diff --git a/.gitignore b/.gitignore
index fa5bcb63..7fe87369 100644
--- a/.gitignore
+++ b/.gitignore
@@ -62,6 +62,7 @@ lerna-debug.log*
 .auto-claude-status
 .claude_settings.json
 .update-metadata.json
+/config.json
 
 # ===========================
 # Python (apps/backend)
diff --git a/apps/frontend/scripts/download-python.cjs b/apps/frontend/scripts/download-python.cjs
index 6c48dc89..86bc47b3 100644
--- a/apps/frontend/scripts/download-python.cjs
+++ b/apps/frontend/scripts/download-python.cjs
@@ -702,9 +702,32 @@ async function downloadPython(targetPlatform, targetArch, options = {}) {
     try {
       const version = verifyPythonBinary(pythonBin);
       console.log(`[download-python] Verified: ${version}`);
-      return { success: true, pythonPath: pythonBin, sitePackagesPath: sitePackagesDir };
-    } catch {
-      console.log(`[download-python] Existing installation is broken, re-downloading...`);
+
+      // Verify critical packages exist (fixes GitHub issue #416)
+      // Without this check, corrupted caches with missing packages would be accepted
+      // Note: Same list exists in python-env-manager.ts - keep them in sync
+      // This validation assumes traditional Python packages with __init__.py (not PEP 420 namespace packages)
+      const criticalPackages = ['claude_agent_sdk', 'dotenv'];
+      const missingPackages = criticalPackages.filter(pkg => {
+        const pkgPath = path.join(sitePackagesDir, pkg);
+        // Check both directory and __init__.py for more robust validation
+        const initFile = path.join(pkgPath, '__init__.py');
+        return !fs.existsSync(pkgPath) || !fs.existsSync(initFile);
+      });
+
+      if (missingPackages.length > 0) {
+        console.log(`[download-python] Critical packages missing or incomplete: ${missingPackages.join(', ')}`);
+        console.log(`[download-python] Reinstalling packages...`);
+        // Remove site-packages to force reinstall, keep Python binary
+        // Flow continues below to re-install packages (skipPackages check at line 794)
+        fs.rmSync(sitePackagesDir, { recursive: true, force: true });
+      } else {
+        console.log(`[download-python] All critical packages verified`);
+        return { success: true, pythonPath: pythonBin, sitePackagesPath: sitePackagesDir };
+      }
+    } catch (err) {
+      const errorMsg = err instanceof Error ? err.message : String(err);
+      console.log(`[download-python] Existing installation is broken: ${errorMsg}`);
       fs.rmSync(platformDir, { recursive: true, force: true });
     }
   }
@@ -784,6 +807,22 @@ async function downloadPython(targetPlatform, targetArch, options = {}) {
       // Install packages
       installPackages(pythonBin, requirementsPath, sitePackagesDir);
 
+      // Verify critical packages were installed before creating marker (fixes #416)
+      // Note: Same list exists in python-env-manager.ts - keep them in sync
+      // This validation assumes traditional Python packages with __init__.py (not PEP 420 namespace packages)
+      const criticalPackages = ['claude_agent_sdk', 'dotenv'];
+      const postInstallMissing = criticalPackages.filter(pkg => {
+        const pkgPath = path.join(sitePackagesDir, pkg);
+        const initFile = path.join(pkgPath, '__init__.py');
+        return !fs.existsSync(pkgPath) || !fs.existsSync(initFile);
+      });
+
+      if (postInstallMissing.length > 0) {
+        throw new Error(`Package installation failed - missing critical packages: ${postInstallMissing.join(', ')}`);
+      }
+
+      console.log(`[download-python] All critical packages verified after installation`);
+
       // Create marker file to indicate successful bundling
       fs.writeFileSync(packagesMarker, JSON.stringify({
         bundledAt: new Date().toISOString(),
diff --git a/apps/frontend/src/main/python-env-manager.ts b/apps/frontend/src/main/python-env-manager.ts
index 778c641a..bd31c72f 100644
--- a/apps/frontend/src/main/python-env-manager.ts
+++ b/apps/frontend/src/main/python-env-manager.ts
@@ -122,19 +122,36 @@ export class PythonEnvManager extends EventEmitter {
       return false;
     }
 
-    // Check for the marker file that indicates successful bundling
-    const markerPath = path.join(sitePackagesPath, '.bundled');
-    if (existsSync(markerPath)) {
-      console.log(`[PythonEnvManager] Found bundle marker, using bundled packages`);
-      return true;
+    // Critical packages that must exist for proper functionality
+    // This fixes GitHub issue #416 where marker exists but packages are missing
+    // Note: Same list exists in download-python.cjs - keep them in sync
+    // This validation assumes traditional Python packages with __init__.py (not PEP 420 namespace packages)
+    const criticalPackages = ['claude_agent_sdk', 'dotenv'];
+
+    // Check each package exists with valid structure (directory + __init__.py)
+    const missingPackages = criticalPackages.filter((pkg) => {
+      const pkgPath = path.join(sitePackagesPath, pkg);
+      const initPath = path.join(pkgPath, '__init__.py');
+      // Package is valid if directory and __init__.py both exist
+      return !existsSync(pkgPath) || !existsSync(initPath);
+    });
+
+    // Log missing packages for debugging
+    for (const pkg of missingPackages) {
+      console.log(
+        `[PythonEnvManager] Missing critical package: ${pkg} at ${path.join(sitePackagesPath, pkg)}`
+      );
     }
 
-    // Fallback: check if key packages exist
-    // This handles cases where the marker might be missing but packages are there
-    const claudeSdkPath = path.join(sitePackagesPath, 'claude_agent_sdk');
-    const dotenvPath = path.join(sitePackagesPath, 'dotenv');
-    if (existsSync(claudeSdkPath) || existsSync(dotenvPath)) {
-      console.log(`[PythonEnvManager] Found key packages, using bundled packages`);
+    // All packages must exist - don't rely solely on marker file
+    if (missingPackages.length === 0) {
+      // Also check marker for logging purposes
+      const markerPath = path.join(sitePackagesPath, '.bundled');
+      if (existsSync(markerPath)) {
+        console.log(`[PythonEnvManager] Found bundle marker and all critical packages`);
+      } else {
+        console.log(`[PythonEnvManager] Found critical packages (marker missing)`);
+      }
       return true;
     }