docs: document fallback_budget_check and enforce_fallback_budget

[runjivu]

Documents fallback_budget_check, added to Router.__init__ in BerriAI/litellm#41379.

Why

Budget is enforced once during auth, against the model group the caller requested. The fallback target is chosen afterwards, inside the router, so on its own that check cannot see the group that actually bills. This bites hardest when the primary is priced at zero: a zero-cost model skips budget checks entirely, so without a second check the request is admitted, falls back to the paid model, and bills in full with no cap applied.

fallback_budget_check is the budget sibling of the existing fallback_access_check and is documented the same way. Unlike enforce_fallback_model_access, it is on by default, because it closes a budget bypass rather than adding an optional restriction. enforce_fallback_budget: false is the opt-out.

Changes

• docs/proxy/config_settings.md: fallback_budget_check row in the router_settings reference, next to fallback_access_check
• docs/proxy/config_settings.md: enforce_fallback_budget row in the general_settings reference, documented as true by default, plus the matching yaml entry
• docs/proxy/reliability.md: new "Enforce Budget on Fallbacks" section, mirroring "Enforce Key Model Access on Fallbacks", with the opt-out snippet

Note on merge order

BerriAI/litellm#41379 cannot go green until this merges. tests/documentation_tests/test_router_settings.py reflects over Router.__init__ and requires every kwarg to appear in the router_settings - Reference table; CI checks this repo's default branch out into docs/my-website, so the code PR's documentation and code-quality jobs fail with:

Exception: Keys not documented in 'router settings - Reference': {'fallback_budget_check'}

Verified locally with this branch checked out at docs/my-website:

$ python ./tests/documentation_tests/test_router_settings.py
All keys are documented in 'router settings - Reference'.

test_env_keys.py, test_api_docs.py and test_circular_imports.py also pass. test_general_setting_keys.py fails on 31 unrelated keys, identically at the code PR's merge base, so it is pre-existing drift and not from either PR.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
litellm	Ready	Preview	Sep 16, 2026 9:37pm UTC