By OpenAILearning Objectives
- Understand what GPT-5.6 is, its Sol / Terra / Luna tier scheme, and how it supersedes GPT-5.5
- 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 is OpenAI's flagship model family, made generally available on July 9, 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. It replaces GPT-5.5 as the model OpenAI points most users toward.
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.
✅Tip
Access GPT-5.6: Live now in ChatGPT for Plus, Pro, Business, and Enterprise, in Codex, and through the OpenAI API. In the API, gpt-5.6 is an alias for gpt-5.6-sol; Terra and Luna are separate endpoints. GPT-5.6 also brings programmatic tool calling to the Responses API. Check platform.openai.com/pricing for the live rate card.
The Three Tiers
| Tier | Role | Context / max output | API price (per million tokens) |
|---|---|---|---|
| Sol | Flagship, deepest reasoning; the gpt-5.6 default | 1.05 million / 128K | $5 in / $30 out |
| Terra | Balanced, strong lower-cost option | 1.05 million / 128K | $2.50 in / $15 out |
| Luna | Fastest, most cost-efficient | 1.05 million / 128K | $1 in / $6 out |
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; Luna is the budget tier at 1 dollar in and 6 dollars out.
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.
⚠️Warning
Read these numbers critically. No independent lab benchmarked GPT-5.6 during its gated preview, so every score above is vendor-reported. Just as important, OpenAI has not published a SWE-bench Pro number for Sol — the benchmark many engineers consider most decision-relevant for real production software work, and the one where the previous generation trailed: Claude Fable 5 led at 80.3 percent versus GPT-5.5's 58.6 percent. Terminal-Bench, Agents' Last Exam, and SWE-bench Pro measure different things and are not interchangeable. Treat GPT-5.6 as a strong agentic model, but verify on your own workload before migrating high-stakes coding jobs.
Pricing
- Flagship, deepest reasoning
- Default alias for the gpt-5.6 endpoint
- Balanced everyday tier
- Strong quality at lower cost
- Fastest, cheapest tier
- Best for high-volume or latency-sensitive traffic
GPT-5.6 is included in ChatGPT for Plus ($20/month), Pro ($200/month), Business, and Enterprise; the tier and reasoning-effort options available depend on the subscription. 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
| Benchmark | GPT-5.6 Sol | GPT-5.5 | Claude Fable 5 |
|---|---|---|---|
| Terminal-Bench 2.1 | 88.8% (ultra 91.9%) | — | 83.4% |
| Agents' Last Exam | 53.6 | — | 40.5 |
| SWE-bench Pro | Not published | 58.6% | 80.3% |
The picture is genuinely mixed. GPT-5.6 sets records on the agentic benchmarks OpenAI chose to publish, but the absence of a SWE-bench Pro score is conspicuous given that Claude led the prior generation on it by a wide margin. For end-to-end resolution of real GitHub issues, Claude Fable 5 remains the safer documented choice until independent SWE-bench Pro numbers for Sol 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
Related Tools
- GPT-5.5 — the prior flagship, now the recent-previous generation
- OpenAI Codex — OpenAI's coding agent, where the Sol Ultra tier lands
- ChatGPT — OpenAI's consumer product, now defaulting to GPT-5.6
- Claude Fable 5 — Anthropic's competing frontier model that still leads on SWE-bench Pro
Lineage
- GPT-5.6 (generally available July 9, 2026) — current 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-5.4API endpoint - GPT-5.5-Cyber (June 22, 2026) — defensive security variant; CyberGym 85.6 percent; anchors OpenAI's 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, replacing GPT-5.5 as OpenAI's flagship
- 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
