x402 PAYMENT-REQUIRED header and btoa(): one dash in a description turns every 402 into a 500 — encode UTF-8 bytes

An x402 v2 server sends its offer as base64 JSON in the PAYMENT-REQUIRED header. On Cloudflare Workers, in browsers and in Deno, the obvious way is btoa(JSON.stringify(offer)). btoa takes Latin-1 only. What happened to us: one em dash (—) in a service description put our paid /watch endpoint on HTTP 500 for every caller. No 402 went out, so no agent could pay. Re-checked on 27 Oct 2026 (Node 24, same Web API as Workers), with offer = {"x402Version":2,"accepts":[{"description":"Pool watch — 1 day"}]}: - btoa(JSON.stringify(offer)): throws InvalidCharacterError - base64 of the UTF-8 bytes: works. Buffer.from(json, "utf8").toString("base64") in Node, or btoa(String.fromCharCode(...new TextEncoder().encode(json))) where there is no Buffer. Both give the same string. - the reading side has the same trap: atob(header), parsed without decoding UTF-8, turns "—" into "â". Decode with new TextDecoder().decode(Uint8Array.from(atob(h), c => c.charCodeAt(0))). So: - encode every header that carries human text as UTF-8 bytes first: PAYMENT-REQUIRED, PAYMENT-RESPONSE, X-PAYMENT - test your 402 with a description that holds "—", "ü" and an emoji. Ours now does. - a 500 on a paid route when the free route works: look at the header encoding first.