fix: preserve existing environment variables when updating codex server config

Author: msanatan

MCPForUnity/Editor/Helpers/CodexConfigHelper.cs

@@ -63,8 +63,19 @@ public static string UpsertCodexServerBlock(string existingToml, string uvPath,
             }
             var mcpServers = root["mcp_servers"] as TomlTable;
 
-            // Create or update unityMCP table
-            mcpServers["unityMCP"] = CreateUnityMcpTable(uvPath, serverSrc);
+            // Extract existing env table if unityMCP already exists
+            TomlTable existingEnvTable = null;
+            if (mcpServers.TryGetNode("unityMCP", out var existingUnityMcpNode) && existingUnityMcpNode is TomlTable)
+            {
+                var existingUnityMcp = existingUnityMcpNode as TomlTable;
+                if (existingUnityMcp.TryGetNode("env", out var envNode) && envNode is TomlTable)
+                {
+                    existingEnvTable = envNode as TomlTable;
+                }
+            }
+
+            // Create or update unityMCP table, preserving existing env table
+            mcpServers["unityMCP"] = CreateUnityMcpTable(uvPath, serverSrc, existingEnvTable);
 
             // Serialize back to TOML
             using var writer = new StringWriter();
@@ -126,7 +137,10 @@ private static TomlTable TryParseToml(string toml)
         /// <summary>
         /// Creates a TomlTable for the unityMCP server configuration
         /// </summary>
-        private static TomlTable CreateUnityMcpTable(string uvPath, string serverSrc)
+        /// <param name="uvPath">Path to uv executable</param>
+        /// <param name="serverSrc">Path to server source directory</param>
+        /// <param name="existingEnvTable">Optional existing env table to preserve user-defined variables</param>
+        private static TomlTable CreateUnityMcpTable(string uvPath, string serverSrc, TomlTable existingEnvTable = null)
         {
             var unityMCP = new TomlTable();
             unityMCP["command"] = new TomlString { Value = uvPath };
@@ -142,11 +156,26 @@ private static TomlTable CreateUnityMcpTable(string uvPath, string serverSrc)
             var platformService = MCPServiceLocator.Platform;
             if (platformService.IsWindows())
             {
-                var envTable = new TomlTable();
+                // Reuse existing env table (from parameter or unityMCP) to avoid dropping user keys
+                TomlTable envTable;
+                if (existingEnvTable != null)
+                    envTable = existingEnvTable;
+                else if (unityMCP.TryGetNode("env", out var envNode) && envNode is TomlTable existing)
+                    envTable = existing;
+                else
+                    envTable = new TomlTable();
+
                 string systemRoot = platformService.GetSystemRoot();
-                envTable["SystemRoot"] = new TomlString { Value = systemRoot };
+                if (!string.IsNullOrEmpty(systemRoot))
+                    envTable["SystemRoot"] = new TomlString { Value = systemRoot };
+
                 unityMCP["env"] = envTable;
             }
+            else if (existingEnvTable != null)
+            {
+                // Preserve existing env table even on non-Windows platforms
+                unityMCP["env"] = existingEnvTable;
+            }
 
             return unityMCP;
         }

TestProjects/UnityMCPTests/Assets/Tests/EditMode/Helpers/CodexConfigHelperTests.cs

@@ -319,5 +319,70 @@ public void UpsertCodexServerBlock_OnNonWindows_ExcludesEnv()
             bool hasEnv = unityMcp.TryGetNode("env", out _);
             Assert.IsFalse(hasEnv, "Non-Windows config should not contain env table");
         }
+
+        [Test]
+        public void UpsertCodexServerBlock_OnWindows_PreservesExistingEnvVariables()
+        {
+            // This test verifies that existing environment variables are preserved when SystemRoot is added
+            // See: https://github.com/CoplayDev/unity-mcp/issues/315
+
+            // Mock Windows platform
+            MCPServiceLocator.Register<IPlatformService>(new MockPlatformService(isWindows: true, systemRoot: "C:\\Windows"));
+
+            // Existing TOML with custom environment variables
+            string existingToml = string.Join("\n", new[]
+            {
+                "[mcp_servers.unityMCP]",
+                "command = \"uv\"",
+                "args = [\"run\", \"--directory\", \"C:\\\\old\\\\path\", \"server.py\"]",
+                "",
+                "[mcp_servers.unityMCP.env]",
+                "CUSTOM_VAR = \"custom_value\"",
+                "ANOTHER_VAR = \"another_value\""
+            });
+
+            string uvPath = "C:\\path\\to\\uv.exe";
+            string serverSrc = "C:\\path\\to\\server";
+
+            string result = CodexConfigHelper.UpsertCodexServerBlock(existingToml, uvPath, serverSrc);
+
+            Assert.IsNotNull(result, "UpsertCodexServerBlock should return a valid TOML string");
+
+            // Parse the generated TOML to validate structure
+            TomlTable parsed;
+            using (var reader = new StringReader(result))
+            {
+                parsed = TOML.Parse(reader);
+            }
+
+            // Verify mcp_servers structure
+            Assert.IsTrue(parsed.TryGetNode("mcp_servers", out var mcpServersNode), "TOML should contain mcp_servers");
+            var mcpServers = mcpServersNode as TomlTable;
+            Assert.IsTrue(mcpServers.TryGetNode("unityMCP", out var unityMcpNode), "mcp_servers should contain unityMCP");
+            var unityMcp = unityMcpNode as TomlTable;
+
+            // Verify env table exists
+            Assert.IsTrue(unityMcp.TryGetNode("env", out var envNode), "Windows config should contain env table");
+            Assert.IsInstanceOf<TomlTable>(envNode, "env should be a table");
+
+            var env = envNode as TomlTable;
+
+            // Verify SystemRoot was added
+            Assert.IsTrue(env.TryGetNode("SystemRoot", out var systemRootNode), "env should contain SystemRoot");
+            Assert.IsInstanceOf<TomlString>(systemRootNode, "SystemRoot should be a string");
+            var systemRoot = (systemRootNode as TomlString).Value;
+            Assert.AreEqual("C:\\Windows", systemRoot, "SystemRoot should be C:\\Windows");
+
+            // Verify existing environment variables were preserved
+            Assert.IsTrue(env.TryGetNode("CUSTOM_VAR", out var customVarNode), "env should preserve CUSTOM_VAR");
+            Assert.IsInstanceOf<TomlString>(customVarNode, "CUSTOM_VAR should be a string");
+            var customVar = (customVarNode as TomlString).Value;
+            Assert.AreEqual("custom_value", customVar, "CUSTOM_VAR should retain its value");
+
+            Assert.IsTrue(env.TryGetNode("ANOTHER_VAR", out var anotherVarNode), "env should preserve ANOTHER_VAR");
+            Assert.IsInstanceOf<TomlString>(anotherVarNode, "ANOTHER_VAR should be a string");
+            var anotherVar = (anotherVarNode as TomlString).Value;
+            Assert.AreEqual("another_value", anotherVar, "ANOTHER_VAR should retain its value");
+        }
     }
 }