diff --git a/client.py b/client.py
index be0b585..f394ebb 100644
--- a/client.py
+++ b/client.py
@@ -7,6 +7,7 @@ Functions for creating and configuring the Claude Agent SDK client.
 
 import json
 import os
+import re
 import shutil
 import sys
 from pathlib import Path
@@ -68,7 +69,7 @@ EXTRA_READ_PATHS_BLOCKLIST = {
     ".netrc",
 }
 
-def compute_mode(model: str) -> str:
+def convert_model_for_vertex(model: str) -> str:
     """
     Convert model name format for Vertex AI compatibility.
 
@@ -89,8 +90,7 @@ def compute_mode(model: str) -> str:
     # Pattern: claude-{name}-{version}-{date} -> claude-{name}-{version}@{date}
     # Example: claude-opus-4-5-20251101 -> claude-opus-4-5@20251101
     # The date is always 8 digits at the end
-    import re
-    match = re.match(r'^(claude-[a-z0-9-]+?)-(\d{8})$', model)
+    match = re.match(r'^(claude-.+)-(\d{8})$', model)
     if match:
         base_name, date = match.groups()
         return f"{base_name}@{date}"
@@ -208,6 +208,7 @@ def get_extra_read_paths() -> list[Path]:
 
     return validated_paths
 
+
 # Feature MCP tools for feature/test management
 FEATURE_MCP_TOOLS = [
     # Core feature operations
@@ -438,7 +439,7 @@ def create_client(
     is_vertex = sdk_env.get("CLAUDE_CODE_USE_VERTEX") == "1"
     is_alternative_api = bool(base_url) or is_vertex
     is_ollama = "localhost:11434" in base_url or "127.0.0.1:11434" in base_url
-    model = compute_mode(model)
+    model = convert_model_for_vertex(model)
     if sdk_env:
         print(f"   - API overrides: {', '.join(sdk_env.keys())}")
         if is_vertex:
diff --git a/test_client.py b/test_client.py
new file mode 100644
index 0000000..48f52c4
--- /dev/null
+++ b/test_client.py
@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+"""
+Client Utility Tests
+====================
+
+Tests for the client module utility functions.
+Run with: python test_client.py
+"""
+
+import os
+import unittest
+
+from client import convert_model_for_vertex
+
+
+class TestConvertModelForVertex(unittest.TestCase):
+    """Tests for convert_model_for_vertex function."""
+
+    def setUp(self):
+        """Save original env state."""
+        self._orig_vertex = os.environ.get("CLAUDE_CODE_USE_VERTEX")
+
+    def tearDown(self):
+        """Restore original env state."""
+        if self._orig_vertex is None:
+            os.environ.pop("CLAUDE_CODE_USE_VERTEX", None)
+        else:
+            os.environ["CLAUDE_CODE_USE_VERTEX"] = self._orig_vertex
+
+    # --- Vertex AI disabled (default) ---
+
+    def test_returns_model_unchanged_when_vertex_disabled(self):
+        os.environ.pop("CLAUDE_CODE_USE_VERTEX", None)
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5-20251101",
+        )
+
+    def test_returns_model_unchanged_when_vertex_set_to_zero(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "0"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5-20251101",
+        )
+
+    def test_returns_model_unchanged_when_vertex_set_to_empty(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = ""
+        self.assertEqual(
+            convert_model_for_vertex("claude-sonnet-4-5-20250929"),
+            "claude-sonnet-4-5-20250929",
+        )
+
+    # --- Vertex AI enabled: standard conversions ---
+
+    def test_converts_opus_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5@20251101",
+        )
+
+    def test_converts_sonnet_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-sonnet-4-5-20250929"),
+            "claude-sonnet-4-5@20250929",
+        )
+
+    def test_converts_haiku_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-3-5-haiku-20241022"),
+            "claude-3-5-haiku@20241022",
+        )
+
+    # --- Vertex AI enabled: already converted or non-matching ---
+
+    def test_already_vertex_format_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5@20251101"),
+            "claude-opus-4-5@20251101",
+        )
+
+    def test_non_claude_model_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("gpt-4o"),
+            "gpt-4o",
+        )
+
+    def test_model_without_date_suffix_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5"),
+            "claude-opus-4-5",
+        )
+
+    def test_empty_string_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(convert_model_for_vertex(""), "")
+
+
+if __name__ == "__main__":
+    unittest.main()