How to Migrate Code and Prompts from Claude Sonnet 4.x to Opus 4.5

Replace the model identifier claude-3-sonnet-20240229 with claude-3-opus-20240229, increase max_tokens from 4,096 to 75,000, and run the migration skill from the anthropics/claude-code repository to automate bulk configuration updates.

Migrate your existing Claude Sonnet 4.x implementations to Opus 4.5 using the official migration plugin in the anthropics/claude-code repository. This process involves updating API parameters to leverage the 75,000 token context window, adjusting cost-calculation logic for the lower per-token pricing, and optionally streamlining prompt verbosity to match Opus's richer response characteristics.

Update Model Identifiers and API Parameters

The primary change when migrating from Claude Sonnet 4.x to Claude Opus 4.5 is the model identifier string. In plugins/claude-opus-4-5-migration/README.md, the specification requires replacing claude-3-sonnet-20240229 with claude-3-opus-20240229 in all API calls. Additionally, remove or update any hardcoded max_tokens values that assume Sonnet's 4,096 token limit, as Opus 4.5 supports up to 75,000 output tokens.

Node.js Fetch Example

import fetch from "node-fetch";

async function callClaudeOpus(messages: any[]) {
  const response = await fetch("https://api.anthropic.com/v1/messages", {
    method: "POST",
    headers: {
      "x-api-key": process.env["ANTHROPIC_API_KEY"],
      "content-type": "application/json",
      "anthropic-version": "2023-06-01",
    },
    body: JSON.stringify({
      model: "claude-3-opus-20240229",      // ← updated from claude-3-sonnet-20240229
      max_tokens: 75000,                     // ← increased from 4096
      temperature: 0.7,
      messages,
    }),
  });

  const data = await response.json();
  return data.content[0].text;
}

The model field and max_tokens parameter are the critical differences. Failing to update the model name results in a 400 Bad Request error from the Anthropic API.

Expand Token Limits and Context Windows

Claude Opus 4.5 dramatically extends the context window from Sonnet 4.x's 4,000 tokens to 75,000 tokens. According to plugins/claude-opus-4-5-migration/skills/claude-opus-4-5-migration/references/prompt-snippets.md, this allows you to:

• Retain full conversation history without truncation
• Pass larger codebases or documentation blocks in a single prompt
• Reduce token waste from previously necessary summarization workarounds

Update your max_prompt_tokens and max_output_tokens configurations in YAML, JSON, or environment variables to utilize the expanded capacity.

YAML Configuration Before and After

# Before (Sonnet 4.x)

model: claude-3-sonnet-20240229
max_prompt_tokens: 4000
max_output_tokens: 4096

# After (Opus 4.5)

model: claude-3-opus-20240229
max_prompt_tokens: 75000
max_output_tokens: 75000

Refactor Prompt Templates for Opus 4.5

The references/prompt-snippets.md file in the migration plugin notes that Opus 4.5 often produces richer responses with less scaffolding. Review your system prompts for verbosity that was necessary to constrain Sonnet's shorter outputs. You can typically:

1. Remove explicit "be concise" instructions that were workarounds for token limits
2. Simplify multi-step reasoning instructions into single, coherent directives
3. Eliminate redundant formatting constraints

Test existing stop_sequences and temperature values, as Opus 4.5's different architecture may respond differently to Sonnet-tuned hyperparameters.

Automate Migration with the Claude-Code Plugin

The anthropics/claude-code repository includes an executable skill at plugins/claude-opus-4-5-migration/skills/claude-opus-4-5-migration/SKILL.md that automates configuration updates. This skill scans supplied configuration files, rewrites model fields, expands token limits, and prints a diff for review.

Run the migration skill from your project root:

claude-code skill run claude-opus-4-5-migration \
  --config ./my-project/config.yaml

The skill definition references plugins/claude-opus-4-5-migration/skills/claude-opus-4-5-migration/references/effort.md for estimating migration scope based on project size.

Update Pricing and Usage Tracking

Adjust cost-calculation logic to reflect Opus 4.5's lower per-token rates. Where Sonnet 4.x cost $0.0015 per 1,000 input tokens, Opus 4.5 pricing is $0.00075 per 1,000 input tokens. Update any budget monitoring scripts or usage dashboards to prevent over-reporting expenses after migration.

Summary

• Replace claude-3-sonnet-20240229 with claude-3-opus-20240229 in all API requests to avoid 400 errors
• Increase max_tokens and max_output_tokens from 4,096 to 75,000 to utilize the expanded context window
• Reference plugins/claude-opus-4-5-migration/README.md for the complete migration checklist
• Run claude-code skill run claude-opus-4-5-migration to automate config file updates using the skill defined in SKILL.md
• Update pricing calculations from $0.0015 to $0.00075 per 1K input tokens for accurate budgeting
• Review prompt verbosity in references/prompt-snippets.md—Opus 4.5 typically requires less scaffolding than Sonnet 4.x

Frequently Asked Questions

What is the exact model identifier for Claude Opus 4.5?

The model identifier is claude-3-opus-20240229. According to the migration plugin source in plugins/claude-opus-4-5-migration/README.md, this string replaces the previous claude-3-sonnet-20240229 identifier used in Sonnet 4.x implementations.

How do I update existing YAML configuration files?

Use the claude-opus-4-5-migration skill or perform a search-and-replace operation. Update the model field to claude-3-opus-20240229, change max_prompt_tokens to 75000, and set max_output_tokens to 4000 or higher. The references/prompt-snippets.md file contains before-and-after examples for common configuration formats.

Does Opus 4.5 require different prompt engineering?

Generally no, but you should simplify prompts that were verbose to work within Sonnet's 4K token constraints. Opus 4.5 generates richer outputs naturally, so you can remove "be concise" instructions and other scaffolding. Always re-test temperature and stop_sequences values, as the larger model may exhibit different behavior with your existing settings.

Where can I find the migration automation scripts?

The automation skill is located at plugins/claude-opus-4-5-migration/skills/claude-opus-4-5-migration/SKILL.md in the anthropics/claude-code repository. Execute it via claude-code skill run claude-opus-4-5-migration to automatically rewrite model identifiers and token limits across your project files.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}