Type Alias Provider 

pub type Provider = String;

Which provider serves a tier’s model — an open string, opaque to warble (mirrors how the IR treats tier; see binding-spec.md). Two well-known values get behavior baked into TierBinding parsing below (ANTHROPIC_PROVIDER, the default; OPENAI_COMPAT_PROVIDER, which requires endpoint), but warble does not validate this field against a fixed provider list — any other string is a valid, warble-unrecognized provider that passes through unchanged. Rejecting a genuinely unsupported provider is the consuming harness/back-end’s job (its per-provider adapter registry), never warble’s — warble stays opaque pass-through.

Aliased Type

pub struct Provider { /* private fields */ }