GPT-5.5 vs GPT-5.6 Sol

These two share a context window, a rate-limit table and the long-context rule. When this page was first written they also shared a price, which left recency and caching as the whole story. Sol's August promotional cut changed that.

Price now favors Sol

GPT-5.6 Sol is cheaper than GPT-5.5 on input, cached input and output, in both context bands and every service tier both publish. The promotion has a published "at least through" date and nothing stated beyond it, so treat Sol's advantage as current rather than permanent.

Caching stopped being GPT-5.5's argument

GPT-5.5 adds no surcharge for writing a prefix into the cache — a written token bills as ordinary input — while Sol bills writes at its cache-write rate instead of its input rate. At launch prices that made Sol's writes dearer. At promotional prices, Sol's cache-write rate and GPT-5.5's ordinary input rate are the same figure, so a write-heavy workload no longer has a cost reason to stay on the older model.

Recency and availability

Sol's knowledge cutoff is newer. And GPT-5.5 leaves Codex for ChatGPT sign-in on October 14, 2026, with GPT-6 Sol as OpenAI's named replacement on paid plans — so for Codex users the real comparison is three-way, and GPT-6 Sol undercuts both on price.

How to decide

On the API, Sol now beats GPT-5.5 on price and recency at once; the remaining reason to stay is behavior you've validated and don't want to re-test. Reasoning effort is the one capability difference: Sol accepts max, GPT-5.5 stops at xhigh.