diff --git a/packages/codemode/interpreter-support.md b/packages/codemode/interpreter-support.md
index b4789e245a97..ba2be07da7e3 100644
--- a/packages/codemode/interpreter-support.md
+++ b/packages/codemode/interpreter-support.md
@@ -104,7 +104,9 @@ the ultimate source of truth. Upstream test262 files run verbatim from `test/tes
       synchronous iterators, and confined synchronous generators. Abrupt completion invokes the iterator's optional `return()`.
 - [x] `for...in` over own keys of plain objects, arrays, strings, and tool references. `null`, `undefined`, and other
       non-objects iterate nothing. An un-awaited promise throws rather than iterating.
-- [ ] `for...in` over inherited enumerable keys (`Object.create(proto)`), and skipping keys deleted during the loop.
+- [x] `for...in` over inherited enumerable keys (`Object.create(proto)`): own keys first, then each prototype's, skipping
+      keys a nearer level already has, even as a non-enumerable property.
+- [ ] `for...in` skipping keys deleted during the loop.
 - [x] Unlabeled `break` and `continue`.
 - [x] `try`, `catch`, optional catch bindings, and `finally`.
 - [x] `throw` with arbitrary values.
@@ -338,8 +340,8 @@ reject }` object.
       (`Object.getPrototypeOf("a") === String.prototype`); `null`, `undefined`, and the two confined symbols throw a
       `TypeError`.
 - [x] `Object.create(proto)` with an object or `null` prototype; any other prototype is a `TypeError`
-      (`Object prototype may only be an Object or null`). Inherited reads, `in`, `hasOwnProperty`, and own-only
-      `Object.keys` follow the chain as in JS, but `for...in` still enumerates own keys only. A second `properties`
+      (`Object prototype may only be an Object or null`). Inherited reads, `in`, `hasOwnProperty`, own-only
+      `Object.keys`, and `for...in` follow the chain as in JS. A second `properties`
       argument other than `undefined` throws a `TypeError`: property descriptors are not supported (there is no
       `Object.defineProperty` either).
 - [x] `Object.freeze`, `Object.seal`, and `Object.preventExtensions`, with `isFrozen`, `isSealed`, and `isExtensible`.
diff --git a/packages/codemode/src/interpreter/interpreter.ts b/packages/codemode/src/interpreter/interpreter.ts
index 8cdab139da5e..555de4ad2408 100644
--- a/packages/codemode/src/interpreter/interpreter.ts
+++ b/packages/codemode/src/interpreter/interpreter.ts
@@ -80,6 +80,7 @@ import {
   IteratorObj,
   type Cursor,
   has,
+  hasOwn,
   hidden,
   hasPrototype,
   keys,
@@ -1097,11 +1098,23 @@ class Frame<R> {
     throw typeError(`${context} must be a function.`, node)
   }
 
-  // for...in over null/undefined iterates nothing, like JS.
+  // for...in over null/undefined iterates nothing, like JS. After the own keys come each prototype's enumerable
+  // string keys, skipping any key a nearer level has, even as a non-enumerable property.
   private enumerableKeys(value: Value, node: AstNode): Array<string> {
     if (value instanceof ToolReference) return [...this.ctx.tools.keys(value.path)]
     if (value === null || value === undefined) return []
-    return keys(enumerableSource(this.ctx, "for...in", value, node))
+    const source = enumerableSource(this.ctx, "for...in", value, node)
+    const chain = [source]
+    // A string's characters stand in an array, but its prototype is String.prototype.
+    let proto = primitivePrototype(this.ctx.builtins, value) ?? source.proto
+    while (proto !== null) {
+      chain.push(proto)
+      proto = proto.proto
+    }
+    return chain.flatMap((target, depth) => {
+      const nearer = chain.slice(0, depth)
+      return keys(target).filter((key) => !nearer.some((level) => hasOwn(level, key)))
+    })
   }
 
