Support Claude's lower-priority mode (/low-priority) when out of tokens

[saphid]

What

Claude Code has started rolling out lower-priority mode (/low-priority) to some subscribers. When you hit your 5-hour session limit, it lets you keep working at lower priority if Anthropic has spare capacity. It still counts against your weekly limit and has its own weekly allowance.

T3 Code users who run out of tokens mid-thread should get the same option, once the Claude Agent SDK exposes it.

Why it can't be built yet

The feature is in the Claude Code binary (seen in 2.1.282), but it's hidden, gated behind a server-side flag, and interactive-only:

• /low-priority is registered with supportsNonInteractive: false, so it's unavailable in SDK sessions like T3's.
• The on/off state lives in memory in the Claude Code process. No setting or env var controls it.
• The SDK (0.3.282) rate_limit_event has no fields for it. The server does return anthropic-ratelimit-unified-slow-* headers (status, offer, retry-after, budget, reset), but they don't reach SDK consumers.

Sending the request header anthropic-usage-limit: slow ourselves through ANTHROPIC_CUSTOM_HEADERS would work around the opt-in, and Claude Code wouldn't handle the "waiting for capacity" retry state. That isn't a reasonable path.

Proposal (once the SDK supports it)

This builds on the usage-limit classification in a6483eeca3, where out-of-tokens failures render as a calm row.

1. Way in: when a Claude turn fails with usage_limit and the account is offered the mode, the row offers "Continue at lower priority · uses weekly limit".
2. Way to see it: a thread or composer indicator such as "Lower priority until {reset}", with the remaining allowance. While the server is waiting for capacity, show that honestly instead of a normal spinner.
3. Way out: a way to turn the mode off, which reverts to waiting for the limit reset. Also surface the "allowance used" and "taking a break" (cooloff) states.
4. Surfaces: web, desktop, and mobile. The state crosses the wire, so it needs a contract in packages/contracts.
5. Providers: Claude only. The other adapters have no equivalent and should report it as unsupported.

The most likely plumbing is the Claude adapter's existing rate-limit handling (ClaudeAdapterV2), fed by new SDK fields plus a control request or setting to toggle the mode.

Blocked on

Anthropic exposing lower-priority mode to Agent SDK consumers. That means a toggle (setting, control request, or allowing /low-priority in SDK sessions) plus the status fields on rate_limit_event. We'd link the upstream anthropics/claude-code request here once it exists.