diff --git a/scripts/append-run-log.test.mjs b/scripts/append-run-log.test.mjs
new file mode 100644
index 00000000..269bffa9
--- /dev/null
+++ b/scripts/append-run-log.test.mjs
@@ -0,0 +1,41 @@
+import test from 'node:test';
+import assert from 'node:assert';
+import path from 'node:path';
+import fs from 'node:fs/promises';
+import { execFile } from 'node:child_process';
+import { promisify } from 'node:util';
+import { fileURLToPath } from 'node:url';
+
+const exec = promisify(execFile);
+const scriptsDir = path.dirname(fileURLToPath(import.meta.url));
+const SCRIPT = path.join(scriptsDir, 'append-run-log.mjs');
+const MARKER = '<!-- Loop appends below this line -->';
+
+test('invalid JSON entry exits 1 with a Usage message', async () => {
+  let code = 0;
+  let stderr = '';
+  try {
+    await exec('node', [SCRIPT, 'not-json']);
+  } catch (err) {
+    code = err.code;
+    stderr = err.stderr;
+  }
+  assert.strictEqual(code, 1, 'Should exit with code 1');
+  assert.match(stderr, /Usage/, 'Should print a Usage message');
+  assert.match(stderr, /valid JSON/, 'Should mention valid JSON');
+});
+
+test('valid minimal JSON entry is appended without throwing', async () => {
+  const tmpDir = await fs.mkdtemp(path.join(scriptsDir, '.test-run-log-'));
+  const logPath = path.join(tmpDir, 'loop-run-log.md');
+  await fs.writeFile(logPath, `# Run Log\n\n${MARKER}\n`);
+
+  const entry = { run_id: new Date().toISOString(), status: 'ok' };
+  const { stdout } = await exec('node', [SCRIPT, JSON.stringify(entry), logPath]);
+  assert.match(stdout, /Appended run/, 'Should confirm the append');
+
+  const content = await fs.readFile(logPath, 'utf8');
+  assert.ok(content.includes(JSON.stringify(entry)), 'Log should contain the new entry');
+
+  await fs.rm(tmpDir, { recursive: true, force: true });
+});