diff --git a/src/client/cross-spawn.test.ts b/src/client/cross-spawn.test.ts
index 11e81bf6..d28c7a1a 100644
--- a/src/client/cross-spawn.test.ts
+++ b/src/client/cross-spawn.test.ts
@@ -72,7 +72,26 @@ describe("StdioClientTransport using cross-spawn", () => {
       "test-command",
       [],
       expect.objectContaining({
-        env: customEnv
+        env: expect.objectContaining(customEnv)
+      })
+    );
+  });
+
+  test("should merge default and custom environment variables", async () => {
+    const customEnv = { CUSTOM_VAR: "custom-value" };
+    const transport = new StdioClientTransport({
+      command: "test-command",
+      env: customEnv
+    });
+
+    await transport.start();
+
+    // verify environment variables are merged correctly
+    expect(mockSpawn).toHaveBeenCalledWith(
+      "test-command",
+      [],
+      expect.objectContaining({
+        env: expect.objectContaining(customEnv)
       })
     );
   });
diff --git a/src/client/stdio.ts b/src/client/stdio.ts
index b83bf27c..0963ca13 100644
--- a/src/client/stdio.ts
+++ b/src/client/stdio.ts
@@ -117,7 +117,7 @@ export class StdioClientTransport implements Transport {
         this._serverParams.command,
         this._serverParams.args ?? [],
         {
-          env: this._serverParams.env ?? getDefaultEnvironment(),
+          env: { ...getDefaultEnvironment(), ...(this._serverParams.env ?? {}) },
           stdio: ["pipe", "pipe", this._serverParams.stderr ?? "inherit"],
           shell: false,
           signal: this._abortController.signal,