Fallback Provider Chain (F3)

Automatic failover to backup LLM providers on transient errors.

How It Works

FallbackChainProvider wraps a primary provider plus an ordered fallback list. On each request:

1. Try the current provider
2. If the error is transient (network failure, 429 rate limit, 5xx, interrupted stream) → advance to the next provider
3. Non-transient errors (auth failure, bad request) propagate immediately — no pointless retries

Config

Default OFF (empty fallback list = primary only). Enable explicitly:

[fallback]
enabled = true

[[fallback.providers]]
provider = "openrouter"
model = "anthropic/claude-sonnet-4"

[[fallback.providers]]
provider = "gemini"
model = "gemini-2.5-pro"

Implementation

• kerux-core/src/client/fallback.rs — FallbackChainProvider + FallbackEntry + is_transient() detection
• Wired in kerux-cli via wrap_with_fallbacks() around the runtime client