## Why
Ultra reasoning may proactively use multiple agents. When
`features.multi_agent_v2.max_concurrent_threads_per_session` is
configured at 8 or higher, explicitly selecting Ultra can allow up to `N
- 1` subagents to work concurrently and increase usage quickly. Showing
the configured limits at selection time makes that tradeoff visible and
points users to the setting that controls it.
## What changed
- Show a warning history cell after the user explicitly selects Ultra
reasoning with a concurrent-thread limit of at least 8.
- Include the configured concurrent-thread count and maximum subagent
count in the warning.
- Apply the warning consistently across model and reasoning pickers,
Plan-mode scope selection, and reasoning shortcuts.
- Keep the trigger limited to the selected reasoning effort and
configured thread limit, independent of how multi-agent v2 is activated.
- Add focused threshold coverage and an `insta` snapshot for the
rendered warning.
## User impact
Users with high multi-agent concurrency receive a concrete warning
immediately after selecting Ultra. Other reasoning efforts, limits below
8, and startup behavior are unchanged.
https://github.com/user-attachments/assets/5795bcb9-432e-42bc-bde3-ee363e9aeb69
## Test plan
- `just test -p codex-tui ultra_reasoning_selection`
- Built the full debug CLI with `cargo build -p codex-cli`.
- Manually verified the debug TUI with a 10-thread limit: switching from
Max to Ultra displayed the warning with 10 concurrent threads and up to
9 subagents.
Shijie Rao
927004c06d