just today, MS announced that gpt-5.3-codex will become Copilot’s new base model. It is also the first LTS model that will be supported until 2027.
https://github.blog/changelog/2026-03-18-gpt-5-3-codex-long-...
https://github.blog/changelog/2026-03-18-gpt-5-3-codex-long-...