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,
diff --git a/packages/cli/src/ui/components/shared/text-buffer.test.ts b/packages/cli/src/ui/components/shared/text-buffer.test.ts
index 807c33df..cbceedbc 100644
--- a/packages/cli/src/ui/components/shared/text-buffer.test.ts
+++ b/packages/cli/src/ui/components/shared/text-buffer.test.ts
@@ -32,6 +32,7 @@ describe('textBufferReducer', () => {
   it('should return the initial state if state is undefined', () => {
     const action = { type: 'unknown_action' } as unknown as TextBufferAction;
     const state = textBufferReducer(initialState, action);
+    expect(state).toHaveOnlyValidCharacters();
     expect(state).toEqual(initialState);
   });
 
@@ -42,6 +43,7 @@ describe('textBufferReducer', () => {
         payload: 'hello\nworld',
       };
       const state = textBufferReducer(initialState, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['hello', 'world']);
       expect(state.cursorRow).toBe(1);
       expect(state.cursorCol).toBe(5);
@@ -55,6 +57,7 @@ describe('textBufferReducer', () => {
         pushToUndo: false,
       };
       const state = textBufferReducer(initialState, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['no undo']);
       expect(state.undoStack.length).toBe(0);
     });
@@ -64,6 +67,7 @@ describe('textBufferReducer', () => {
     it('should insert a character', () => {
       const action: TextBufferAction = { type: 'insert', payload: 'a' };
       const state = textBufferReducer(initialState, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['a']);
       expect(state.cursorCol).toBe(1);
     });
@@ -72,6 +76,7 @@ describe('textBufferReducer', () => {
       const stateWithText = { ...initialState, lines: ['hello'] };
       const action: TextBufferAction = { type: 'insert', payload: '\n' };
       const state = textBufferReducer(stateWithText, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['', 'hello']);
       expect(state.cursorRow).toBe(1);
       expect(state.cursorCol).toBe(0);
@@ -88,6 +93,7 @@ describe('textBufferReducer', () => {
       };
       const action: TextBufferAction = { type: 'backspace' };
       const state = textBufferReducer(stateWithText, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['']);
       expect(state.cursorCol).toBe(0);
     });
@@ -101,6 +107,7 @@ describe('textBufferReducer', () => {
       };
       const action: TextBufferAction = { type: 'backspace' };
       const state = textBufferReducer(stateWithText, action);
+      expect(state).toHaveOnlyValidCharacters();
       expect(state.lines).toEqual(['helloworld']);
       expect(state.cursorRow).toBe(0);
       expect(state.cursorCol).toBe(5);
@@ -115,12 +122,14 @@ describe('textBufferReducer', () => {
         payload: 'test',
       };
       const stateAfterInsert = textBufferReducer(initialState, insertAction);
+      expect(stateAfterInsert).toHaveOnlyValidCharacters();
       expect(stateAfterInsert.lines).toEqual(['test']);
       expect(stateAfterInsert.undoStack.length).toBe(1);
 
       // 2. Undo
       const undoAction: TextBufferAction = { type: 'undo' };
       const stateAfterUndo = textBufferReducer(stateAfterInsert, undoAction);
+      expect(stateAfterUndo).toHaveOnlyValidCharacters();
       expect(stateAfterUndo.lines).toEqual(['']);
       expect(stateAfterUndo.undoStack.length).toBe(0);
       expect(stateAfterUndo.redoStack.length).toBe(1);
@@ -128,6 +137,7 @@ describe('textBufferReducer', () => {
       // 3. Redo
       const redoAction: TextBufferAction = { type: 'redo' };
       const stateAfterRedo = textBufferReducer(stateAfterUndo, redoAction);
+      expect(stateAfterRedo).toHaveOnlyValidCharacters();
       expect(stateAfterRedo.lines).toEqual(['test']);
       expect(stateAfterRedo.undoStack.length).toBe(1);
       expect(stateAfterRedo.redoStack.length).toBe(0);
@@ -144,6 +154,7 @@ describe('textBufferReducer', () => {
       };
       const action: TextBufferAction = { type: 'create_undo_snapshot' };
       const state = textBufferReducer(stateWithText, action);
+      expect(state).toHaveOnlyValidCharacters();
 
       expect(state.lines).toEqual(['hello']);
       expect(state.cursorRow).toBe(0);
@@ -157,16 +168,19 @@ describe('textBufferReducer', () => {
 });
 
