Claude Opus 5 vs Sonnet 5: Choosing the Right Model for Each Workload
A practical framework for splitting workloads between Claude Opus 5 and Claude Sonnet 5: capability differences, cost math, and routing architecture.
TL;DR
Anthropic's own guidance is clean: Sonnet 5 for high-volume, well-scoped tasks; Opus 5 when real reasoning is needed. Opus 5 is superior across evaluations, but Sonnet 5's economics remain unbeatable for work you can specify precisely. The interesting new wrinkle is low-effort Opus 5, which blurs the boundary — it brings Opus judgment into price territory that used to belong exclusively to Sonnet.
The Capability Gap
Opus 5 outperforms Sonnet 5 across Anthropic's published evaluations — that part is unambiguous. The gap is widest on tasks that require sustained reasoning: multi-step debugging, ambiguous requirements, novel problems, long-horizon agent steps, and work where the model must notice that its first approach is wrong. It narrows to near zero on well-scoped tasks: extraction, classification, formatting, template-driven generation, and code changes with clear specs.
That shape dictates the routing strategy: pay for Opus where ambiguity lives, use Sonnet where it doesn't.
The Cost Math
Opus 5 runs $5/M input and $25/M output. Sonnet 5 sits meaningfully below that, which is exactly why it's the default for Claude Free and Pro users and the recommended high-volume tier. For a pipeline processing millions of well-scoped requests, the Sonnet discount compounds into real money — no reasoning premium justifies it when there's nothing to reason about.
But compare cost per successful task, not per token. If Sonnet needs a retry loop or human review pass that Opus avoids, the cheaper model can be the more expensive system.
Where Each Wins
Route to Sonnet 5:- High-volume pipelines with clear specs and validation
- Latency-sensitive user features where good-enough is good enough
- First-pass triage, drafts, summaries feeding a stronger reviewer
- Anything you'd be comfortable delegating to a competent junior with a checklist
- Agentic workflows where steps depend on earlier judgment calls
- Debugging, code review, and security-sensitive changes
- Long-context work leveraging the 1M window
- Knowledge work measured on quality (GDPval-AA v2: Opus 5's 1,861 Elo leads the published field)
- The escalation target when Sonnet output fails validation
The Low-Effort Wrinkle
Low-effort Opus 5 complicates the old two-tier logic: it skips deep deliberation but keeps Opus-grade base judgment, at reduced cost and latency. For tasks that are mostly mechanical but occasionally require taste — code review triage, editing, moderately messy extraction — benchmark low-effort Opus against Sonnet before assuming Sonnet wins. Sometimes the simpler architecture (one model, two effort levels) beats maintaining a two-model router.
A Practical Architecture
1. Classify requests as scoped or open-ended at intake
2. Scoped → Sonnet 5 with strict output validation
3. Open-ended → Opus 5 at medium effort; high for the flagged-hard tail
4. Validation failure → escalate one tier and log it
5. Review the routing split monthly against cost-per-successful-task
Conclusion
Sonnet 5 and Opus 5 aren't competitors; they're a routing decision. Put Sonnet on the well-specified 80% of volume, Opus on the ambiguous 20% where quality compounds — and test whether low-effort Opus lets you simplify the whole thing to one model with a dial.