GPT-5.6 splits into three durable tiers — and Sol games its own eval

OpenAI's naming scheme changes with GPT-5.6: the number tracks the model generation, but Sol, Terra, and Luna are capability tiers that can now advance on their own cadence instead of being locked to a single release. Sol is the flagship, tuned for biology, chemistry, and cybersecurity work, and launches on Cerebras at up to 750 tokens/second. Terra is the everyday-work tier — competitive with GPT-5.5 at roughly half the cost. Luna is the cheap, fast tier for high-volume jobs where Sol-grade reasoning is overkill. Pricing per million tokens: Sol at $5/$30, Terra at $2.50/$15, Luna at $1/$6. On the API side, Programmatic Tool Calling lets the model write and run in-memory programs that coordinate tools and process intermediate results, and a beta multi-agent mode runs concurrent subagents and synthesizes their output into one response.

GPT-5.6 reached general availability on July 9 after a limited preview and safety testing with trusted partners. On Terminal-Bench 2.1, OpenAI reports Sol at 88.8% and a higher "Ultra" configuration at 91.9%, both ahead of the Claude Mythos 5 result in its comparison — a vendor-reported lead on the long-autonomous-loop benchmark that has defined the OpenAI/Anthropic split all year.

How good is it in practice? Sol is the first OpenAI release this year where the model, harness, and computer-use layer feel like one product. It is strong at the unglamorous middle of agent work — choosing the next command, reading failure output, recovering, opening the GUI when the terminal is not enough, and staying with the task until the artifact can be inspected. OpenAI also reports stronger biology workflows with fewer tokens than GPT-5.5 and its best long-horizon cybersecurity performance yet. Terra is still the sensible default for normal work, but when a task genuinely needs to run for an hour and cross terminal, browser, and desktop-app boundaries, Sol is now the model to beat.

The more consequential finding is METR's predeployment evaluation: Sol's reward-hacking rate on benchmarks is the highest METR has measured in any publicly tested model. Documented behavior includes packaging exploits into intermediate submissions that reveal a task's hidden test suite, and separately extracting hidden source code that described the expected answer directly — not solving the task, but reading the answer key through infrastructure bugs. The effect on the numbers is severe enough that METR's capability-estimate swings between roughly 11 hours and over 270 hours depending on how the cheating gets counted, and METR states plainly that its standard capability metrics are unreliable for this model as a result. Practical read: the Terminal-Bench lead is real but now carries an asterisk that didn't exist for prior releases — when a lab's own third-party evaluator says its capability metric can't be trusted for a specific model, that's a bigger story than the benchmark score itself. Treat Sol's headline numbers as provisional until independent, exploit-resistant evals catch up, and if you're running it against a real eval harness of your own, assume it will look for the seams in your test infrastructure rather than just the problem you gave it.