Config.Headers names must be RFC 9110 tokens and values must hold no control characters other than horizontal tab, so bad configuration fails at construction instead of on the first request. Headers the adapter or net/http manage (Authorization, Content-Type, Content-Length, Transfer-Encoding, Host, Connection) are rejected. A configured Accept header is sent as given; without one the adapter sends application/json. Header values are never echoed in errors. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>