Lesson 8: AI Model Selection

Time: 25 minutes Goal: Optimize cost and quality by choosing the right model

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Claude Models

Model	Best For	Cost	Speed
Opus 4.5	Architecture, complex analysis	$$$	Slower
Sonnet 4.5	Daily coding, features, tests	$$	Fast
Haiku 3.5	Quick lookups, simple edits	$	Fastest

The most expensive model isn't always best. Using Opus for a typo is like a sledgehammer for a thumbtack.

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When to Use Each Model

Opus 4.5

✅ Architecture decisions
✅ Complex multi-file refactoring
✅ System design
✅ Security reviews
✅ Comprehensive specs

Example commands:

/specweave:increment "..."  # Uses Opus for planning
/specweave:qa 0001          # Uses Opus for analysis

Sonnet 4.5

✅ Feature implementation
✅ Writing tests
✅ Bug fixes
✅ Code reviews
✅ Daily development

Example commands:

/specweave:do   # Uses Sonnet for execution

Haiku 3.5

✅ Quick lookups
✅ Simple file edits
✅ Bulk find-and-replace
✅ Syntax checks

Example:

"What's the export name in utils/constants.ts?"
"Change button color from blue to green"

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SpecWeave Default Selection

SpecWeave picks automatically:

/specweave:increment    → Opus (complex planning)
/specweave:do           → Sonnet (balanced execution)
/specweave:qa           → Opus (deep analysis)
/specweave:sync-progress → Haiku (fast sync)

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Cost Optimization

Strategy 1: Progressive Escalation

Start cheap, escalate as needed:

Step 1: Haiku exploration
  "What files handle authentication?"
  Cost: ~$0.001

Step 2: Sonnet analysis
  "Explain the auth flow"
  Cost: ~$0.02

Step 3: Opus architecture (if needed)
  "Redesign for multi-tenant"
  Cost: ~$0.15

Strategy 2: Batch Similar Tasks

❌ Expensive: 10 separate Opus calls
  "Fix typo in file1.ts"
  "Fix typo in file2.ts"
  ...

✅ Cheap: 1 Haiku bulk operation
  "Fix all typos: file1.ts, file2.ts, ..."

Strategy 3: Explore First

# Cheap exploration with Haiku
/specweave:do --explore-only

# Then implement with Sonnet
/specweave:do --continue

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Real-World Example

Task: "Add payment processing"

Phase	Model	Cost	Action
Research	Haiku	$0.005	Find existing code
Planning	Opus	$0.20	Create increment
Implementation	Sonnet	$1.20	Build 8 tasks
Total		$1.40	vs $3.00 all-Opus

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Configuration

Default Models

In .specweave/config.json:

{
  "ai": {
    "defaultModel": "sonnet",
    "planningModel": "opus",
    "explorationModel": "haiku"
  }
}

Per-Increment Override

In spec.md frontmatter:

---
increment: 0001-complex-refactor
ai:
  model: opus  # Use Opus throughout
---

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Quick Reference

┌─────────────────────────────────────────────────────────┐
│              WHICH MODEL?                               │
├─────────────────────────────────────────────────────────┤
│                                                         │
│  "I need to..."                                         │
│                                                         │
│  Design / Architect     ────────────► OPUS 4.5         │
│  Complex refactor                                       │
│  Security review                                        │
│                                                         │
│  Write features         ────────────► SONNET 4.5       │
│  Fix bugs                                               │
│  Write tests                                            │
│                                                         │
│  Quick lookup           ────────────► HAIKU 3.5        │
│  Simple edit                                            │
│  Bulk operation                                         │
│                                                         │
└─────────────────────────────────────────────────────────┘

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Glossary Terms Used

• LLM — Large Language Model
• Inference — Using trained models

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Key Takeaways

1. Start cheap (Haiku), escalate as needed
2. Use Opus for planning, Sonnet for execution
3. Batch similar operations
4. Cache exploration results

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What's Next?

Learn how to fix common issues.

:next → Lesson 9: Troubleshooting