diff --git a/docs/cli/sandbox.md b/docs/cli/sandbox.md
index e27587abf0..0c3a1dd341 100644
--- a/docs/cli/sandbox.md
+++ b/docs/cli/sandbox.md
@@ -227,6 +227,29 @@ export SANDBOX_FLAGS="--flag1 --flag2=value"
 $env:SANDBOX_FLAGS="--flag1 --flag2=value"
 ```
 
+### Route custom environment variables
+
+Use the `SANDBOX_ENV` environment variable to explicitly route custom
+environment variables into the sandbox. This is a comma-separated list of
+`KEY=VALUE` pairs.
+
+**macOS/Linux**
+
+```bash
+export SANDBOX_ENV="MY_VAR=hello,ANOTHER_VAR=world"
+gemini -p "echo \$MY_VAR \$ANOTHER_VAR"
+```
+
+**Windows (PowerShell)**
+
+```powershell
+$env:SANDBOX_ENV="MY_VAR=hello,ANOTHER_VAR=world"
+gemini -p "echo %MY_VAR% %ANOTHER_VAR%"
+```
+
+Environment variables listed in `SANDBOX_ENV` are also available to tools run by
+the agent.
+
 ## Linux UID/GID handling
 
 The sandbox automatically handles user permissions on Linux. Override these
diff --git a/packages/cli/src/config/settings.ts b/packages/cli/src/config/settings.ts
index 7eec1c61b8..6431d8add4 100644
--- a/packages/cli/src/config/settings.ts
+++ b/packages/cli/src/config/settings.ts
@@ -85,6 +85,7 @@ const AUTH_ENV_VAR_WHITELIST = [
   'GOOGLE_API_KEY',
   'GOOGLE_CLOUD_PROJECT',
   'GOOGLE_CLOUD_LOCATION',
+  'SANDBOX_ENV',
 ];
 
 /**
diff --git a/packages/cli/src/utils/sandbox.test.ts b/packages/cli/src/utils/sandbox.test.ts
index ef972a4a0b..cc1f31421e 100644
--- a/packages/cli/src/utils/sandbox.test.ts
+++ b/packages/cli/src/utils/sandbox.test.ts
@@ -514,6 +514,41 @@ describe('sandbox', () => {
       );
     });
 
+    it('should handle SANDBOX_ENV in macOS seatbelt', async () => {
+      vi.mocked(os.platform).mockReturnValue('darwin');
+      process.env['SANDBOX_ENV'] = 'MY_VAR=hello,ANOTHER_VAR=world';
+      const config: SandboxConfig = createMockSandboxConfig({
+        command: 'sandbox-exec',
+        image: 'some-image',
+      });
+
+      interface MockProcess extends EventEmitter {
+        stdout: EventEmitter;
+        stderr: EventEmitter;
+      }
+      const mockSpawnProcess = new EventEmitter() as MockProcess;
+      mockSpawnProcess.stdout = new EventEmitter();
+      mockSpawnProcess.stderr = new EventEmitter();
+      vi.mocked(spawn).mockReturnValue(
+        mockSpawnProcess as unknown as ReturnType<typeof spawn>,
+      );
+
+      const promise = start_sandbox(config);
+      setTimeout(() => mockSpawnProcess.emit('close', 0), 10);
+      await promise;
+
+      // Check that SANDBOX_ENV variables are passed in the sh -c command
+      expect(spawn).toHaveBeenCalledWith(
+        'sandbox-exec',
+        expect.arrayContaining([
+          'sh',
+          '-c',
+          expect.stringContaining('MY_VAR=hello ANOTHER_VAR=world'),
+        ]),
+        expect.any(Object),
+      );
+    });
+
     it('should pass through GOOGLE_GEMINI_BASE_URL and GOOGLE_VERTEX_BASE_URL', async () => {
       const config: SandboxConfig = createMockSandboxConfig({
         command: 'docker',
diff --git a/packages/cli/src/utils/sandbox.ts b/packages/cli/src/utils/sandbox.ts
index dbd2ec64e3..4f027e1ba0 100644
--- a/packages/cli/src/utils/sandbox.ts
+++ b/packages/cli/src/utils/sandbox.ts
@@ -141,18 +141,48 @@ export async function start_sandbox(
       }
 
       const finalArgv = cliArgs;
+      const shCommandParts = [
+        `SANDBOX=sandbox-exec`,
+        `NODE_OPTIONS="${nodeOptions}"`,
+      ];
 
-      args.push(
-        '-f',
-        profileFile,
-        'sh',
-        '-c',
-        [
-          `SANDBOX=sandbox-exec`,
-          `NODE_OPTIONS="${nodeOptions}"`,
-          ...finalArgv.map((arg) => quote([arg])),
-        ].join(' '),
-      );
+      // copy additional environment variables from SANDBOX_ENV
+      if (process.env['SANDBOX_ENV']) {
+        let currentEnv = '';
+        for (let part of process.env['SANDBOX_ENV'].split(',')) {
+          part = part.trim();
+          if (!part) continue;
+          if (part.includes('=')) {
+            if (currentEnv) {
+              debugLogger.log(`SANDBOX_ENV: ${currentEnv}`);
+              const [k, ...vParts] = currentEnv.split('=');
+              const v = vParts.join('=');
+              shCommandParts.push(`${k}=${quote([v])}`);
+            }
+            currentEnv = part;
+          } else {
+            if (currentEnv) {
+              currentEnv += ',' + part;
+            } else {
+              debugLogger.log(`SANDBOX_ENV: ${part} (forwarded)`);
