Normalized payment failure surfaced to the caller (agent, LLM, or human).

Every payment error path collapses to a small closed set of reasons so the surface can render a clear sentence and decide whether a retry is worth it, instead of leaking a raw HTTP tuple or an inspect/1 dump. from/1 maps any internal error term (quote miss, terminal intent status, HTTP error, spend gate denial, validation error) onto a %Failure{}.

The struct carries:

• :reason -- one of the closed reason atoms below.
• :message -- a human sentence safe to show a user.
• :retryable? -- whether an immediate retry could plausibly succeed.
• :detail -- the original error term, kept for logs and debugging.

String.Chars returns the message; Inspect renders reason: message, so a generic surface that to_strings or inspects the error still shows prose.