Codex mobile and Desktop disagree on Fast mode state

What version of the Codex App are you using (From “About Codex” dialog)?

Version 26.513.31313 (2867)

What subscription do you have?

Not included here; please ask if needed.

What platform is your computer?

macOS Darwin 25.4.0 arm64

What issue are you seeing?

Codex mobile and Codex Desktop do not appear to agree on the Fast mode / Speed setting.

Observed user-visible behavior:

• In the mobile app, I can set Codex to use Fast mode, and the mobile UI sometimes says Fast mode is enabled.
• In Codex Desktop, the UI says I am not using Fast mode / shows Standard.
• It is unclear whether this is only a UI/state sync problem or whether the actual request service tier differs between mobile and Desktop.

Local Desktop state also shows a split source of truth:

# ~/.codex/config.toml
model = "gpt-5.5"
model_reasoning_effort = "medium"
service_tier = "fast"

# ~/.codex/.codex-global-state.json, electron-persisted-atom-state
default-service-tier = null
has-user-changed-service-tier = true
has-seen-fast-mode-announcement = true
has-seen-fast-mode-home-banner = true
fast-mode-personalized-estimate = { estimatedSavedMs: 9280002, rolloutCountWithCompletedTurns: 94, computedAtMs: 1779163536400 }
codex-mobile-has-connected-device = true

That state matches the Desktop UI showing Standard/off even though config.toml still requests service_tier = "fast" and mobile has been set to Fast.

What steps can reproduce the bug?

1. Connect/use Codex from the mobile app and Codex Desktop on the same account / connected device setup.
2. In the mobile app, set Codex Speed/Fast mode to Fast.
3. Open Codex Desktop.
4. Observe the Desktop Speed/Fast mode UI.
5. Check local Desktop state:

• ~/.codex/config.toml contains service_tier = "fast"
• ~/.codex/.codex-global-state.json has electron-persisted-atom-state.default-service-tier = null

1. The mobile UI and Desktop UI do not present the same Fast mode state.

What is the expected behavior?

One of these should be true:

• Fast mode is an account/connected-device preference and mobile/Desktop should show the same current state.
• Fast mode is intentionally per-client/per-device, and the UI should make that explicit.
• If Desktop cannot determine the effective service tier, it should show an explicit unknown/unsupported state rather than confidently showing Standard/off.

Also, Desktop should expose the actual effective service tier for the current turn so users can tell whether they are using Fast mode or only seeing stale UI state.

Additional information

This may overlap with the Desktop persistence/config drift class of bugs, but the mobile/Desktop disagreement makes it hard to know the real effective state.

Related issues found before filing:

• #20769: Codex App resets Speed from Fast to Standard after restart
• #14689: Codex App is misreporting the state of /fast mode
• #17772 / #22028: Fast mode disappeared from Codex App / CLI UI

The key difference here is the cross-client symptom: mobile can show Fast while Desktop shows Standard/off for what appears to be the same connected Codex setup.