Fable scheduling

How llmux picks the subscription that serves a Fable-family request (model matching the fable family, vendor prefixes included — e.g. claude-fable-5, claude-fable-5[1m]). Everything else is a non-Fable request and uses the main lane unchanged.

Code: src/scheduler/select.rs (pick_scoped, fable_score, fable_ranked, gate_scoped), src/scheduler/mod.rs (fable_current, manual_pin, lease_for_scoped), src/routing.rs (is_fable_model).

Why a separate lane

Anthropic subscriptions carry a model-scoped Fable weekly bucket on top of the account-wide 5h/7d windows. An account can be Fable-exhausted while perfectly serviceable for every other model, so Fable traffic keeps its own sticky slot (fable_current) and its own gates — benching an account for Fable never benches it for the rest.

The Fable weekly gauge

Two feeds, freshest-wins, same slot:

1. Usage poll — limits[] rows with kind == "weekly_scoped", labelled by the model display name ("Fable"). Carries upstream's own severity / is_active labels.
2. Response headers — anthropic-ratelimit-unified-7d_oi-{utilization,reset} ride every response through the account, so the gauge stays live between polls and a cold gauge heals on the first request through that account. (Severity is derived from utilization for display only.)

A window past its resets_at reads as utilization 0 (no constraint). A never-observed bucket is cold — unknown, not zero and not exhausted.

Selection, in gate order

1. Hard gates (always): auth-failed, operator pause (absolute — also refused at lease time), account-wide cooldown, and for Fable requests a Fable-scoped cooldown (set by a 429 on a Fable request) or a preemptively constraining bucket (utilization ≥ the fable_weekly_max ceiling, default 0.98 — severity labels never bench an account, only utilization does).
2. Manual pin (operator override, 5 minutes): a manual switch pins BOTH lanes to the chosen account. Thresholds, staleness and the preemptive Fable exclusion do not break the pin — "send it until it actually errors." A recorded 429 / auth failure clears it immediately; otherwise it lapses on its own.
3. Perishability ranking — fable_score = servable_now × urgency:
   • servable_now = min(5h headroom, 7d headroom, Fable headroom) — a Fable request burns all three budgets, the binding one wins.
   • urgency = week / time-to-Fable-reset (hyperbolic, clamped to 100×): the required burn rate. A full bucket resetting in 3h (≈56×) is burned before a 93% bucket resetting in 1d14h (≈4.4×) — use-it-or-lose-it. The main lane's linear ramp is deliberately NOT reused here: 38h vs 3h differ by only ×1.19 on it, inside the stickiness margin, which is exactly the live defect this replaced.
   • A cold bucket has no known perishability → urgency 1.0×. It is held by anchoring (below), never hunted.
4. Anchoring + stickiness: the lane anchors on fable_current, seeded from the account-wide current when the slot is empty — so with no clearly better candidate (score must exceed the anchor's by SWITCH_MARGIN, 25%) Fable traffic stays on the account the operator sees as current, cold bucket included, preserving prompt-cache locality. Round-robin mode keeps its absolute-stickiness contract.
5. Healing: the periodic evaluation tick re-evaluates the Fable slot every cycle (claude group), so a paused / hard-blocked fable current is abandoned within one tick — it used to stay pinned forever.

429 handling (scope-aware)

A 429 on a Fable request without retry-after parks the Fable scope only (the account keeps serving non-Fable traffic); it escalates to an account-wide park only when the 5h/7d windows themselves corroborate. A retry-after 429 is an upstream mandate and always parks account-wide. Any 429 on a manually pinned account also releases the pin.

Operator levers

• Manual switch (TUI switcher / POST /llmux/switch) — 5-minute pin, both lanes, overrides thresholds.
• Pause (p / POST /llmux/pause-account) — absolute bench, both lanes, until resumed.
• llmux reset-usage (POST /llmux/reset-usage) — force every gauge back to cold after a provider-side quota reset; feeds repopulate from headers/polls.
• Ceilings — scheduler.fable_weekly_max (global) and per-account account_limits set where the preemptive exclusion engages.