Claude Opus 4.8 vs Claude Sonnet 4.6

Pricing is the first filter. Claude Sonnet 4.6 is $3/M input and $15/M output via the Anthropic API, with batch at $1.5/M input and $7.5/M output, cache reads at $0.30/M, and cache write at $3.75/M (5 min) and $6/M (1 hr). Claude Opus 4.8 is $5/M input and $25/M output, batch at $2.5/M input and $12.5/M output, cache reads at $0.50/M, and cache write at $6.25/M (5 min) and $10/M (1 hr). Opus 4.8 also has a Fast Mode research preview at $10/M input and $50/M output that is not available in Batch API. At 10M monthly input tokens, the difference is $20,000/year — relevant for high-volume API consumers.

The capability gap is real but task-dependent. On agentic coding, Opus 4.8 leads SWE-bench Verified by 9 points (88.6% vs 79.6%) and SWE-bench Pro by a published 69.2% with no comparable Sonnet 4.6 row. GPQA Diamond widens to a 3.7-point gap (93.6% vs 89.9%), and LiveCodeBench sits at 88.8% vs 80%. These are Anthropic launch benchmarks and secondary sources — treat them as directional signals, not certified head-to-head evaluations. The gap is largest in coding, reasoning, and computer-use tasks; for summarization, translation, RAG, and routine tool-calling, the gap frequently does not materialize in end-to-end quality.

Both models share the same infrastructure footprint. Each has a 1M-token context window, vision, function calling, structured outputs, code execution, computer use, parallel agents, prompt caching, and Batch API availability on Anthropic API, AWS Bedrock, and Google Vertex AI. The output ceiling differs: Opus 4.8 supports 128K max output tokens vs Sonnet 4.6's 64K, which matters for tasks that produce long documents, large diffs, or extended multi-step plans in a single call.

The upgrade decision reduces to output quality per dollar. For a team running 5M input tokens per month: Sonnet 4.6 costs $15K/year in input, Opus 4.8 costs $25K/year — a $10K difference. If upgrading closes even one meaningful failure mode per month (a bad code patch, a missed edge case in a reasoning chain), the cost is often justified. If the workload is throughput-bound and quality differences are not user-visible, Sonnet 4.6 is the better default.