diff --git a/src/hooks/rules-injector/rule-scan-cache.test.ts b/src/hooks/rules-injector/rule-scan-cache.test.ts
new file mode 100644
index 000000000..778cf341a
--- /dev/null
+++ b/src/hooks/rules-injector/rule-scan-cache.test.ts
@@ -0,0 +1,91 @@
+import { afterEach, beforeEach, describe, expect, it, mock } from "bun:test";
+import { existsSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+function createImportSuffix(): string {
+  return `?test=${Date.now()}-${Math.random()}`;
+}
+
+describe("createRuleScanCache", () => {
+  afterEach(() => {
+    mock.restore();
+  });
+
+  it("returns undefined before set, returns stored value, and clears entries", async () => {
+    // given
+    const { createRuleScanCache } = await import(`./rule-scan-cache${createImportSuffix()}`);
+    const cache = createRuleScanCache();
+    const value = ["a", "b"];
+
+    // when
+    const initialValue = cache.get("k1");
+    cache.set("k1", value);
+    const storedValue = cache.get("k1");
+    cache.clear();
+    const clearedValue = cache.get("k1");
+
+    // then
+    expect(initialValue).toBeUndefined();
+    expect(storedValue).toEqual(value);
+    expect(clearedValue).toBeUndefined();
+  });
+});
+
+describe("findRuleFiles with scan cache", () => {
+  let testRoot = "";
+  let homeDir = "";
+  let projectRoot = "";
+  let currentFile = "";
+  let expectedRuleFile = "";
+  let expectedRuleDir = "";
+
+  beforeEach(() => {
+    testRoot = join(tmpdir(), `rule-scan-cache-test-${Date.now()}`);
+    homeDir = join(testRoot, "home");
+    projectRoot = join(testRoot, "project");
+    currentFile = join(projectRoot, "src", "index.ts");
+    expectedRuleDir = join(projectRoot, ".github", "instructions");
+    expectedRuleFile = join(expectedRuleDir, "typescript.instructions.md");
+
+    mkdirSync(join(projectRoot, ".git"), { recursive: true });
+    mkdirSync(join(projectRoot, "src"), { recursive: true });
+    mkdirSync(homeDir, { recursive: true });
+    writeFileSync(currentFile, "export const value = 1;\n");
+  });
+
+  afterEach(() => {
+    mock.restore();
+    if (existsSync(testRoot)) {
+      rmSync(testRoot, { recursive: true, force: true });
+    }
+  });
+
+  it("reuses cached directory scan results for identical inputs", async () => {
+    // given
+    const findRuleFilesRecursive = mock((directoryPath: string, results: string[]) => {
+      if (directoryPath === expectedRuleDir) {
+        results.push(expectedRuleFile);
+      }
+    });
+
+    mock.module("./rule-file-scanner", () => ({
+      findRuleFilesRecursive,
+      safeRealpathSync: (filePath: string) => filePath,
+    }));
+
+    const { createRuleScanCache } = await import(`./rule-scan-cache${createImportSuffix()}`);
+    const { findRuleFiles } = await import(`./rule-file-finder${createImportSuffix()}`);
+    const cache = createRuleScanCache();
+
+    // when
+    const firstCandidates = findRuleFiles(projectRoot, homeDir, currentFile, undefined, cache);
+    const firstInvocationCount = findRuleFilesRecursive.mock.calls.length;
+    const secondCandidates = findRuleFiles(projectRoot, homeDir, currentFile, undefined, cache);
+
+    // then
+    expect(firstCandidates).toEqual(secondCandidates);
+    expect(firstInvocationCount).toBeGreaterThan(0);
+    expect(findRuleFilesRecursive).toHaveBeenCalledTimes(firstInvocationCount);
+  });
+});