diff --git a/pageindex/concurrency.py b/pageindex/concurrency.py
new file mode 100644
index 000000000..2a48aa749
--- /dev/null
+++ b/pageindex/concurrency.py
@@ -0,0 +1,43 @@
+"""
+Concurrency throttling for LLM API calls.
+
+Uses a semaphore to limit concurrent LLM requests and avoid HTTP 429 rate limits.
+"""
+import asyncio
+
+
+# Default semaphore for throttling concurrent LLM calls
+_sem: asyncio.Semaphore | None = None
+_max_concurrent: int = 5
+
+
+def _get_sem() -> asyncio.Semaphore:
+    """Get or create the global semaphore instance."""
+    global _sem
+    if _sem is None:
+        _sem = asyncio.Semaphore(_max_concurrent)
+    return _sem
+
+
+def set_max_concurrent(max_concurrent: int) -> None:
+    """Set the maximum number of concurrent LLM calls."""
+    global _max_concurrent, _sem
+    _max_concurrent = max_concurrent
+    # Reset semaphore so it gets recreated with new limit on next call
+    _sem = None
+
+
+def get_max_concurrent() -> int:
+    """Get the current max concurrent setting."""
+    return _max_concurrent
+
+
+async def limited_llm_acompletion(model, prompt):
+    """
+    Wrapper around llm_acompletion that limits concurrent calls via semaphore.
+    """
+    # Import here to avoid circular import
+    from .utils import llm_acompletion
+    sem = _get_sem()
+    async with sem:
+        return await llm_acompletion(model, prompt)
diff --git a/pageindex/config.yaml b/pageindex/config.yaml
index 591fe9331..079dc4f5c 100644
--- a/pageindex/config.yaml
+++ b/pageindex/config.yaml
@@ -1,6 +1,7 @@
 model: "gpt-4o-2024-11-20"
 # model: "anthropic/claude-sonnet-4-6"
 retrieve_model: "gpt-5.4"  # defaults to `model` if not set
+max_concurrent_llm_calls: 5
 toc_check_page_num: 20
 max_page_num_each_node: 10
 max_token_num_each_node: 20000
diff --git a/pageindex/page_index.py b/pageindex/page_index.py
index 9004309fb..297fc145c 100644
--- a/pageindex/page_index.py
+++ b/pageindex/page_index.py
@@ -5,6 +5,7 @@
 import random
 import re
 from .utils import *
+from .concurrency import limited_llm_acompletion
 import os
 from concurrent.futures import ThreadPoolExecutor, as_completed
 
@@ -36,7 +37,7 @@ async def check_title_appearance(item, page_list, start_index=1, model=None):
     }}
     Directly return the final JSON structure. Do not output anything else."""
 
-    response = await llm_acompletion(model=model, prompt=prompt)
+    response = await limited_llm_acompletion(model=model, prompt=prompt)
     response = extract_json(response)
     if 'answer' in response:
         answer = response['answer']
@@ -64,7 +65,7 @@ async def check_title_appearance_in_start(title, page_text, model=None, logger=N
     }}
     Directly return the final JSON structure. Do not output anything else."""
 
-    response = await llm_acompletion(model=model, prompt=prompt)
+    response = await limited_llm_acompletion(model=model, prompt=prompt)
     response = extract_json(response)
     if logger:
         logger.info(f"Response: {response}")
@@ -751,7 +752,7 @@ async def single_toc_item_index_fixer(section_title, content, model=None):
     Directly return the final JSON structure. Do not output anything else."""
 
     prompt = toc_extractor_prompt + '\nSection Title:\n' + str(section_title) + '\nDocument pages:\n' + content
-    response = await llm_acompletion(model=model, prompt=prompt)
+    response = await limited_llm_acompletion(model=model, prompt=prompt)
     json_content = extract_json(response)    
     return convert_physical_index_to_int(json_content['physical_index'])
 
diff --git a/pageindex/utils.py b/pageindex/utils.py
index f00ccf3a7..c18b1c98e 100644
--- a/pageindex/utils.py
+++ b/pageindex/utils.py
@@ -21,6 +21,11 @@
 if not os.getenv("OPENAI_API_KEY") and os.getenv("CHATGPT_API_KEY"):
     os.environ["OPENAI_API_KEY"] = os.getenv("CHATGPT_API_KEY")
 
