OpenAI Codex Guide: Code Generation & Practical Tips

A newer model is smarter but more expensive, or so the thinking usually goes. Yet comparing GPT-6 Astra with GPT-5.6 Sol on unit price alone settles nothing, because Astra finishes the same job on fewer tokens. In measurements by the independent benchmarking firm Artificial Analysis, running Astra at its weakest setting, low, costs almost exactly what Sol costs at high for one task ($0.82 against $0.81) while scoring higher (46 against 42), on one third the output tokens and a quarter of the wait before the reply starts. Working from the figures as of September 18, 2026, this article covers cost, tokens and speed at each level of reasoning effort, the breakdown that explains why a 2.5x unit price still lands on the same total, what the numbers look like once Sol's promotional price ends, the way the gap in price narrows on coding agent work, and the steps for measuring all of it on your own work.

OpenAI released GPT-6 Astra on September 3, 2026. The API is generally available with no gating, the model ID is gpt-6-astra, the context window is 1,050,000 tokens, pricing is $10 in / $50 out per million tokens, and the knowledge cutoff is April 30, 2026. But "it launched" and "your plan can use it" are two different things: on ChatGPT Plus it does not appear in the regular chat interface, and the way in is ChatGPT Work and Codex. This article covers availability by plan, how to think about pricing when a 2x unit price can still be cheaper per task, the benchmarks OpenAI actually named, the cybersecurity capability that reached "Critical" under the Preparedness Framework for the first time and the line OpenAI drew around it, the four things that break when you migrate (sampling parameters removed, Responses API, no none reasoning effort, the renamed cache setting), and a comparison with Claude Opus 5, Fable 5.1, and Gemini 3.8 Flash — restricted throughout to what could be confirmed against primary sources. It also explains why we are not running the "Astra 74.1% vs. Opus 5 96%" style comparison table that is circulating elsewhere.

Arriving July 9, just two and a half months after April 2026's GPT-5.5, GPT-5.6 is not merely a performance bump — the biggest change is the reorganization from a single flagship into a three-model lineup: Luna/Terra/Sol. What matters most to many GPT-5.5 users is that the mid-tier Terra delivers GPT-5.5-class quality at a far lower price: it launched at half price, and the July 30, 2026 price cut took it to 40% of GPT-5.5's unit price ($2/$12). For those wanting more performance there is the top-tier Sol (same $5/$30, with SWE-Bench Pro rising an estimated 58.6→64.6%); for those wanting lower costs there is Terra. This article lays out, based on official announcements and independent analysis, what the three-model shift means, a spec-at-a-glance table, Terra's price-to-performance, generational benchmark gains (same-metric SWE-Bench Pro +6pt, but TerminalBench differs in version 2.0→2.1 and much general reasoning is undisclosed), 5.6's new features (Programmatic Tool Calling, max effort, ChatGPT Work, GPT-Live, GitHub Copilot support), real cost (roughly $1,100→$440 for 100M input/20M output a month), and which model you should migrate to (cut the bill by 60% with Terra, then move only performance-critical work to Sol).

OpenAI made GPT-5.6 generally available on July 9, 2026, replacing the old "standard + Pro" structure with a three-model lineup: Luna (fast, low-cost, $0.20/$1.20), Terra (balanced, $2/$12, GPT-5.5-class at 40% of Sol's unit price), and Sol (flagship, $5/$30) — prices as revised on July 30, 2026. Sol takes first place on Agents' Last Exam (53.6) and the Coding Agent Index (80), and its 88.8% on TerminalBench 2.1 edges out Claude Fable 5 (86.0%) — yet on production-grade SWE-Bench Pro, Claude Fable 5 leads decisively at 80.0% against Sol's 64.6%. This article covers the differences between the three models, pricing, benchmarks, new features (Programmatic Tool Calling, ChatGPT Work, the full-duplex GPT-Live voice model), availability by ChatGPT plan, a comparison with Claude (Fable 5 / Opus 4.8), and how to choose by use case — all grounded in OpenAI's official announcement and independent benchmarks.

OpenAI shipped "ChatGPT 5.5 (GPT-5.5)" on April 23, 2026. Pitched as "a new class of intelligence for real work and AI agents," it scored 82.7% on Terminal-Bench 2.0 — pulling ahead of Claude Opus 4.7 (69.4%) and Gemini 3.1 Pro (68.5%) to reclaim the top spot. But API pricing doubled vs GPT-5.4 ($5/$30 per MTok), and Claude Opus 4.7 still beats it on SWE-Bench Pro. This article gives you the full picture — features, benchmarks, pricing, plan availability, head-to-head with Claude and Gemini, and how to pick — all grounded in official sources.