Opus 4.5 Migration

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Opus 4.5 Migration Guide

One-shot migration from Sonnet 4.0, Sonnet 4.5, or Opus 4.1 to Opus 4.5.

Migration Workflow

1. Search codebase for model strings and API calls
2. Update model strings to Opus 4.5 (see platform-specific strings below)
3. Remove unsupported beta headers
4. Add effort parameter set to "high" (see Reference below)
5. Summarize all changes made
6. Tell the user: "If you encounter any issues with Opus 4.5, let me know and I can help adjust your prompts."

Model String Updates

Identify which platform the codebase uses, then replace model strings accordingly.

Unsupported Beta Headers

Remove the context-1m-2025-08-07 beta header if present—it is not yet supported with Opus 4.5. Leave a comment noting this:

# Note: 1M context beta (context-1m-2025-08-07) not yet supported with Opus 4.5

Target Model Strings (Opus 4.5)

Source Model Strings to Replace

Do NOT migrate: Any Haiku models (e.g., claude-haiku-4-5-20251001).

Prompt Adjustments

Opus 4.5 has known behavioral differences from previous models. Only apply these fixes if the user explicitly requests them or reports a specific issue. By default, just update model strings.

Integration guidelines: When adding snippets, don't just append them to prompts. Integrate them thoughtfully:

• Use XML tags (e.g., <code_guidelines>, <tool_usage>) to organize additions
• Match the style and structure of the existing prompt
• Place snippets in logical locations (e.g., coding guidelines near other coding instructions)
• If the prompt already uses XML tags, add new content within appropriate existing tags or create consistent new ones

1. Tool Overtriggering

Opus 4.5 is more responsive to system prompts. Aggressive language that prevented undertriggering on previous models may now cause overtriggering.

Apply if: User reports tools being called too frequently or unnecessarily.

Find and soften:

• CRITICAL: → remove or soften
• You MUST... → You should...
• ALWAYS do X → Do X
• NEVER skip... → Don't skip...
• REQUIRED → remove or soften

Only apply to tool-triggering instructions. Leave other uses of emphasis alone.

2. Over-Engineering Prevention

Opus 4.5 tends to create extra files, add unnecessary abstractions, or build unrequested flexibility.

Apply if: User reports unwanted files, excessive abstraction, or unrequested features.

Author a short snippet in the prompt's own voice telling the model to do only what was asked: no new files unless necessary (prefer editing existing ones), no abstractions or configurability for hypothetical future needs, no refactoring adjacent code as part of a fix.

3. Code Exploration

Opus 4.5 can be overly conservative about exploring code, proposing solutions without reading files.

Apply if: User reports the model proposing fixes without inspecting relevant code.

Author a snippet requiring the model to read the relevant files before proposing a change, and to trace a symbol to its definition rather than inferring its behaviour from the call site -- explicitly permitting the exploration cost.

4. Frontend Design

Apply if: User requests improved frontend design quality or reports generic-looking outputs.

Author an aesthetics snippet that names the failure mode concretely: avoid the default-template look (centered hero, purple gradient, uniformly rounded cards, Inter everywhere), commit to a deliberate type scale and spacing rhythm, and justify colour choices against the product's existing palette.

5. Thinking Sensitivity

When extended thinking is not enabled (the default), Opus 4.5 is particularly sensitive to the word "think" and its variants. Extended thinking is enabled only if the API request contains a thinking parameter.

Apply if: User reports issues related to "thinking" while extended thinking is not enabled (no thinking parameter in request).

Replace "think" with alternatives like "consider," "believe," or "evaluate."

Reference

This skill bundles no reference files. The snippets above are described rather than quoted on purpose -- write them in the voice and structure of the prompt you are editing, per the integration guidelines, instead of pasting a canonical block.

The effort parameter is a request-level setting; set it to "high" alongside the model string. For its exact spelling on the platform in front of you -- first-party API, Bedrock, Vertex or Foundry, each of which nests request parameters differently -- read the current API reference rather than guessing the field path.