diff --git a/tests/test_resource_guard.py b/tests/test_resource_guard.py
new file mode 100644
index 00000000..1aef1b9f
--- /dev/null
+++ b/tests/test_resource_guard.py
@@ -0,0 +1,105 @@
+"""
+Unit tests for services/resource_guard.py
+
+Resource exhaustion protection tests.
+"""
+import pytest
+from unittest.mock import patch
+from services.resource_guard import ResourceGuard, ResourceExhaustedError
+
+
+class TestResourceGuard:
+    """Tests for ResourceGuard class."""
+
+    def test_check_time_within_limit(self):
+        """Does not raise when elapsed time is below timeout_seconds."""
+        guard = ResourceGuard(timeout_seconds=10)
+        # Mock time.time to return the same value (no elapsed time)
+        with patch("services.resource_guard.time.time", return_value=1000.0):
+            guard.start_time = 1000.0  # Simulate guard created at t=1000
+            # Mock time.time to return t=1005 (5 seconds elapsed, within limit)
+            with patch("services.resource_guard.time.time", return_value=1005.0):
+                guard.check_time()  # Should not raise
+
+    def test_check_time_exceeds_timeout(self):
+        """Raises ResourceExhaustedError when elapsed time exceeds timeout."""
+        guard = ResourceGuard(timeout_seconds=10)
+        with patch("services.resource_guard.time.time", return_value=1000.0):
+            guard.start_time = 1000.0  # Created at t=1000
+            # Elapsed: 15 seconds > 10 second timeout
+            with patch("services.resource_guard.time.time", return_value=1015.0):
+                with pytest.raises(ResourceExhaustedError) as exc_info:
+                    guard.check_time()
+                assert "Processing timeout exceeded" in str(exc_info.value)
+
+    def test_increment_rows_within_limit(self):
+        """Does not raise when rows_processed is below max_rows."""
+        guard = ResourceGuard(max_rows=100000)
+        guard.increment_rows(50000)  # Within limit
+        assert guard.rows_processed == 50000
+
+    def test_increment_rows_at_boundary(self):
+        """Does not raise when rows_processed equals max_rows."""
+        guard = ResourceGuard(max_rows=100000)
+        guard.increment_rows(100000)  # Exactly at limit
+        assert guard.rows_processed == 100000
+
+    def test_increment_rows_exceeds_max(self):
+        """Raises ResourceExhaustedError when rows exceed max_rows."""
+        guard = ResourceGuard(max_rows=100000)
+        with pytest.raises(ResourceExhaustedError) as exc_info:
+            guard.increment_rows(100001)
+        assert "Maximum row count" in str(exc_info.value)
+        assert "100000" in str(exc_info.value)
+
+    def test_increment_rows_multiple_calls_accumulate(self):
+        """Multiple increments accumulate row count correctly."""
+        guard = ResourceGuard(max_rows=100000)
+        guard.increment_rows(30000)
+        guard.increment_rows(30000)
+        guard.increment_rows(30000)
+        assert guard.rows_processed == 90000
+        # One more increment of 10001 would exceed
+        with pytest.raises(ResourceExhaustedError):
+            guard.increment_rows(10001)
+
+    def test_check_resource_limits_calls_both(self):
+        """check_resource_limits enforces both time and row limits."""
+        guard = ResourceGuard(max_rows=100000, timeout_seconds=10)
+
+        # Time exceeded: should raise
+        with patch("services.resource_guard.time.time", return_value=1000.0):
+            guard.start_time = 1000.0
+            with patch("services.resource_guard.time.time", return_value=1020.0):
+                with pytest.raises(ResourceExhaustedError) as exc_info:
+                    guard.check_resource_limits(100)
+                assert "Processing timeout exceeded" in str(exc_info.value)
+
+    def test_check_resource_limits_rows_exceeded(self):
+        """check_resource_limits raises when row limit is exceeded."""
+        guard = ResourceGuard(max_rows=100000, timeout_seconds=10)
+
+        # Row limit exceeded (time is fine)
+        with patch("services.resource_guard.time.time", return_value=1000.0):
+            guard.start_time = 1000.0
+            with patch("services.resource_guard.time.time", return_value=1001.0):
+                with pytest.raises(ResourceExhaustedError) as exc_info:
+                    guard.check_resource_limits(100001)
+                assert "Maximum row count" in str(exc_info.value)
+
+    def test_check_resource_limits_within_both_limits(self):
+        """check_resource_limits passes when both limits are respected."""
+        guard = ResourceGuard(max_rows=100000, timeout_seconds=10)
+        with patch("services.resource_guard.time.time", return_value=1000.0):
+            guard.start_time = 1000.0
+            with patch("services.resource_guard.time.time", return_value=1005.0):
+                guard.check_resource_limits(50000)  # Should not raise
+                assert guard.rows_processed == 50000
+
+    def test_resource_exhausted_error_message_format(self):
+        """ResourceExhaustedError has the expected message format."""
+        error = ResourceExhaustedError("Processing timeout exceeded")
+        assert str(error) == "Processing timeout exceeded"
+
+        error2 = ResourceExhaustedError("Maximum row count (150000) exceeded")
+        assert "150000" in str(error2)