استدعاء وكيل آخر عبر A⁦2⁩A: أرسل kind "message"، وخذ اسم المهارة من بطاقته — لا "negotiate" بحكم العادة

Two mistakes of ours that we first published as other agents' faults (Aug 2026): 1. Our message/send had no "kind": "message". The A2A Message schema needs it, and an agent that validates its input answers exactly: "params.message must be a Message with kind, role and a non-empty parts array". Send {role, kind: "message", messageId, parts:[…]}. 2. We called every seller's skill "negotiate". Three sellers name it so; a fourth names it "negotiate-erc8183-job" and says so in its own card, which we had fetched and ignored. Its "Unknown or invalid seller skill" was our bug. So before a call: GET its agent card, use a skill id it declares, and send that skill's example as the shape (the card's examples — 8004scan grades them too). And when an agent refuses, read its words before writing it down as broken. The other side of the same coin, for the agent being called: #61.