diff --git a/bin/moshcode.mjs b/bin/moshcode.mjs
index cb500cdb..775ef2be 100755
--- a/bin/moshcode.mjs
+++ b/bin/moshcode.mjs
@@ -49,7 +49,7 @@ function parseMax(value) {
     throw new Error(`moshcode run: --max must be a positive integer, got ${JSON.stringify(value)}`);
   }
   const max = Number(value);
-  if (!Number.isInteger(max) || max < 1) {
+  if (!Number.isSafeInteger(max) || max < 1) {
     throw new Error(`moshcode run: --max must be a positive integer, got ${JSON.stringify(value)}`);
   }
   return max;
diff --git a/src/tui.mjs b/src/tui.mjs
index ecbdf78c..e8a7bc75 100644
--- a/src/tui.mjs
+++ b/src/tui.mjs
@@ -325,11 +325,11 @@ async function runFile(args) {
     const a = args[i];
     if (a === "--max" || a === "-n") {
       const v = Number(args[++i]);
-      if (!Number.isInteger(v) || v < 1) { console.log(err(`--max needs a positive integer`)); return; }
+      if (!Number.isSafeInteger(v) || v < 1) { console.log(err(`--max needs a positive integer`)); return; }
       max = v;
     } else if (a.startsWith("--max=")) {
       const v = Number(a.slice("--max=".length));
-      if (!Number.isInteger(v) || v < 1) { console.log(err(`--max needs a positive integer`)); return; }
+      if (!Number.isSafeInteger(v) || v < 1) { console.log(err(`--max needs a positive integer`)); return; }
       max = v;
     } else if (a === "--dry-run") {
       dryRun = true;
diff --git a/test/run-options.test.mjs b/test/run-options.test.mjs
index a0355965..36f2245f 100644
--- a/test/run-options.test.mjs
+++ b/test/run-options.test.mjs
@@ -83,7 +83,7 @@ test("run accepts equals-form max option", async () => {
 });
 
 test("run rejects non-decimal max values", async () => {
-  for (const value of ["1e1", "0x2"]) {
+  for (const value of ["1e1", "0x2", "9007199254740992"]) {
     const result = await run([`--max=${value}`, "--dry-run"]);
 
     assert.equal(result.status, 1);
diff --git a/test/tui.test.mjs b/test/tui.test.mjs
index 999d6d56..f52186f8 100644
--- a/test/tui.test.mjs
+++ b/test/tui.test.mjs
@@ -59,6 +59,14 @@ test("TUI /run rejects unknown options before reading a script file", async () =
   assert.doesNotMatch(result.stdout, /can't read --dryrun/);
 });
 
+test("TUI /run rejects unsafe iteration limits", async () => {
+  const result = await runTui("/run --max=9007199254740992\n/quit\n");
+
+  assert.equal(result.status, 0);
+  assert.match(result.stdout, /--max needs a positive integer/);
+  assert.doesNotMatch(result.stdout, /usage: \/run/);
+});
+
 test("TUI /run passes positional args through to moshscript argv", () => {
   const dir = mkdtempSync(join(tmpdir(), "moshcode-tui-"));
   mkdirSync(join(dir, "space dir"));