GPT-5.6 Sol vs. Claude Fable: Why OpenAI’s new model crushes my benchmark

What It Covers

Host Claire runs a weighted benchmark — 70% personal taste, 30% LLM judge — comparing GPT-5.6 Sol, Terra, Luna, Claude Fable 5, and Sonnet 5 across PRD writing, prototyping, debugging, and agentic voice tasks, concluding that GPT-5.6 Sol outperforms Fable on practical output quality despite costing half the price.

Key Questions Answered

• •Model pricing strategy: GPT-5.6 Sol costs $5 per million input tokens and $30 per million output tokens, roughly half the price of Claude Fable 5 at $10 input and $50 output. For high-volume API work, Sol delivers comparable or superior results at significantly lower cost, making it the default choice for teams watching inference spend.
• •Task-specific model routing: No single model wins every category. GPT-5.6 Sol leads on prototyping and browser use, Terra performs best for clean PRD writing, and Sonnet 5 produces the most human-sounding agentic voice responses. Routing tasks to the right model within the GPT-5.6 family and Claude Sonnet yields better results than defaulting to one model for everything.
• •Prototype functionality as eval criterion: When benchmarking front-end prototypes, prioritize functional interactivity over visual polish. Sol consistently produced clickable, fully interactive dashboards and apps in one shot, while Fable outputs were visually serviceable but contained layout gaps, non-semantic color use, and broken interactive elements — making functionality a more reliable differentiator than aesthetics alone.
• •Breaking model self-imposed constraints: Fable tends to over-engineer solutions and resist deviating from its own architectural decisions, which can lock entire workflows to a single compatible model. Switching to Sol or Codex and explicitly instructing the model to abandon prior constraints unlocked stalled projects, including a prototyping tool that previously only ran on GPT-5.5.
• •Browser automation via Codex: Using the `@Chrome` command inside Codex with GPT-5.6 enables autonomous browser tasks on authenticated sessions — processing hundreds of LinkedIn messages, testing web apps, and completing forms without manual intervention. This combination functions as a practical browser agent requiring only a logged-in Chrome instance and a plain-language instruction.