diff --git a/.changeset/fix-env-mutation.md b/.changeset/fix-env-mutation.md
new file mode 100644
index 000000000000..92454ae660df
--- /dev/null
+++ b/.changeset/fix-env-mutation.md
@@ -0,0 +1,5 @@
+---
+'@ai/core': patch
+---
+
+fix (ai/mcp): prevent mutation of customEnv
diff --git a/packages/ai/mcp-stdio/get-environment.test.ts b/packages/ai/mcp-stdio/get-environment.test.ts
new file mode 100644
index 000000000000..1ea0bb900766
--- /dev/null
+++ b/packages/ai/mcp-stdio/get-environment.test.ts
@@ -0,0 +1,13 @@
+import { describe, it, expect } from 'vitest';
+import { getEnvironment } from './get-environment';
+
+describe('getEnvironment', () => {
+  it('should not mutate the original custom environment object', () => {
+    const customEnv = { CUSTOM_VAR: 'custom_value' };
+
+    const result = getEnvironment(customEnv);
+
+    expect(customEnv).toStrictEqual({ CUSTOM_VAR: 'custom_value' });
+    expect(result).not.toBe(customEnv);
+  });
+});
diff --git a/packages/ai/mcp-stdio/get-environment.ts b/packages/ai/mcp-stdio/get-environment.ts
index 843d7edb41e9..d22b7f854caf 100644
--- a/packages/ai/mcp-stdio/get-environment.ts
+++ b/packages/ai/mcp-stdio/get-environment.ts
@@ -24,7 +24,7 @@ export function getEnvironment(
         ]
       : ['HOME', 'LOGNAME', 'PATH', 'SHELL', 'TERM', 'USER'];
 
-  const env: Record<string, string> = customEnv ?? {};
+  const env: Record<string, string> = customEnv ? { ...customEnv } : {};
 
   for (const key of DEFAULT_INHERITED_ENV_VARS) {
     const value = globalThis.process.env[key];