2 files changed, 65 insertions, 0 deletions
diff --git a/packages/cli/src/test-utils/customMatchers.ts b/packages/cli/src/test-utils/customMatchers.ts
new file mode 100644
index 00000000..c0b4df6b
--- /dev/null
+++ b/packages/cli/src/test-utils/customMatchers.ts
@@ -0,0 +1,63 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/// <reference types="vitest/globals" />
+
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { expect } from 'vitest';
+import type { TextBuffer } from '../ui/components/shared/text-buffer.js';
+
+// RegExp to detect invalid characters: backspace, and ANSI escape codes
+// eslint-disable-next-line no-control-regex
+const invalidCharsRegex = /[\b\x1b]/;
+
+function toHaveOnlyValidCharacters(this: vi.Assertion, buffer: TextBuffer) {
+  const { isNot } = this;
+  let pass = true;
+  const invalidLines: Array<{ line: number; content: string }> = [];
+
+  for (let i = 0; i < buffer.lines.length; i++) {
+    const line = buffer.lines[i];
+    if (line.includes('\n')) {
+      pass = false;
+      invalidLines.push({ line: i, content: line });
+      break; // Fail fast on newlines
+    }
+    if (invalidCharsRegex.test(line)) {
+      pass = false;
+      invalidLines.push({ line: i, content: line });
+    }
+  }
+
+  return {
+    pass,
+    message: () =>
+      `Expected buffer ${isNot ? 'not ' : ''}to have only valid characters, but found invalid characters in lines:\n${invalidLines
+        .map((l) => `  [${l.line}]: "${l.content}"`) /* This line was changed */
+        .join('\n')}`,
+    actual: buffer.lines,
+    expected: 'Lines with no line breaks, backspaces, or escape codes.',
+  };
+}
+
+expect.extend({
+  toHaveOnlyValidCharacters,
+});
+
+// Extend Vitest's `expect` interface with the custom matcher's type definition.
+declare module 'vitest' {
+  interface Assertion<T> {
+    toHaveOnlyValidCharacters(): T;
+  }
+  interface AsymmetricMatchersContaining {
+    toHaveOnlyValidCharacters(): void;
+  }
+}
diff --git a/packages/cli/src/test-utils/mockCommandContext.ts b/packages/cli/src/test-utils/mockCommandContext.ts
index cf450484..4137dbff 100644
--- a/packages/cli/src/test-utils/mockCommandContext.ts
+++ b/packages/cli/src/test-utils/mockCommandContext.ts
@@ -53,8 +53,10 @@ export const createMockCommandContext = (
       setPendingItem: vi.fn(),
       loadHistory: vi.fn(),
       toggleCorgiMode: vi.fn(),
+      toggleVimEnabled: vi.fn(),
     },
     session: {
+      sessionShellAllowlist: new Set<string>(),
       stats: {
         sessionStartTime: new Date(),
         lastPromptTokenCount: 0,