 // Helper to get the state from the hook
-const getBufferState = (result: { current: TextBuffer }) => ({
-  text: result.current.text,
-  lines: [...result.current.lines], // Clone for safety
-  cursor: [...result.current.cursor] as [number, number],
-  allVisualLines: [...result.current.allVisualLines],
-  viewportVisualLines: [...result.current.viewportVisualLines],
-  visualCursor: [...result.current.visualCursor] as [number, number],
-  visualScrollRow: result.current.visualScrollRow,
-  preferredCol: result.current.preferredCol,
-});
+const getBufferState = (result: { current: TextBuffer }) => {
+  expect(result.current).toHaveOnlyValidCharacters();
+  return {
+    text: result.current.text,
+    lines: [...result.current.lines], // Clone for safety
+    cursor: [...result.current.cursor] as [number, number],
+    allVisualLines: [...result.current.allVisualLines],
+    viewportVisualLines: [...result.current.viewportVisualLines],
+    visualCursor: [...result.current.visualCursor] as [number, number],
+    visualScrollRow: result.current.visualScrollRow,
+    preferredCol: result.current.preferredCol,
+  };
+};
 
 describe('useTextBuffer', () => {
   let viewport: Viewport;
@@ -1152,6 +1166,22 @@ Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots
       expect(state.text).toBe('fiXrd');
       expect(state.cursor).toEqual([0, 3]); // After 'X'
     });
+
+    it('should replace a single-line range with multi-line text', () => {
+      const { result } = renderHook(() =>
+        useTextBuffer({
+          initialText: 'one two three',
+          viewport,
+          isValidPath: () => false,
+        }),
+      );
+      // Replace "two" with "new\nline"
+      act(() => result.current.replaceRange(0, 4, 0, 7, 'new\nline'));
+      const state = getBufferState(result);
+      expect(state.lines).toEqual(['one new', 'line three']);
+      expect(state.text).toBe('one new\nline three');
+      expect(state.cursor).toEqual([1, 4]); // cursor after 'line'
+    });
   });
 
   describe('Input Sanitization', () => {
@@ -1159,7 +1189,7 @@ Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots
       const { result } = renderHook(() =>
         useTextBuffer({ viewport, isValidPath: () => false }),
       );
-      const textWithAnsi = '\x1B[31mHello\x1B[0m';
+      const textWithAnsi = '\x1B[31mHello\x1B[0m \x1B[32mWorld\x1B[0m';
       act(() =>
         result.current.handleInput({
           name: '',
@@ -1170,7 +1200,7 @@ Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots
           sequence: textWithAnsi,
         }),
       );
-      expect(getBufferState(result).text).toBe('Hello');
+      expect(getBufferState(result).text).toBe('Hello World');
     });
 
     it('should strip control characters from input', () => {
@@ -1425,6 +1455,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const result = textBufferReducer(initialState, action);
+      expect(result).toHaveOnlyValidCharacters();
 
       // After deleting line2, we should have line1 and line3, with cursor on line3 (now at index 1)
       expect(result.lines).toEqual(['line1', 'line3']);
@@ -1452,6 +1483,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const result = textBufferReducer(initialState, action);
+      expect(result).toHaveOnlyValidCharacters();
 
       // Should delete line2 and line3, leaving line1 and line4
       expect(result.lines).toEqual(['line1', 'line4']);
@@ -1479,6 +1511,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const result = textBufferReducer(initialState, action);
+      expect(result).toHaveOnlyValidCharacters();
 
       // Should clear the line content but keep the line
       expect(result.lines).toEqual(['']);
@@ -1506,6 +1539,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const result = textBufferReducer(initialState, action);
+      expect(result).toHaveOnlyValidCharacters();
 
       // Should delete the last line completely, not leave empty line
       expect(result.lines).toEqual(['line1']);
@@ -1534,6 +1568,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const afterDelete = textBufferReducer(initialState, deleteAction);
+      expect(afterDelete).toHaveOnlyValidCharacters();
 
