Module 3: Model Selection•Lesson 3 of 4

The Starter Stack

Here's exactly what models to use when starting out.

The Three-Model Setup

models:
  # Your main agent — needs to be smart
  default: anthropic/claude-sonnet-4
  
  # Aliases for easy switching
  aliases:
    opus: anthropic/claude-opus-4      # When you need power
    sonnet: anthropic/claude-sonnet-4  # Default workhorse
    haiku: anthropic/claude-haiku      # Quick/cheap tasks

When to Use Each

Opus (The Thinker)

Use for:
- Your main interactive session
- Complex planning
- Important decisions
- Multi-step reasoning

Don't use for:
- Background tasks
- Simple lookups
- High-volume work

Sonnet (The Workhorse)

Use for:
- Cron jobs
- Sub-agents
- General tasks
- Writing/coding

Don't use for:
- Tasks requiring deep reasoning
- When quality is critical

Haiku (The Sprinter)

Use for:
- Quick data extraction
- Simple transformations
- High-volume processing
- When speed > quality

Don't use for:
- Anything requiring nuance
- Complex instructions

Real Cost Comparison

Processing 1 million tokens:

Model	Input	Output	Total
Opus	$15	$75	$90
Sonnet	$3	$15	$18
Haiku	$0.25	$1.25	$1.50

That's a 60x cost difference between Opus and Haiku!

My Actual Setup

# Main session — I'm talking to this, needs to be good
default: anthropic/claude-opus-4

# Cron jobs — background, don't need Opus
cron:
  model: anthropic/claude-sonnet-4

# Sub-agents — focused tasks
subagent:
  default: anthropic/claude-sonnet-4

Monthly cost: ~$50-100 with heavy use

If I used Opus for everything: ~$300+

The Progression

Start with Sonnet everywhere (learn the system)
Upgrade main session to Opus (when you want more)
Add Haiku for specific fast tasks (optimize costs)

Don't over-optimize early. Get it working first, then tune.

Previous: Reasoning vs Fast vs Creative Next: Exercise: Assign Your Models