diff --git a/packages/loop-js/src/engine/guard.test.ts b/packages/loop-js/src/engine/guard.test.ts
new file mode 100644
index 0000000..9a5fed6
--- /dev/null
+++ b/packages/loop-js/src/engine/guard.test.ts
@@ -0,0 +1,87 @@
+import { describe, expect, test } from "bun:test"
+import { ERROR_CAP } from "./config.ts"
+import { drivePhase, withErrorCap, type Guards } from "./guard.ts"
+import { Interruption, type ExecutorEvent, type PhaseStream } from "./executor.ts"
+
+const usage = (usd: number) => ({
+  inputTokens: 0,
+  outputTokens: 0,
+  cachedInputTokens: 0,
+  usd,
+})
+
+const makeGuards = (budget = 100): Guards => ({
+  cancel: new AbortController().signal,
+  controller: new AbortController(),
+  budget: { spent: 0, cap: budget },
+})
+
+const wait = (ms: number) => new Promise<void>((resolve) => setTimeout(resolve, ms))
+
+describe("drivePhase guard race table", () => {
+  test("cuts off a phase whose next step hangs when the timer fires", async () => {
+    const guards = makeGuards()
+    guards.roundDeadline = Date.now() + 10
+    const phase: PhaseStream<{ reason: "done" }> = (async function* () {
+      await new Promise<void>(() => {})
+      return { reason: "done" }
+    })()
+
+    await expect(drivePhase(phase, guards, async () => {})).rejects.toMatchObject({
+      cause: "error",
+      detail: "round timeout",
+    })
+    expect(guards.controller.signal.aborted).toBe(true)
+  }, 10_000)
+
+  test("lets cancel beat a clean result while the final step drains", async () => {
+    const cancel = new AbortController()
+    const guards = makeGuards()
+    guards.cancel = cancel.signal
+    const phase: PhaseStream<{ reason: "done" }> = (async function* () {
+      await wait(10)
+      return { reason: "done" }
+    })()
+    setTimeout(() => cancel.abort(), 1)
+
+    await expect(drivePhase(phase, guards, async () => {})).rejects.toMatchObject({
+      cause: "cancel",
+      detail: "aborted",
+    })
+  })
+
+  test("stops after one budget-overshooting step", async () => {
+    const guards = makeGuards(5)
+    let returned = false
+    const phase: PhaseStream<{ reason: "done" }> = (async function* () {
+      yield { kind: "cost", usage: usage(3) } satisfies ExecutorEvent
+      yield { kind: "cost", usage: usage(4) } satisfies ExecutorEvent
+      returned = true
+      return { reason: "done" }
+    })()
+
+    await expect(drivePhase(phase, guards, async () => {})).rejects.toMatchObject({
+      cause: "budget",
+      detail: "usd 7.00 > cap 5",
+    })
+    expect(guards.budget.spent).toBe(7)
+    expect(returned).toBe(false)
+  })
+})
+
+test("retries only ERROR_CAP consecutive errors before rethrowing the last one", async () => {
+  const guards = makeGuards()
+  let attempts = 0
+
+  await expect(
+    withErrorCap(
+      guards,
+      1,
+      async () => {
+        attempts++
+        throw new Interruption("error", `failure ${attempts}`)
+      },
+    ),
+  ).rejects.toMatchObject({ cause: "error", detail: `failure ${ERROR_CAP}` })
+  expect(attempts).toBe(ERROR_CAP)
+})