diff --git a/packages/codemode/interpreter-support.md b/packages/codemode/interpreter-support.md
index 7cc781ed1bc..65d637e8db9 100644
--- a/packages/codemode/interpreter-support.md
+++ b/packages/codemode/interpreter-support.md
@@ -31,8 +31,8 @@ ultimate source of truth.
 - [x] `null`, `undefined`, booleans, finite and non-finite numbers, and strings.
 - [x] Array literals, including holes and spread from arrays, strings, Maps, Sets, URLSearchParams, custom synchronous
       iterators, and synchronous generators.
-- [x] Object literals with shorthand, computed string/number keys, and spread from plain data objects; `null` and
-      `undefined` are no-ops, while arrays are rejected.
+- [x] Object literals with shorthand, computed string/number keys, and spread following ToObject: data objects and
+      arrays copy own enumerable keys, strings copy index keys, and other values contribute nothing.
 - [x] Template literals with interpolation.
 - [x] Regular-expression literals.
 - [x] `NaN` and `Infinity` globals.
@@ -70,7 +70,7 @@ ultimate source of truth.
 - [x] `for`, `while`, and `do...while`.
 - [x] `for...of` over arrays, strings, Maps, Sets, URLSearchParams, custom synchronous iterators, and confined
       synchronous generators. Abrupt completion invokes the iterator's optional `return()`.
-- [x] `for...in` over own keys of plain objects, arrays, and tool references.
+- [x] `for...in` over own keys of plain objects, arrays, strings, and tool references; other values iterate nothing.
 - [x] Unlabeled `break` and `continue`.
 - [x] `try`, `catch`, optional catch bindings, and `finally`.
 - [x] `throw` with arbitrary values.
@@ -210,7 +210,9 @@ ultimate source of truth.
       primitive wrapper objects (`Object(1)`) are rejected explicitly.
 - [x] Computed property names and object spread.
 - [x] `Object.keys`, `Object.values`, `Object.entries`, `Object.hasOwn`, `Object.assign`, and `Object.fromEntries`, with
-      synchronous iterator support for `fromEntries`.
+      synchronous iterator support for `fromEntries`. Sources follow ToObject: strings enumerate by index, other
+      primitives and wrappers contribute nothing, and `null`/`undefined` throw. `Object.assign` accepts array
+      targets for index keys only; a primitive target is a `TypeError` rather than a boxed object.
 - [x] `Object.keys` over arrays and tool references.
 - [x] Object identity is preserved by in-CodeMode Object helpers.
 - [x] `__proto__`, `constructor`, and `prototype` are ordinary own data keys. `x.constructor` without an own key resolves
diff --git a/packages/codemode/src/interpreter/references.ts b/packages/codemode/src/interpreter/references.ts
index 5bdb4c88e96..318e5a70656 100644
--- a/packages/codemode/src/interpreter/references.ts
+++ b/packages/codemode/src/interpreter/references.ts
@@ -108,3 +108,12 @@ export const typeofValue = (value: unknown): string => {
   if (value instanceof ToolReference) return value.path.length > 0 ? "function" : "object"
   return typeof value
 }
+
+const MAX_ARRAY_LENGTH = 4_294_967_295
+
+export const parseArrayIndex = (key: string | number): number | undefined => {
+  const property = String(key)
+  if (!/^(0|[1-9]\d*)$/.test(property)) return undefined
+  const index = Number(property)
+  return index < MAX_ARRAY_LENGTH ? index : undefined
+}
diff --git a/packages/codemode/src/interpreter/runtime.ts b/packages/codemode/src/interpreter/runtime.ts
index 4a1e1dbb865..ea77fcc55dd 100644
--- a/packages/codemode/src/interpreter/runtime.ts
+++ b/packages/codemode/src/interpreter/runtime.ts
@@ -75,6 +75,7 @@ import {
   containsOpaqueReference,
   describeValue,
   isRuntimeReference,
+  parseArrayIndex,
   rejectCircularInsertion,
   typeofValue,
 } from "./references.js"
@@ -85,18 +86,10 @@ import { numberMethods } from "../stdlib/number.js"
 import { constructRegExp, regexpMethods, regexpProperties } from "../stdlib/regexp.js"
 import { stringMethods } from "../stdlib/string.js"
 import { uriArgument, urlMethods, urlProperties, urlSearchParamsMethods, urlWritableProperties } from "../stdlib/url.js"
+import { enumerableSource } from "../stdlib/object.js"
 import { coerceToNumber, coerceToString, compoundOperators, errorBrandName } from "../stdlib/value.js"
 import { Values } from "../values.js"
 
