Add env var test

tests/deprecated/test_server_init_kwargs.py

@@ -1,9 +1,11 @@
+import os
 import warnings
 from unittest.mock import patch
 
 import pytest
 
 from fastmcp import FastMCP
+from fastmcp.settings import Settings
 
 # reset deprecation warnings for this module
 pytestmark = pytest.mark.filterwarnings("default::DeprecationWarning")
@@ -303,3 +305,31 @@ class TestDeprecatedServerInitKwargs:
         # This verifies the stacklevel is working as intended (pointing to constructor)
         warning = deprecation_warnings[0]
         assert "server.py" in warning.filename
+
+
+class TestDeprecatedEnvironmentVariables:
+    """Test deprecated environment variable prefixes."""
+
+    def test_fastmcp_server_env_var_deprecation_warning(self):
+        """Test that FASTMCP_SERVER_ environment variables emit deprecation warnings."""
+        env_var_name = "FASTMCP_SERVER_HOST"
+        original_value = os.environ.get(env_var_name)
+
+        try:
+            os.environ[env_var_name] = "192.168.1.1"
+
+            with pytest.warns(
+                DeprecationWarning,
+                match=r"Using `FASTMCP_SERVER_` environment variables is deprecated\. Use `FASTMCP_` instead\.",
+            ):
+                settings = Settings()
+
+            # Verify the setting is still applied
+            assert settings.host == "192.168.1.1"
+
+        finally:
+            # Clean up environment variable
+            if original_value is not None:
+                os.environ[env_var_name] = original_value
+            else:
+                os.environ.pop(env_var_name, None)