GPT-5.5 Pro

GPT-5.5 Pro is the previous generation's high tier, still in the index because previous-generation flagships are frequently the best value in any lineup. They are mature, well-documented, and priced below whatever replaced them, while remaining more capable than the current mid-tier on many tasks.

The interesting question with any model in this position is not whether it is the best available, but whether the newer tier is better enough to justify migrating.

Stability and documentation. A model that has been in production for a while has known failure modes, community-tested prompts, and behaviour your existing prompts were tuned against. That is worth real money — migrating a prompt library to a new model is not free, and the new model is not automatically better at your specific task.

Capability remains high for reasoning and long-form work.

It is behind on the things newer generations improved: tool use, structured output reliability, and cost per unit of capability. It will also reach end-of-life before the current generation, so anything built on it has a migration in its future.

For new projects the case is weaker — you would be adopting a model with a known expiry date.

Keep GPT-5.5 Pro for workloads already running well on it, and re-test rather than assuming a newer tier is an upgrade for your specific task. For anything new, start on the current generation — the migration cost is cheaper paid now than later.