-const MAX_ARRAY_LENGTH = 4_294_967_295
-
-const parseArrayIndex = (key: string | number): number | undefined => {
-  const property = String(key)
-  if (!/^(0|[1-9]\d*)$/.test(property)) return undefined
-  const index = Number(property)
-  return index < MAX_ARRAY_LENGTH ? index : undefined
-}
-
 const calleeDescription = (callee: Expression | Super | undefined): string => {
   if (callee?.type === "Identifier") return callee.name
   if (callee?.type === "MemberExpression") {
@@ -852,17 +845,11 @@ class Frame<R> {
     throw new InterpreterRuntimeError(`${context} must be a function.`, node).as("TypeError")
   }
 
-  private enumerableKeys(value: unknown): Array<string> | undefined {
-    if (value instanceof ToolReference) {
-      return [...this.runtime.toolKeys(value.path)]
-    }
-    if (Array.isArray(value)) {
-      return Object.keys(value)
-    }
-    if (value !== null && typeof value === "object" && !isRuntimeReference(value)) {
-      return Object.keys(value)
-    }
-    return undefined
+  // for...in over null/undefined iterates nothing, like JS.
+  private enumerableKeys(value: unknown, node: AstNode): Array<string> {
+    if (value instanceof ToolReference) return [...this.runtime.toolKeys(value.path)]
+    if (value === null || value === undefined) return []
+    return Object.keys(enumerableSource("for...in", value, node))
   }
 
   private evaluateForInStatement(
@@ -878,13 +865,7 @@ class Frame<R> {
       if (declared?.lexical) self.predeclarePattern(declared.pattern, declared.mutable, left)
       const right = yield* self.evaluateExpression(node.right)
 
-      const keys = self.enumerableKeys(right)
-      if (keys === undefined) {
-        throw new InterpreterRuntimeError(
-          "for...in requires a plain object, array, or tools reference. Use for...of for arrays/strings/Maps/Sets, or Object.keys(value) for a key list.",
-          node,
-        )
-      }
+      const keys = self.enumerableKeys(right, node.right)
 
       if (left.type !== "Identifier" && left.type !== "VariableDeclaration") {
         throw new InterpreterRuntimeError("Unsupported for...in binding.", left)
@@ -1916,16 +1897,10 @@ class Frame<R> {
       for (const property of node.properties) {
         if (property.type === "SpreadElement") {
           const spread = yield* self.evaluateExpression(property.argument)
-          if (spread === null || spread === undefined || Values.isValue(spread)) continue
-          if (typeof spread !== "object" || Array.isArray(spread) || isRuntimeReference(spread)) {
-            throw new InterpreterRuntimeError(
-              `Object spread requires a data object, received ${describeValue(spread)}.`,
-              property,
-              "InvalidDataValue",
-            )
-          }
-          for (const [key, value] of Object.entries(spread)) objectValue[key] = value
-          copyIteratorSymbols(spread, objectValue)
+          if (spread === null || spread === undefined) continue
+          const from = enumerableSource("Object spread", spread, property)
+          for (const [key, value] of Object.entries(from)) objectValue[key] = value
+          if (typeof from === "object") copyIteratorSymbols(from, objectValue)
           continue
         }
 
diff --git a/packages/codemode/src/stdlib/object.ts b/packages/codemode/src/stdlib/object.ts
index 44da0c3be48..47fd029dfb0 100644
--- a/packages/codemode/src/stdlib/object.ts
+++ b/packages/codemode/src/stdlib/object.ts
@@ -5,6 +5,8 @@ import { type AstNode, AsyncIteratorSymbol, InterpreterRuntimeError, IteratorSym
 import {
   containsOpaqueReference,
   describeValue,
+  isRuntimeReference,
+  parseArrayIndex,
   rejectCircularInsertion,
   typeofValue,
 } from "../interpreter/references.js"
@@ -14,28 +16,53 @@ import { Values } from "../values.js"
 import { groupBy } from "./collections.js"
 import { coerceToString } from "./value.js"
 
