OpenAI introduces GPT-5.6 to challenge Claude Mythos 5

OpenAI Group PBC today introduced GPT-5.6, a new series of large language models that it says can outperform Claude Mythos 5 across certain coding tasks.

The most advanced algorithm in the lineup is known as Sol. It’s available alongside a midrange option called Terra and an entry-level model dubbed Luna.

All three artificial intelligence models come with two modes that weren’t included in GPT-5.5. The first is a “max” setting that increases the amount of time GPT-5.6 spends on a task to boost reasoning quality. Additionally, OpenAI has developed an “ultra” mode that can spin up multiple subagents to do work in parallel.

The company describes Sol as the most capable LLM it has built to date. The model scored 88.8% on a popular AI benchmark called TerminalBench-2.1 that includes 89 complex programming tasks. When the company enabled the “ultra” setting, Sol’s score increased to 91.9%. Anthropic PBC’s flagship Claude Mythos 5 model managed 88%.

Claude Mythos 5 was preceded by a model called Mythos Preview that made its debut in April. According to Anthropic, the latter LLM has identified more than 10,000 high-severity and critical software vulnerabilities. OpenAI says that Sol nearly matches Mythos Preview’s performance on a cybersecurity research benchmark called ExploitBench.

The GPT-5.6 series also brings efficiency improvements. OpenAI had Sol tackle GeneBench v1, a collection of scientific data analysis tasks that it released in April. The model matched the performance of the company’s previous flagship LLM using fewer tokens.

Sol includes guardrails designed to prevent it from supporting malicious activities such as developing hacking campaigns. If the controls fail to prevent the LLM from generating harmful output, a specialized large reasoning model filters the prompt response before it reaches the user.

OpenAI says the GPT-5.6 series can not only block risky requests but also fend off cyberattacks. The company ran a series of red-teaming exercises to find universal jailbreaks, hacking tactics that can be used to create not one but multiple malicious prompts.

Some of the tests were carried out automatically using “700,000 A100-equivalent GPU hours.” OpenAI used the test findings to improve its new model lineup’s security.

Terra and Luna, the two lower-end GPT-5.6 models that debuted alongside Sol, trade off some output quality for increased cost-efficiency. Sol is priced at $5 per million input tokens and $30 per million output tokens. Terra costs half as much, while Luna offers 80% lower rates.

At the request of the U.S. government, OpenAI is limiting GPT-6.5 access to a ”small group of trusted partners” on launch. The company plans to move the LLM series into general availability in a few weeks. Additionally, OpenAI will bring Sol to newly public Cerebras Systems Inc.’s WSE-3 wafer-size AI chip.

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