diff --git a/src/hooks/rules-injector/test-isolation.test.ts b/src/hooks/rules-injector/test-isolation.test.ts
new file mode 100644
index 000000000..44e71ad0a
--- /dev/null
+++ b/src/hooks/rules-injector/test-isolation.test.ts
@@ -0,0 +1,25 @@
+import { beforeEach, describe, expect, it } from "bun:test";
+import { existsSync, readdirSync } from "node:fs";
+import { RULES_INJECTOR_STORAGE } from "./constants";
+
+function readStorageEntries(): readonly string[] {
+	if (!existsSync(RULES_INJECTOR_STORAGE)) return [];
+	return readdirSync(RULES_INJECTOR_STORAGE);
+}
+
+describe("rules injector test isolation", () => {
+	beforeEach(() => {
+		const leakedEntries = readStorageEntries();
+		expect(leakedEntries).toEqual([]);
+	});
+
+	it("#given the shared test setup runs #then persisted injected-rule state starts empty", () => {
+		// given: test-setup beforeEach has already run
+
+		// when
+		const entries = readStorageEntries();
+
+		// then
+		expect(entries).toEqual([]);
+	});
+});
diff --git a/test-setup.ts b/test-setup.ts
index c8e8f8d42..223f9f30e 100644
--- a/test-setup.ts
+++ b/test-setup.ts
@@ -4,6 +4,7 @@ import { rmSync } from "node:fs"
 import { _resetForTesting as resetClaudeSessionState } from "./src/features/claude-code-session-state/state"
 import { _resetTaskToastManagerForTesting as resetTaskToastManager } from "./src/features/task-toast-manager/manager"
 import { _resetForTesting as resetModelFallbackState } from "./src/hooks/model-fallback/hook"
+import { RULES_INJECTOR_STORAGE } from "./src/hooks/rules-injector/constants"
 import { _resetMemCacheForTesting as resetConnectedProvidersCache } from "./src/shared/connected-providers-cache"
 import { getOmoOpenCodeCacheDir } from "./src/shared/data-path"
 import { releaseAllPromptAsyncReservationsForTesting } from "./src/shared/prompt-async-gate"
@@ -17,11 +18,16 @@ function cleanupOmoCacheDir(cacheDir: string): void {
   rmSync(cacheDir, { recursive: true, force: true })
 }
 
+function cleanupRulesInjectorStorage(): void {
+  rmSync(RULES_INJECTOR_STORAGE, { recursive: true, force: true })
+}
+
 beforeEach(() => {
   environmentSnapshot = { ...process.env }
   workingDirectorySnapshot = process.cwd()
   process.env.OMO_DISABLE_POSTHOG = "true"
   cleanupOmoCacheDir(getOmoOpenCodeCacheDir())
+  cleanupRulesInjectorStorage()
   resetClaudeSessionState()
   resetTaskToastManager()
   resetModelFallbackState()
@@ -53,6 +59,7 @@ afterEach(() => {
 
   cleanupOmoCacheDir(currentCacheDir)
   cleanupOmoCacheDir(getOmoOpenCodeCacheDir())
+  cleanupRulesInjectorStorage()
   resetTaskToastManager()
   resetConnectedProvidersCache()
   releaseAllPromptAsyncReservationsForTesting()