OpenAI on June 26, 2026 announced the GPT-5.6 series (Sol, Terra and Luna), including the flagship GPT-5.6 Sol, in a limited preview. Positioned as top-tier in coding, science and cybersecurity, it ships with benchmarks compared directly against Anthropic's Claude Mythos class.
June 26, 2026 · OpenAI
GPT-5.6 arrives: three models led by the agentic flagship "Sol"
A limited preview of the GPT-5.6 series — top-tier Sol , balanced Terra and fast, low-cost Luna — pushes autonomous agentic work in coding, science and cybersecurity, positioned as a direct answer to Anthropic's Claude.
30h
Sol reached a GPU-optimization speedup in ~30 hours…
64h
…that Opus took 64 hours to achieve — deeper, with less code.
3
Models in the series: Sol, Terra, Luna.
Autonomous research task — time to same speedup
Lower is better. Sol reached the target in less than half the time.
Pricing per 1M tokens
Output cost drives the tiers — Terra matches prior-gen quality at Sonnet-tier value.
GPT-5.6 Sol · flagship $30 out / $5 in
Claude Opus 4.8 · generally available $25 out / $5 in
GPT-5.6 Terra · balanced $15 out / $2.50 in
GPT-5.6 Luna · fast, low-cost $6 out / $1 in
Bar width scaled to output price per 1M tokens.
Praised
Persistence — keeps improving even after repeated failures
Digs deeper with less code on hard tasks
Weeks-long red-teaming and real-world attack hardening
Cautious
"Better than Mythos, but not far ahead of Opus 4.8"
Still a preview — real evaluation yet to come
Government-linked access restrictions frustrate some developers
Independent evaluation
METR assessed GPT-5.6 Sol as not reaching its automated AI R&D threshold.
Released as a limited preview for trusted partners with a governance-focused rollout, including model sharing with the U.S. government — the full picture of its capability gains awaits broader verification after general availability.
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