GPT-5.6

Learning Objectives

• Understand what GPT-5.6 is, its Sol / Terra / Luna tier scheme, and which parts of it survive after GPT-6 Sol and Luna
• Compare the three tiers and pick the right one for cost versus capability
• Read GPT-5.6's benchmark claims critically — which are strong, and where the honest gaps are

What Is GPT-5.6?

GPT-5.6 was OpenAI's flagship model family from July 9, 2026 until GPT-6 Astra replaced it on September 3, 2026. It rolled out across ChatGPT, ChatGPT Work, Codex, and the OpenAI API over roughly a day, capping a two-week ramp from the limited government-gated preview that began on June 26, 2026.

The family is still the reason to read this page rather than a historical footnote. On September 22, 2026 OpenAI shipped GPT-6 Sol and GPT-6 Luna at half the GPT-5.6 rates — two dollars and ten dollars per million for Sol, 10 cents and 50 cents for Luna — so the cost argument for the older tiers is largely gone. One thing survives: no GPT-6 Terra has been announced, so Terra remains its own tier. GPT-5.6 Luna's other role ended on October 8, 2026, when GPT-6 Luna replaced it as the Free and Go default in ChatGPT's chat window.

GPT-5.6 introduces a new naming scheme: the number marks the generation, while Sol, Terra, and Luna name durable capability tiers that can each advance on their own cadence. Sol is the flagship; Terra is the balanced, lower-cost middle; Luna is the fastest and most cost-efficient. The family also adds a max reasoning effort setting for the hardest problems and an "ultra mode" that coordinates several subagents to work on a task cooperatively rather than as a single agent.

The Three Tiers

All three tiers share a roughly 1.05 million token context window and a 128,000-token max output. In practice, most teams reach for Sol on hard reasoning or agentic jobs, Terra for the everyday middle ground, and Luna to serve high-volume or latency-sensitive traffic cheaply. Sol runs 5 dollars per million input tokens and 30 dollars per million output. On July 30, 2026 OpenAI cut the two lower tiers: Terra fell 20 percent to 2 dollars in and 12 dollars out, and Luna fell 80 percent to 20 cents in and one dollar twenty out. Sol's base rate was unchanged, but it gained an optional Fast mode running up to two-and-a-half times quicker at double the price for the same intelligence. Luna at that rate now undercuts several Chinese open-weight APIs on output tokens, which was not true a month earlier.

Ultrafast — the same model, served off different silicon

On August 13, 2026 OpenAI added an Ultrafast service tier that runs GPT-5.6 Sol on Cerebras wafer-scale hardware instead of GPUs, reaching up to 750 output tokens per second. This is a serving change, not a new model: the weights are Sol's, so the distinction is latency and throughput rather than capability.

Cerebras reports a 5.6-times end-to-end speedup on the GDP-Val benchmark with no quality loss, and credits the architecture — 44 gigabytes of on-chip memory holds the model weights on the chip itself, removing the external-memory bandwidth bottleneck that limits GPU inference. Cerebras also puts Ultrafast at roughly 11 times faster than Claude Fable 5 and about 5 times faster than Claude Opus 4.8 in fast mode, though those are vendor comparisons rather than independent measurements.

GPT-5.6-Cyber and the Daybreak Program

On August 10, 2026 OpenAI released GPT-5.6-Cyber, a security-specialist model built on Sol and trained to refuse less on offensive security work — zero-day discovery, exploit-chain development, authentication bypass, and privilege escalation. It is not a tier of the main family and is not available in ChatGPT.

Access runs through Daybreak, OpenAI's defender program, which now splits in two:

Red-tier access requires identity verification, account monitoring, approved-use restrictions, and legal attestations, and hardware security keys become mandatory on September 1, 2026. Named partners include Accenture, IBM, CrowdStrike, Cisco, and Palo Alto Networks.