-const requireObject = (name: string, input: unknown, node: AstNode): Record<string, unknown> => {
-  if (Array.isArray(input)) return input as unknown as Record<string, unknown>
-  if (Values.isValue(input)) return {}
-  const prototype = input === null || typeof input !== "object" ? undefined : Object.getPrototypeOf(input)
-  if (prototype !== null && prototype !== Object.prototype) {
+// ToObject for enumeration. Strings return themselves: the host's Object.keys/entries/hasOwn index a
+// primitive string directly. Numbers, booleans, wrappers, and functions have no own enumerable keys.
+export const enumerableSource = (label: string, value: unknown, node: AstNode): Record<string, unknown> => {
+  if (value === null || value === undefined) {
+    throw new InterpreterRuntimeError(`${label} cannot convert ${describeValue(value)} to an object.`, node).as(
+      "TypeError",
+    )
+  }
+  if (value instanceof Values.Promise) {
     throw new InterpreterRuntimeError(
-      `Object.${name} expects a data object or array, received ${describeValue(input)}.`,
+      `${label} received an un-awaited Promise; await it before inspecting the result.`,
       node,
       "InvalidDataValue",
     )
   }
-  return input as Record<string, unknown>
+  if (value instanceof ToolReference) {
+    throw new InterpreterRuntimeError(
+      `${label} cannot read tool references: they are not plain data. Use Object.keys(tools) for names, or search({ query }) for signatures.`,
+      node,
+      "InvalidDataValue",
+    )
+  }
+  if (typeof value === "string") return value as unknown as Record<string, unknown>
+  if (typeof value !== "object" || Values.isValue(value) || isRuntimeReference(value)) return {}
+  return value as Record<string, unknown>
 }
 
 export const objectAssign = (args: Array<unknown>, node: AstNode): unknown => {
   const target = args[0]
-  if (target === null || typeof target !== "object" || Array.isArray(target) || Values.isValue(target)) {
-    throw new InterpreterRuntimeError("Object.assign expects a data object target.", node)
+  // JS would box a primitive target; wrappers and primitives cannot hold fields here.
+  if (target === null || typeof target !== "object" || Values.isValue(target) || isRuntimeReference(target)) {
+    throw new InterpreterRuntimeError(
+      `Object.assign expects a data object or array target, received ${describeValue(target)}.`,
+      node,
+    ).as("TypeError")
   }
   const out = target as Record<string, unknown>
   const seen = new Set<object>()
   const guardedSet = (key: PropertyKey, item: unknown): void => {
+    // Arrays hold only indexed elements, as with direct assignment; Reflect.set would otherwise
+    // reach Array's length and Object.prototype's __proto__ setter.
+    if (Array.isArray(out) && (typeof key === "symbol" || parseArrayIndex(key) === undefined)) {
+      throw new InterpreterRuntimeError(
+        `Object.assign cannot assign '${String(key)}' to an array: only array indexes may be assigned.`,
+        node,
+      ).as("TypeError")
+    }
     rejectCircularInsertion(out, item, "Object.assign result", node, seen)
     if (!Reflect.set(out, key, item))
       throw new InterpreterRuntimeError(`Object.assign could not assign property '${String(key)}'.`, node).as(
@@ -43,18 +70,15 @@ export const objectAssign = (args: Array<unknown>, node: AstNode): unknown => {
       )
   }
   for (const source of args.slice(1)) {
-    if (source === null || source === undefined || Values.isValue(source)) continue
-    if (typeof source !== "object" || Array.isArray(source)) {
-      throw new InterpreterRuntimeError("Object.assign expects data objects.", node)
+    if (source === null || source === undefined) continue
+    const from = enumerableSource("Object.assign(...)", source, node)
+    if (typeof from !== "object") {
+      for (const [key, item] of Object.entries(from)) guardedSet(key, item)
+      continue
     }
-    for (const key of Reflect.ownKeys(source)) {
-      if (typeof key === "string") {
-        if (Object.prototype.propertyIsEnumerable.call(source, key)) guardedSet(key, Reflect.get(source, key))
-        continue
-      }
-      if (key !== AsyncIteratorSymbol && key !== IteratorSymbol) continue
-      if (!Object.prototype.propertyIsEnumerable.call(source, key)) continue
-      guardedSet(key, Reflect.get(source, key))
+    for (const key of Reflect.ownKeys(from)) {
+      if (typeof key === "symbol" && key !== AsyncIteratorSymbol && key !== IteratorSymbol) continue
+      if (Object.prototype.propertyIsEnumerable.call(from, key)) guardedSet(key, Reflect.get(from, key))
     }
   }
   return out
@@ -121,10 +145,7 @@ const rejectTools = (name: string, args: Array<unknown>, node: AstNode): void =>
 }
 
 const objectStatic = (name: string, impl: (args: Array<unknown>, node: AstNode) => unknown) =>
-  sync(`Object.${name}`, (args, node) => {
-    rejectTools(name, args, node)
-    return impl(args, node)
-  })
+  sync(`Object.${name}`, impl)
 
