Claude Sonnet 5 vs GPT-5.2

Pricing math on a real workload

For a 10M input / 2M output token job, Claude Sonnet 5 costs $40.00 and GPT-5.2 costs $45.50 on OpenKey — both already include the flat 3% fee on top of provider list price (Sonnet 5: $2.00/$10.00 per M tokens provider-side becomes $2.06/$10.30 on OpenKey; GPT-5.2's $1.75/$14.00 becomes $1.8025/$14.42). GPT-5.2's input token is actually cheaper per-million ($1.8025 vs $2.06, an 0.14x ratio favoring GPT-5.2), but its completion price of $14.42/M is 40% higher than Sonnet 5's $10.30/M. That completion gap is what flips the total cost in Sonnet 5's favor on any workload with meaningful output volume — code generation, long responses, agent traces.

Coding and agent benchmarks

Claude Sonnet 5 posts an Artificial Analysis coding index of 71.5 and agentic index of 46.7, with an overall intelligence index of 53.4. GPT-5.2 doesn't have Artificial Analysis numbers in this dataset, but it has Design Arena rankings across 13 categories: strongest in Godot game dev (rank 13, 1183 elo, 48.1% win rate) and full-stack agent work (rank 21, 1110 elo), weaker in 3D (rank 60) and website generation (rank 34, 1237 elo, 54.5% win rate). Because the two models report different benchmark suites here, you can't do a direct elo-to-index comparison — but Sonnet 5's coding index of 71.5 is a strong signal on its own terms.

Context and long-document work

Sonnet 5's 1,000,000-token context window is 2.5x GPT-5.2's 400,000. Both cap output at 128,000 completion tokens. If you're feeding in large codebases, long transcripts, or multi-document RAG context in a single call, Sonnet 5 has more room before you need to chunk or summarize. GPT-5.2's smaller window is still large enough for most single-repo or single-document tasks, but for anything approaching book-length or multi-repo context, Sonnet 5 is the one that won't force you to split requests.

Reasoning controls and parameters

Both models expose adjustable reasoning effort as a supported parameter. Sonnet 5 offers five tiers — low, medium, high, xhigh, max — defaulting to medium. GPT-5.2 offers low, medium, high, xhigh, and none, also defaulting to medium, with a `none` option to disable reasoning entirely for latency-sensitive calls. GPT-5.2 additionally supports a `seed` parameter for more reproducible outputs, which Sonnet 5's parameter list doesn't include. Sonnet 5 supports a `verbosity` parameter GPT-5.2 lacks, useful for controlling response length directly instead of through prompting.

When to pick each

Use Claude Sonnet 5 for coding-heavy and agentic pipelines where output volume is high and context needs to be large — its lower completion price and bigger window both pay off. Use GPT-5.2 when your workload is input-heavy with short outputs (its per-token input price is lower), when you need the `seed` parameter for deterministic testing, or when you want a `none` reasoning mode to skip the reasoning tax on simple calls. Both models are available on OpenKey with one API key and a flat 3% fee over provider list price, so switching between them for A/B testing doesn't require separate accounts.

Questions

Which model is cheaper for a typical 10M-input/2M-output job?

Claude Sonnet 5, at $40.00 total on OpenKey versus $45.50 for GPT-5.2. The gap comes from completion pricing: Sonnet 5 runs $10.30 per million output tokens on OpenKey against GPT-5.2's $14.42, even though GPT-5.2's input price is slightly lower at $1.8025 per million.

Which model has the bigger context window?

Claude Sonnet 5, with 1,000,000 tokens versus GPT-5.2's 400,000 — a 2.5x difference. Both cap output at 128,000 completion tokens, so the gap only matters for how much you can feed in, not how much you get back.

Does GPT-5.2 have coding benchmark data comparable to Sonnet 5's coding index?

Not directly. GPT-5.2 reports Design Arena elo rankings across 13 categories (e.g., rank 21 in full-stack, 1110 elo), while Sonnet 5 reports an Artificial Analysis coding index of 71.5. The two benchmark suites aren't the same scale, so there's no apples-to-apples number here.

Can I use both models through the same API key?

Yes. Both Claude Sonnet 5 and GPT-5.2 are available on OpenKey with one API key across all 329 models from 52 labs, and pricing is provider list price plus a flat 3% fee — no separate contracts with Anthropic or OpenAI needed.