diff --git a/bin/moshcode.mjs b/bin/moshcode.mjs
index c1d3f48..c28a940 100755
--- a/bin/moshcode.mjs
+++ b/bin/moshcode.mjs
@@ -224,7 +224,10 @@ async function main() {
   }
   if (cmd === "install") {
     const target = rest.find((a) => !a.startsWith("-"))?.toLowerCase();
-    const entry = target && (ENGINES[target] || TOOLS[target]);
+    // Own properties only — `install constructor` must print usage, not resolve
+    // to something off Object.prototype and crash on its missing install spec.
+    const entry = target
+      && ((Object.hasOwn(ENGINES, target) && ENGINES[target]) || (Object.hasOwn(TOOLS, target) && TOOLS[target]));
     if (!target || !entry) {
       console.error(`usage: moshcode install <engine|tool>\nengines:\n${engineList()}\ntools:\n${toolList()}`);
       process.exit(target ? 1 : 0);
diff --git a/src/engines.mjs b/src/engines.mjs
index d4e72d0..b42851c 100644
--- a/src/engines.mjs
+++ b/src/engines.mjs
@@ -78,7 +78,10 @@ const ALIASES = { cc: "claude", "claude-code": "claude", openai: "codex", gpt: "
 export function resolveEngine(token) {
   if (!token) return null;
   const t = String(token).trim().toLowerCase();
-  const key = ENGINES[t] ? t : ALIASES[t];
+  // Own properties only: ENGINES/ALIASES are plain object literals, so a name
+  // like `constructor` or `__proto__` would otherwise resolve to something off
+  // Object.prototype and be handed on as an engine with no bin/install.
+  const key = Object.hasOwn(ENGINES, t) ? t : Object.hasOwn(ALIASES, t) ? ALIASES[t] : null;
   return key ? [key, ENGINES[key]] : null;
 }
 
@@ -145,7 +148,7 @@ const AI_EXEC = {
 
 /** argv that runs `prompt` headlessly on `engine` (throws if it has no headless mode). */
 export function aiExecArgs(engine, prompt) {
-  const fn = AI_EXEC[engine];
+  const fn = Object.hasOwn(AI_EXEC, engine) ? AI_EXEC[engine] : null;
   if (!fn) throw new Error(`moshscript: ai() has no headless mode for "${engine}"`);
   return fn(String(prompt));
 }
@@ -154,7 +157,7 @@ export function aiExecArgs(engine, prompt) {
 export function pickAiEngine(preferred) {
   const order = preferred ? [preferred] : ["claude", "codex", "opencode", "gemini", "aider"];
   for (const key of order) {
-    if (ENGINES[key] && AI_EXEC[key] && isInstalled(ENGINES[key].bin)) return key;
+    if (Object.hasOwn(ENGINES, key) && Object.hasOwn(AI_EXEC, key) && isInstalled(ENGINES[key].bin)) return key;
   }
   return null;
 }
diff --git a/src/tools.mjs b/src/tools.mjs
index f52c308..e0848da 100644
--- a/src/tools.mjs
+++ b/src/tools.mjs
@@ -38,7 +38,10 @@ export const TOOLS = {
 export function resolveTool(token) {
   if (!token) return null;
   const key = String(token).trim().toLowerCase();
-  return TOOLS[key] ? [key, TOOLS[key]] : null;
+  // Own properties only: TOOLS is a plain object literal, so a name like
+  // `constructor` or `__proto__` would otherwise resolve to something off
+  // Object.prototype and be handed on as a tool with no bin/install.
+  return Object.hasOwn(TOOLS, key) ? [key, TOOLS[key]] : null;
 }
 
