diff --git a/Cargo.lock b/Cargo.lock
index 237a0e9..addc0b1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -830,6 +830,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2",
+ "tempfile",
  "thiserror 2.0.17",
  "tokio",
  "toml",
diff --git a/Cargo.toml b/Cargo.toml
index 9eeaf65..a9a6e37 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,3 +16,6 @@ sha2 = "0.10.8"
 thiserror = "2.0.11"
 tokio = { version = "1.48.0", features = ["full"] }
 toml = "0.8.19"
+
+[dev-dependencies]
+tempfile = "3.15"
diff --git a/src/cache.rs b/src/cache.rs
index 8164ce5..e8b8507 100644
--- a/src/cache.rs
+++ b/src/cache.rs
@@ -6,22 +6,40 @@ use std::fs;
 use std::path::Path;
 use std::time::{SystemTime, UNIX_EPOCH};
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
+pub struct FileMetadata {
+    size: u64,
+    modified: u64,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct CacheEntry {
     pub response: OrganizationPlan,
     pub timestamp: u64,
-    pub file_hashes: HashMap<String, String>,
+    pub file_metadata: HashMap<String, FileMetadata>,
 }
 
 #[derive(Serialize, Deserialize, Debug)]
 pub struct Cache {
     entries: HashMap<String, CacheEntry>,
+    max_entries: usize,
+}
+
+impl Default for Cache {
+    fn default() -> Self {
+        Self::new()
+    }
 }
 
 impl Cache {
     pub fn new() -> Self {
+        Self::with_max_entries(1000)
+    }
+
+    pub fn with_max_entries(max_entries: usize) -> Self {
         Self {
             entries: HashMap::new(),
+            max_entries,
         }
     }
 
@@ -63,16 +81,15 @@ impl Cache {
 
     pub fn get_cached_response(&self, filenames: &[String], base_path: &Path) -> Option<OrganizationPlan> {
         let cache_key = self.generate_cache_key(filenames);
-        
+
         if let Some(entry) = self.entries.get(&cache_key) {
-            // Check if files have changed by comparing hashes
             let mut all_files_unchanged = true;
-            
+
             for filename in filenames {
                 let file_path = base_path.join(filename);
-                if let Ok(current_hash) = Self::hash_file(&file_path) {
-                    if let Some(cached_hash) = entry.file_hashes.get(filename) {
-                        if current_hash != *cached_hash {
+                if let Ok(current_metadata) = Self::get_file_metadata(&file_path) {
+                    if let Some(cached_metadata) = entry.file_metadata.get(filename) {
+                        if current_metadata != *cached_metadata {
                             all_files_unchanged = false;
                             break;
                         }
@@ -81,71 +98,80 @@ impl Cache {
                         break;
                     }
                 } else {
-                    // File doesn't exist or can't be read
                     all_files_unchanged = false;
                     break;
                 }
             }
-            
+
             if all_files_unchanged {
                 println!("Using cached response (timestamp: {})", entry.timestamp);
                 return Some(entry.response.clone());
             }
         }
-        
+
         None
     }
 
-    pub fn cache_response(&mut self, filenames: &[String], response: OrganizationPlan, base_path: &Path) {
+    pub fn cache_response(
+        &mut self,
+        filenames: &[String],
+        response: OrganizationPlan,
+        base_path: &Path,
+    ) {
         let cache_key = self.generate_cache_key(filenames);
-        let mut file_hashes = HashMap::new();
-        
-        // Hash all files for future change detection
+        let mut file_metadata = HashMap::new();
+
         for filename in filenames {
             let file_path = base_path.join(filename);
-            if let Ok(hash) = Self::hash_file(&file_path) {
-                file_hashes.insert(filename.clone(), hash);
+            if let Ok(metadata) = Self::get_file_metadata(&file_path) {
+                file_metadata.insert(filename.clone(), metadata);
             }
         }
-        
+
         let timestamp = SystemTime::now()
             .duration_since(UNIX_EPOCH)
             .unwrap_or_default()
             .as_secs();
-        
+
         let entry = CacheEntry {
             response,
             timestamp,
-            file_hashes,
+            file_metadata,
         };
-        
+
         self.entries.insert(cache_key, entry);
+
+        if self.entries.len() > self.max_entries {
+            self.evict_oldest();
+        }
+
         println!("Cached response for {} files", filenames.len());
     }
 
