Squashed 'third_party/pi/' content from commit d981de122

git-subtree-dir: third_party/pi
git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

packages/coding-agent/test/suite/regressions/7290-json-stream-linear.test.ts

@@ -0,0 +1,44 @@
+import { fauxAssistantMessage } from "@earendil-works/pi-ai";
+import { afterEach, describe, expect, it } from "vitest";
+import { toJsonEvent } from "../../../src/modes/json-event.ts";
+import { createHarness, type Harness } from "../harness.ts";
+
+describe("regression #7290: JSON event streams stay linear", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	async function measureUpdateBytes(text: string): Promise<number> {
+		const harness = await createHarness();
+		harnesses.push(harness);
+		harness.setResponses([fauxAssistantMessage(text)]);
+
+		await harness.session.prompt("respond");
+
+		const sessionUpdates = harness.eventsOfType("message_update");
+		for (const update of sessionUpdates) {
+			expect(update).toHaveProperty("message");
+			expect(update.assistantMessageEvent).toHaveProperty("partial");
+		}
+
+		const updates = sessionUpdates.map((event) => toJsonEvent(event));
+		expect(updates.length).toBeGreaterThan(0);
+		for (const update of updates) {
+			expect(update).not.toHaveProperty("message");
+			expect(update.assistantMessageEvent).not.toHaveProperty("partial");
+		}
+		return updates.reduce((bytes, event) => bytes + Buffer.byteLength(JSON.stringify(event)), 0);
+	}
+
+	it("emits delta-only message updates whose size scales linearly", async () => {
+		const smallBytes = await measureUpdateBytes("x".repeat(2_000));
+		const largeBytes = await measureUpdateBytes("x".repeat(4_000));
+
+		expect(largeBytes).toBeGreaterThan(smallBytes);
+		expect(largeBytes / smallBytes).toBeLessThan(2.2);
+	});
+});