diff --git a/llama_stack/core/stack.py b/llama_stack/core/stack.py
index 87a3978c1..f734d0285 100644
--- a/llama_stack/core/stack.py
+++ b/llama_stack/core/stack.py
@@ -225,7 +225,10 @@ def replace_env_vars(config: Any, path: str = "") -> Any:
 
         try:
             result = re.sub(pattern, get_env_var, config)
-            return _convert_string_to_proper_type(result)
+            # Only apply type conversion if substitution actually happened
+            if result != config:
+                return _convert_string_to_proper_type(result)
+            return result
         except EnvVarError as e:
             raise EnvVarError(e.var_name, e.path) from None
 
diff --git a/tests/unit/server/test_replace_env_vars.py b/tests/unit/server/test_replace_env_vars.py
index 0dda682c0..14b3b7231 100644
--- a/tests/unit/server/test_replace_env_vars.py
+++ b/tests/unit/server/test_replace_env_vars.py
@@ -88,3 +88,10 @@ def test_nested_structures(setup_env_vars):
     }
     expected = {"key1": "test_value", "key2": ["default", "conditional"], "key3": {"nested": None}}
     assert replace_env_vars(data) == expected
+
+
+def test_explicit_strings_preserved(setup_env_vars):
+    # Explicit strings that look like numbers/booleans should remain strings
+    data = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
+    expected = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
+    assert replace_env_vars(data) == expected