     fn generate_cache_key(&self, filenames: &[String]) -> String {
         let mut sorted_filenames = filenames.to_vec();
         sorted_filenames.sort();
-        
+
         let mut hasher = Sha256::new();
         for filename in &sorted_filenames {
             hasher.update(filename.as_bytes());
             hasher.update(b"|");
         }
-        
+
         hex::encode(hasher.finalize())
     }
 
-    fn hash_file(file_path: &Path) -> Result<String, Box<dyn std::error::Error>> {
-        if !file_path.exists() {
-            return Err("File does not exist".into());
-        }
-        
-        let mut hasher = Sha256::new();
-        let content = fs::read(file_path)?;
-        hasher.update(content);
-        
-        Ok(hex::encode(hasher.finalize()))
+    fn get_file_metadata(file_path: &Path) -> Result<FileMetadata, Box<dyn std::error::Error>> {
+        let metadata = fs::metadata(file_path)?;
+        let modified = metadata
+            .modified()?
+            .duration_since(UNIX_EPOCH)?
+            .as_secs();
+
+        Ok(FileMetadata {
+            size: metadata.len(),
+            modified,
+        })
     }
 
     pub fn cleanup_old_entries(&mut self, max_age_seconds: u64) {
@@ -153,16 +179,243 @@ impl Cache {
             .duration_since(UNIX_EPOCH)
             .unwrap_or_default()
             .as_secs();
-        
+
         let initial_count = self.entries.len();
-        
+
         self.entries.retain(|_, entry| {
             current_time - entry.timestamp < max_age_seconds
         });
-        
+
         let removed_count = initial_count - self.entries.len();
         if removed_count > 0 {
             println!("Cleaned up {} old cache entries", removed_count);
         }
+
+        if self.entries.len() > self.max_entries {
+            self.compact_cache();
+        }
     }
-}
\ No newline at end of file
+
+    fn evict_oldest(&mut self) {
+        if let Some(oldest_key) = self
+            .entries
+            .iter()
+            .min_by_key(|(_, entry)| entry.timestamp)
+            .map(|(k, _)| k.clone())
+        {
+            self.entries.remove(&oldest_key);
+            println!("Evicted oldest cache entry to maintain limit");
+        }
+    }
+
+    fn compact_cache(&mut self) {
+        while self.entries.len() > self.max_entries {
+            self.evict_oldest();
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::files::FileCategory;
+    use std::fs::File;
+    use std::io::Write;
+
+    #[test]
+    fn test_cache_new() {
+        let cache = Cache::new();
+        assert_eq!(cache.max_entries, 1000);
+        assert_eq!(cache.entries.len(), 0);
+    }
+
+    #[test]
+    fn test_cache_with_max_entries() {
+        let cache = Cache::with_max_entries(5);
+        assert_eq!(cache.max_entries, 5);
+    }
+
+    #[test]
+    fn test_cache_default() {
+        let cache = Cache::default();
+        assert_eq!(cache.max_entries, 1000);
+    }
+
+    #[test]
+    fn test_cache_response_and_retrieve() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let base_path = temp_dir.path();
+
+        let mut cache = Cache::new();
+        let filenames = vec!["file1.txt".to_string(), "file2.txt".to_string()];
+
+        for filename in &filenames {
+            let file_path = base_path.join(filename);
+            let mut file = File::create(&file_path).unwrap();
+            file.write_all(b"test content").unwrap();
+        }
+
+        let response = OrganizationPlan {
+            files: vec![FileCategory {
+                filename: "file1.txt".to_string(),
+                category: "Documents".to_string(),
+                sub_category: "Text".to_string(),
+            }],
+        };
+
+        cache.cache_response(&filenames, response.clone(), base_path);
+
+        let cached = cache.get_cached_response(&filenames, base_path);
+        assert!(cached.is_some());
+        assert_eq!(cached.unwrap().files[0].category, "Documents");
+    }
+
+    #[test]
+    fn test_cache_response_file_changed() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let base_path = temp_dir.path();
+
+        let mut cache = Cache::new();
+        let filenames = vec!["file1.txt".to_string()];
+
+        let file_path = base_path.join("file1.txt");
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"original content").unwrap();
+
+        let response = OrganizationPlan {
+            files: vec![FileCategory {
+                filename: "file1.txt".to_string(),
