

1. Remove prefills and migrate structured output config before any traffic shift.
2. Shadow Opus 4.6 on production prompts that matter to you.
3. Canary traffic with Opus 4.5 still configured as a fallback.
4. Promote only after schema pass rate, latency, and task completion stay stable.

## Sources

* [Anthropic migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide)
* [Introducing Claude Opus 4.6](https://www.anthropic.com/news/claude-opus-4-6)