 // Object constructs identically with or without new, like JS. Only `keys` copies its result into the
 // program; `values`, `entries`, `assign`, and `fromEntries` hand back the program's own values.
@@ -139,17 +160,19 @@ export const objectGlobal = <R>(runner: Runner<R>, toolKeys: (path: ReadonlyArra
         toProgram(
           args[0] instanceof ToolReference
             ? [...toolKeys(args[0].path)]
-            : Object.keys(requireObject("keys", args[0], node)),
+            : Object.keys(enumerableSource("Object.keys(...)", args[0], node)),
           "Object.keys result",
         ),
       ),
-      values: objectStatic("values", (args, node) => Object.values(requireObject("values", args[0], node))),
+      values: objectStatic("values", (args, node) =>
+        Object.values(enumerableSource("Object.values(...)", args[0], node)),
+      ),
       entries: objectStatic("entries", (args, node) =>
-        Object.entries(requireObject("entries", args[0], node)).map(([key, item]) => [key, item]),
+        Object.entries(enumerableSource("Object.entries(...)", args[0], node)).map(([key, item]) => [key, item]),
       ),
       hasOwn: objectStatic("hasOwn", (args, node) =>
         Object.hasOwn(
-          requireObject("hasOwn", args[0], node),
+          enumerableSource("Object.hasOwn(...)", args[0], node),
           args[1] === AsyncIteratorSymbol || args[1] === IteratorSymbol ? args[1] : String(args[1]),
         ),
       ),
diff --git a/packages/codemode/test/enumeration.test.ts b/packages/codemode/test/enumeration.test.ts
index 3636bbef280..4e92b2d197b 100644
--- a/packages/codemode/test/enumeration.test.ts
+++ b/packages/codemode/test/enumeration.test.ts
@@ -85,18 +85,16 @@ describe("Object.keys over arrays", () => {
     expect(await value(`return Object.keys({ a: 1, b: 2 })`)).toEqual(["a", "b"])
   })
 
-  test("non-object inputs name what was received", async () => {
-    expect((await error(`return Object.keys("nope")`)).message).toContain(
-      "Object.keys expects a data object or array, received a string.",
-    )
-    expect((await error(`return Object.entries(42)`)).message).toContain("received a number.")
-    expect((await error(`return Object.values(null)`)).message).toContain("received null.")
+  test("non-object inputs follow ToObject, and nullish inputs name what was received", async () => {
+    expect(
+      await value(`return [Object.keys("ab"), Object.entries(42), Object.keys(() => 1), Object.keys(true)]`),
+    ).toEqual([["0", "1"], [], [], []])
+    expect(await value(`try { Object.values(null) } catch (e) { return [e.name, e.message] }`)).toEqual([
+      "TypeError",
+      "Object.values(...) cannot convert null to an object.",
+    ])
     expect((await error(`return Object.keys(tools.github.list_issues({ value: "x" }))`)).message).toContain(
-      "received an un-awaited Promise.",
-    )
-    expect((await error(`return Object.entries(() => 1)`)).message).toContain("received a function.")
-    expect((await error(`return { ...[1] }`)).message).toContain(
-      "Object spread requires a data object, received an array.",
+      "received an un-awaited Promise",
     )
     expect((await error(`const { a } = new Map(); return a`)).message).toContain("received a Map.")
     expect((await error(`return Array.from(7)`)).message).toContain("received a number.")
@@ -161,11 +159,21 @@ describe("for...in", () => {
     ).toEqual(["github.list_issues", "github.get_issue", "memory.search", "playwright.navigate"])
   })
 
