diff --git a/src/main/java/cod/ast/ASTVisitor.java b/src/main/java/cod/ast/ASTVisitor.java
index 41ff2af..c6ade16 100644
--- a/src/main/java/cod/ast/ASTVisitor.java
+++ b/src/main/java/cod/ast/ASTVisitor.java
@@ -297,9 +297,8 @@ private static String startPerfTimer(DebugSystem.Level level, String operation)
     if (!isTimerEnabled(level)) {
       return null;
     }
-    String timerName = operation + "#" + Thread.currentThread().getId() + ":" + System.nanoTime();
-    DebugSystem.startTimer(level, timerName);
-    return timerName;
+    DebugSystem.startTimer(level, operation);
+    return operation;
   }
 
   private static void stopPerfTimer(String timerName) {
diff --git a/src/main/java/cod/debug/DebugSystem.java b/src/main/java/cod/debug/DebugSystem.java
index d1bd3e8..185047c 100644
--- a/src/main/java/cod/debug/DebugSystem.java
+++ b/src/main/java/cod/debug/DebugSystem.java
@@ -32,9 +32,11 @@ public int getLevel() {
     private static SimpleDateFormat timeFormat = new SimpleDateFormat("HH:mm:ss.SSS");
     private static final boolean BENCHMARK_MODE = parseBenchmarkMode();
     
-    // Level-based timers (new)
-    private static Map<String, Long> levelTimers = new HashMap<String, Long>();
-    private static Map<String, Level> timerLevels = new HashMap<String, Level>();
+    // Level-based timers (support nested same-name timers per thread)
+    private static final ThreadLocal<Map<String, Deque<Long>>> levelTimerStacks =
+            ThreadLocal.withInitial(HashMap::new);
+    private static final ThreadLocal<Map<String, Deque<Level>>> timerLevelStacks =
+            ThreadLocal.withInitial(HashMap::new);
 
     private static boolean parseBenchmarkMode() {
         String raw = System.getProperty("cod.benchmark.mode");
@@ -50,6 +52,9 @@ public static boolean isBenchmarkMode() {
 
     public static void setLevel(Level level) {
         currentLevel = level;
+        if (level == Level.OFF) {
+            clearTimerStateForCurrentThread();
+        }
     }
 
     public static void setShowTimestamp(boolean show) {
@@ -112,20 +117,40 @@ public static void startTimer(String name) {
     // New level-based timer
     public static void startTimer(Level level, String name) {
         if (shouldLog(level)) {
-            levelTimers.put(name, System.nanoTime());
-            timerLevels.put(name, level);
+            Map<String, Deque<Long>> startsByName = levelTimerStacks.get();
+            Deque<Long> starts = startsByName.computeIfAbsent(name, k -> new ArrayDeque<>());
+            starts.push(System.nanoTime());
+
+            Map<String, Deque<Level>> levelsByName = timerLevelStacks.get();
+            Deque<Level> levels = levelsByName.computeIfAbsent(name, k -> new ArrayDeque<>());
+            levels.push(level);
         }
     }
 
     // Unified stopTimer - works for both original and level-based timers
     public static double stopTimer(String name) {
-        // Check level-based timers first
-        Long levelStart = levelTimers.remove(name);
-        if (levelStart != null) {
-            Level originalLevel = timerLevels.remove(name);
+        // Check level-based timers first (LIFO for nested same-name timers)
+        Map<String, Deque<Long>> startsByName = levelTimerStacks.get();
+        Deque<Long> starts = startsByName.get(name);
+        if (starts != null && !starts.isEmpty()) {
+            long levelStart = starts.pop();
+            if (starts.isEmpty()) {
+                startsByName.remove(name);
+            }
+
+            Map<String, Deque<Level>> levelsByName = timerLevelStacks.get();
+            Deque<Level> levels = levelsByName.get(name);
+            Level originalLevel = null;
+            if (levels != null && !levels.isEmpty()) {
+                originalLevel = levels.pop();
+                if (levels.isEmpty()) {
+                    levelsByName.remove(name);
+                }
+            }
+
             long durationNs = System.nanoTime() - levelStart;
             double durationMs = durationNs / 1_000_000.0;
-            
+
             if (originalLevel != null && shouldLog(originalLevel)) {
                 log(originalLevel, "PERF", String.format("%s took %.3f ms", name, durationMs));
             }
@@ -148,9 +173,10 @@ public static double stopTimer(String name) {
 
     public static double getTimerDuration(String name) {
         // Check level-based timers first
-        Long levelStart = levelTimers.get(name);
-        if (levelStart != null) {
-            long durationNs = System.nanoTime() - levelStart;
+        Map<String, Deque<Long>> startsByName = levelTimerStacks.get();
+        Deque<Long> starts = startsByName.get(name);
+        if (starts != null && !starts.isEmpty()) {
+            long durationNs = System.nanoTime() - starts.peek();
             return durationNs / 1_000_000.0;
         }
         
