diff --git a/tests/test_async_tokenization.py b/tests/test_async_tokenization.py
new file mode 100644
index 00000000000..537b14a8b07
--- /dev/null
+++ b/tests/test_async_tokenization.py
@@ -0,0 +1,286 @@
+"""
+Tests for async tokenization feature
+
+Verifies that large inputs use threadpool tokenization to prevent
+event loop blocking, while small inputs use fast inline tokenization.
+"""
+
+import asyncio
+import sys
+import os
+
+import pytest
+
+sys.path.insert(0, os.path.abspath("../.."))
+
+import litellm
+from litellm.litellm_core_utils.token_counter import (
+    async_token_counter,
+    _calculate_input_size_bytes,
+    _should_use_async_tokenization,
+)
+
+
+def test_calculate_input_size_bytes():
+    """Test input size calculation"""
+    # Text input
+    text = "Hello world"
+    size = _calculate_input_size_bytes(text=text, messages=None)
+    assert size == len(text.encode('utf-8'))
+    
+    # Messages input
+    messages = [
+        {"role": "user", "content": "Test message"}
+    ]
+    size = _calculate_input_size_bytes(text=None, messages=messages)
+    assert size > 0
+    
+    # List of text
+    text_list = ["Hello", "World"]
+    size = _calculate_input_size_bytes(text=text_list, messages=None)
+    assert size > 0
+
+
+def test_should_use_async_tokenization():
+    """Test async tokenization threshold logic"""
+    # Save original setting
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    
+    try:
+        # Disabled by default
+        litellm.async_tokenizer_threshold_bytes = None
+        assert _should_use_async_tokenization(1_000_000) is False
+        
+        # Below threshold
+        litellm.async_tokenizer_threshold_bytes = 500_000
+        assert _should_use_async_tokenization(100_000) is False
+        
+        # Above threshold
+        assert _should_use_async_tokenization(600_000) is True
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+
+
+def test_token_counter_with_disabled_flag():
+    """Test that disable_token_counter still works"""
+    original_flag = litellm.disable_token_counter
+    
+    try:
+        litellm.disable_token_counter = True
+        
+        result = litellm.token_counter(
+            model="gpt-4o-mini",
+            text="This should return 0"
+        )
+        
+        assert result == 0, "Should return 0 when disabled"
+        
+    finally:
+        litellm.disable_token_counter = original_flag
+
+
+@pytest.mark.asyncio
+async def test_async_token_counter_small_input():
+    """Test that small inputs use inline tokenization (fast path)"""
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    
+    try:
+        # Set high threshold so this input stays inline
+        litellm.async_tokenizer_threshold_bytes = 1_000_000
+        
+        text = "Hello, this is a small input"
+        result = await async_token_counter(
+            model="gpt-4o-mini",
+            text=text
+        )
+        
+        # Should get a valid token count
+        assert result > 0
+        assert isinstance(result, int)
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+
+
+@pytest.mark.asyncio
+async def test_async_token_counter_large_input():
+    """Test that large inputs use threadpool (non-blocking path)"""
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    
+    try:
+        # Set low threshold to force threadpool
+        litellm.async_tokenizer_threshold_bytes = 1000  # 1KB
+        
+        # Large input (100KB)
+        large_text = "x" * 100_000
+        result = await async_token_counter(
+            model="gpt-4o-mini",
+            text=large_text
+        )
+        
+        # Should still get valid token count
+        assert result > 0
+        assert isinstance(result, int)
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+
+
+@pytest.mark.asyncio
+async def test_async_tokenization_timeout():
+    """Test that tokenization timeout works"""
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    original_timeout = litellm.tokenizer_timeout_seconds
+    
+    try:
+        # Enable async tokenization with very short timeout
+        litellm.async_tokenizer_threshold_bytes = 1000
+        litellm.tokenizer_timeout_seconds = 0.001  # 1ms - will timeout for large inputs
+        
+        # Very large input that will take longer than 1ms to tokenize
+        large_text = "x" * 1_000_000  # 1MB
+        result = await async_token_counter(
+            model="gpt-4o-mini",
+            text=large_text
+        )
+        
+        # Should return 0 on timeout (graceful degradation)
+        assert result == 0, "Should return 0 on timeout"
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+        litellm.tokenizer_timeout_seconds = original_timeout
+
+
+@pytest.mark.asyncio
+async def test_event_loop_remains_responsive():
+    """
+    Test that event loop stays responsive during tokenization of large inputs.
