Claude Sonnet 5 vs Opus 4.8 for coding and AI agents.

Sonnet 5 narrows the gap between Anthropic's general production model and Opus-class capability. Both support a 1M-token context window, 128k maximum output, adaptive thinking, effort controls, tool use, and the core Claude platform. The decision therefore depends less on feature checklists and more on outcome quality, latency, and price.

For most new coding and agent workflows, Sonnet is the correct first evaluation. Opus earns its place when complex judgment, long-running execution, difficult debugging, or enterprise analysis produces a measurable improvement.

Source note: Facts were checked against Anthropic's Sonnet 5 announcement, Sonnet migration documentation, and Opus 4.8 announcement.

Sonnet 5 vs Opus 4.8 specifications

Choose Sonnet 5 as the production default

Sonnet is well suited to code generation, codebase navigation, data analysis, content and document work, visual understanding, agentic tool use, and multi-step enterprise workflows. Its lower cost matters twice: direct token spend is lower, and the same budget supports larger evaluation sets and more production volume.

Sonnet also responds faster in standard mode. That makes it a better fit for interactive assistants and tool loops where each model turn compounds latency. If Sonnet passes the quality threshold, moving to Opus adds cost without creating value.

Choose Opus 4.8 for the hard tail

Opus fits cases that require stronger judgment over long execution: large-scale refactoring, complex systems engineering, difficult debugging, advanced research, vision-heavy work, computer use, and autonomous workflows where one poor decision can derail many later steps.

Do not migrate the entire workload based on a handful of impressive examples. Identify the segment Sonnet handles inconsistently, then compare accepted completion, steps, corrections, and total cost. Many systems should use Sonnet as the default and Opus as a targeted escalation.

Routing Sonnet to Opus

A model router can escalate based on risk class, task type, validation failure, uncertainty, repeated tool errors, or exhausted step budget. Static routing is easier to audit: known difficult tasks go directly to Opus. Dynamic routing can reduce cost further but must not let an unvalidated confidence score make consequential decisions.

Log the initial model, escalation reason, context transferred, final model, token use, latency, validation result, and reviewer outcome. That data shows whether routing saves money or only adds complexity.

Cost comparison example

For 20 million input tokens and 5 million output tokens per month, Sonnet 5 introductory base cost is $90. The announced standard price produces $135. Opus 4.8 costs $225 for the same token volume. The correct comparison still includes retries, output length, review, and accepted completion rate.

Use the Claude API cost calculator for your own request and token assumptions.

Evaluation checklist

1. Create a task set with routine cases and the difficult tail.
2. Run both models with equivalent tools, context, effort intent, and acceptance criteria.
3. Measure accepted completion, error severity, steps, latency, input, output, and human correction.
4. Test Sonnet-only, Opus-only, and routed configurations.
5. Choose the lowest-cost configuration that meets the reliability requirement.
6. Repeat the evaluation before future model migrations.

Frequently asked questions