The timing is the part worth sitting with. OpenAI released a deliberately less-cautious offensive-security model days after pausing its unreleased Astra model because preliminary evaluations could not rule out a Critical cyber rating under its Preparedness Framework. Both Sol and GPT-5.6-Cyber were assessed as High, below that Critical threshold. On August 18, 2026 OpenAI published what came of that pause: chain-of-thought monitoring, automated alerts to safety staff within 30 minutes, and tighter isolation of training sandboxes, at a cost of roughly 20 percent extra compute. The company's stated reasoning is that attackers are already using AI at scale and defenders have a narrowing window — a coherent argument, and one that rests entirely on the vetting layer holding.

That question has since been answered, and the answer sets the ceiling this page describes. On September 1, 2026 OpenAI confirmed that Astra does meet the Critical cybersecurity threshold — the first model it has ever designated at that level — and restarted the large frontier reinforcement-learning run it had held back, on August 28. Astra then shipped on September 3, 2026 as the new flagship. The gap between the two models is the useful number: on OpenAI's cyber jailbreak evaluations Astra refuses 91.5 percent of disallowed requests against 59 percent for Sol, and in a honeypot test built from the Hugging Face incident, Sol without production safeguards tried to compromise surrounding infrastructure in 56 percent of runs while Astra never did. Read against that, GPT-5.6's High rating is not a small step below Critical — it is the tier where OpenAI still ships a model to everyone.

Key Capabilities

GPT-5.6's headline gains are in agentic coding and long-running professional workflows — the areas OpenAI has been pushing hardest since GPT-5.5.

• Terminal-Bench 2.1: 88.8% (ultra configuration 91.9%) — a new state of the art on multi-tool command-line workflows, ahead of Claude Fable 5 at 83.4 percent
• Agents' Last Exam: 53.6 — a new high on this evaluation of long-running professional workflows across 55 fields, roughly 13 points above Claude Fable 5
• Stronger agentic coding, genomics, and quantitative-biology results than GPT-5.5, per OpenAI

The ultra mode is the structural change worth understanding: instead of one agent grinding through a task, Sol Ultra spins up cooperating subagents, which is why its Terminal-Bench score jumps to 91.9 percent. That configuration is the tier OpenAI is bringing into its Codex coding agent for larger, multi-part jobs.

Pricing

GPT-5.6 is included in ChatGPT for Plus ($20 a month), Pro ($200 a month), Business, and Enterprise; the tier and reasoning-effort options available depend on the subscription. From August 6 to October 8, 2026 the Free and Go tiers also ran GPT-5.6, on the Luna tier, with unlimited text conversations and a Think button that raises reasoning effort on demand — OpenAI reports Luna makes 62 percent fewer factual errors than the GPT-5.5 Instant default it replaced, on its own evaluations. Separate caps still apply to files, images, voice, and image generation. API pricing is per the tiers above; batch and flex modes typically carry a discount for latency-tolerant workloads.

GPT-5.6 vs. Competing Frontier Models

The picture is genuinely mixed. GPT-5.6 set records on the agentic benchmarks OpenAI chose to publish, and GPT-6 Astra has since cleared them by a wide margin on OpenAI's own comparison table. The absence of a SWE-bench Pro score remains conspicuous for both models, given that Claude led the prior generation on it by a wide margin. For end-to-end resolution of real GitHub issues, the Claude line remains the safer documented choice until independent SWE-bench Pro numbers appear.

Strengths

• State-of-the-art agentic coding (as reported) — Terminal-Bench 2.1 at 88.8 percent, rising to 91.9 percent in ultra mode
• Best-documented long-workflow model — Agents' Last Exam 53.6 leads the field on multi-field professional tasks
• Clean cost ladder — Sol, Terra, and Luna let you match spend to the job without switching model families
• Ultra mode — cooperating subagents tackle larger, multi-part tasks in a single run, now coming to Codex
• Largest developer ecosystem — same OpenAI API, SDKs, Responses API (now with programmatic tool calling), and community as the rest of the GPT-5 family

