diff --git a/src/hooks/rules-injector/hook.ts b/src/hooks/rules-injector/hook.ts
index f46af4570..2c37c9b4c 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 } from "./cache";
 import { createRuleInjectionProcessor } from "./injector";
+import { clearProjectRootCache } from "./project-root-finder";
 
 interface ToolExecuteInput {
   tool: string;
@@ -75,6 +76,7 @@ export function createRulesInjectorHook(
       if (sessionInfo?.id) {
         clearSessionCache(sessionInfo.id);
       }
+      clearProjectRootCache();
     }
 
     if (event.type === "session.compacted") {
@@ -83,6 +85,7 @@ export function createRulesInjectorHook(
       if (sessionID) {
         clearSessionCache(sessionID);
       }
+      clearProjectRootCache();
     }
   };
 
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 {