How to Use Claude Opus 4.5 (Dec 2025)

Note: This guide covers the December 2025 Opus 4.5 release. Anthropic has since released Claude Opus 4.6 with improved quality and a 1M token context window. For current model guidance, see our Claude vs GPT vs Gemini comparison and the Claude API profile.

Quick Reference

Using Opus 4.5 in the Claude App

1. Sign in at claude.ai with a Pro, Max, Team, or Enterprise account.
2. Start a new chat and select Opus from the model picker.
3. Paste your task or attach files/images. Keep total input under ~200K tokens.
4. For coding tasks, include the relevant files and be explicit about what you want changed.

Calling Opus 4.5 via the API

Endpoint: https://api.anthropic.com/v1/messages

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-4-5-20251101",
    "max_tokens": 800,
    "messages": [
      {
        "role": "user",
        "content": "Review this function for edge cases:\n```python\n...code...\n```"
      }
    ]
  }'

Tips for API usage:

• Keep total tokens (prompt + response) within 200K
• Use the system parameter to set style and constraints---keep it stable to benefit from prompt caching
• For structured actions, include a tools array and let Claude return tool_use calls
• Enable prompt caching or batch routes for repeated prompts to reduce cost

Using Opus 4.5 on Cloud Partners

Partner pricing differs from the direct Claude API. Check each provider's rate card.

Prompting Best Practices

These apply to all Claude models, not just Opus 4.5:

1. Be explicit about goals, constraints, and the files or APIs involved.
2. Use structure: Bullet lists and numbered steps reduce ambiguity.
3. Trim context: Include only the relevant file sections. Stay within the context window.
4. Plan first: For multi-step work, ask for a short plan before executing.
5. Use tool calls when you need structured actions instead of free-text responses.
6. Cache system prompts to reduce cost and latency on repeated calls.

Opus 4.5 vs Sonnet 4.5

Sources (December 2025)

• Claude pricing and models: anthropic.com/pricing
• Claude API reference: docs.anthropic.com
• Claude tool use docs: docs.anthropic.com/docs/en/build-with-claude

For current model guidance, see our beginner's guide to AI models and directory.