GPT-5.6 Sol, Terra & Luna: Complete Guide to OpenAI's New Model Family (2026)

What is GPT-5.6?

GPT-5.6 is OpenAI's latest model family, released on July 9, 2026. It introduces a new naming system where the number (5.6) identifies the generation, while the tier names — Sol, Terra, and Luna — identify durable capability levels that can advance on their own cadence.

Price update (July 30, 2026): OpenAI cut Luna prices by 80% and Terra by 20%, citing efficiency improvements and competitive pressure. The prices above reflect the new rates.

Specifications

Prompt Caching

All three models support prompt caching with a 30-minute minimum cache life and explicit cache breakpoints:

Luna: The 80% Price Cut Story

The biggest news in the GPT-5.6 family is Luna's dramatic price reduction on July 30, 2026:

OpenAI attributed the cuts to a 20% reduction in serving costs and 15% improvement in token-generation efficiency. Media reports also point to competitive pressure from Chinese AI labs like DeepSeek and ByteDance, which have been aggressively pricing their models.

At $0.20/$1.20 per 1M tokens, Luna is now OpenAI's most affordable model ever — competitive with DeepSeek V4 Flash ($0.14/$0.28) while offering access to OpenAI's ecosystem.

Sol Fast Mode

For users who need speed, Sol now offers a Fast mode at 2x the standard price, delivering up to 2.5x faster generation. This is available through the API and Codex, and is particularly useful for interactive coding sessions.

Capabilities

Sol: The Flagship

GPT-5.6 Sol is OpenAI's most capable model, designed for complex tasks across coding, research, cybersecurity, and agentic workflows.

Key benchmark results:

Sol Pro is a separate model variant available in ChatGPT for more difficult, longer-running workflows. It extends Sol's capabilities for tasks that benefit from additional compute.

Max reasoning effort (levels: none through max) gives Sol more time to reason deeply on complex problems. The "max" level is the most thorough but also the most expensive.

Ultra mode goes beyond a single agent by leveraging sub-agents to accelerate complex work. This is available in the ChatGPT interface and represents OpenAI's most powerful configuration.

Terra: The Balanced Choice

Terra is designed for everyday professional work. At $2/$12 per 1M tokens (after the 20% price cut), it offers competitive performance with GPT-5.5 at half the price. It supports reasoning efforts up to "high" and is suitable for:

• Content creation and editing
• Data analysis and visualization
• Code generation and review
• Research synthesis

Luna: The Budget Champion

Luna is the fast and affordable option. At $0.20/$1.20 per 1M tokens, it's 80% cheaper than its launch price and competitive with the most affordable models on the market. It supports reasoning efforts up to "medium" and excels at:

• High-volume text processing
• Simple coding tasks
• Classification and extraction
• Customer support automation
• Batch processing

New API Features

GPT-5.6 introduces several new capabilities through the API:

Programmatic Tool Calling

Models can compose and run JavaScript that orchestrates tool calls. This bridges the gap between individual tool calls and full terminal sessions, enabling more complex autonomous workflows.

Multi-Agent

The API now supports spinning up sub-agents for parallel, focused work. This is a significant step toward practical multi-agent systems, baked directly into the core API rather than requiring custom orchestration.

Prompt Cache Breakpoints

Inspired by Claude's approach, developers can now explicitly set cache breakpoints rather than relying on automatic detection. Combined with a 30-minute minimum cache life, this gives developers more control over caching costs.

Image Detail Control

The new detail: original option on image requests avoids resizing images before processing, preserving maximum visual information for analysis tasks.

GPT-5.6 in ChatGPT

In the ChatGPT interface, the three models map to different user experiences:

Sol Pro is a ChatGPT-specific variant that extends Sol's capabilities for demanding workflows. Max is a reasoning effort slider (none → max). Ultra activates sub-agent mode for complex multi-step tasks.

Competitor Comparison

GPT-5.6 Sol is the most expensive model here, but it leads on agentic benchmarks. Luna, at $0.20/$1.20, is the most affordable entry point into the GPT-5.6 ecosystem and directly competes with DeepSeek V4 Flash on price.

Who Should Use Which Model?

Summary

GPT-5.6 represents OpenAI's most comprehensive model family to date. Sol sets new benchmarks in agentic performance, Terra offers a balanced middle ground, and Luna — now 80% cheaper — is OpenAI's most affordable model ever.

The pricing landscape has shifted dramatically. Luna at $0.20/$1.20 directly competes with DeepSeek V4 Flash and other low-cost models, while Sol maintains OpenAI's position at the frontier of agentic capabilities. The new API features — Programmatic Tool Calling, Multi-agent support, and Prompt Cache Breakpoints — give developers tools that weren't available before.

For users already in the OpenAI ecosystem, the GPT-5.6 family offers a clear upgrade path. For new users, the choice between Sol, Terra, and Luna depends on the balance of capability and cost your workflow demands.