diff --git a/pyproject.toml b/pyproject.toml
index 750b5ac..d27f2c6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -17,4 +17,5 @@ dependencies = [
     "ruff>=0.9.4",
     "tomli>=2.2.1",
     "typing-extensions>=4.12.2",
+    "pytest>=7.0.0",
 ]
diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
new file mode 100644
index 0000000..52e9834
--- /dev/null
+++ b/tests/test_exceptions.py
@@ -0,0 +1,52 @@
+"""Unit tests for custom exceptions."""
+import pytest
+from exceptions import (
+    PromptTooLongError,
+    DimensionTooSmallError,
+    NoImagesGeneratedError,
+    ImageGenerationError,
+    APIError,
+)
+
+
+class TestCustomExceptions:
+    """Test suite for custom exception classes."""
+
+    def test_prompt_too_long_error(self):
+        """Test PromptTooLongError can be raised and caught."""
+        with pytest.raises(PromptTooLongError):
+            raise PromptTooLongError()
+
+    def test_dimension_too_small_error(self):
+        """Test DimensionTooSmallError can be raised and caught."""
+        with pytest.raises(DimensionTooSmallError):
+            raise DimensionTooSmallError()
+
+    def test_no_images_generated_error(self):
+        """Test NoImagesGeneratedError can be raised and caught."""
+        with pytest.raises(NoImagesGeneratedError):
+            raise NoImagesGeneratedError()
+
+    def test_api_error(self):
+        """Test APIError can be raised and caught."""
+        with pytest.raises(APIError):
+            raise APIError()
+
+    def test_image_generation_error_with_message(self):
+        """Test ImageGenerationError stores message and model index."""
+        error = ImageGenerationError("Test error message", model_index=2)
+        assert str(error) == "Test error message"
+        assert error.model_index == 2
+
+    def test_image_generation_error_defaults(self):
+        """Test ImageGenerationError defaults model_index to 0."""
+        error = ImageGenerationError("Test")
+        assert error.model_index == 0
+
+    def test_prompt_too_long_error_inheritance(self):
+        """Test PromptTooLongError inherits from AppCommandError."""
+        assert issubclass(PromptTooLongError, Exception)
+
+    def test_dimension_too_small_error_inheritance(self):
+        """Test DimensionTooSmallError inherits from AppCommandError."""
+        assert issubclass(DimensionTooSmallError, Exception)