 /** Tool entries annotated with native executable install status. */
diff --git a/src/tui.mjs b/src/tui.mjs
index e5374af..ab9b08f 100644
--- a/src/tui.mjs
+++ b/src/tui.mjs
@@ -252,7 +252,9 @@ async function openShell(rawCmd) {
 
 function installTarget(key) {
   return new Promise((resolve) => {
-    const target = ENGINES[key] || TOOLS[key];
+    // Own properties only — `/install constructor` must print the unknown-target
+    // line, not resolve to something off Object.prototype and crash the pit.
+    const target = (Object.hasOwn(ENGINES, key) && ENGINES[key]) || (Object.hasOwn(TOOLS, key) && TOOLS[key]);
     if (!target) { console.log(err(`unknown engine or tool "${key}"`)); return resolve(); }
     console.log(info(`installing ${key}: ${target.install.cmd} ${target.install.args.join(" ")}`));
     console.log(hr());
diff --git a/test/engines.test.mjs b/test/engines.test.mjs
index 54d47db..437e77d 100644
--- a/test/engines.test.mjs
+++ b/test/engines.test.mjs
@@ -14,7 +14,7 @@ import { fileURLToPath } from "node:url";
 import { spawn } from "node:child_process";
 import test from "node:test";
 
-import { ENGINES, agentLaunchArgs, exitReason, ranOk, runCmd } from "../src/engines.mjs";
+import { ENGINES, agentLaunchArgs, aiExecArgs, exitReason, pickAiEngine, ranOk, resolveEngine, runCmd } from "../src/engines.mjs";
 
 const BIN = fileURLToPath(new URL("../bin/moshcode.mjs", import.meta.url));
 // The autonomous-session bypass flags each engine declares (engine.agentArgs).
@@ -157,3 +157,12 @@ test("ranOk and exitReason cover clean exits, bad codes, and spawn errors", asyn
   assert.equal(ranOk(missing), false);
   assert.match(exitReason(missing), /ENOENT|not found|spawn/i);
 });
+
+test("engine lookup ignores inherited Object.prototype members", () => {
+  // ENGINES/ALIASES/AI_EXEC are plain object literals, so an unknown name that
+  // matches an Object.prototype member must not resolve as a real engine.
+  assert.equal(resolveEngine("constructor"), null);
+  assert.equal(resolveEngine("__proto__"), null);
+  assert.equal(pickAiEngine("constructor"), null);
+  assert.throws(() => aiExecArgs("constructor", "hi"), /no headless mode for "constructor"/);
+});
diff --git a/test/tools.test.mjs b/test/tools.test.mjs
index 8286b7c..9e6ec7c 100644
--- a/test/tools.test.mjs
+++ b/test/tools.test.mjs
@@ -184,3 +184,19 @@ fs.writeFileSync(process.env.SHELL_CAPTURE, JSON.stringify(process.argv.slice(2)
     assert.deepEqual(JSON.parse(readFileSync(capture, "utf8")), ["-c", script]);
   });
 }
+
+test("tool lookup ignores inherited Object.prototype members", () => {
+  // TOOLS is a plain object literal: `TOOLS.constructor` is truthy but is not a
+  // tool, so resolving it would hand a bin-less, install-less entry downstream.
+  assert.equal(resolveTool("constructor"), null);
+  assert.equal(resolveTool("__proto__"), null);
+  assert.equal(resolveTool("valueOf"), null);
+});
+
+test("moshcode install reports an Object.prototype name as unknown", async () => {
+  const result = await run(["install", "constructor"]);
+
+  assert.equal(result.status, 1);
+  assert.match(result.stderr, /usage: moshcode install <engine\|tool>/);
+  assert.doesNotMatch(result.stderr, /TypeError/);
+});
diff --git a/test/tui.test.mjs b/test/tui.test.mjs
index a3a4de9..42b9214 100644
--- a/test/tui.test.mjs
+++ b/test/tui.test.mjs
@@ -105,3 +105,15 @@ test("TUI /shell and !cmd run an identical command identically", posixShell, asy
   assert.equal(viaBang.status, 0);
   assert.deepEqual(bracketed(viaSlash.stdout), bracketed(viaBang.stdout));
 });
+
+// ENGINES/TOOLS are plain object literals, so an unknown target that happens to
+// name an Object.prototype member used to resolve truthy and reach `.install`,
+// killing the whole pit with a raw TypeError instead of printing the usual
+// unknown-target line.
+test("TUI /install rejects an Object.prototype name instead of crashing the pit", async () => {
+  const result = await runTui("/install constructor\n/quit\n");
+
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+  assert.match(result.stdout, /unknown engine or tool "constructor"/);
+  assert.doesNotMatch(result.stderr, /TypeError/);
+});