diff --git a/src/commands.mjs b/src/commands.mjs
index 189dc44..7cacf74 100644
--- a/src/commands.mjs
+++ b/src/commands.mjs
@@ -36,9 +36,21 @@ async function deliver(type, data) {
 }
 
 export function defaultCommands() {
+  const expectNoArgs = (name, args) => {
+    if (args.length > 0) {
+      throw new Error(`moshscript: ${name}() does not take arguments`);
+    }
+  };
+
   return {
-    code: (ctx) => ctx.out("  ⌨  code()    → compiling features (no bugs)…"),
-    mosh: (ctx) => ctx.out("  🤘 mosh()    → opening the pit"),
+    code: (ctx, args) => {
+      expectNoArgs("code", args);
+      ctx.out("  ⌨  code()    → compiling features (no bugs)…");
+    },
+    mosh: (ctx, args) => {
+      expectNoArgs("mosh", args);
+      ctx.out("  🤘 mosh()    → opening the pit");
+    },
     notify: async (ctx, args) => {
       const msg = args.length ? args.join(" ") : "moshcode ping 🤘";
       ctx.out(`  🔔 notify()  → ${msg}`);
@@ -46,10 +58,20 @@ export function defaultCommands() {
       const res = await deliver("moshscript.notify", { message: msg, iter: ctx.iter });
       for (const r of res) if (!r.ok) ctx.out(`     ! notify ${r.target} failed (${r.status || r.error})`);
     },
-    repeat: (ctx) => ctx.out("  ↻  repeat()  → back to the top"),
+    repeat: (ctx, args) => {
+      expectNoArgs("repeat", args);
+      ctx.out("  ↻  repeat()  → back to the top");
+    },
     // handy extras
     say: (ctx, args) => ctx.out(`  💬 ${args.join(" ")}`),
-    sleep: async (_ctx, args) => { const ms = Number(args[0] || 0); if (ms > 0) await new Promise((r) => setTimeout(r, ms)); },
-    stop: (ctx) => { ctx.vars.alive = false; ctx.out("  ⏹  stop()    → alive = false"); },
+    sleep: async (_ctx, args) => {
+      const ms = Number(args[0] || 0);
+      if (ms > 0) await new Promise((r) => setTimeout(r, ms));
+    },
+    stop: (ctx, args) => {
+      expectNoArgs("stop", args);
+      ctx.vars.alive = false;
+      ctx.out("  ⏹  stop()    → alive = false");
+    },
   };
 }
diff --git a/test/commands.test.mjs b/test/commands.test.mjs
new file mode 100644
index 0000000..0ad9f08
--- /dev/null
+++ b/test/commands.test.mjs
@@ -0,0 +1,36 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { defaultCommands } from "../src/commands.mjs";
+
+function createCtx() {
+  return {
+    dryRun: true,
+    iter: 0,
+    vars: { alive: true },
+    lines: [],
+    out(line) {
+      this.lines.push(line);
+    },
+  };
+}
+
+for (const name of ["code", "mosh", "repeat", "stop"]) {
+  test(`${name}() rejects unexpected arguments`, async () => {
+    const commands = defaultCommands();
+    await assert.rejects(
+      async () => commands[name](createCtx(), ["extra"]),
+      new RegExp(`moshscript: ${name}\\(\\) does not take arguments`)
+    );
+  });
+}
+
+test("notify() still accepts message arguments", async () => {
+  const commands = defaultCommands();
+  const ctx = createCtx();
+
+  await commands.notify(ctx, ["hello", "there"]);
+
+  assert.equal(ctx.lines.length, 1);
+  assert.match(ctx.lines[0], /hello there/);
+});