diff --git a/evals/subprocess-xml.eval.ts b/evals/subprocess-xml.eval.ts
new file mode 100644
index 0000000000..44dfbf3bd6
--- /dev/null
+++ b/evals/subprocess-xml.eval.ts
@@ -0,0 +1,67 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect } from 'vitest';
+import { evalTest } from './test-helper.js';
+
+describe('Subprocess XML tagging behavior', () => {
+  evalTest('ALWAYS_PASSES', {
+    name: 'should detect successful command execution with exit code 0',
+    prompt:
+      "Run 'echo Hello' and tell me if it succeeded. Only say 'Yes' or 'No'.",
+    assert: async (rig, result) => {
+      await rig.waitForToolCall('run_shell_command');
+      expect(result.toLowerCase()).toContain('yes');
+
+      const lastRequest = rig.readLastApiRequest();
+      expect(lastRequest?.attributes?.request_text).toContain(
+        '<exit_code>0</exit_code>',
+      );
+    },
+  });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should detect failed command execution with non-zero exit code',
+    prompt:
+      "Run 'ls non_existent_file_12345' and tell me if it failed. Only say 'Yes' or 'No'.",
+    assert: async (rig, result) => {
+      await rig.waitForToolCall('run_shell_command');
+      expect(result.toLowerCase()).toContain('yes');
+
+      const lastRequest = rig.readLastApiRequest();
+      expect(lastRequest?.attributes?.request_text).toMatch(
+        /<exit_code>[1-9]\d*<\/exit_code>/,
+      );
+    },
+  });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should correctly parse content from <output> tag',
+    prompt: "Run 'echo UNIQUE_STRING_99' and tell me what the output was.",
+    assert: async (rig, result) => {
+      await rig.waitForToolCall('run_shell_command');
+      expect(result).toContain('UNIQUE_STRING_99');
+    },
+  });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should correctly parse error messages from <error> tag',
+    // We force a process-level error by trying to execute a directory
+    prompt:
+      "Try to execute the current directory './' as a command and tell me what the error message was.",
+    assert: async (rig, result) => {
+      await rig.waitForToolCall('run_shell_command');
+      // The error message usually contains "Permission denied" or "is a directory"
+      expect(result.toLowerCase()).toMatch(/permission denied|is a directory/);
+
+      const lastRequest = rig.readLastApiRequest();
+      expect(lastRequest?.attributes?.request_text).toContain('<output>');
+      expect(lastRequest?.attributes?.request_text).toContain(
+        '<exit_code>126</exit_code>',
+      );
+    },
+  });
+});