diff --git a/src/commands/index.rs b/src/commands/index.rs
index 16cd4142..f2ece876 100644
--- a/src/commands/index.rs
+++ b/src/commands/index.rs
@@ -129,3 +129,30 @@ pub fn build_index(
 
     Ok(())
 }
+
+#[test]
+fn build_index_creates_db_and_registers_files() {
+    let mut cfg = Config::default();
+    cfg.performance.worker_threads = Some(1);
+    cfg.performance.channel_multiplier = 1;
+    cfg.performance.batch_size = 2;
+
+    let td = tempfile::tempdir().unwrap();
+    let project_dir = td.path().join("proj");
+    fs::create_dir(&project_dir).unwrap();
+    let f_txt = project_dir.join("readme.txt");
+    fs::write(&f_txt, "hello").unwrap();
+
+    let db_path = td.path().join("proj.sqlite");
+
+    build_index("proj", &project_dir, &db_path, &cfg).expect("index build should succeed");
+
+    // ── Assert ────────────────────────────────────────────────────────────────
+    assert!(db_path.is_file(), "SQLite file must exist");
+
+    let pool = Indexer::init(&db_path).unwrap();
+    let idx = Indexer::from_pool("proj", &pool).unwrap();
+    let files = idx.get_files("proj").unwrap();
+    assert_eq!(files.len(), 1, "exactly one file indexed");
+    assert_eq!(files[0], f_txt);
+}
diff --git a/src/commands/scan.rs b/src/commands/scan.rs
index 6db5db73..00b46d90 100644
--- a/src/commands/scan.rs
+++ b/src/commands/scan.rs
@@ -165,3 +165,35 @@ pub fn scan_with_index_parallel(
 
     Ok(diags)
 }
+
+#[test]
+fn scan_with_index_parallel_uses_existing_index_without_rescanning() {
+    let mut cfg = Config::default();
+    cfg.performance.worker_threads = Some(1);
+    cfg.performance.channel_multiplier = 1;
+    cfg.performance.batch_size = 2;
+
+    let td = tempfile::tempdir().unwrap();
+    let project_dir = td.path().join("proj");
+    std::fs::create_dir(&project_dir).unwrap();
+    std::fs::write(project_dir.join("foo.txt"), "abc").unwrap();
+
+    let (project_name, db_path) = get_project_info(&project_dir, td.path()).unwrap();
+    crate::commands::index::build_index(&project_name, &project_dir, &db_path, &cfg).unwrap();
+
+    let pool = Indexer::init(&db_path).unwrap();
+
+    assert_eq!(
+        Indexer::from_pool(&project_name, &pool)
+            .unwrap()
+            .get_files(&project_name)
+            .unwrap()
+            .len(),
+        1
+    );
+
+    let diags = scan_with_index_parallel(&project_name, Arc::clone(&pool), &cfg)
+        .expect("scan should succeed");
+
+    assert!(diags.is_empty());
+}
diff --git a/src/patterns/mod.rs b/src/patterns/mod.rs
index 55ce40bc..c429eed6 100644
--- a/src/patterns/mod.rs
+++ b/src/patterns/mod.rs
@@ -60,6 +60,7 @@ impl FromStr for Severity {
 
 /// One AST pattern with a tree-sitter query and meta-data.
 #[derive(Debug, Clone, Serialize)]
+#[derive(PartialEq)]
 pub struct Pattern {
     /// Unique identifier (snake-case preferred).
     pub id: &'static str,
@@ -113,3 +114,35 @@ pub fn load(lang: &str) -> Vec<Pattern> {
     let key = lang.to_ascii_lowercase();
     REGISTRY.get(key.as_str()).copied().unwrap_or(&[]).to_vec()
 }
+
+#[test]
+fn severity_as_db_str_roundtrip() {
+  for &s in &[Severity::High, Severity::Medium, Severity::Low] {
+    let db = s.as_db_str();
+    assert!(matches!(db, "HIGH" | "MEDIUM" | "LOW"));
+    
+    assert_eq!(db.parse::<Severity>().unwrap(), s);
+    assert_eq!(db.to_lowercase().parse::<Severity>().unwrap(), s);
+  }
+}
+
+#[test]
+fn severity_display_contains_uppercase_name() {
+  assert!(Severity::High.to_string().contains("HIGH"));
+  assert!(Severity::Medium.to_string().contains("MEDIUM"));
+  assert!(Severity::Low.to_string().contains("LOW"));
+}
+
+#[test]
+fn load_returns_correct_pattern_slices() {
+  let rust = load("rust");
+  assert!(!rust.is_empty(), "Rust patterns should be loaded");
+
+  let ts   = load("typescript");
+  let tsx  = load("tsx");
+  assert_eq!(ts, tsx, "alias ‘tsx’ must map to TypeScript patterns");
+  
+  assert_eq!(load("RUST"), rust);
+  
+  assert!(load("brainfuck").is_empty());
+}