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GPT-5.6 in the MesoSim AI Assistant

· 3 min read
Telescope

On July 9, 2026, OpenAI released the GPT-5.6 model family. On the Artificial Analysis Coding Agent Index, GPT-5.6 Sol and Terra outperform Anthropic's Claude Fable 5 while having a lower estimated cost.

We have updated both the integrated MesoSim AI Assistant and the ChatGPT-hosted assistant to use GPT-5.6. The update improves response times and the accuracy of answers about MesoSim and options trading in general.

Our development experience with Anthropic and OpenAI models

For our development efforts we used Anthropic's Sonnet and Opus models in mid-2025. We switched to GPT-5 Codex in late 2025 because Sonnet's performance did not meet our needs, while Opus used too many tokens. It was cumbersome to design with Opus, implement with Sonnet and yet still have to wait for quota resets - even on their highest plan.

With the release of Claude Fable 5, we re-evaluated Anthropic's offering. We found Fable to be a capable model, but it remained very token-hungry. Opus was more affordable than Fable, but at higher reasoning efforts it often overcomplicated solutions. We did, however, appreciate Claude Code's multi-subagent code-review approach.

During our evaluation of GPT-5.6 Sol and Terra, we observed that:

  • Sol outperforms Fable at a lower cost. It is now our preferred model for complex research tasks.
  • Terra clearly outperforms both Opus and GPT-5.4 in our day-to-day work while remaining highly affordable.
  • Ultra reasoning provides the multi-subagent code-review workflow that we enjoyed in Claude Code.

Based on our evaluation, we are continuing to use OpenAI models and recommend them to our FundPro customers.

You can use the updated integrated assistant at mesosim.io/ai.
If you do not yet have a MesoSim subscription, you can still use the ChatGPT-hosted MesoSim AI Assistant.

To learn more, see our guide to AI-assisted development with MesoSim and see what developers are saying about GPT-5.6 and Fable 5 on HN.