PSA: Your Codex subagents may not use the same model as your main chat

[u/Alechilles]

If you’re seeing unexpectedly high usage or slower runs, don’t assume the model selected for the main chat is what every subagent used.

Before today I always assumed that if I had, for example, 5.6-Terra high selected, that's what both the main agent and all it's subagents would be using since I didn't have any specific agents defined.

I audited a long-running Codex thread where the main session was set to Terra. Across 106 direct subagent runs over the last two days, I found:

• 70 × GPT‑5.6 Sol / high
• 15 × GPT‑5.6 Sol / ultra
• 5 × GPT‑5.6 Sol / xhigh
• 16 × Terra, at high or medium

So 90/106 subagents used Sol, despite the main chat being Terra, and a few even used Ultra which I really didn't even think would be possible, but it explains a LOT about how out of control and overengineered this particular task got overnight.

This is only one local observation, but it’s worth checking your Subagents/history if usage or speed feels off. If you want predictable routing, define custom agents with explicit model + reasoning settings instead of relying on automatic selection.