       // After deleting all lines, should have one empty line
       expect(afterDelete.lines).toEqual(['']);
@@ -1547,6 +1582,7 @@ describe('textBufferReducer vim operations', () => {
       };
 
       const afterPaste = textBufferReducer(afterDelete, pasteAction);
+      expect(afterPaste).toHaveOnlyValidCharacters();
 
       // All lines including the first one should be present
       expect(afterPaste.lines).toEqual(['new1', 'new2', 'new3', 'new4']);
diff --git a/packages/cli/src/ui/components/shared/text-buffer.ts b/packages/cli/src/ui/components/shared/text-buffer.ts
index d2d9087a..cf5ce889 100644
--- a/packages/cli/src/ui/components/shared/text-buffer.ts
+++ b/packages/cli/src/ui/components/shared/text-buffer.ts
@@ -271,26 +271,23 @@ export const replaceRangeInternal = (
     .replace(/\r/g, '\n');
   const replacementParts = normalisedReplacement.split('\n');
 
-  // Replace the content
-  if (startRow === endRow) {
-    newLines[startRow] = prefix + normalisedReplacement + suffix;
+  // The combined first line of the new text
+  const firstLine = prefix + replacementParts[0];
+
+  if (replacementParts.length === 1) {
+    // No newlines in replacement: combine prefix, replacement, and suffix on one line.
+    newLines.splice(startRow, endRow - startRow + 1, firstLine + suffix);
   } else {
-    const firstLine = prefix + replacementParts[0];
-    if (replacementParts.length === 1) {
-      // Single line of replacement text, but spanning multiple original lines
-      newLines.splice(startRow, endRow - startRow + 1, firstLine + suffix);
-    } else {
-      // Multi-line replacement text
-      const lastLine = replacementParts[replacementParts.length - 1] + suffix;
-      const middleLines = replacementParts.slice(1, -1);
-      newLines.splice(
-        startRow,
-        endRow - startRow + 1,
-        firstLine,
-        ...middleLines,
-        lastLine,
-      );
-    }
+    // Newlines in replacement: create new lines.
+    const lastLine = replacementParts[replacementParts.length - 1] + suffix;
+    const middleLines = replacementParts.slice(1, -1);
+    newLines.splice(
+      startRow,
+      endRow - startRow + 1,
+      firstLine,
+      ...middleLines,
+      lastLine,
+    );
   }
 
   const finalCursorRow = startRow + replacementParts.length - 1;
diff --git a/packages/cli/src/ui/components/shared/vim-buffer-actions.test.ts b/packages/cli/src/ui/components/shared/vim-buffer-actions.test.ts
index f268bb1e..8f7f72ab 100644
--- a/packages/cli/src/ui/components/shared/vim-buffer-actions.test.ts
+++ b/packages/cli/src/ui/components/shared/vim-buffer-actions.test.ts
@@ -36,7 +36,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(2);
         expect(result.preferredCol).toBeNull();
       });
@@ -49,7 +49,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(0);
       });
 
@@ -61,7 +61,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(4); // On last character '1' of 'line1'
       });
@@ -74,7 +74,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(1); // On 'b' after 5 left movements
       });
@@ -88,6 +88,7 @@ describe('vim-buffer-actions', () => {
           type: 'vim_move_right' as const,
           payload: { count: 1 },
         });
+        expect(state).toHaveOnlyValidCharacters();
         expect(state.cursorRow).toBe(1);
         expect(state.cursorCol).toBe(0); // Should be on 'f'
 
@@ -96,6 +97,7 @@ describe('vim-buffer-actions', () => {
           type: 'vim_move_left' as const,
           payload: { count: 1 },
         });
+        expect(state).toHaveOnlyValidCharacters();
         expect(state.cursorRow).toBe(0);
         expect(state.cursorCol).toBe(10); // Should be on 'd', not past it
       });
@@ -110,7 +112,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(5);
       });
 
@@ -122,7 +124,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(4); // Last character of 'hello'
       });
 