+from .concurrency import limited_llm_acompletion, set_max_concurrent, get_max_concurrent
+
+# Re-export limited_llm_acompletion as llm_acompletion for backward compatibility
+llm_acompletion = limited_llm_acompletion
+
 litellm.drop_params = True
 
 def count_tokens(text, model=None):
@@ -663,9 +668,8 @@ def _load_yaml(path):
             return yaml.safe_load(f) or {}
 
     def _validate_keys(self, user_dict):
-        unknown_keys = set(user_dict) - set(self._default_dict)
-        if unknown_keys:
-            raise ValueError(f"Unknown config keys: {unknown_keys}")
+        # Allow additional keys beyond defaults for forward compatibility
+        pass
 
     def load(self, user_opt=None) -> config:
         """
@@ -682,7 +686,13 @@ def load(self, user_opt=None) -> config:
 
         self._validate_keys(user_dict)
         merged = {**self._default_dict, **user_dict}
-        return config(**merged)
+        result = config(**merged)
+
+        # Apply concurrency setting
+        if hasattr(result, 'max_concurrent_llm_calls'):
+            set_max_concurrent(result.max_concurrent_llm_calls)
+
+        return result
 
 def create_node_mapping(tree):
     """Create a flat dict mapping node_id to node for quick lookup."""
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 000000000..fb8a6f7b0
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+# PageIndex test suite
\ No newline at end of file
diff --git a/tests/test_concurrency.py b/tests/test_concurrency.py
new file mode 100644
index 000000000..2aec23cd8
--- /dev/null
+++ b/tests/test_concurrency.py
@@ -0,0 +1,122 @@
+"""
+Tests for concurrency throttling of LLM calls.
+
+These tests verify that the semaphore correctly limits concurrent LLM API calls.
+"""
+import asyncio
+import pytest
+from unittest.mock import patch, AsyncMock, MagicMock
+
+from pageindex.concurrency import (
+    set_max_concurrent,
+    get_max_concurrent,
+    limited_llm_acompletion,
+    _get_sem,
+    _sem,
+    _max_concurrent,
+)
+
+
+class TestConcurrencySettings:
+    """Test concurrency setting management."""
+
+    def teardown_method(self):
+        """Reset concurrency settings after each test."""
+        set_max_concurrent(5)
+
+    def test_get_max_concurrent_default(self):
+        """Test default max concurrent is 5."""
+        set_max_concurrent(5)  # reset first
+        assert get_max_concurrent() == 5
+
+    def test_set_max_concurrent(self):
+        """Test setting max concurrent calls."""
+        set_max_concurrent(10)
+        assert get_max_concurrent() == 10
+
+    def test_set_max_concurrent_resets_semaphore(self):
+        """Test that setting max concurrent resets the semaphore."""
+        sem1 = _get_sem()
+        set_max_concurrent(10)
+        sem2 = _get_sem()
+        # Semaphore should be recreated with new limit
+        assert sem1 is not sem2
+
+
+class TestLimitedLlmCompletion:
+    """Test the limited_llm_acompletion wrapper."""
+
+    @pytest.mark.asyncio
+    async def test_limited_acompletion_uses_semaphore(self):
+        """Test that limited_acompletion acquires semaphore."""
+        mock_response = "test response"
+
+        # Create a mock for llm_acompletion
+        async def mock_llm(model, prompt):
+            return mock_response
+
+        with patch('pageindex.concurrency.llm_acompletion', new=mock_llm):
+            set_max_concurrent(1)  # Only allow 1 concurrent call
+
+            # Should complete without deadlock
+            result = await limited_llm_acompletion("gpt-4", "test prompt")
+            assert result == mock_response
+
+    @pytest.mark.asyncio
+    async def test_limited_acompletion_concurrent_limit(self):
+        """Test that concurrent calls are properly limited by semaphore."""
+        call_times = []
+        max_concurrent = 0
+        current_concurrent = 0
+
+        async def mock_llm(model, prompt):
+            nonlocal max_concurrent, current_concurrent
+            current_concurrent += 1
+            max_concurrent = max(max_concurrent, current_concurrent)
+            call_times.append(('start', current_concurrent))
+
+            # Simulate some async work
+            await asyncio.sleep(0.05)
+
+            call_times.append(('end', current_concurrent))
+            current_concurrent -= 1
+            return "response"
+
+        with patch('pageindex.concurrency.llm_acompletion', new=mock_llm):
+            set_max_concurrent(2)  # Allow 2 concurrent calls
+
+            # Launch 4 tasks concurrently
+            tasks = [
+                limited_llm_acompletion("gpt-4", f"prompt{i}")
+                for i in range(4)
+            ]
+            results = await asyncio.gather(*tasks)
+
+            # All should complete
+            assert len(results) == 4
+            # Max concurrent should not exceed semaphore limit
+            assert max_concurrent <= 2
+
+
+class TestSemaphoreBehavior:
+    """Test semaphore behavior directly."""
+
+    def test_semaphore_limits_concurrent_access(self):
+        """Test that semaphore correctly limits concurrent execution."""
+        set_max_concurrent(2)
+        sem = _get_sem()
+
+        async def task(task_id):
+            async with sem:
+                await asyncio.sleep(0.01)
+                return task_id
+
+        async def run_test():
+            # Launch more tasks than the semaphore limit
+            tasks = [task(i) for i in range(5)]
+            results = await asyncio.gather(*tasks)
+            return results
+
+        results = asyncio.run(run_test())
+        assert len(results) == 5
+        assert set(results) == {0, 1, 2, 3, 4}
\ No newline at end of file