diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..572ba5f
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2024-11-20 - Command Injection in Tool Validation
+**Vulnerability:** Found a command injection vulnerability in `toolExists` within `Sources/Cacheout/Models/CacheCategory.swift` where user-controlled input (`tool`) was interpolated into a shell wrapper: `shell("/usr/bin/which \(tool)")`.
+**Learning:** String interpolation in shell commands (`bash -c`) evaluates variables dynamically in the shell, opening severe command injection vectors if the input contains spaces, pipelines, or glob characters.
+**Prevention:** Always avoid shell wrappers (`bash -c`) when possible. Use direct `Process` execution (e.g., `/usr/bin/env` with `arguments = ["which", tool]`) where dynamic arguments are passed safely as an array, entirely bypassing the shell's evaluation step.
diff --git a/Sources/Cacheout/Models/CacheCategory.swift b/Sources/Cacheout/Models/CacheCategory.swift
index 7b3d942..ba20985 100644
--- a/Sources/Cacheout/Models/CacheCategory.swift
+++ b/Sources/Cacheout/Models/CacheCategory.swift
@@ -186,8 +186,23 @@ struct CacheCategory: Identifiable, Hashable {
     }
 
     private func toolExists(_ tool: String) -> Bool {
-        let result = shell("/usr/bin/which \(tool)")
-        return result != nil && !result!.isEmpty
+        let process = Process()
+        process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
+        process.arguments = ["which", tool]
+        process.standardOutput = FileHandle.nullDevice
+        process.standardError = FileHandle.nullDevice
+        process.environment = [
+            "PATH": "/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin",
+            "HOME": FileManager.default.homeDirectoryForCurrentUser.path
+        ]
+
+        do {
+            try process.run()
+            process.waitUntilExit()
+            return process.terminationStatus == 0
+        } catch {
+            return false
+        }
     }
 
     private func runProbe(_ command: String) -> String? {