+              const val = process.env[part];
+              if (val !== undefined) {
+                shCommandParts.push(`${part}=${quote([val])}`);
+              }
+            }
+          }
+        }
+        if (currentEnv) {
+          debugLogger.log(`SANDBOX_ENV: ${currentEnv}`);
+          const [k, ...vParts] = currentEnv.split('=');
+          const v = vParts.join('=');
+          shCommandParts.push(`${k}=${quote([v])}`);
+        }
+      }
+
+      shCommandParts.push(...finalArgv.map((arg) => quote([arg])));
+
+      args.push('-f', profileFile, 'sh', '-c', shCommandParts.join(' '));
       // start and set up proxy if GEMINI_SANDBOX_PROXY_COMMAND is set
       const proxyCommand = process.env['GEMINI_SANDBOX_PROXY_COMMAND'];
       let proxyProcess: ChildProcess | undefined = undefined;
@@ -616,18 +646,29 @@ export async function start_sandbox(
 
     // copy additional environment variables from SANDBOX_ENV
     if (process.env['SANDBOX_ENV']) {
-      for (let env of process.env['SANDBOX_ENV'].split(',')) {
-        if ((env = env.trim())) {
-          if (env.includes('=')) {
-            debugLogger.log(`SANDBOX_ENV: ${env}`);
-            args.push('--env', env);
+      let currentEnv = '';
+      for (let part of process.env['SANDBOX_ENV'].split(',')) {
+        part = part.trim();
+        if (!part) continue;
+        if (part.includes('=')) {
+          if (currentEnv) {
+            debugLogger.log(`SANDBOX_ENV: ${currentEnv}`);
+            args.push('--env', currentEnv);
+          }
+          currentEnv = part;
+        } else {
+          if (currentEnv) {
+            currentEnv += ',' + part;
           } else {
-            throw new FatalSandboxError(
-              'SANDBOX_ENV must be a comma-separated list of key=value pairs',
-            );
+            debugLogger.log(`SANDBOX_ENV: ${part} (forwarded)`);
+            args.push('--env', part);
           }
         }
       }
+      if (currentEnv) {
+        debugLogger.log(`SANDBOX_ENV: ${currentEnv}`);
+        args.push('--env', currentEnv);
+      }
     }
 
     // copy NODE_OPTIONS
@@ -979,19 +1020,31 @@ async function start_lxc_sandbox(
 
     // Forward SANDBOX_ENV key=value pairs
     if (process.env['SANDBOX_ENV']) {
-      for (let env of process.env['SANDBOX_ENV'].split(',')) {
-        if ((env = env.trim())) {
-          if (env.includes('=')) {
-            envArgs.push('--env', env);
+      let currentEnv = '';
+      for (let part of process.env['SANDBOX_ENV'].split(',')) {
+        part = part.trim();
+        if (!part) continue;
+        if (part.includes('=')) {
+          if (currentEnv) {
+            envArgs.push('--env', currentEnv);
+          }
+          currentEnv = part;
+        } else {
+          if (currentEnv) {
+            currentEnv += ',' + part;
           } else {
-            throw new FatalSandboxError(
-              'SANDBOX_ENV must be a comma-separated list of key=value pairs',
-            );
+            // LXC doesn't automatically forward from host, so we look it up
+            const val = process.env[part];
+            if (val !== undefined) {
+              envArgs.push('--env', `${part}=${val}`);
+            }
           }
         }
       }
+      if (currentEnv) {
+        envArgs.push('--env', currentEnv);
+      }
     }
-
     // Forward NODE_OPTIONS (e.g. from --inspect flags)
     const existingNodeOptions = process.env['NODE_OPTIONS'] || '';
     const allNodeOptions = [
diff --git a/packages/core/src/services/shellExecutionService.ts b/packages/core/src/services/shellExecutionService.ts
index 0bd825db17..a9d9b97f18 100644
--- a/packages/core/src/services/shellExecutionService.ts
+++ b/packages/core/src/services/shellExecutionService.ts
@@ -425,6 +425,20 @@ export class ShellExecutionService {
       GIT_PAGER: shellExecutionConfig.pager ?? 'cat',
     };
 
+    // Forward SANDBOX_ENV key=value pairs
+    if (process.env['SANDBOX_ENV']) {
+      for (let env of process.env['SANDBOX_ENV'].split(',')) {
+        if ((env = env.trim())) {
+          const index = env.indexOf('=');
+          if (index > 0) {
+            const key = env.substring(0, index);
+            const value = env.substring(index + 1);
+            baseEnv[key] = value;
+          }
+        }
+      }
+    }
+
     if (!isInteractive) {
       // Ensure all GIT_CONFIG_* variables are preserved even if they were redacted
       for (const key of gitConfigKeys) {