+                category: "Documents".to_string(),
+                sub_category: "Text".to_string(),
+            }],
+        };
+
+        cache.cache_response(&filenames, response.clone(), base_path);
+
+        std::thread::sleep(std::time::Duration::from_millis(100));
+
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"modified content longer than original").unwrap();
+
+        let cached = cache.get_cached_response(&filenames, base_path);
+        assert!(cached.is_none());
+    }
+
+    #[test]
+    fn test_cache_save_and_load() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let cache_path = temp_dir.path().join("cache.json");
+        let base_path = temp_dir.path();
+
+        let mut cache = Cache::new();
+        let filenames = vec!["file1.txt".to_string()];
+
+        let file_path = base_path.join("file1.txt");
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"test").unwrap();
+
+        let response = OrganizationPlan {
+            files: vec![FileCategory {
+                filename: "file1.txt".to_string(),
+                category: "Documents".to_string(),
+                sub_category: "Text".to_string(),
+            }],
+        };
+
+        cache.cache_response(&filenames, response, base_path);
+        cache.save(&cache_path).unwrap();
+
+        let loaded_cache = Cache::load_or_create(&cache_path);
+        assert_eq!(loaded_cache.entries.len(), 1);
+    }
+
+    #[test]
+    fn test_cache_cleanup_old_entries() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let base_path = temp_dir.path();
+
+        let mut cache = Cache::new();
+        let filenames = vec!["file1.txt".to_string()];
+
+        let file_path = base_path.join("file1.txt");
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"test").unwrap();
+
+        let response = OrganizationPlan {
+            files: vec![FileCategory {
+                filename: "file1.txt".to_string(),
+                category: "Documents".to_string(),
+                sub_category: "Text".to_string(),
+            }],
+        };
+
+        cache.cache_response(&filenames, response, base_path);
+
+        cache.cleanup_old_entries(0);
+        assert_eq!(cache.entries.len(), 0);
+    }
+
+    #[test]
+    fn test_cache_max_entries_eviction() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let base_path = temp_dir.path();
+
+        let mut cache = Cache::with_max_entries(2);
+
+        for i in 1..=3 {
+            let filename = format!("file{}.txt", i);
+            let file_path = base_path.join(&filename);
+            let mut file = File::create(&file_path).unwrap();
+            file.write_all(b"test").unwrap();
+
+            let response = OrganizationPlan {
+                files: vec![FileCategory {
+                    filename: filename.clone(),
+                    category: "Documents".to_string(),
+                    sub_category: "Text".to_string(),
+                }],
+            };
+
+            cache.cache_response(&vec![filename], response, base_path);
+        }
+
+        assert_eq!(cache.entries.len(), 2);
+    }
+
+    #[test]
+    fn test_cache_serialization() {
+        let cache = Cache::new();
+        let json = serde_json::to_string(&cache).unwrap();
+        let deserialized: Cache = serde_json::from_str(&json).unwrap();
+        assert_eq!(cache.max_entries, deserialized.max_entries);
+    }
+
+    #[test]
+    fn test_file_metadata_equality() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("test.txt");
+
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"test content").unwrap();
+
+        let metadata1 = Cache::get_file_metadata(&file_path).unwrap();
+        let metadata2 = Cache::get_file_metadata(&file_path).unwrap();
+
+        assert_eq!(metadata1, metadata2);
+    }
+
+    #[test]
+    fn test_cache_key_generation() {
+        let cache = Cache::new();
+        let filenames1 = vec!["a.txt".to_string(), "b.txt".to_string()];
+        let filenames2 = vec!["b.txt".to_string(), "a.txt".to_string()];
+        let filenames3 = vec!["c.txt".to_string()];
+
+        let key1 = cache.generate_cache_key(&filenames1);
+        let key2 = cache.generate_cache_key(&filenames2);
+        let key3 = cache.generate_cache_key(&filenames3);
+
+        assert_eq!(key1, key2);
+        assert_ne!(key1, key3);
+    }
+}
diff --git a/src/config.rs b/src/config.rs
index 7af6409..e93dd31 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -269,3 +269,86 @@ fn expand_home(path: &str) -> String {
 
