GPT-6.1 Sol makes endpoint compatibility part of the cost comparison

The Sol update

OpenAI released GPT-6.1 Sol on September 29, 2026. Its documentation positions it as a lower-cost option for complex work compared with Astra and specifies that tool calling uses the Responses API; Chat Completions support does not include tool calling.

That endpoint distinction can matter more to an existing application than a headline capability comparison.

Compatibility must cover the whole workflow

A successful text response does not prove that an agent's tools, streaming events, or state handling will work through the same route. A migration test needs to exercise the operations the product actually uses.

Reasoning settings also deserve review. Supported values can change across model families, and an invalid inherited setting may fail before the model sees the task.

Use a representative integration test

Include one plain response, one tool-using task, one visual input, and one controlled failure. Check parsing, usage accounting, and retry behavior alongside output quality. Keep the old route available during evaluation. A cost-saving migration is complete only when it preserves the application's required behavior and produces accepted work with a lower overall operational burden.