Fix token statistics on abort for Anthropic provider

- Add handling for message_start event to capture initial token usage
- Fix message_delta to use assignment (=) instead of addition (+=)
  since Anthropic sends cumulative token counts, not incremental
- Add comprehensive tests for all providers (Google, OpenAI Completions,
  OpenAI Responses, Anthropic)
- Document OpenAI limitation: token stats only available at stream end

Fixes issue where aborted streams had zero token counts despite
Anthropic sending input tokens in the initial message_start event.

packages/ai/test/tokens.test.ts

@@ -0,0 +1,80 @@
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.js";
+import { stream } from "../src/stream.js";
+import type { Api, Context, Model, OptionsForApi } from "../src/types.js";
+
+async function testTokensOnAbort<TApi extends Api>(llm: Model<TApi>, options: OptionsForApi<TApi> = {}) {
+	const context: Context = {
+		messages: [
+			{
+				role: "user",
+				content: "Write a long poem with 10 stanzas about the beauty of nature.",
+				timestamp: Date.now(),
+			},
+		],
+	};
+
+	const controller = new AbortController();
+	const response = stream(llm, context, { ...options, signal: controller.signal });
+
+	let abortFired = false;
+	for await (const event of response) {
+		if (!abortFired && (event.type === "text_delta" || event.type === "thinking_delta")) {
+			abortFired = true;
+			setTimeout(() => controller.abort(), 3000);
+		}
+	}
+
+	const msg = await response.result();
+
+	expect(msg.stopReason).toBe("aborted");
+
+	// OpenAI providers only send usage in the final chunk, so when aborted they have no token stats
+	// Anthropic and Google send usage information early in the stream
+	if (llm.api === "openai-completions" || llm.api === "openai-responses") {
+		expect(msg.usage.input).toBe(0);
+		expect(msg.usage.output).toBe(0);
+	} else {
+		expect(msg.usage.input).toBeGreaterThan(0);
+		expect(msg.usage.output).toBeGreaterThan(0);
+		expect(msg.usage.cost.input).toBeGreaterThan(0);
+		expect(msg.usage.cost.total).toBeGreaterThan(0);
+	}
+}
+
+describe("Token Statistics on Abort", () => {
+	describe.skipIf(!process.env.GEMINI_API_KEY)("Google Provider", () => {
+		const llm = getModel("google", "gemini-2.5-flash");
+
+		it("should include token stats when aborted mid-stream", async () => {
+			await testTokensOnAbort(llm, { thinking: { enabled: true } });
+		}, 10000);
+	});
+
+	describe.skipIf(!process.env.OPENAI_API_KEY)("OpenAI Completions Provider", () => {
+		const llm: Model<"openai-completions"> = {
+			...getModel("openai", "gpt-4o-mini")!,
+			api: "openai-completions",
+		};
+
+		it("should include token stats when aborted mid-stream", async () => {
+			await testTokensOnAbort(llm);
+		}, 10000);
+	});
+
+	describe.skipIf(!process.env.OPENAI_API_KEY)("OpenAI Responses Provider", () => {
+		const llm = getModel("openai", "gpt-5-mini");
+
+		it("should include token stats when aborted mid-stream", async () => {
+			await testTokensOnAbort(llm);
+		}, 20000);
+	});
+
+	describe.skipIf(!process.env.ANTHROPIC_API_KEY && !process.env.ANTHROPIC_OAUTH_TOKEN)("Anthropic Provider", () => {
+		const llm = getModel("anthropic", "claude-opus-4-1-20250805");
+
+		it("should include token stats when aborted mid-stream", async () => {
+			await testTokensOnAbort(llm, { thinkingEnabled: true, thinkingBudgetTokens: 2048 });
+		}, 10000);
+	});
+});