@@ -134,7 +136,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(1);
         expect(result.cursorCol).toBe(0);
       });
@@ -146,7 +148,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_up' as const, payload: { count: 2 } };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(3);
       });
@@ -156,7 +158,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_up' as const, payload: { count: 5 } };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
       });
 
@@ -165,7 +167,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_up' as const, payload: { count: 1 } };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(5); // End of 'short'
       });
@@ -180,7 +182,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(2);
         expect(result.cursorCol).toBe(2);
       });
@@ -193,7 +195,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(1);
       });
     });
@@ -207,7 +209,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(6); // Start of 'world'
       });
 
@@ -219,7 +221,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(12); // Start of 'test'
       });
 
@@ -231,7 +233,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(5); // Start of ','
       });
     });
@@ -245,7 +247,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(6); // Start of 'world'
       });
 
@@ -257,7 +259,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(0); // Start of 'hello'
       });
     });
@@ -271,7 +273,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(4); // End of 'hello'
       });
 
@@ -283,7 +285,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(10); // End of 'world'
       });
     });
@@ -294,7 +296,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_to_line_start' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(0);
       });
 
@@ -303,7 +305,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_to_line_end' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(10); // Last character of 'hello world'
       });
 
@@ -312,7 +314,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_to_first_nonwhitespace' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(3); // Position of 'h'
       });
 
@@ -321,7 +323,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_to_first_line' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(0);
       });
@@ -331,7 +333,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_move_to_last_line' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(2);
         expect(result.cursorCol).toBe(0);
       });
@@ -344,7 +346,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(1); // 0-indexed
         expect(result.cursorCol).toBe(0);
       });
@@ -357,7 +359,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(1); // Last line
       });
     });
@@ -373,7 +375,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hllo');
         expect(result.cursorCol).toBe(1);
       });
@@ -386,7 +388,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('ho');
         expect(result.cursorCol).toBe(1);
       });
@@ -399,7 +401,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hel');
         expect(result.cursorCol).toBe(3);
       });
@@ -412,7 +414,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hello');
         expect(result.cursorCol).toBe(5);
       });
@@ -427,7 +429,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('world test');
         expect(result.cursorCol).toBe(0);
       });
@@ -440,7 +442,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('test');
         expect(result.cursorCol).toBe(0);
       });
@@ -453,7 +455,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hello ');
         expect(result.cursorCol).toBe(6);
       });
@@ -468,7 +470,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hello test');
         expect(result.cursorCol).toBe(6);
       });
@@ -481,7 +483,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('test');
         expect(result.cursorCol).toBe(0);
       });
@@ -496,7 +498,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines).toEqual(['line1', 'line3']);
         expect(result.cursorRow).toBe(1);
         expect(result.cursorCol).toBe(0);
@@ -510,7 +512,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines).toEqual(['line3']);
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(0);
@@ -524,7 +526,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines).toEqual(['']);
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(0);
@@ -537,7 +539,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_delete_to_end_of_line' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hello');
         expect(result.cursorCol).toBe(5);
       });
@@ -547,7 +549,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_delete_to_end_of_line' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hello');
       });
     });
@@ -560,7 +562,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_insert_at_cursor' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorRow).toBe(0);
         expect(result.cursorCol).toBe(2);
       });
@@ -572,7 +574,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_append_at_cursor' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(3);
       });
 
@@ -581,7 +583,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_append_at_cursor' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(5);
       });
     });
@@ -592,7 +594,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_append_at_line_end' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(11);
       });
     });
@@ -603,7 +605,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_insert_at_line_start' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(2);
       });
 
