OpenAI DevDay - Decisions API and GPT-6.1 Sol. New Sonnet 5.5 and Haiku 5.5 models, Mistral Large 4, Gemini 4 Argon.

Gemini 4 Argon. Announced, but no one has it yet. After a long pause, on October 1 Google showed its new flagship: SOTA-level on benchmarks in coding and security. But there is no access yet, so far only the US government and selected testers. https://blog.google/innovation-and-ai/models-and-research/gemini-models/gemini-4-argon/

Mistral Large 4 (Le Chonk). Their first release in a long time, but still far from SOTA models, lagging by about half a year, if not more. So far only paid access via API, with open weights promised for the end of the month. Vibe CLI for coding still runs on Medium 3.5 and Devstral 2 / Codestral, with Large 4 only callable via API. https://mistral.ai/news/mistral-large-4

OpenAI DevDay 2026 — what was shown for coders
https://openai.com/index/devday-2026-recap/
GPT-6.1 Sol — the main model for code. Almost Astra-level in coding and computer use, at a fifth of the price, but reasoning cannot be turned off completely, the lowest level is low. Already set as default in Codex and in Work. https://openai.com/index/introducing-gpt-6-1-sol/

Ultrafast — a paid fast tier. Up to eight times faster output in Codex, up to six times faster in the API, up to three hundred tokens per second. Astra Ultrafast is already available, Sol Ultrafast — coming soon. A new Pro 500 plan was introduced for it.

Sign in with ChatGPT — you can spend your plan in partner services, including Devin, Vercel, Notion.

Codex improvements: cloud work from laptop or phone, shared team environments, updated CLI with voice, /agents view, worktrees, and better scrolling of long sessions. Separately — in-app change review and Codex Security Cloud for checking repos and new pull requests with ready-made fixes.

OpenAI launched the Decisions API on GPT-6 Luna, a direct answer to Jev from TypeSafe - not a conversation, but a quick answer: how true a statement is, what to pick from a list, how to score against a rubric. It runs ten times faster than a regular Responses API response. Unlike Jev, the Decisions API also understands images, runs on the full GPT-6 Luna, so it handles complex cases better, but is more expensive. https://developers.openai.com/api/docs/guides/decisions

Claude Sonnet 5.5 and Haiku 5.5
Sonnet 5.5 came out September 28, Haiku 5.5 — October 7. Sonnet 5.5 is for daily coding work, Haiku 5.5 helps it with small tasks.

Sonnet 5.5 in some computer-use and terminal benchmarks even beats the larger Opus 5.5, but in the announcement its benchmark numbers are from Max mode, which uses significantly more tokens and is generally not recommended for everyday use. https://www.anthropic.com/claude-sonnet-5-5

Haiku 5.5 sometimes matches Sonnet 5.5, compresses conversations well, finds what you need in a pile of logs, cleans up junk before the big model. While the conversation is short, pricing is on par with GPT-6 Luna, but gets significantly more expensive after that. https://www.anthropic.com/claude-haiku-5-5

In Claude Code the short names sonnet and opus now point to 5.5. There is an opusplan mode: Opus makes the plan, Sonnet executes it. Subagents are recommended to keep on Haiku: exploration, compaction, first review of changes.

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