cloudflare/ai#672 reports that AI Gateway's /run rejects an Anthropic Messages body whose system is an array of text blocks: 7003: Model execution failed (User Input Error): Invalid value at system: Invalid input: expected string, received array. createAI's pi-ai provider sends exactly that body for an Anthropic catalog id, so every request with a system prompt may fail.

import { createAI } from "agents/models/pi-ai";
const ai = createAI({ binding: env.AI });
ai.stream(ai("anthropic/claude-opus-4.8"), { systemPrompt: "Be terse.", messages });
// binding.run("anthropic/claude-opus-4.8", { system: [{ type: "text", text: "Be terse.", cache_control: { type: "ephemeral" } }], ... }, { openaiEndpoint: "/v1/messages" })

The body is pi-ai's own, passed through by packages/agents/src/models/pi-ai/wires/anthropic.ts. Not checked live: the old package didn't send openaiEndpoint, and createAI does, so /run may validate it differently. If it does fail, either /run should accept the array (it's valid Anthropic) or the wire should join text-only blocks into a string on /run only. The AI SDK provider isn't affected: string ids go over chat completions.

From cloudflare/ai#672.