GPT Models Explained: From GPT-4 to GPT-6

What is the newest GPT model?

GPT-6 Astra, released September 3, 2026, is OpenAI's flagship and most capable model. GPT-6 Sol and GPT-6 Luna followed on September 22, GPT-6.1 Sol on September 29, and GPT-6.1 Terra, the newest release, on October 26. Sol is the best everyday default; Astra is for the hardest, longest tasks; Terra is a fast, efficient model for computer use and high-volume work that OpenAI says performs on par with Astra at a tenth of its price per token; Luna is the cheapest.

Source: OpenAI: GPT-6 Astra, a new generation of intelligence; OpenAI: Introducing GPT-6 Sol and Luna; OpenAI: Introducing GPT-6.1 Sol; OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026)

Which GPT model does ChatGPT use?

It depends on your plan. GPT-6 Astra is available on Plus, Pro, Business and Enterprise. Free and Go users use GPT-6 Luna. Paid users can pick models from the model menu, and ChatGPT may route some requests automatically.

Source: OpenAI: GPT-6 Astra, a new generation of intelligence; Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026)

How much do the latest GPT models cost in the API?

Per million tokens: GPT-6 Astra is $10 input and $50 output, GPT-6.1 Sol is $2 and $10, GPT-6.1 Terra is $1 and $5, and GPT-6 Luna is $0.10 and $0.50. See our LLM pricing page for comparisons with Claude and Gemini.

Source: OpenAI API: Pricing; OpenAI: GPT-6 Astra, a new generation of intelligence; OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026)

When was GPT-5 released, and is it still available?

GPT-5 launched in ChatGPT on August 7, 2025, with Instant and Thinking modes, and replaced GPT-4o, o3, o4-mini, GPT-4.1 and GPT-4.5 for signed-in users. It was retired from ChatGPT on February 13, 2026. The original GPT-5 API snapshots are scheduled to shut down on December 11, 2026.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026); OpenAI Help: Model release notes; OpenAI API: Deprecations

What were GPT-5.1, 5.2, 5.3 and 5.4?

GPT-5.1 arrived on November 12, 2025 and left ChatGPT on March 11, 2026. GPT-5.2 (Instant, Thinking and Pro) arrived on December 11, 2025 and left ChatGPT on June 12, 2026. GPT-5.3 Instant became the default on March 3, 2026, and GPT-5.4 Thinking and Pro arrived on March 5, 2026.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026); OpenAI Help: Model release notes

What happened to GPT-5.5?

GPT-5.5 Thinking and Pro arrived on April 23, 2026, and GPT-5.5 Instant became the default on May 5. OpenAI retired GPT-5.5 from ChatGPT, ChatGPT Work and Codex on October 14, 2026; API access was not affected.

Source: OpenAI Help: Model release notes; Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026)

What is GPT-5.6?

GPT-5.6 launched on July 9, 2026 as a family of three: Sol (frontier), Terra (mid-tier) and Luna (fast and cheap). GPT-5.6 Sol came to eligible paid ChatGPT plans, and OpenAI's API deprecation notices pointed most retired models to GPT-5.6 replacements until the GPT-6 family arrived.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026); OpenAI API: Deprecations

Can I still use GPT-4 in ChatGPT?

No. GPT-4 arrived in ChatGPT on March 14, 2023 and was replaced by GPT-4o on April 30, 2025. GPT-4o, GPT-4.1, GPT-4.1 mini and o4-mini were then retired from ChatGPT on February 13, 2026.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026); OpenAI Help: Model release notes

Is GPT-4 still in the API?

The original gpt-4 model was scheduled to shut down in the API on October 23, 2026. Remaining GPT-4o and GPT-4.1 snapshots are scheduled to shut down on January 14, 2027, with GPT-6 Luna as OpenAI's recommended replacement. Check OpenAI's deprecations page before building on any older model.

Source: OpenAI API: Deprecations

Why do people still search for GPT-4?

GPT-4 was the model that made ChatGPT famous, and many tutorials, courses and app settings still mention it. If a tool asks you to choose GPT-4, pick a current model such as GPT-6.1 Sol or GPT-6 Luna instead.

How often does OpenAI retire models?

Often. OpenAI wrote when retiring GPT-5.2 in June 2026 that models will generally remain available in ChatGPT for about 90 days after a successor is released. API models get formal deprecation notices with shutdown dates and recommended replacements.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026); OpenAI API: Deprecations

What happens to my chats when a model is retired?

Existing conversations move to the model that replaced it. For example, chats on GPT-5.1 moved to GPT-5.3 Instant or GPT-5.4 Thinking when GPT-5.1 was retired in March 2026.

Source: Whizi: ChatGPT versions, all models from GPT-3.5 to GPT-6 with dates (read Oct 18, 2026)

Which GPT model is best for everyday work?

GPT-6.1 Sol balances quality, speed and cost for writing, analysis and coding. Use GPT-6 Astra for long, complex, multi-step tasks where quality matters most, and GPT-6 Luna for quick, high-volume or budget work.

Source: OpenAI: Introducing GPT-6.1 Sol; OpenAI: GPT-6 Astra, a new generation of intelligence

Which GPT model should developers build on?

Start with GPT-6.1 Sol, test GPT-6 Luna for cost-sensitive steps, and reserve Astra for the hardest calls. Avoid pinning to models with announced shutdown dates, and watch the deprecations page.

Source: OpenAI API: Deprecations; OpenAI API: Pricing