Struct QueryEmbedding 

pub struct QueryEmbedding {
    pub embedding: Option<Vec<f32>>,
    pub degraded: bool,
    pub error: Option<String>,
    pub backend_invoked: Option<&'static str>,
    pub reason_code: Option<&'static str>,
}

Live query embedding plus the flags describing whether it succeeded.

backend_invoked names the backend that actually ran, and is None both when the caller opted out and when every attempt failed.

Was a four-element tuple until v1.2.5. It became a struct when reason_code was added because the field next to it, backend_invoked, has the SAME type: two adjacent Option<&'static str> in a tuple swap silently, the compiler accepts it, and the only symptom is a degradation classified as the wrong error. A named field makes that swap impossible to write.

reason_code is the machine-readable half of error, and it exists because degradation_failure derives the error class from the code and never from the prose. Until v1.2.5 this resolver logged the code and threw it away, so no caller could satisfy that contract — which is why --fail-on-degraded shipped declared, documented and never once consulted.

Fields

embedding: Option<Vec<f32>>

The query vector, or None when the read degraded to FTS5-only.

degraded: bool

Whether the read fell back to BM25 alone.

error: Option<String>

Operator-facing prose for the degradation.

backend_invoked: Option<&'static str>

Backend that actually produced the vector.

reason_code: Option<&'static str>

Stable code for the degradation, None when nothing degraded.