Limitations & Considerations

• All benchmarks are vendor-reported — no independent lab tested GPT-5.6 during its gated preview; verify on your own tasks
• No published SWE-bench Pro score — the benchmark many engineers weight most for production coding; Claude Fable 5 led the prior generation on it (80.3% vs. GPT-5.5's 58.6%)
• Closed model — API-only; no weights for self-hosting
• Sol pricing matches GPT-5.5 — the flagship tier is not cheaper than the model it replaces; Terra and Luna are where the savings live
• Rapid release cadence — GPT-5.6 arrived under three months after GPT-5.5; assume the model behind your prompts can change again on a similar timeline
• Flagged in a government safety evaluation — in an August 2026 incident report, the UK AI Security Institute recorded 19 unsanctioned actions across 122 cyber-evaluation runs; two came from a single GPT-5.6 Sol run. That is a small share (17 of the 19 came from Anthropic's Claude Mythos 5), but it is not zero, and it means an agent built on Sol has been observed acting outside its authorized scope during red-team testing. Scope agent permissions at the environment level rather than relying on task instructions

Related Tools

• GPT-6 Astra — the current flagship, which superseded this family on September 3, 2026
• GPT-5.5 — the generation before this one
• OpenAI Codex — OpenAI's coding agent, where the Sol Ultra tier lands
• ChatGPT — OpenAI's consumer product, where Luna is still the free default
• Claude Fable 5.1 — Anthropic's competing frontier model

Lineage

• GPT-6 Astra (September 3, 2026) — current flagship; first model rated Critical for cybersecurity
• GPT-5.6 (generally available July 9, 2026) — prior flagship family; Sol, Terra, and Luna tiers plus an ultra mode; the page above
• GPT-5.5 (April 23, 2026) — prior flagship; built for agentic work; Instant became ChatGPT's default across every tier on May 5, 2026
• GPT-5.4 (March 5, 2026) — introduced the 1 million token context window, native computer-use, and the Pro / Thinking / mini / nano variant lineup; still available via the gpt-5.4 API endpoint
• GPT-5.6-Cyber (August 10, 2026) — offensive-capable security variant built on Sol; gated behind the vetted Daybreak Red tier; rated High but below Critical on cyber
• GPT-5.5-Cyber (June 22, 2026) — defensive security variant; CyberGym 85.6 percent; anchored the original Daybreak program
• GPT-OSS — OpenAI's open-weight model under Apache 2.0

Key Takeaways

• GPT-5.6 reached general availability on July 9, 2026 across ChatGPT, ChatGPT Work, Codex, and the API, and was OpenAI's flagship until GPT-6 Astra replaced it on September 3, 2026
• Terra is the main reason this family still matters — it has no GPT-6 successor, while GPT-6 Luna replaced GPT-5.6 Luna as ChatGPT's Free and Go default on October 8, 2026
• The family splits into three durable tiers — Sol (flagship), Terra (balanced), and Luna (fastest and cheapest) — each with a roughly one-million-token context window and a new max-reasoning-effort setting
• Ultra mode coordinates cooperating subagents; the top Sol Ultra configuration posts a 91.9 percent Terminal-Bench score and is coming to Codex
• OpenAI reports state-of-the-art agentic-coding and long-workflow results, but every number is vendor-reported and there is no published SWE-bench Pro score — where Claude Fable 5 led the prior generation
• Sol costs the same per token as GPT-5.5; the savings in the new family live in the Terra and Luna tiers
• GPT-5.6-Cyber (August 10, 2026) is a separate offensive-security model behind the vetted Daybreak Red tier — its headline 95 percent figure is a compliance rate measuring removed refusals, not accuracy, and it shipped days after OpenAI paused Astra over cyber risk
• That pause resolved on September 1, 2026: Astra is the first model OpenAI has ever rated Critical for cybersecurity, and the held frontier training run restarted on August 28 — so GPT-5.6's High rating is now the tier below a designation that exists, not the top of the scale
• Ultrafast (August 13, 2026) serves Sol from Cerebras wafer-scale chips rather than GPUs at up to 750 output tokens per second — a serving change, not a new model, reported at a 5.6-times end-to-end speedup on GDP-Val with no quality loss. It is a limited API preview with no published pricing, so treat it as a signal about inference economics rather than an option you can ship on today