Function effective_upstream_auth_header 

pub fn effective_upstream_auth_header(
    config: &HarnessConfig,
) -> (String, String)

The header name and scheme the upstream credential is sent with.

upstream_auth_header / upstream_auth_scheme default to authorization / Bearer, which is right for OpenAI and every OpenAI-compatible server. The Anthropic and Gemini native APIs read the key from their own headers (x-api-key, x-goog-api-key) with no scheme prefix, so when the operator left both settings at their defaults the provider’s native header is used instead. An explicit non-default setting is always honoured as written.