diff --git a/src/client/cross-spawn.test.ts b/src/client/cross-spawn.test.ts
index 11e81bf6..98454a9a 100644
--- a/src/client/cross-spawn.test.ts
+++ b/src/client/cross-spawn.test.ts
@@ -1,4 +1,4 @@
-import { StdioClientTransport } from "./stdio.js";
+import { getDefaultEnvironment, StdioClientTransport } from "./stdio.js";
 import spawn from "cross-spawn";
 import { JSONRPCMessage } from "../types.js";
 import { ChildProcess } from "node:child_process";
@@ -72,7 +72,10 @@ describe("StdioClientTransport using cross-spawn", () => {
       "test-command",
       [],
       expect.objectContaining({
-        env: customEnv
+        env: {
+          ...customEnv,
+          ...getDefaultEnvironment()
+        }
       })
     );
   });
diff --git a/src/client/stdio.test.ts b/src/client/stdio.test.ts
index e6ccb347..cc3731fb 100644
--- a/src/client/stdio.test.ts
+++ b/src/client/stdio.test.ts
@@ -1,10 +1,21 @@
 import { JSONRPCMessage } from "../types.js";
-import { StdioClientTransport, StdioServerParameters } from "./stdio.js";
+import { StdioClientTransport, StdioServerParameters, DEFAULT_INHERITED_ENV_VARS, getDefaultEnvironment } from "./stdio.js";
 
 const serverParameters: StdioServerParameters = {
   command: "/usr/bin/tee",
 };
 
+
+let spawnEnv: Record<string, string> | undefined;
+
+jest.mock('cross-spawn', () => {
+  const originalSpawn = jest.requireActual('cross-spawn');
+  return jest.fn((command, args, options) => {
+    spawnEnv = options.env;
+    return originalSpawn(command, args, options);
+  });
+});
+
 test("should start then close cleanly", async () => {
   const client = new StdioClientTransport(serverParameters);
   client.onerror = (error) => {
@@ -60,32 +71,51 @@ test("should read messages", async () => {
   await client.close();
 });
 
-test("should work with actual node mcp server", async () => {
-  const client = new StdioClientTransport({
-    command: "npx",
-    args: ["-y", "@wrtnlabs/calculator-mcp"],
-  });
-  
+test("should properly set default environment variables in spawned process", async () => {
+  const client = new StdioClientTransport(serverParameters);
+
   await client.start();
   await client.close();
+
+  // Get the default environment variables
+  const defaultEnv = getDefaultEnvironment();
+
+  // Verify that all default environment variables are present
+  for (const key of DEFAULT_INHERITED_ENV_VARS) {
+    if (process.env[key] && !process.env[key].startsWith("()")) {
+      expect(spawnEnv).toHaveProperty(key);
+      expect(spawnEnv![key]).toBe(process.env[key]);
+      expect(spawnEnv![key]).toBe(defaultEnv[key]);
+    }
+  }
 });
 
-test("should work with actual node mcp server and empty env", async () => {
-  const client = new StdioClientTransport({
-    command: "npx",
-    args: ["-y", "@wrtnlabs/calculator-mcp"],
-    env: {},
-  });
-  await client.start();
-  await client.close();
-});
+test("should override default environment variables with custom ones", async () => {
+  const customEnv = {
+    HOME: "/custom/home",
+    PATH: "/custom/path",
+    USER: "custom_user"
+  };
 
-test("should work with actual node mcp server and custom env", async () => {
   const client = new StdioClientTransport({
-    command: "npx",
-    args: ["-y", "@wrtnlabs/calculator-mcp"],
-    env: {TEST_VAR: "test-value"},
+    ...serverParameters,
+    env: customEnv
   });
+
   await client.start();
   await client.close();
+
+  // Verify that custom environment variables override default ones
+  for (const [key, value] of Object.entries(customEnv)) {
+    expect(spawnEnv).toHaveProperty(key);
+    expect(spawnEnv![key]).toBe(value);
+  }
+
+  // Verify that other default environment variables are still present
+  for (const key of DEFAULT_INHERITED_ENV_VARS) {
+    if (!(key in customEnv) && process.env[key] && !process.env[key].startsWith("()")) {
+      expect(spawnEnv).toHaveProperty(key);
+      expect(spawnEnv![key]).toBe(process.env[key]);
+    }
+  }
 });