@@ -207,4 +233,9 @@ private static boolean shouldLog(Level level) {
     public static Level getLevel() {
         return currentLevel;
     }
-}
\ No newline at end of file
+
+    public static void clearTimerStateForCurrentThread() {
+        levelTimerStacks.remove();
+        timerLevelStacks.remove();
+    }
+}
diff --git a/src/main/java/cod/interpreter/context/ExecutionContext.java b/src/main/java/cod/interpreter/context/ExecutionContext.java
index 32ef1e9..ee66cd2 100644
--- a/src/main/java/cod/interpreter/context/ExecutionContext.java
+++ b/src/main/java/cod/interpreter/context/ExecutionContext.java
@@ -657,9 +657,8 @@ private static String startPerfTimer(DebugSystem.Level level, String operation)
         if (!isTimerEnabled(level)) {
             return null;
         }
-        String timerName = operation + "#" + Thread.currentThread().getId() + ":" + System.nanoTime();
-        DebugSystem.startTimer(level, timerName);
-        return timerName;
+        DebugSystem.startTimer(level, operation);
+        return operation;
     }
 
     private static void stopPerfTimer(String timerName) {
diff --git a/src/main/java/cod/interpreter/handler/ExpressionHandler.java b/src/main/java/cod/interpreter/handler/ExpressionHandler.java
index ac11971..402fdd4 100644
--- a/src/main/java/cod/interpreter/handler/ExpressionHandler.java
+++ b/src/main/java/cod/interpreter/handler/ExpressionHandler.java
@@ -30,7 +30,7 @@ public ExpressionHandler(TypeHandler typeSystem, InterpreterVisitor dispatcher)
     // === Core Expression Evaluation ===
     
     public Object handleBinaryOp(BinaryOp node, ExecutionContext ctx) {
-        String timer = startPerfTimer(DebugSystem.Level.DEBUG, "expression.handleBinaryOp");
+        String timer = startPerfTimer(DebugSystem.Level.TRACE, "expression.handleBinaryOp");
         try {
             if (node == null) {
                 throw new InternalError("handleBinaryOp called with null node");
@@ -640,9 +640,8 @@ private static String startPerfTimer(DebugSystem.Level level, String operation)
         if (!isTimerEnabled(level)) {
             return null;
         }
-        String timerName = operation + "#" + Thread.currentThread().getId() + ":" + System.nanoTime();
-        DebugSystem.startTimer(level, timerName);
-        return timerName;
+        DebugSystem.startTimer(level, operation);
+        return operation;
     }
 
     private static void stopPerfTimer(String timerName) {
diff --git a/src/main/java/cod/range/NaturalArray.java b/src/main/java/cod/range/NaturalArray.java
index 03eb748..0eb9a6b 100644
--- a/src/main/java/cod/range/NaturalArray.java
+++ b/src/main/java/cod/range/NaturalArray.java
@@ -1785,9 +1785,8 @@ private static String startPerfTimer(DebugSystem.Level level, String operation)
         if (!isTimerEnabled(level)) {
             return null;
         }
-        String timerName = operation + "#" + Thread.currentThread().getId() + ":" + System.nanoTime();
-        DebugSystem.startTimer(level, timerName);
-        return timerName;
+        DebugSystem.startTimer(level, operation);
+        return operation;
     }
 
     private static void stopPerfTimer(String timerName) {
diff --git a/src/main/java/cod/semantic/ImportResolver.java b/src/main/java/cod/semantic/ImportResolver.java
index 40c82f1..0824d4b 100644
--- a/src/main/java/cod/semantic/ImportResolver.java
+++ b/src/main/java/cod/semantic/ImportResolver.java
@@ -1797,9 +1797,8 @@ private static String startPerfTimer(DebugSystem.Level level, String operation)
         if (!isTimerEnabled(level)) {
             return null;
         }
-        String timerName = operation + "#" + Thread.currentThread().getId() + ":" + System.nanoTime();
-        DebugSystem.startTimer(level, timerName);
-        return timerName;
+        DebugSystem.startTimer(level, operation);
+        return operation;
     }
 
     private static void stopPerfTimer(String timerName) {