diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..395ab52
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2024-05-24 - String Interpolation in Shell Commands
+**Vulnerability:** Command injection vulnerability identified in `toolExists` within `CacheCategory.swift`, which utilized a shell wrapper (`/bin/bash -c`) with string interpolation to pass dynamic arguments: `shell("/usr/bin/which \(tool)")`.
+**Learning:** String interpolation combined with shell wrappers allows arbitrary command injection if the interpolated variable contains shell metacharacters. Even inside `toolExists`, where `requiresTool` inputs seemed controlled internally, the code pattern itself was inherently unsafe.
+**Prevention:** Avoid using shell wrappers (`/bin/bash -c`) and string interpolation for executing dynamic commands. Always use direct execution of the binary using `Process` with arguments safely separated into the `process.arguments` array.
diff --git a/Sources/Cacheout/Models/CacheCategory.swift b/Sources/Cacheout/Models/CacheCategory.swift
index 7b3d942..818f93d 100644
--- a/Sources/Cacheout/Models/CacheCategory.swift
+++ b/Sources/Cacheout/Models/CacheCategory.swift
@@ -186,8 +186,20 @@ 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/which")
+        process.arguments = [tool]
+        process.standardOutput = FileHandle.nullDevice
+        process.standardError = FileHandle.nullDevice
+
+        do {
+            try process.run()
+            process.waitUntilExit()
+            return process.terminationStatus == 0
+        } catch {
+            return false
+        }
     }
 
     private func runProbe(_ command: String) -> String? {