     path.to_string()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_config_serialization() {
+        let config = Config {
+            api_key: "test_key_12345".to_string(),
+            download_folder: PathBuf::from("/test/path"),
+        };
+
+        let toml_str = toml::to_string_pretty(&config).unwrap();
+        assert!(toml_str.contains("test_key_12345"));
+
+        let deserialized: Config = toml::from_str(&toml_str).unwrap();
+        assert_eq!(config.api_key, deserialized.api_key);
+        assert_eq!(config.download_folder, deserialized.download_folder);
+    }
+
+    #[test]
+    fn test_validate_api_key_valid() {
+        assert!(validate_api_key("AIzaSyB1234567890123456789012345678"));
+        assert!(validate_api_key("AIzaSyB123456789012345678901234567890"));
+    }
+
+    #[test]
+    fn test_validate_api_key_invalid() {
+        assert!(!validate_api_key(""));
+        assert!(!validate_api_key("invalid_key"));
+        assert!(!validate_api_key("BizaSyB1234567890123456789012345678"));
+        assert!(!validate_api_key("short"));
+    }
+
+    #[test]
+    fn test_validate_folder_path_valid() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        assert!(validate_folder_path(temp_dir.path()));
+    }
+
+    #[test]
+    fn test_validate_folder_path_invalid() {
+        assert!(!validate_folder_path(Path::new("/nonexistent/path/that/does/not/exist")));
+
+        let temp_file = tempfile::NamedTempFile::new().unwrap();
+        assert!(!validate_folder_path(temp_file.path()));
+    }
+
+    #[test]
+    fn test_expand_home_with_tilde() {
+        if let Some(base_dirs) = BaseDirs::new() {
+            let home = base_dirs.home_dir();
+            let expanded = expand_home("~/test/path");
+            assert!(expanded.starts_with(home.to_string_lossy().as_ref()));
+            assert!(expanded.contains("test/path"));
+        }
+    }
+
+    #[test]
+    fn test_expand_home_without_tilde() {
+        let expanded = expand_home("/absolute/path");
+        assert_eq!(expanded, "/absolute/path");
+
+        let expanded = expand_home("relative/path");
+        assert_eq!(expanded, "relative/path");
+    }
+
+    #[test]
+    fn test_get_default_downloads_folder() {
+        let path = get_default_downloads_folder();
+        assert!(path.ends_with("Downloads"));
+    }
+
+    #[test]
+    fn test_config_empty_api_key_error() {
+        let config = Config {
+            api_key: String::new(),
+            download_folder: PathBuf::from("/test/path"),
+        };
+
+        assert!(config.api_key.is_empty());
+    }
+}
diff --git a/src/files.rs b/src/files.rs
index 776a5dc..326fd3a 100644
--- a/src/files.rs
+++ b/src/files.rs
@@ -258,3 +258,133 @@ pub fn read_file_sample(path: &Path, max_chars: usize) -> Option<String> {
     // Try to convert to UTF-8. If it fails (binary data), return None.
     String::from_utf8(buffer).ok()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::{self, File};
+    use std::io::Write;
+
+    #[test]
+    fn test_is_text_file_with_text_extensions() {
+        assert!(is_text_file(Path::new("test.txt")));
+        assert!(is_text_file(Path::new("test.rs")));
+        assert!(is_text_file(Path::new("test.py")));
+        assert!(is_text_file(Path::new("test.md")));
+        assert!(is_text_file(Path::new("test.json")));
+    }
+
+    #[test]
+    fn test_is_text_file_with_binary_extensions() {
+        assert!(!is_text_file(Path::new("test.exe")));
+        assert!(!is_text_file(Path::new("test.bin")));
+        assert!(!is_text_file(Path::new("test.jpg")));
