Claude Code + Opus 4.5: The Complete Guide

Everyone’s freaking out about Opus 4.5. And honestly? They should be.

I’ve been using Claude Code since it launched. The jump to Opus 4.5 is the biggest improvement I’ve seen. It’s not just faster or smarter. It thinks differently.

Here’s everything you need to know to get the most out of it.

What Actually Changed

Opus 4.5 isn’t just a better version of Opus 4. It’s a fundamental upgrade in how the model handles complex tasks.

Multi-step reasoning: Previous versions would sometimes lose track of what they were doing in the middle of a complex task. Opus 4.5 maintains context better across long projects.

Code quality: The code it writes is cleaner. Better naming conventions. More modular. Fewer bugs that require fixing later.

Understanding intent: It’s better at understanding what you actually want, not just what you literally said. This matters when you’re describing things in plain English.

Self-correction: When something goes wrong, it catches its own mistakes more often before you have to point them out.

How to Access Opus 4.5 in Claude Code

If you have Claude Pro, you already have access. But there are some things worth knowing:

Selecting the model: In Claude Code, you can specify which model to use. For complex coding tasks, always pick Opus 4.5. Sonnet is faster and cheaper for simple stuff.

Token limits: Opus 4.5 has a massive context window. You can feed it entire codebases and it’ll understand how everything connects. Don’t be shy about giving it more context.

When to use what:

• Simple changes, quick fixes -> Sonnet 4.5
• New features, complex logic -> Opus 4.5
• Debugging tricky issues -> Opus 4.5
• Refactoring large files -> Opus 4.5

The Prompt Patterns That Work Best

Opus 4.5 responds well to certain prompting styles. Here’s what I’ve found works:

Pattern 1: Context First

Before asking for code, give context about your project. What’s the tech stack? What’s the purpose? Who’s using it?

“I’m building a habit tracker app using React and Supabase. It’s for personal use, so no need for authentication. I want to add a feature that shows weekly stats. Here’s my current code structure…”

The more context, the better the output.

Pattern 2: Describe the Why

Don’t just say what you want. Say why you want it.

“I need a loading spinner (because users are complaining about blank screens during data fetches)”

The “why” helps Claude make better decisions about implementation details.

Pattern 3: Iterative Refinement

Start broad, then get specific. Don’t try to specify everything in one prompt.

1. “Build me a basic dashboard layout”
2. “Add a sidebar with navigation”
3. “Make the charts update in real-time”
4. “Add dark mode toggle”

Each step builds on the previous one. Claude remembers context.

Real Example: Building a Full Feature

Let me walk through how I used Opus 4.5 to add a complex feature to an existing app.

The ask: Add a notification system that sends daily reminders based on user preferences.

My first prompt:

“I need to add daily notifications to my habit tracker. Users should be able to set a reminder time, and they should get a push notification at that time reminding them to log their habits. I’m using Supabase for backend.”

What Claude did:

• Asked clarifying questions about the notification delivery method
• Proposed using Supabase Edge Functions for scheduling
• Wrote the database schema for storing preferences
• Created the frontend UI for setting reminder times
• Wrote the Edge Function that sends notifications
• Set up the cron job to trigger the function

Total time: about 45 minutes of back and forth. What would have taken me days took less than an hour.

Common Mistakes to Avoid

Mistake 1: Prompts that are too vague

Bad: “Make it better”

Good: “The button feels too small on mobile. Increase padding and make the text larger.”

Mistake 2: Not providing existing code

Claude works best when it can see your current code. Always share the relevant files when asking for changes.

Mistake 3: Trying to do too much at once

Break complex features into steps. It’s tempting to describe your entire vision in one prompt, but you’ll get better results with iteration.

Mistake 4: Ignoring the suggestions

Opus 4.5 often suggests improvements you didn’t ask for. “While I’m here, I noticed X could be improved…” Pay attention to these. They’re usually good.

The CLAUDE.md File Trick

Here’s something that took my Claude Code workflow to another level: the CLAUDE.md file.

Create a file called CLAUDE.md in your project root. In it, describe:

• What your project does
• The tech stack
• Coding conventions you follow
• Common patterns in your codebase
• Any gotchas or things to avoid

Claude reads this file automatically and uses it as context for every conversation. It’s like giving Claude a brief before every task.

Example CLAUDE.md:

# Project: Habit Tracker

## Stack
- React 18 with TypeScript
- Supabase for backend
- Tailwind for styling
- Deployed on Vercel

## Conventions
- Use functional components only
- All state in hooks, no Redux
- File names: kebab-case
- Component names: PascalCase

## Important
- Always use the useAuth hook for user data
- Database types are in /types/database.ts
- Run `npm run typecheck` before committing

This saves so much repetition across sessions.

Token Efficiency Tips

Even with Opus 4.5’s large context window, tokens cost money. Here’s how to be efficient:

• Only share relevant files. Don’t dump your entire codebase if you’re just fixing one component.
• Use summaries for context. Instead of the full file, sometimes a description works: “I have a UserProfile component that fetches from /api/user and displays name, email, and avatar.”
• Start new sessions for unrelated tasks. Context from old tasks clutters new ones.
• Use Sonnet for simple stuff. Save Opus for when you need it.

What Opus 4.5 Still Can’t Do

Let’s be real about limitations:

It’s not always right. Claude can and will make mistakes, especially on novel problems or unusual tech stacks. Always review the code.

Security requires human review. Don’t trust AI-generated code for authentication, payment processing, or anything security-critical without careful review.

It doesn’t know your business. Claude can write the code, but it can’t tell you if you’re building the right thing. That’s still on you.

Performance optimization needs measurement. Claude can suggest optimizations, but you need to actually benchmark to know if they work.

The Super Cycle Is Real

People are calling this the “Claude Code + Opus 4.5 super cycle” and I think they’re right.

The combination of improved model capabilities with the Claude Code interface creates something genuinely new. It’s not just faster development. It’s a different way of building software.

Ideas that used to require a team now require one person and good prompts. MVPs that took months now take weekends.

If you’re not using these tools yet, you’re already falling behind. The gap between people who use AI for coding and people who don’t is widening every week.

The good news? It’s not too late to start. Open Claude Code, start a project, and see what happens.

You’ll be surprised what you can build.