Opt-in usage meter in the composer: session % left, click opens /usage-limits

[vitalyiegorov]

I drive Claude and Codex all day in T3 Code, and the one number I check constantly is how much of the session window is left. Claude Code puts it in the status line, Codex CLI in /status. In T3 Code it is two steps away: type /usage-limits, or open Usage → Limits. By the time I check, the turn I just sent is the one that hits the wall.

This has been asked for a few times already: #8520 (composer toolbar), #11563 (top bar), #6993 (Claude Desktop style), and the original #5539 / #6896 that led to #9875. Two PRs tried to put it in the composer and were closed: #11243 (hover popover, permanent control, expired windows drawn as full) and #10802 (bar below the chat). #10707 (sidebar footer) and #10841 (Usage icon preview) are still open. The close note on #11243 asked for the interaction to be reconsidered before more implementation, so I am asking here first.

What I am proposing is deliberately smaller than any of those:

• Off by default. A switch under Settings → General, next to the context window meter, which is the same opt-in shape.
• One bar, one number. The session window's percent left for the selected provider. Providers without a session window fall back to the window with the least left. Amber at 20%, red at 5%.
• Not a new surface. Clicking it does exactly what /usage-limits does. No hover panel, only a plain tooltip with the window name and reset countdown.
• Stale is unknown, not full. When the window's reset time has passed without a fresh reading, the bar goes empty and the label shows a dash. The number never comes back until the provider reports again.
• Nothing new on the wire. It reads the usageLimits snapshot the config stream already carries, the same one Usage → Limits renders. Repaints at most once a minute on the shared clock.
• Tests cover the logic, not markup. Window selection, fallback, and expiry live in packages/shared with unit tests. No static-render assertions.

Sits after the mode controls in the footer, and is the first thing to overflow in the resting strip. Mobile can follow with the same shared helper if this lands.

Settings:

Full composer:

Resting composer:

The implementation is up as #11890, about 200 lines including tests, so you can see the exact shape. If the placement or the opt-in is wrong I will move it or close it, no hard feelings.