-  test("unsupported values fail with a hint at for...of and Object.keys", async () => {
-    for (const expression of [`"text"`, "new Map([[1, 2]])", "new Set([1])", "42", "null"]) {
-      const failure = await error(`for (const key in ${expression}) {}; return "no"`)
-      expect(failure.message).toContain("for...in requires a plain object, array, or tools reference")
-      expect(failure.message).toContain("Use for...of for arrays/strings/Maps/Sets, or Object.keys(value)")
-    }
+  test("non-object values enumerate like JS: strings by index, everything else nothing", async () => {
+    expect(
+      await value(`
+        const out = []
+        for (const key in "ab") out.push(key)
+        for (const key in 42) out.push(key)
+        for (const key in null) out.push(key)
+        for (const key in undefined) out.push(key)
+        for (const key in new Map([[1, 2]])) out.push(key)
+        for (const key in Math) out.push(key)
+        return out
+      `),
+    ).toEqual(["0", "1"])
+    expect((await error(`for (const key in tools.github.list_issues({ value: "x" })) {}`)).message).toContain(
+      "un-awaited Promise",
+    )
   })
 })
diff --git a/packages/codemode/test/object-coercion-test262.test.ts b/packages/codemode/test/object-coercion-test262.test.ts
new file mode 100644
index 00000000000..e8713904c39
--- /dev/null
+++ b/packages/codemode/test/object-coercion-test262.test.ts
@@ -0,0 +1,217 @@
+/**
+ * Portions adapted from Test262 at revision 250f204f23a9249ff204be2baec29600faae7b75:
+ * - test/built-ins/Object/keys/15.2.3.14-1-1.js
+ * - test/built-ins/Object/keys/15.2.3.14-1-2.js
+ * - test/built-ins/Object/keys/15.2.3.14-1-3.js
+ * - test/built-ins/Object/keys/15.2.3.14-1-4.js
+ * - test/built-ins/Object/keys/15.2.3.14-1-5.js
+ * - test/built-ins/Object/entries/primitive-strings.js
+ * - test/built-ins/Object/entries/primitive-numbers.js
+ * - test/built-ins/Object/entries/primitive-booleans.js
+ * - test/built-ins/Object/values/primitive-strings.js
+ * - test/built-ins/Object/values/primitive-numbers.js
+ * - test/built-ins/Object/values/primitive-booleans.js
+ * - test/built-ins/Object/hasOwn/toobject_null.js
+ * - test/built-ins/Object/hasOwn/toobject_undefined.js
+ * - test/built-ins/Object/hasOwn/hasown_nonexistent.js
+ * - test/built-ins/Object/assign/Source-String.js
+ * - test/built-ins/Object/assign/Source-Null-Undefined.js
+ * - test/built-ins/Object/assign/target-Array.js
+ * - test/built-ins/Object/assign/Target-Null.js
+ * - test/built-ins/Object/assign/Target-Undefined.js
+ * - test/built-ins/Object/assign/Target-Object.js
+ * - test/built-ins/Object/assign/Override.js
+ * - test/built-ins/Object/assign/ObjectOverride-sameproperty.js
+ *
+ * Copyright (c) 2012 Ecma International. All rights reserved.
+ * Copyright (C) 2015 Jordan Harband. All rights reserved.
+ * Copyright 2015 Microsoft Corporation. All rights reserved.
+ * Copyright 2021 Jamie Kyle. All rights reserved.
+ * Test262 portions are governed by the BSD license in LICENSE.test262.
+ *
+ * Boxed-primitive cases (`Object.assign("a")`, `Object.assign(1, …)`) are omitted: CodeMode has no
+ * wrapper objects, so a primitive target is a TypeError rather than a boxed result. `Override.js`
+ * checks `Object.keys(result).length` instead of `Object.getOwnPropertyNames`. `target-Array.js`
+ * omits its named-key (`-0`, `1.5`, `4294967295`), `length`, and Proxy assertions: arrays here hold
+ * only indexed elements, so those keys are a TypeError (pinned below) rather than array properties.
+ */
+import { describe, expect, test } from "bun:test"
+import { Effect } from "effect"
+import { CodeMode } from "../src/index.js"
+
+const value = async (code: string) => {
+  const result = await Effect.runPromise(CodeMode.execute({ code, tools: {} }))
+  if (!result.ok) throw new Error(`expected success, got ${result.error.kind}: ${result.error.message}`)
+  return result.value
+}
+
+const throwsTypeError = (expression: string) =>
+  value(`try { ${expression}; return "no throw" } catch (error) { return error.name }`)
+
+describe("Object.keys Test262 parity", () => {
+  test("test/built-ins/Object/keys/15.2.3.14-1-{1,2,3}.js: primitives are coerced", async () => {
+    expect(await value(`return [Object.keys(0), Object.keys(true), Object.keys("abc")]`)).toEqual([
+      [],
+      [],
+      ["0", "1", "2"],
+    ])
+  })
+
+  test("test/built-ins/Object/keys/15.2.3.14-1-{4,5}.js: null and undefined throw TypeError", async () => {
+    expect(await throwsTypeError(`Object.keys(null)`)).toBe("TypeError")
+    expect(await throwsTypeError(`Object.keys(undefined)`)).toBe("TypeError")
+  })
+})
+
+describe("Object.entries and Object.values Test262 parity", () => {
+  test("test/built-ins/Object/entries/primitive-strings.js", async () => {
+    expect(
+      await value(`
+        const result = Object.entries('abc')
+        return [Array.isArray(result), result.length, result[0][0], result[0][1], result[1][0], result[1][1], result[2][0], result[2][1]]
+      `),
+    ).toEqual([true, 3, "0", "a", "1", "b", "2", "c"])
+  })
+
+  test("test/built-ins/Object/entries/primitive-numbers.js", async () => {
+    expect(
+      await value(`
+        return [0, -0, Infinity, -Infinity, NaN, Math.PI].map((number) => Object.entries(number).length)
+      `),
+    ).toEqual([0, 0, 0, 0, 0, 0])
+  })
