fix(ai): clamp MiniMax shared budget max tokens

closes #6061

packages/ai/test/estimate.test.ts

@@ -0,0 +1,74 @@
+import { describe, expect, it } from "vitest";
+import {
+	type AssistantMessage,
+	type Context,
+	calculateContextTokens,
+	estimateContextTokens,
+	estimateMessageTokens,
+	type Usage,
+} from "../src/index.ts";
+
+function usage(totalTokens: number): Usage {
+	return {
+		input: totalTokens,
+		output: 0,
+		cacheRead: 0,
+		cacheWrite: 0,
+		totalTokens,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+	};
+}
+
+function assistant(totalTokens: number, stopReason: AssistantMessage["stopReason"] = "stop"): AssistantMessage {
+	return {
+		role: "assistant",
+		content: [{ type: "text", text: "done" }],
+		api: "anthropic-messages",
+		provider: "test-provider",
+		model: "test-model",
+		usage: usage(totalTokens),
+		stopReason,
+		timestamp: Date.now(),
+	};
+}
+
+describe("token estimation", () => {
+	it("calculates context tokens from usage", () => {
+		expect(calculateContextTokens({ ...usage(0), input: 10, output: 20, cacheRead: 3, cacheWrite: 4 })).toBe(37);
+		expect(calculateContextTokens(usage(123))).toBe(123);
+	});
+
+	it("estimates message tokens from content", () => {
+		expect(estimateMessageTokens({ role: "user", content: "x".repeat(400), timestamp: 0 })).toBe(100);
+	});
+
+	it("uses the latest valid assistant usage as baseline and estimates only trailing messages", () => {
+		const context: Context = {
+			systemPrompt: "system text that should already be counted by provider usage",
+			messages: [
+				{ role: "user", content: "x".repeat(10_000), timestamp: 0 },
+				assistant(50_000),
+				{ role: "user", content: "x".repeat(400), timestamp: 0 },
+			],
+		};
+
+		expect(estimateContextTokens(context)).toMatchObject({
+			tokens: 50_100,
+			usageTokens: 50_000,
+			trailingTokens: 100,
+			lastUsageIndex: 1,
+		});
+	});
+
+	it("ignores error/aborted assistant usage", () => {
+		const context: Context = {
+			messages: [assistant(50_000, "error"), { role: "user", content: "x".repeat(400), timestamp: 0 }],
+		};
+
+		expect(estimateContextTokens(context)).toMatchObject({
+			tokens: 101,
+			usageTokens: 0,
+			lastUsageIndex: null,
+		});
+	});
+});