diff --git a/scripts/ci-test-ts.test.ts b/scripts/ci-test-ts.test.ts
new file mode 100644
index 000000000..2b73cb41a
--- /dev/null
+++ b/scripts/ci-test-ts.test.ts
@@ -0,0 +1,73 @@
+import { describe, expect, test } from "bun:test";
+import { describeChunkFailure } from "./ci-test-ts.ts";
+
+// The two ways a chunk reaches SIGKILL are indistinguishable by exit code, so
+// these drive real subprocesses to produce a genuine 137 rather than asserting
+// against a hand-written constant.
+async function spawnExitCode(script: string): Promise<number> {
+	const proc = Bun.spawn(["sh", "-c", script], { stdout: "ignore", stderr: "ignore" });
+	return await proc.exited;
+}
+
+// Re-hosts the sequential runner's failure tail: spawn, watchdog, attribute.
+// `runTestCommand` itself is not injectable (it builds argv from the repo
+// layout), so the decision under test is driven directly.
+async function runWithWatchdog(script: string, timeoutMs: number): Promise<string> {
+	const proc = Bun.spawn(["sh", "-c", script], { stdout: "ignore", stderr: "ignore" });
+	let timedOut = false;
+	const killTimer = setTimeout(() => {
+		timedOut = true;
+		proc.kill("SIGKILL");
+	}, timeoutMs);
+	const exitCode = await proc.exited;
+	clearTimeout(killTimer);
+	return describeChunkFailure(exitCode, timedOut);
+}
+
+describe("describeChunkFailure", () => {
+	test("a real SIGKILL that the watchdog did not cause is attributed to the OOM killer", async () => {
+		const exitCode = await spawnExitCode("kill -9 $$");
+		expect(exitCode).toBe(137);
+
+		const message = describeChunkFailure(exitCode, false);
+		expect(message).toContain("OOM killer");
+		expect(message).toContain("chunkSize");
+		// The old wording carried no cause at all; it must not come back.
+		expect(message).not.toBe("failed with exit code 137");
+	});
+
+	test("a watchdog kill is attributed to the watchdog, not to memory", async () => {
+		const message = await runWithWatchdog("sleep 30", 150);
+		expect(message).toContain("chunk watchdog");
+		expect(message).toContain("OMP_TEST_CHUNK_TIMEOUT");
+		expect(message).not.toContain("OOM killer");
+	});
+
+	test("the two SIGKILL causes produce different messages from the same exit code", async () => {
+		const oomKilled = describeChunkFailure(137, false);
+		const watchdogKilled = describeChunkFailure(137, true);
+		expect(oomKilled).not.toBe(watchdogKilled);
+	});
+
+	test("an ordinary test failure keeps the plain wording", async () => {
+		const exitCode = await spawnExitCode("exit 1");
+		expect(exitCode).toBe(1);
+		expect(describeChunkFailure(exitCode, false)).toBe("failed with exit code 1");
+	});
+
+	test("a bun crash exit keeps the plain wording so the retry log still reads naturally", () => {
+		expect(describeChunkFailure(134, false)).toBe("failed with exit code 134");
+		expect(describeChunkFailure(139, false)).toBe("failed with exit code 139");
+	});
+
+	test("the watchdog message reports the configured timeout", () => {
+		const previous = Bun.env.OMP_TEST_CHUNK_TIMEOUT;
+		Bun.env.OMP_TEST_CHUNK_TIMEOUT = "42";
+		try {
+			expect(describeChunkFailure(137, true)).toContain("42s");
+		} finally {
+			if (previous === undefined) delete Bun.env.OMP_TEST_CHUNK_TIMEOUT;
+			else Bun.env.OMP_TEST_CHUNK_TIMEOUT = previous;
+		}
+	});
+});