fix(core): allow environment variable expansion and explicit overrides for MCP servers (#18837)

packages/core/src/tools/mcp-client.test.ts

@@ -1704,6 +1704,40 @@ describe('mcp-client', () => {
       expect(callArgs.env!['GEMINI_CLI_EXT_VAR']).toBeUndefined();
     });
 
+    it('should expand environment variables in mcpServerConfig.env and not redact them', async () => {
+      const mockedTransport = vi
+        .spyOn(SdkClientStdioLib, 'StdioClientTransport')
+        .mockReturnValue({} as SdkClientStdioLib.StdioClientTransport);
+
+      const originalEnv = process.env;
+      process.env = {
+        ...originalEnv,
+        GEMINI_TEST_VAR: 'expanded-value',
+      };
+
+      try {
+        await createTransport(
+          'test-server',
+          {
+            command: 'test-command',
+            env: {
+              TEST_EXPANDED: 'Value is $GEMINI_TEST_VAR',
+              SECRET_KEY: 'intentional-secret-123',
+            },
+          },
+          false,
+          EMPTY_CONFIG,
+        );
+
+        const callArgs = mockedTransport.mock.calls[0][0];
+        expect(callArgs.env).toBeDefined();
+        expect(callArgs.env!['TEST_EXPANDED']).toBe('Value is expanded-value');
+        expect(callArgs.env!['SECRET_KEY']).toBe('intentional-secret-123');
+      } finally {
+        process.env = originalEnv;
+      }
+    });
+
     describe('useGoogleCredentialProvider', () => {
       beforeEach(() => {
         // Mock GoogleAuth client

packages/core/src/tools/mcp-client.ts

@@ -70,6 +70,7 @@ import {
   sanitizeEnvironment,
   type EnvironmentSanitizationConfig,
 } from '../services/environmentSanitization.js';
+import { expandEnvVars } from '../utils/envExpansion.js';
 import {
   GEMINI_CLI_IDENTIFICATION_ENV_VAR,
   GEMINI_CLI_IDENTIFICATION_ENV_VAR_VALUE,
@@ -783,9 +784,16 @@ function createTransportRequestInit(
   mcpServerConfig: MCPServerConfig,
   headers: Record<string, string>,
 ): RequestInit {
+  const expandedHeaders: Record<string, string> = {};
+  if (mcpServerConfig.headers) {
+    for (const [key, value] of Object.entries(mcpServerConfig.headers)) {
+      expandedHeaders[key] = expandEnvVars(value, process.env);
+    }
+  }
+
   return {
     headers: {
-      ...mcpServerConfig.headers,
+      ...expandedHeaders,
       ...headers,
     },
   };
@@ -1970,15 +1978,33 @@ export async function createTransport(
   }
 
   if (mcpServerConfig.command) {
+    // 1. Sanitize the base process environment to prevent unintended leaks of system-wide secrets.
+    const sanitizedEnv = sanitizeEnvironment(process.env, {
+      ...sanitizationConfig,
+      enableEnvironmentVariableRedaction: true,
+    });
+
+    const finalEnv: Record<string, string> = {
+      [GEMINI_CLI_IDENTIFICATION_ENV_VAR]:
+        GEMINI_CLI_IDENTIFICATION_ENV_VAR_VALUE,
+    };
+    for (const [key, value] of Object.entries(sanitizedEnv)) {
+      if (value !== undefined) {
+        finalEnv[key] = value;
+      }
+    }
+
+    // Expand and merge explicit environment variables from the MCP configuration.
+    if (mcpServerConfig.env) {
+      for (const [key, value] of Object.entries(mcpServerConfig.env)) {
+        finalEnv[key] = expandEnvVars(value, process.env);
+      }
+    }
+
     let transport: Transport = new StdioClientTransport({
       command: mcpServerConfig.command,
       args: mcpServerConfig.args || [],
-      env: {
-        ...sanitizeEnvironment(process.env, sanitizationConfig),
-        ...(mcpServerConfig.env || {}),
-        [GEMINI_CLI_IDENTIFICATION_ENV_VAR]:
-          GEMINI_CLI_IDENTIFICATION_ENV_VAR_VALUE,
-      } as Record<string, string>,
+      env: finalEnv,
       cwd: mcpServerConfig.cwd,
       stderr: 'pipe',
     });