GPT-5.6 Sol: Better AND cheaper than Fable

2026-07-09 · Source: How I AI · Field: Technology & Digital — Artificial Intelligence & Machine Learning, Software Development & Engineering · Depth: Intermediate, extended

Summary

OpenAI has released three new GPT-5.6 models: Soul (frontier), Terra (balanced), and Luna (cheap, high-volume). An analyst, "Clairvo," extensively evaluated these models, primarily GPT-5.6 Soul, against Anthropic's Fable and Sonnet 5 using a custom "How I AI benchmark." This benchmark assesses PRD writing, prototyping, code debugging, and agentic voice, combining an LLM judge (GPT 5.5) with a 70% "Clairvo taste test" weighting. GPT-5.6 Soul emerged as the top performer, particularly for functional, unique designs and practical problem-solving. It is also more affordable than Fable, priced at \$5 per million input tokens and \$30 per million output tokens, compared to Fable's \$10 and \$50 respectively. Terra was favored for clean PRD writing, and Sonnet 5 for agentic voice.

Key takeaway

For AI Product Managers or Engineers building complex applications, consider integrating GPT-5.6 Soul for its demonstrated ability to produce functional, unique prototypes and its practical problem-solving approach. Its lower API pricing (\$5 input, \$30 output per million tokens) also makes it a cost-effective alternative to Fable, especially for tasks requiring creative design or breaking through architectural rigidity. Evaluate GPT-5.6 Terra for straightforward business writing and Sonnet 5 for agentic voice applications.

Key insights

GPT-5.6 Soul offers superior practical effectiveness, design creativity, and affordability compared to Fable for complex development tasks.

Principles

• Prioritize practical effectiveness over theoretical intelligence.
• Value unique, functional designs in AI-generated outputs.
• Direct, crisp writing enhances AI collaboration.

Method

The "How I AI benchmark" evaluates models across PRD writing, prototyping, code debugging, and agentic voice, using a GPT 5.5 LLM judge and a 70% human "Clairvo taste test" weighting.

In practice

• Use GPT-5.6 Soul for complex UI/UX prototyping.
• Employ GPT-5.6 Terra for streamlined business writing.
• Automate social video clipping with GPT-5.6.

Topics

• GPT-5.6 Soul
• LLM Benchmarking
• AI Prototyping
• Model Pricing
• Agentic AI
• AI-assisted Development

Best for: CTO, VP of Engineering/Data, Machine Learning Engineer, AI Engineer, AI Product Manager, Director of AI/ML

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Editorial summary, takeaway, and curation by AIssential. Original article published by How I AI.