diff --git a/src/hooks/rules-injector/hook.ts b/src/hooks/rules-injector/hook.ts
index 781acc4a3..b3a09ca12 100644
--- a/src/hooks/rules-injector/hook.ts
+++ b/src/hooks/rules-injector/hook.ts
@@ -3,6 +3,7 @@ import { createDynamicTruncator } from "../../shared/dynamic-truncator";
 import { getRuleInjectionFilePath } from "./output-path";
 import { createSessionCacheStore, createSessionRuleScanCacheStore } from "./cache";
 import { createRuleInjectionProcessor } from "./injector";
+import { clearProjectRootCache } from "./project-root-finder";
 
 interface ToolExecuteInput {
   tool: string;
@@ -83,6 +84,7 @@ export function createRulesInjectorHook(
       if (sessionInfo?.id) {
         clearSessionState(sessionInfo.id);
       }
+      clearProjectRootCache();
     }
 
     if (event.type === "session.compacted") {
@@ -91,6 +93,7 @@ export function createRulesInjectorHook(
       if (sessionID) {
         clearSessionState(sessionID);
       }
+      clearProjectRootCache();
     }
   };
 
diff --git a/src/hooks/rules-injector/project-root-finder.test.ts b/src/hooks/rules-injector/project-root-finder.test.ts
new file mode 100644
index 000000000..35d442290
--- /dev/null
+++ b/src/hooks/rules-injector/project-root-finder.test.ts
@@ -0,0 +1,47 @@
+import { afterEach, describe, expect, it, mock } from "bun:test";
+
+describe("findProjectRoot", () => {
+  afterEach(async () => {
+    const actualFileSystem = await import("node:fs");
+    mock.module("node:fs", () => actualFileSystem);
+  });
+
+  it("memoizes repeated lookups for the same start path and resets on cache clear", async () => {
+    // given
+    const actualFileSystem = await import("node:fs");
+    const projectRoot = "/workspace/project";
+    const startPath = `${projectRoot}/src/file.ts`;
+    const packageJsonPath = `${projectRoot}/package.json`;
+
+    const existsSyncSpy = mock((path: string) => path === packageJsonPath);
+    const statSyncSpy = mock(() => ({ isDirectory: () => false }));
+
+    mock.module("node:fs", () => ({
+      ...actualFileSystem,
+      existsSync: existsSyncSpy,
+      statSync: statSyncSpy,
+    }));
+
+    const { clearProjectRootCache, findProjectRoot } = await import(
+      `./project-root-finder.ts?memoization=${Date.now()}`
+    );
+
+    // when
+    const firstResult = findProjectRoot(startPath);
+    const firstExistsSyncCallCount = existsSyncSpy.mock.calls.length;
+
+    const secondResult = findProjectRoot(startPath);
+    const secondExistsSyncCallCount = existsSyncSpy.mock.calls.length;
+
+    clearProjectRootCache();
+    const thirdResult = findProjectRoot(startPath);
+
+    // then
+    expect(firstResult).toBe(projectRoot);
+    expect(secondResult).toBe(projectRoot);
+    expect(thirdResult).toBe(projectRoot);
+    expect(firstExistsSyncCallCount).toBeGreaterThan(0);
+    expect(secondExistsSyncCallCount).toBe(firstExistsSyncCallCount);
+    expect(existsSyncSpy).toHaveBeenCalledTimes(firstExistsSyncCallCount * 2);
+  });
+});
diff --git a/src/hooks/rules-injector/project-root-finder.ts b/src/hooks/rules-injector/project-root-finder.ts
index da697f0d9..ea552e0c9 100644
--- a/src/hooks/rules-injector/project-root-finder.ts
+++ b/src/hooks/rules-injector/project-root-finder.ts
@@ -2,6 +2,12 @@ import { existsSync, statSync } from "node:fs";
 import { dirname, join } from "node:path";
 import { PROJECT_MARKERS } from "./constants";
 
+const projectRootCache = new Map<string, string | null>();
+
+export function clearProjectRootCache(): void {
+  projectRootCache.clear();
+}
+
 /**
  * Find project root by walking up from startPath.
  * Checks for PROJECT_MARKERS (.git, pyproject.toml, package.json, etc.)
@@ -10,6 +16,16 @@ import { PROJECT_MARKERS } from "./constants";
  * @returns Project root path or null if not found
  */
 export function findProjectRoot(startPath: string): string | null {
+  if (projectRootCache.has(startPath)) {
+    return projectRootCache.get(startPath) ?? null;
+  }
+
+  const projectRoot = findProjectRootWithoutCache(startPath);
+  projectRootCache.set(startPath, projectRoot);
+  return projectRoot;
+}
+
+function findProjectRootWithoutCache(startPath: string): string | null {
   let current: string;
 
   try {