   private *evaluateForInStatement(node: ForInStatement, labels?: ReadonlySet<string>): Eval<StatementResult> {
diff --git a/packages/codemode/test/enumeration.test.ts b/packages/codemode/test/enumeration.test.ts
index b9daf647c059..c46b6048d5be 100644
--- a/packages/codemode/test/enumeration.test.ts
+++ b/packages/codemode/test/enumeration.test.ts
@@ -176,4 +176,52 @@ describe("for...in", () => {
       "un-awaited Promise",
     )
   })
+
+  const forIn = (target: string, setup = "") =>
+    value(`${setup}; const out = []; for (const key in ${target}) out.push(key); return out`)
+
+  test("visits inherited enumerable keys after own keys", async () => {
+    expect(await forIn("o", "const p = { inh: 1 }; const o = Object.create(p); o.own = 2")).toEqual(["own", "inh"])
+  })
+
+  test("walks every level, skipping keys a nearer level already has", async () => {
+    expect(
+      await forIn(
+        "o",
+        `const top = { a: 1, top: 1 }
+        const mid = Object.create(top); mid.a = 2; mid.mid = 2
+        const o = Object.create(mid); o.own = 3; o.mid = 3`,
+      ),
+    ).toEqual(["own", "mid", "a", "top"])
+  })
+
+  test("orders integer-like keys first on each level", async () => {
+    expect(
+      await forIn("o", `const p = { b: 1, 10: 1, 2: 1 }; const o = Object.create(p); o.z = 1; o[5] = 1; o[1] = 1`),
+    ).toEqual(["1", "5", "z", "2", "10", "b"])
+  })
+
+  test("a non-enumerable nearer property shadows an enumerable inherited one", async () => {
+    // An array's own length and Array.prototype.push are non-enumerable.
+    const setup = "Object.prototype.length = 1; Object.prototype.push = 2; Object.prototype.extra = 3"
+    expect(await forIn("[5]", setup)).toEqual(["0", "extra"])
+    expect(await forIn("Object.create([5])", setup)).toEqual(["0", "extra"])
+  })
+
+  test("a null prototype ends the walk", async () => {
+    expect(await forIn("o", "const o = Object.create(null); o.a = 1")).toEqual(["a"])
+    expect(await forIn("Object.create(null)")).toEqual([])
+  })
+
+  test("arrays and strings only visit their indexes, since built-in methods are non-enumerable", async () => {
+    expect(await forIn("[7, 8]")).toEqual(["0", "1"])
+    expect(await forIn(`"ab"`)).toEqual(["0", "1"])
+  })
+
+  test("visits enumerable properties a program adds to a built-in prototype", async () => {
+    expect(await forIn("[7, 8]", "Array.prototype.extra = 1")).toEqual(["0", "1", "extra"])
+    expect(await forIn(`"ab"`, "Array.prototype.extra = 1")).toEqual(["0", "1"])
+    expect(await forIn(`"ab"`, "String.prototype.extra = 1")).toEqual(["0", "1", "extra"])
+    expect(await forIn("5", "Number.prototype.extra = 1")).toEqual(["extra"])
+  })
 })
diff --git a/packages/codemode/test/parity.test.ts b/packages/codemode/test/parity.test.ts
index 02f5d9eaa24e..72b00a447a53 100644
--- a/packages/codemode/test/parity.test.ts
+++ b/packages/codemode/test/parity.test.ts
@@ -1382,7 +1382,7 @@ describe("Object.getPrototypeOf and Object.create", () => {
     expect((await error(`Object.getPrototypeOf(Symbol.iterator)`)).message).toContain("cannot convert a symbol")
   })
 
-  test("Object.create links the prototype: inherited reads, in, and own-only keys", async () => {
+  test("Object.create links the prototype: inherited reads, in, own-only keys, and inherited for...in", async () => {
     expect(
       await value(`
         const p = { greet(name) { return "hi " + name }, a: 1 }
@@ -1392,7 +1392,7 @@ describe("Object.getPrototypeOf and Object.create", () => {
         for (const key in c) seen.push(key)
         return ["greet" in c, Object.keys(c), c.hasOwnProperty("a"), c.a, c.greet(c.name), Object.getPrototypeOf(c) === p, Object.getPrototypeOf(Object.create(null)), seen]
       `),
-    ).toEqual([true, ["name"], false, 1, "hi x", true, null, ["name"]])
+    ).toEqual([true, ["name"], false, 1, "hi x", true, null, ["name", "greet", "a"]])
   })
 
   test("Object.create rejects non-object prototypes and property descriptors", async () => {