+        assert!(!is_text_file(Path::new("test.pdf")));
+    }
+
+    #[test]
+    fn test_is_text_file_case_insensitive() {
+        assert!(is_text_file(Path::new("test.TXT")));
+        assert!(is_text_file(Path::new("test.RS")));
+        assert!(is_text_file(Path::new("test.Py")));
+    }
+
+    #[test]
+    fn test_file_batch_from_path() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let dir_path = temp_dir.path();
+
+        File::create(dir_path.join("file1.txt")).unwrap();
+        File::create(dir_path.join("file2.rs")).unwrap();
+        fs::create_dir(dir_path.join("subdir")).unwrap();
+
+        let batch = FileBatch::from_path(dir_path.to_path_buf());
+        assert_eq!(batch.count(), 2);
+        assert!(batch.filenames.contains(&"file1.txt".to_string()));
+        assert!(batch.filenames.contains(&"file2.rs".to_string()));
+    }
+
+    #[test]
+    fn test_file_batch_from_path_nonexistent() {
+        let batch = FileBatch::from_path(PathBuf::from("/nonexistent/path"));
+        assert_eq!(batch.count(), 0);
+    }
+
+    #[test]
+    fn test_read_file_sample() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("test.txt");
+
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"Hello, World!").unwrap();
+
+        let content = read_file_sample(&file_path, 1000);
+        assert_eq!(content, Some("Hello, World!".to_string()));
+    }
+
+    #[test]
+    fn test_read_file_sample_with_limit() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("test.txt");
+
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(b"Hello, World! This is a long text.").unwrap();
+
+        let content = read_file_sample(&file_path, 5);
+        assert_eq!(content, Some("Hello".to_string()));
+    }
+
+    #[test]
+    fn test_read_file_sample_binary_file() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let file_path = temp_dir.path().join("test.bin");
+
+        let mut file = File::create(&file_path).unwrap();
+        file.write_all(&[0x00, 0xFF, 0x80, 0x90]).unwrap();
+
+        let content = read_file_sample(&file_path, 1000);
+        assert_eq!(content, None);
+    }
+
+    #[test]
+    fn test_read_file_sample_nonexistent() {
+        let content = read_file_sample(Path::new("/nonexistent/file.txt"), 1000);
+        assert_eq!(content, None);
+    }
+
+    #[test]
+    fn test_organization_plan_serialization() {
+        let plan = OrganizationPlan {
+            files: vec![
+                FileCategory {
+                    filename: "test.txt".to_string(),
+                    category: "Documents".to_string(),
+                    sub_category: "Text".to_string(),
+                },
+            ],
+        };
+
+        let json = serde_json::to_string(&plan).unwrap();
+        assert!(json.contains("test.txt"));
+        assert!(json.contains("Documents"));
+
+        let deserialized: OrganizationPlan = serde_json::from_str(&json).unwrap();
+        assert_eq!(deserialized.files[0].filename, "test.txt");
+    }
+
+    #[test]
+    fn test_file_category_serialization() {
+        let fc = FileCategory {
+            filename: "file.rs".to_string(),
+            category: "Code".to_string(),
+            sub_category: "Rust".to_string(),
+        };
+
+        let json = serde_json::to_string(&fc).unwrap();
+        let deserialized: FileCategory = serde_json::from_str(&json).unwrap();
+
+        assert_eq!(fc.filename, deserialized.filename);
+        assert_eq!(fc.category, deserialized.category);
+        assert_eq!(fc.sub_category, deserialized.sub_category);
+    }
+}