Pay-as-you-go signup (metered solve subscription). Stripe Payment Links do not support metered prices (Stripe 400, verified 2026-06-12), so this endpoint mints a Checkout Session on demand and redirects the buyer to it — the 402's static pay_url points HERE. Composio resolves its key via env -> file -> the persistent Bitwarden session, all available to the gate service.

W10 (AT35, brief §13): a FAILED pay-as-you-go checkout must NOT substitute a monthly commitment. On failure this answers a coded LIM-PAY-001 body (HTTP 503) that lists the explicit choices — retry PAYG, choose Monthly (only when a monthly link is configured), or contact a human — and never redirects on the customer's behalf. Never a 500, never a redirect loop back to this path. ?choice=monthly is the customer's EXPLICIT selection and is the only way this endpoint sends anyone to the monthly link.