Fix Anthropic empty thinking signature replay

closes #4464

packages/ai/test/anthropic-empty-thinking-signature-compat.test.ts

@@ -0,0 +1,88 @@
+import { describe, expect, it } from "vitest";
+import { streamSimple } from "../src/stream.ts";
+import type { AssistantMessage, Context, Model } from "../src/types.ts";
+
+interface AnthropicPayload {
+	messages?: Array<{
+		role: string;
+		content: Array<{ type: string; text?: string; thinking?: string; signature?: string }>;
+	}>;
+}
+
+class PayloadCaptured extends Error {
+	constructor() {
+		super("payload captured");
+		this.name = "PayloadCaptured";
+	}
+}
+
+function makeModel(allowEmptySignature?: boolean): Model<"anthropic-messages"> {
+	return {
+		id: "mimo-v2.5-pro",
+		name: "MiMo-V2.5-Pro",
+		api: "anthropic-messages",
+		provider: "xiaomi-token-plan-ams",
+		baseUrl: "http://127.0.0.1:9/anthropic",
+		reasoning: true,
+		input: ["text"],
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 1048576,
+		maxTokens: 1024,
+		...(allowEmptySignature === undefined ? {} : { compat: { allowEmptySignature } }),
+	};
+}
+
+function makeContext(thinkingSignature: string): Context {
+	const assistant: AssistantMessage = {
+		role: "assistant",
+		content: [{ type: "thinking", thinking: "internal reasoning", thinkingSignature }],
+		provider: "xiaomi-token-plan-ams",
+		api: "anthropic-messages",
+		model: "mimo-v2.5-pro",
+		timestamp: Date.now(),
+		usage: {
+			input: 0,
+			output: 0,
+			cacheRead: 0,
+			cacheWrite: 0,
+			totalTokens: 0,
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+		},
+		stopReason: "stop",
+	};
+	return {
+		messages: [
+			{ role: "user", content: "first", timestamp: Date.now() },
+			assistant,
+			{ role: "user", content: "second", timestamp: Date.now() },
+		],
+	};
+}
+
+async function capturePayload(model: Model<"anthropic-messages">, context: Context): Promise<AnthropicPayload> {
+	let capturedPayload: AnthropicPayload | undefined;
+	const stream = streamSimple(model, context, {
+		apiKey: "fake-key",
+		onPayload: (payload) => {
+			capturedPayload = payload as AnthropicPayload;
+			throw new PayloadCaptured();
+		},
+	});
+	await stream.result();
+	if (!capturedPayload) throw new Error("Expected payload capture before request");
+	return capturedPayload;
+}
+
+describe("Anthropic empty thinking signature compat", () => {
+	it("converts empty-signature thinking to text by default", async () => {
+		const payload = await capturePayload(makeModel(), makeContext(""));
+		const assistant = payload.messages?.find((message) => message.role === "assistant");
+		expect(assistant?.content).toEqual([{ type: "text", text: "internal reasoning" }]);
+	});
+
+	it("preserves empty-signature thinking when allowEmptySignature is enabled", async () => {
+		const payload = await capturePayload(makeModel(true), makeContext(" "));
+		const assistant = payload.messages?.find((message) => message.role === "assistant");
+		expect(assistant?.content).toEqual([{ type: "thinking", thinking: "internal reasoning", signature: "" }]);
+	});
+});