+    
+    """
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    
+    try:
+        # Enable async tokenization
+        litellm.async_tokenizer_threshold_bytes = 10_000  # 10KB threshold
+        
+        # Create a heartbeat task to monitor event loop
+        heartbeat_count = 0
+        stop_event = asyncio.Event()
+        
+        async def heartbeat():
+            nonlocal heartbeat_count
+            while not stop_event.is_set():
+                heartbeat_count += 1
+                await asyncio.sleep(0.01)  # 10ms heartbeat
+        
+        heartbeat_task = asyncio.create_task(heartbeat())
+        
+        # Tokenize large input (should use threadpool, not block event loop)
+        large_text = "Test message. " * 50_000  # ~700KB
+        
+        result = await async_token_counter(
+            model="gpt-4o-mini",
+            text=large_text
+        )
+        
+        # Stop heartbeat
+        stop_event.set()
+        await heartbeat_task
+        
+        # Verify tokenization worked
+        assert result > 0, "Tokenization should succeed"
+        
+        # Verify event loop was responsive
+        # If event loop was blocked, heartbeat_count would be 0 or very low
+        # If responsive, should be >= 5 (tokenization takes some time)
+        assert heartbeat_count >= 5, f"Event loop should remain responsive, got {heartbeat_count} heartbeats"
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+
+
+@pytest.mark.asyncio
+async def test_concurrent_tokenization():
+    """
+    Test multiple concurrent tokenizations don't block each other.
+    
+    This simulates the real-world scenario from Issue #9145.
+    """
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    
+    try:
+        # Enable async tokenization
+        litellm.async_tokenizer_threshold_bytes = 10_000
+        
+        # Create multiple large inputs
+        large_texts = [
+            f"Request {i}: " + ("test " * 10_000)  # ~50KB each
+            for i in range(5)
+        ]
+        
+        import time
+        start = time.time()
+        
+        tasks = [
+            async_token_counter(model="gpt-4o-mini", text=text)
+            for text in large_texts
+        ]
+        
+        results = await asyncio.gather(*tasks)
+        duration = time.time() - start
+        
+        assert all(r > 0 for r in results), "All tokenizations should succeed"
+        
+        # Should complete reasonably fast (concurrent, not sequential)
+        # If they ran sequentially, would take much longer
+        assert duration < 10, f"Should complete concurrently, took {duration:.2f}s"
+        
+    finally:
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+
+
+def test_threadpool_max_workers_respected():
+    """Test that threadpool max_workers setting is respected"""
+    import litellm.litellm_core_utils.token_counter as tc_module
+    
+    original_threshold = litellm.async_tokenizer_threshold_bytes
+    original_max_workers = litellm.tokenizer_threadpool_max_workers
+    original_pool = tc_module._tokenizer_threadpool
+    
+    try:
+        # Clean up any existing threadpool
+        if tc_module._tokenizer_threadpool is not None:
+            tc_module._tokenizer_threadpool.shutdown(wait=False)
+            tc_module._tokenizer_threadpool = None
+        
+        # Set new configuration
+        litellm.async_tokenizer_threshold_bytes = 1000
+        litellm.tokenizer_threadpool_max_workers = 8
+        
+        # Get threadpool (should create new one with 8 workers)
+        pool = tc_module._get_tokenizer_threadpool()
+        
+        assert pool is not None, "Threadpool should be created"
+        assert pool._max_workers == 8, f"Expected 8 workers, got {pool._max_workers}"
+        
+    finally:
+        # Cleanup
+        if tc_module._tokenizer_threadpool is not None:
+            tc_module._tokenizer_threadpool.shutdown(wait=False)
+        tc_module._tokenizer_threadpool = original_pool
+        litellm.async_tokenizer_threshold_bytes = original_threshold
+        litellm.tokenizer_threadpool_max_workers = original_max_workers
+
+
+if __name__ == "__main__":
+    test_calculate_input_size_bytes()
+    test_should_use_async_tokenization()
+    test_token_counter_with_disabled_flag()
+    asyncio.run(test_async_token_counter_small_input())
+    asyncio.run(test_async_token_counter_large_input())
+    asyncio.run(test_async_tokenization_timeout())
+    asyncio.run(test_event_loop_remains_responsive())
+    asyncio.run(test_concurrent_tokenization())
+    test_threadpool_max_workers_respected()