diff --git a/src/auto-reply/usage-bar/translator.test.ts b/src/auto-reply/usage-bar/translator.test.ts
index 42d0b72fcf4e..27b84f1d1224 100644
--- a/src/auto-reply/usage-bar/translator.test.ts
+++ b/src/auto-reply/usage-bar/translator.test.ts
@@ -65,6 +65,17 @@ describe("usage-bar verbs", () => {
     expect(render([{ text: "{m|alias:models}" }], { m: "some-new-model" })).toBe("some-new-model");
   });
 
+  it("alias — prototype keys (toString, constructor) do not match inherited properties", () => {
+    // When a model is named "toString" or "constructor", the `in` operator
+    // would match Object.prototype inherited properties and return
+    // Object.prototype.toString (a function) instead of the raw key.
+    // After the fix (Object.hasOwn), these should echo through unchanged.
+    expect(render([{ text: "{m|alias:models}" }], { m: "toString" })).toBe("toString");
+    expect(render([{ text: "{m|alias:models}" }], { m: "constructor" })).toBe("constructor");
+    expect(render([{ text: "{m|alias:models}" }], { m: "valueOf" })).toBe("valueOf");
+    expect(render([{ text: "{m|alias:models}" }], { m: "__proto__" })).toBe("__proto__");
+  });
+
   it("fallback when path is missing/empty", () => {
     expect(render([{ text: "{identity.emoji|🤖} hi" }], {})).toBe("🤖 hi");
     expect(render([{ text: "{identity.emoji|🤖} hi" }], { identity: { emoji: "🩺" } })).toBe(
@@ -87,6 +98,18 @@ describe("usage-bar segment forms", () => {
     expect(render(seg, { state: {} })).toBe("");
   });
 
+  it("map — prototype keys (toString, constructor) do not match inherited properties", () => {
+    // When the map key is "toString" or "constructor", the `in` operator
+    // would incorrectly match Object.prototype inherited properties and
+    // return undefined (Object.prototype.toString is a function, not a
+    // string case value) instead of falling through to _default.
+    const seg = [
+      { map: "state.mode", cases: { toString: "should-not-match", _default: "fallback" } },
+    ];
+    expect(render(seg, { state: { mode: "toString" } })).toBe("should-not-match");
+    expect(render(seg, { state: { mode: "constructor" } })).toBe("fallback");
+  });
+
   it("each with item_scales picks a scale per window by position", () => {
     const seg = [
       {
diff --git a/src/auto-reply/usage-bar/translator.ts b/src/auto-reply/usage-bar/translator.ts
index 2802e2187dc0..3846d75c96bf 100644
--- a/src/auto-reply/usage-bar/translator.ts
+++ b/src/auto-reply/usage-bar/translator.ts
@@ -130,11 +130,11 @@ function applyVerb(name: string, args: string[], value: unknown, vocab: Vocab):
       const table =
         args[0] && isObject(aliases[args[0]]) ? (aliases[args[0]] as Record<string, unknown>) : {};
       const key = String(value);
-      if (key in table) {
+      if (Object.hasOwn(table, key)) {
         return table[key];
       }
       const lower = key.toLowerCase();
-      return lower in table ? table[lower] : value;
+      return Object.hasOwn(table, lower) ? table[lower] : value;
     }
     case "meter": {
       const width = args[0] ? Number.parseInt(args[0], 10) || 5 : 5;
@@ -200,7 +200,7 @@ function renderSegment(seg: Segment, ctx: unknown, vocab: Vocab): string | null
     const v = getPath(ctx, String(seg.map));
     const key = typeof v === "boolean" ? String(v) : String(v);
     const cases = isObject(seg.cases) ? seg.cases : {};
-    const hit = key in cases ? cases[key] : cases["_default"];
+    const hit = Object.hasOwn(cases, key) ? cases[key] : cases["_default"];
     return typeof hit === "string" ? hit : null;
   }
   if ("each" in seg) {