@@ -612,34 +614,32 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_insert_at_line_start' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(3);
       });
     });
 
     describe('vim_open_line_below', () => {
-      it('should insert newline at end of current line', () => {
+      it('should insert a new line below the current one', () => {
         const state = createTestState(['hello world'], 0, 5);
         const action = { type: 'vim_open_line_below' as const };
 
         const result = handleVimAction(state, action);
-
-        // The implementation inserts newline at end of current line and cursor moves to column 0
-        expect(result.lines[0]).toBe('hello world\n');
-        expect(result.cursorRow).toBe(0);
-        expect(result.cursorCol).toBe(0); // Cursor position after replaceRangeInternal
+        expect(result).toHaveOnlyValidCharacters();
+        expect(result.lines).toEqual(['hello world', '']);
+        expect(result.cursorRow).toBe(1);
+        expect(result.cursorCol).toBe(0);
       });
     });
 
     describe('vim_open_line_above', () => {
-      it('should insert newline before current line', () => {
+      it('should insert a new line above the current one', () => {
         const state = createTestState(['hello', 'world'], 1, 2);
         const action = { type: 'vim_open_line_above' as const };
 
         const result = handleVimAction(state, action);
-
-        // The implementation inserts newline at beginning of current line
-        expect(result.lines).toEqual(['hello', '\nworld']);
+        expect(result).toHaveOnlyValidCharacters();
+        expect(result.lines).toEqual(['hello', '', 'world']);
         expect(result.cursorRow).toBe(1);
         expect(result.cursorCol).toBe(0);
       });
@@ -651,7 +651,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_escape_insert_mode' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(2);
       });
 
@@ -660,7 +660,7 @@ describe('vim-buffer-actions', () => {
         const action = { type: 'vim_escape_insert_mode' as const };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.cursorCol).toBe(0);
       });
     });
@@ -676,7 +676,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('world test');
         expect(result.cursorCol).toBe(0);
       });
@@ -691,7 +691,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('');
         expect(result.cursorCol).toBe(0);
       });
@@ -706,7 +706,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hel world');
         expect(result.cursorCol).toBe(3);
       });
@@ -719,7 +719,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         expect(result.lines[0]).toBe('hellorld'); // Deletes ' wo' (3 chars to the right)
         expect(result.cursorCol).toBe(5);
       });
@@ -732,7 +732,7 @@ describe('vim-buffer-actions', () => {
         };
 
         const result = handleVimAction(state, action);
-
+        expect(result).toHaveOnlyValidCharacters();
         // The movement 'j' with count 2 changes 2 lines starting from cursor row
         // Since we're at cursor position 2, it changes lines starting from current row
         expect(result.lines).toEqual(['line1', 'line2', 'line3']); // No change because count > available lines
@@ -751,7 +751,7 @@ describe('vim-buffer-actions', () => {
       };
 
       const result = handleVimAction(state, action);
-
+      expect(result).toHaveOnlyValidCharacters();
       expect(result.cursorRow).toBe(0);
       expect(result.cursorCol).toBe(0);
     });
@@ -761,7 +761,7 @@ describe('vim-buffer-actions', () => {
       const action = { type: 'vim_move_to_line_end' as const };
 
       const result = handleVimAction(state, action);
-
+      expect(result).toHaveOnlyValidCharacters();
       expect(result.cursorCol).toBe(0); // Should be last character position
     });
 
@@ -773,7 +773,7 @@ describe('vim-buffer-actions', () => {
       };
 
       const result = handleVimAction(state, action);
-
+      expect(result).toHaveOnlyValidCharacters();
       // Should move to next line with content
       expect(result.cursorRow).toBe(2);
       expect(result.cursorCol).toBe(0);
@@ -789,7 +789,7 @@ describe('vim-buffer-actions', () => {
       };
 
       const result = handleVimAction(state, action);
-
+      expect(result).toHaveOnlyValidCharacters();
       expect(result.undoStack).toHaveLength(2); // Original plus new snapshot
     });
   });
diff --git a/packages/cli/test-setup.ts b/packages/cli/test-setup.ts
new file mode 100644
index 00000000..a419c873
--- /dev/null
+++ b/packages/cli/test-setup.ts
@@ -0,0 +1,7 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import './src/test-utils/customMatchers.js';
diff --git a/packages/cli/vitest.config.ts b/packages/cli/vitest.config.ts
index 8f67a0be..5a3f99fe 100644
--- a/packages/cli/vitest.config.ts
+++ b/packages/cli/vitest.config.ts
@@ -18,6 +18,7 @@ export default defineConfig({
     outputFile: {
       junit: 'junit.xml',
     },
+    setupFiles: ['./test-setup.ts'],
     coverage: {
       enabled: true,
       provider: 'v8',