2.1 — Selecting Claude Models on Trade-Offs

CCAR-P Domain 2 · Task 2.1

What You Need to Know

Model selection = which model clears this task’s quality bar at the lowest cost that meets the latency budget. Never “which model is best”.

Five Constraint Axes

1. Capability (measured quality bar)
2. Latency (TTFT + throughput; compounds in chains)
3. Cost (price × tokens × volume → per-task unit economics)
4. Context window / output limits
5. Lifecycle & platform availability

Current Model Tiers

• Frontier — hardest reasoning / long-running agents (Fable 5.1)
• Heavyweight — complex agentic / enterprise (Opus 5.5)
• Balanced — everyday speed + capability (Sonnet 5.5)
• Fast/low-cost — high-volume classification, extraction, routing (Haiku 5.5)

Right-Sizing Per Stage

Efficiency-first (start Haiku, upgrade where measured gap) or capability-first (start Opus, optimise down). Right-size per stage, not per application. Try the effort parameter before switching tiers.

Escalation Cascades

Default to small model; escalate on low confidence / failed validation / explicit trigger. Near-big-model quality at near-small-model average cost.

Lifecycle Discipline

Pin model versions. Monitor deprecation (≥60 days notice). Re-run evals before migrating. Never let an alias silently move production.

Exam Traps

• Running everything on the biggest model “to be safe”
• Switching to smallest model without re-running quality evals
• Letting production run on an alias or soon-to-retire model
• Upgrading model because outputs are inconsistent (fix instructions first)