diff --git a/src/tools.mjs b/src/tools.mjs
index e0848da..c0f34c3 100644
--- a/src/tools.mjs
+++ b/src/tools.mjs
@@ -122,6 +122,9 @@ export function sleep(ms) {
  * Retry a function with exponential backoff
  */
 export async function retry(fn, maxAttempts = 3, baseDelay = 1000) {
+  if (!Number.isInteger(maxAttempts) || maxAttempts < 1) {
+    throw new Error('retry maxAttempts must be a positive integer');
+  }
   for (let attempt = 1; attempt <= maxAttempts; attempt++) {
     try {
       return await fn();
diff --git a/test/tools.test.mjs b/test/tools.test.mjs
index 7ec43ac..f0fb1b3 100644
--- a/test/tools.test.mjs
+++ b/test/tools.test.mjs
@@ -15,7 +15,7 @@ import { fileURLToPath } from "node:url";
 import { spawn } from "node:child_process";
 import test from "node:test";
 
-import { TOOLS, resolveTool, toolList } from "../src/tools.mjs";
+import { TOOLS, resolveTool, retry, toolList } from "../src/tools.mjs";
 
 const BIN = fileURLToPath(new URL("../bin/moshcode.mjs", import.meta.url));
 
@@ -204,3 +204,25 @@ test("moshcode install reports an Object.prototype name as unknown", async () =>
   assert.match(result.stderr, /usage: moshcode install <engine\|tool>/);
   assert.doesNotMatch(result.stderr, /TypeError/);
 });
+
+test("retry rejects non-positive attempt limits", async () => {
+  let calls = 0;
+
+  await assert.rejects(
+    retry(() => { calls++; }, 0, 1),
+    /maxAttempts must be a positive integer/,
+  );
+  assert.equal(calls, 0);
+});
+
+test("retry retries until a later attempt succeeds", async () => {
+  let calls = 0;
+  const result = await retry(() => {
+    calls++;
+    if (calls < 2) throw new Error("not yet");
+    return "ok";
+  }, 2, 1);
+
+  assert.equal(result, "ok");
+  assert.equal(calls, 2);
+});