OpenAI and Anthropic released new models within an hour of each other on 22 September 2026, triggering what Simon Willison calls a price war among frontier AI vendors.
OpenAI's GPT-6 Sol and GPT-6 Luna arrived priced at roughly half their GPT-5.6 equivalents. GPT-6 Luna now costs $0.10 per million input tokens and $0.50 per million output tokens — among the cheapest models OpenAI has shipped, beaten only by smaller Nano-tier models. GPT-6 Sol dropped to $2/$10 per million tokens, matching what GPT-5.6 Terra used to cost, effectively removing any reason to keep using Terra. Willison also notes GPT-5.6 pricing was scheduled to rise 25% in November, meaning GPT-6's discount looks even larger against that baseline.
Anthropic's Claude Opus 5.5 got a 20% price cut versus Opus 5.0 through 4.8, moving from $5/$25 to $4/$20 per million input/output tokens. Cache-read pricing fell 60%, which matters directly for agentic workloads where, per Willison, over 90% of input tokens in longer conversations are served from cache. Anthropic also said cheaper Sonnet 5.5 and Haiku 5.5 models are coming, and specifically flagged Haiku's need to compete with GPT-6 Luna's pricing.
Grok 4.7, released the prior day, now sits between the two: it undercut GPT-5.6 Sol substantially at launch but is now roughly matched by GPT-6 Sol on input pricing.
Willison reports mixed results testing the models with his standard pelican-drawing benchmark: Claude Opus 5.5 at its "max" reasoning setting failed twice, hitting the 128,000-token output limit while still reasoning about a simple SVG request, costing $2.56 and about 20 minutes per failed attempt. He has since switched his own default coding tools to GPT-6 Sol and Claude Opus 5.5, and upgraded a public Datasette demo to GPT-6 Luna.