Claude Code · Opus + Sonnet pattern

Plan mode = think.
Do mode = act.

Two different modes, two different AI models, one clean handover. Here's the whole story.

Section 1 — The two modes

What each mode is allowed to do

Claude Code has a safety switch. In Plan mode it can only look and think. In Do mode it can actually change things.

🧠

Plan Mode

Claude reads your code, asks questions, and writes a plan — but touches nothing.

Can do

Cannot do

When to use

🔨

Do Mode (default)

Claude reads and acts. It edits files, runs tests, installs packages — with your approval.

Can do

Cannot do (without asking)

When to use

Section 2 — The Opus → Sonnet handover

Two AI models, one job

Claude Code uses a smarter-but-slower model to plan, then a faster-but-cheaper model to execute. The plan file is the bridge.

Section 4 — The handover explained

What the handover actually is

It sounds fancy. It's not. Here's the whole truth.

📄 A markdown file. Opus writes a plain text plan — headings, bullet points, file paths to touch. That's it. No binary format, no magic.
👍 Your approval. Opus stops and shows you the plan. You say "looks good." That "yes" is the gate. Nothing happens until you approve.
🙈 Sonnet doesn't see Opus's thinking. It only reads the final plan file. All the exploration, dead ends, and reasoning Opus did? Gone. Only the clean plan survives.
💸 Why? Cost + clarity. Sending Opus's full context to Sonnet would be slow and expensive. The plan file forces the plan to be clear enough to stand alone.

Section 5 — The analogy

Think of it like building a house

🏛️

Architect = Opus

Walks the land, asks what you need, draws up the blueprints. Doesn't touch a single brick. Smart, slow, charges a lot per hour.

Blueprint = plan.md

📐

Blueprint = the plan file

The one thing that crosses from planning to building. You (the client) approve it before anything gets built. No blueprint = no building.

= ExitPlanMode + your "yes"

🔨

Builder = Sonnet

Reads the blueprint, gets to work. Doesn't need to know why the architect made those choices — just needs clear instructions. Fast, efficient.

Edits files, runs tests, ships

Section 6 — Quick reference

Cheat sheet

🧠 Use Plan mode when…

The task is large or risky
You're new to the codebase
You want to review before anything changes
You'd regret a mistake
Multiple files will be touched

🔨 Use Do mode when…

The task is small and clear
You already have a plan
You trust the AI's judgment
You want it done fast
It's reversible (git is your safety net)

🚀 The golden path

Start in Plan mode
Let Opus explore
Read the plan, ask questions
Approve → ExitPlanMode
Sonnet executes, you verify

Plan mode = think.Do mode = act.