+
+  test("test/built-ins/Object/entries/primitive-booleans.js", async () => {
+    expect(
+      await value(`
+        const trueResult = Object.entries(true)
+        const falseResult = Object.entries(false)
+        return [Array.isArray(trueResult), trueResult.length, Array.isArray(falseResult), falseResult.length]
+      `),
+    ).toEqual([true, 0, true, 0])
+  })
+
+  test("test/built-ins/Object/values/primitive-strings.js", async () => {
+    expect(
+      await value(`
+        const result = Object.values('abc')
+        return [Array.isArray(result), result.length, result[0], result[1], result[2]]
+      `),
+    ).toEqual([true, 3, "a", "b", "c"])
+  })
+
+  test("test/built-ins/Object/values/primitive-numbers.js", async () => {
+    expect(
+      await value(`
+        return [0, -0, Infinity, -Infinity, NaN, Math.PI].map((number) => Object.values(number).length)
+      `),
+    ).toEqual([0, 0, 0, 0, 0, 0])
+  })
+
+  test("test/built-ins/Object/values/primitive-booleans.js", async () => {
+    expect(
+      await value(`
+        const trueResult = Object.values(true)
+        const falseResult = Object.values(false)
+        return [Array.isArray(trueResult), trueResult.length, Array.isArray(falseResult), falseResult.length]
+      `),
+    ).toEqual([true, 0, true, 0])
+  })
+})
+
+describe("Object.hasOwn Test262 parity", () => {
+  test("test/built-ins/Object/hasOwn/toobject_{null,undefined}.js", async () => {
+    expect(await throwsTypeError(`Object.hasOwn(null, 'foo')`)).toBe("TypeError")
+    expect(await throwsTypeError(`Object.hasOwn(undefined, 'foo')`)).toBe("TypeError")
+  })
+
+  test("test/built-ins/Object/hasOwn/hasown_nonexistent.js", async () => {
+    expect(await value(`const o = {}; return Object.hasOwn(o, "foo")`)).toBe(false)
+  })
+})
+
+describe("Object.assign Test262 parity", () => {
+  test("test/built-ins/Object/assign/Source-String.js", async () => {
+    expect(
+      await value(`
+        const target = new Object()
+        const result = Object.assign(target, "123")
+        return [result[0], result[1], result[2]]
+      `),
+    ).toEqual(["1", "2", "3"])
+  })
+
+  test("test/built-ins/Object/assign/Source-Null-Undefined.js", async () => {
+    expect(
+      await value(`
+        const target = new Object()
+        const result = Object.assign(target, undefined, null)
+        return result === target
+      `),
+    ).toBe(true)
+  })
+
+  test("test/built-ins/Object/assign/target-Array.js", async () => {
+    expect(
+      await value(`
+        const target = [7, 8, 9]
+        let result = Object.assign(target, [1])
+        const first = [result === target, [...result]]
+        const sparseArraySource = []
+        sparseArraySource[2] = 3
+        result = Object.assign(target, sparseArraySource)
+        const second = [result === target, [...result]]
+        result = Object.assign(target, { 4: 0 })
+        return [...first, ...second, result === target, result.length, result[3] === undefined, result[4]]
+      `),
+    ).toEqual([true, [1, 8, 9], true, [1, 8, 3], true, 5, true, 0])
+  })
+
+  test("array targets accept only array indexes (deviation from target-Array.js)", async () => {
+    expect(
+      await value(`
+        const target = [7]
+        const out = []
+        for (const source of [{ length: 0 }, { x: 1 }, { "1.5": 1 }, { "-0": 1 }, { ["__proto__"]: null }]) {
+          try { Object.assign(target, source) } catch (error) { out.push(error.name) }
+        }
+        return [out, [...target], target.length, Object.keys(target)]
+      `),
+    ).toEqual([["TypeError", "TypeError", "TypeError", "TypeError", "TypeError"], [7], 1, ["0"]])
+  })
+
+  test("test/built-ins/Object/assign/Target-{Null,Undefined}.js", async () => {
+    expect(await throwsTypeError(`Object.assign(null, { a: 1 })`)).toBe("TypeError")
+    expect(await throwsTypeError(`Object.assign(undefined, { a: 1 })`)).toBe("TypeError")
+  })
+
+  test("test/built-ins/Object/assign/Target-Object.js", async () => {
+    expect(
+      await value(`
+        const target = { foo: 1 }
+        const result = Object.assign(target, { a: 2 })
+        return [result.foo, result.a]
+      `),
+    ).toEqual([1, 2])
+  })
+
+  test("test/built-ins/Object/assign/Override.js", async () => {
+    expect(
+      await value(`
+        const target = { a: 1 }
+        const result = Object.assign(target, "1a2c3", { a: "c" }, undefined, { b: 6 }, null, 125, { a: 5 })
+        return [Object.keys(result).length, result.a, result[0], result[1], result[2], result[3], result[4], result.b]
+      `),
+    ).toEqual([7, 5, "1", "a", "2", "c", "3", 6])
+  })
+
+  test("test/built-ins/Object/assign/ObjectOverride-sameproperty.js", async () => {
+    expect(
+      await value(`
+        const target = { a: 1 }
+        const result = Object.assign(target, { a: 2 }, { a: "c" })
+        return result.a
+      `),
+    ).toBe("c")
+  })
+})
diff --git a/packages/codemode/test/parity.test.ts b/packages/codemode/test/parity.test.ts
index 9fc13debe04..4257f1e89e1 100644
--- a/packages/codemode/test/parity.test.ts
+++ b/packages/codemode/test/parity.test.ts
@@ -118,9 +118,12 @@ describe("H6: object spread of null/undefined is a no-op", () => {
     expect(await value(`const o = { a: 1 }; return { ...o, b: 2 }`)).toEqual({ a: 1, b: 2 })
   })
 
