diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
new file mode 100644
index 000000000..d4cee052a
--- /dev/null
+++ b/.github/copilot-instructions.md
@@ -0,0 +1,8 @@
+# GitHub Copilot Instructions
+
+## Testing
+
+- Use the `node:test` framework and `assert` for assertions.
+- When generating tests, add them to the existing test file for the relevant module.
+- Use a new describe block if the function being tested does not already have one.
+- Follow existing conventions for test names, structure, and assertions.
diff --git a/src/commands/tileindex-validate/__test__/tileindex.validate.test.ts b/src/commands/tileindex-validate/__test__/tileindex.validate.test.ts
index 550632b59..86849131c 100644
--- a/src/commands/tileindex-validate/__test__/tileindex.validate.test.ts
+++ b/src/commands/tileindex-validate/__test__/tileindex.validate.test.ts
@@ -16,6 +16,7 @@ import { createTiff, Url } from '../../common.ts';
 import {
   commandTileIndexValidate,
   extractTiffLocations,
+  getSize,
   getTileName,
   GridSizeFromString,
   groupByTileName,
@@ -28,6 +29,32 @@ import { FakeCogTiff } from './tileindex.validate.data.ts';
 
 /* eslint-disable @typescript-eslint/no-explicit-any */
 
+describe('getSize', () => {
+  it('should return correct width and height for positive extent', () => {
+    const extent: [number, number, number, number] = [10, 20, 30, 40];
+    const size = getSize(extent);
+    assert.deepEqual(size, { width: 20, height: 20 });
+  });
+
+  it('should return zero width and height for zero-size extent', () => {
+    const extent: [number, number, number, number] = [5, 5, 5, 5];
+    const size = getSize(extent);
+    assert.deepEqual(size, { width: 0, height: 0 });
+  });
+
+  it('should handle negative coordinates correctly', () => {
+    const extent: [number, number, number, number] = [-10, -20, 10, 20];
+    const size = getSize(extent);
+    assert.deepEqual(size, { width: 20, height: 40 });
+  });
+
+  it('should handle reversed coordinates (min > max)', () => {
+    const extent: [number, number, number, number] = [30, 40, 10, 20];
+    const size = getSize(extent);
+    assert.deepEqual(size, { width: -20, height: -20 });
+  });
+});
+
 function convertTileName(fileName: string, gridSize: GridSize): string | null {
   const mapTileIndex = MapSheet.getMapTileIndex(fileName);
   if (mapTileIndex == null) return null;