Bounded HTTP adapter failures, always nonretryable. message/1 includes only the reason and random correlation token; inspection excludes source details.

Closed reasons: :invalid_options (missing/unknown/duplicate/invalid choices), :invalid_request (unfaithful or unusable framework value), :not_final_step (signing no longer last), :cross_origin_redirect (origin policy), :unsupported_delivery (delivery cannot withhold bytes), :unsupported_component (transport cannot faithfully supply a component), :body_unavailable (required bytes not retained), :digest_conflict (caller digest disagrees), :limit (retention/resource bound), :signing_failed (custody or signing rejection), and :response_rejected (verification failed). Plug also reports :not_captured, :already_verified, :parser_order, :untrusted_proxy, and :request_rejected (with a bounded core source error). Plug uses :unsupported_component when required or covered exact-target evidence is unavailable, including target-dependent related-request signing. source is a bounded core, custody, or message error, never a framework exception.

An unexpected exception, throw, or exit inside an adapter stage returns that stage's fallback reason without source details: :invalid_options at :attach, :invalid_request at :capture, :signing_failed at :sign, and :response_rejected at :verify. Plug init/1 errors are raised with the plug's own stage (:capture, :verify, or :sign).

attempt counts final signing attempts. Req starts at one and reports a response error under the attempt that produced it; Finch always uses one. Plug uses zero for capture, request verification, and option errors, and one for response signing. Req attach errors use zero.