-  test("spreading an array into an object still errors", async () => {
-    const err = await error(`return { ...[1,2], a: 1 }`)
-    expect(err.kind).toBe("InvalidDataValue")
+  test("spreading an array or string into an object copies index keys, like JS", async () => {
+    expect(await value(`return { ...[1,2], a: 1 }`)).toEqual({ 0: 1, 1: 2, a: 1 })
+    expect(await value(`return { ..."ab", ...5, ...true, ...(() => 1), ...new Map([[1, 2]]) }`)).toEqual({
+      0: "a",
+      1: "b",
+    })
   })
 })
 
diff --git a/packages/codemode/test/stdlib.test.ts b/packages/codemode/test/stdlib.test.ts
index f6af2b8a1cf..86f631e6129 100644
--- a/packages/codemode/test/stdlib.test.ts
+++ b/packages/codemode/test/stdlib.test.ts
@@ -1130,7 +1130,7 @@ describe("CodeMode values at intra-CodeMode checkpoints", () => {
     const diagnostic = await error(`return Object.keys(Promise.resolve({ a: 1 }))`)
     expect(diagnostic.kind).toBe("InvalidDataValue")
     expect(diagnostic.message).toContain("await")
-    expect((await error(`return Object.keys(Math)`)).kind).toBe("InvalidDataValue")
+    expect(await value(`return Object.keys(Math)`)).toEqual([])
   })
 
   test("Object.assign keeps Maps usable", async () => {