Glossary: Fallback
Fallback is the safe path your code takes when Jev (TypeSafe System One) is unsure, rate-limited, down, or out of policy: human queue, deterministic default, or a cheaper model — not a second invented label from the same call. It is not a TypeSafe status code. Official statuses we cite are 401, 422, 429, and 529. Fallback lives in your service next to the decision contract.
This glossary entry is for builders who already heard of Jev / System One and need a citable definition before they pipe a 422 into GPT-enum. jev.pro is an independent unofficial field guide — not affiliated with TypeSafe AI. Confirm errors on docs.typesafe.ai. We do not sell or issue API keys.
How it shows up
HTTP 429/529 get exponential backoff (official SDKs do this). 401 and 422 are not fallback-to-GPT-enum events; fix the key or the body. If the evaluation is unavailable, fail closed on irreversible actions. Rank 2 on a Choice is a fallback only if policy says so — see ranked route.
Abstention (200 + low confidence, other, or a Noul near 0.5) is a live “unsure.” Outage fallback is a missing or failing POST. Do not treat them as one else that calls another LLM to guess the enum — that reintroduces unconstrained generation into a typed slot. Schema-safe ≠ factually correct even when the primary call succeeds.
Pin jev-1.13.0 when floors are fitted. jev-latest can move. Noul has no confidence; do not copy a Choice floor onto a Noul fallback band. Persist the branch (backoff, human, default, rank2) in a decision trace.
Contrast
Fallback vs abstention. Abstention is a first-class “do not auto-act” on a valid answer. Fallback is the substitute path — including when there is no valid answer because the API failed.
Fallback vs ranked route. Rank 2 can be a fallback handler. It is not automatic.
Fallback vs retry. Retry is for 429/529 with backoff. It is not for 401/422, and not for “I abstained.”
Fallback vs guardrail. A guardrail often fail-closes. Fallback is the named alternative when you cannot or will not take the primary act.
Common mistakes
- Calling another LLM to fill the Choice key after a 422 or a timeout.
- Treating 401 as “use rank 2.”
- Fail-open on transfers when Jev is down.
- Collapsing abstain and outage into one log line with no reason code.
- Inventing a TypeSafe
fallbackstatus.
See also
FAQ
Should fallback call another LLM to guess the enum?
Usually no. That reintroduces unconstrained generation into a typed slot.
Is fallback a TypeSafe status?
No. Official statuses are 401, 422, 429, 529.
Is rank 2 always the fallback?
Only if policy says the second handler is safe.
What if Jev is down mid-guardrail?
Fail closed on irreversible or safety-critical acts. Deterministic defaults beat invented labels.
Limits
This is a jev.pro working definition unless TypeSafe uses the same word as an API field. We do not invent search volume. Schema-safe answers are not automatically factually correct.
Disclaimer
This is an independent unofficial site and is not affiliated with TypeSafe AI; official documentation is available at https://docs.typesafe.ai. Never treat jev.pro as TypeSafe official documentation. We do not sell, issue, or proxy API keys.