A budget bypass that ships off by default stays open for every deployment
that does not know to look for the flag, so `enforce_fallback_budget` now
defaults to true and `general_settings.enforce_fallback_budget: false` is
the opt-out for anyone who wants the old unguarded behaviour back.
BREAKING CHANGE: a paid fallback target is now refused for callers who are
over their key or user `max_budget`. Deployments relying on fallbacks to
keep serving over-budget callers must set enforce_fallback_budget: false.
Budget is enforced once during auth, against the requested model group.
`_is_model_cost_zero` waives every budget check for a zero-cost group, and the
router then picks a fallback target afterwards, inside `run_async_fallback`,
where nothing re-checks budget. A free model with a paid fallback therefore
bills with no budget gate at all.
Add `fallback_budget_check`, the budget sibling of the existing
`fallback_access_check`: a predicate awaited per fallback target that skips
targets the caller cannot pay for. The primary attempt is untouched, so a
zero-cost model is never blocked by budget and only the paid fallback is
refused.
Counter reads pass `max_budget` so `get_current_spend` verifies against
authoritative recorded spend, matching the auth-time key and user checks; a
counter restored from an older snapshot reads as a hit rather than a clean
miss, so without it a stale-low value would keep admitting paid fallbacks.
A zero-cost fallback target is always allowed, and a team key does not inherit
the key owner's personal budget unless `apply_user_budget_to_team_keys` is set,
matching `_PROXY_MaxBudgetLimiter`.
Scope is key and user budgets. Team, team-member, end-user, org, global and
per-model budgets are not covered yet: those auth-path functions enforce rather
than report, so reusing them would fire threshold alerts and take spend
reservations for a target that is then skipped. Two limitations of that scope
are documented in the module docstring: the check reads the spend counter
rather than reserving against it, so concurrent fallbacks can cross a cap
together; and a request reaching the router without
`metadata["user_api_key_auth"]` is not restricted. Both are shared with
`fallback_model_access.py`.
Opt-in via `general_settings.enforce_fallback_budget`.
Relates to #41344
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
