diff --git a/packages/codemode/interpreter-support.md b/packages/codemode/interpreter-support.md
index dbcdf211eb7..84cab537556 100644
--- a/packages/codemode/interpreter-support.md
+++ b/packages/codemode/interpreter-support.md
@@ -124,8 +124,8 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
 - [x] `Boolean`, `Number`, `String`, `parseInt`, `parseFloat`, `isFinite`, `isNaN`, and URI helpers as callbacks.
 - [x] Built-in method references as callbacks, such as `values.map(Math.abs)`, `records.map(JSON.stringify)`,
       `items.forEach(console.log)`, and `Promise.resolve(-1).then(Math.abs)`. Extra callback arguments a built-in
-      does not consume are ignored, like JS; consumed arguments stay strictly validated (`Math.floor` still rejects a
-      string). A detached method loses its receiver, as in JS: `values.filter("abc".includes)` is a `TypeError`
+      does not consume are ignored, like JS, and consumed arguments coerce, like JS (`"3.7".replace(/\d\.\d/,
+    Math.floor)` is `"3"`). A detached method loses its receiver, as in JS: `values.filter("abc".includes)` is a `TypeError`
       because `includes` is called without a string `this`.
 - [x] Constructors work as callbacks with JS call semantics: `Error` types construct (`messages.map(Error)`),
       and new-requiring constructors (`Map`, `Set`, `URL`, `URLSearchParams`, `Headers`, `Promise`) throw a `TypeError`,
@@ -205,8 +205,8 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
       applied to functions and namespaces; JavaScript coerces them, the interpreter rejects non-data operands.
 - [ ] ToPrimitive on object operands: operators, `Error(message)`, `Date` arguments, and `parseInt` radix should call
       `valueOf`/`toString` in spec order and surface their throws.
-- [ ] Property keys follow ToPropertyKey: `x[null]`, `x[true]`, and objects (via `toString`) become string keys; only
-      strings and numbers are accepted.
+- [x] Property keys follow ToPropertyKey: `x[null]`, `x[true]`, and objects (via their built-in string form) become
+      string keys.
 
 ## Promises and tools
 
@@ -319,10 +319,12 @@ reject }` object.
 - [x] Assigning `length` to truncate or extend an array; invalid lengths throw `RangeError`.
 - [x] Non-index own properties on arrays (`arr.foo = 1`, `arr.constructor = null`). They are excluded from the JSON
       form, like `JSON.stringify`.
-- [ ] Argument coercion for `indexOf`, `lastIndexOf`, `includes`, `fill`, `flat`, `copyWithin`, and the `join`
-      separator: JavaScript applies ToIntegerOrInfinity/ToString (including `valueOf`, strings, and `undefined`), the
-      interpreter requires numbers and strings. `indexOf()` and `lastIndexOf()` with no argument already search for
-      `undefined`; `includes()` still requires a value.
+- [x] Numeric arguments coerce as in JS (ToIntegerOrInfinity): `indexOf(x, "1")`, `slice("1", "3")`, `at(null)`,
+      `flat(1.9)`, `with(1.5, v)`, `Math.max("3", "2")`, `parseInt("11", "2")`, `(1.5).toFixed("2")`,
+      `String.fromCharCode("65")`, and the Uint8Array equivalents. `join(sep)` and `JSON.parse(text)` apply ToString
+      (`join(null)` is `"1null2"`, `JSON.parse(123)` is `123`). `Array.from({ length: "2" })` applies ToLength; a
+      promise source still throws with an `await` hint rather than JS's silent `[]`. A program object's own
+      `valueOf`/`toString` is not consulted yet (see ToPrimitive above).
 
 ## Strings
 
diff --git a/packages/codemode/src/interpreter/objects.ts b/packages/codemode/src/interpreter/objects.ts
index 84134eaa0e8..a46c5c3c5f1 100644
--- a/packages/codemode/src/interpreter/objects.ts
+++ b/packages/codemode/src/interpreter/objects.ts
@@ -422,6 +422,12 @@ export const coerceToNumber = (value: Value): number => {
   return value instanceof ToolReference ? Number.NaN : Number(value)
 }
 
+/** ToIntegerOrInfinity: NaN is 0, fractions truncate. */
+export const coerceToInteger = (value: Value): number => {
+  const number = coerceToNumber(value)
+  return Number.isNaN(number) ? 0 : Math.trunc(number)
+}
+
 /** Values that cannot cross the data boundary: opaque machinery and host-backed wrappers. */
 export const isRuntimeReference = (value: Value): boolean =>
   value instanceof Opaque || value instanceof Wrapper || value instanceof ToolReference
diff --git a/packages/codemode/src/stdlib/array.ts b/packages/codemode/src/stdlib/array.ts
index b8110568a7c..922d9a1473b 100644
--- a/packages/codemode/src/stdlib/array.ts
+++ b/packages/codemode/src/stdlib/array.ts
@@ -10,6 +10,8 @@ import {
   GeneratorObj,
   hostIterator,
   Obj,
+  PromiseObj,
+  coerceToInteger,
   coerceToNumber,
   coerceToString,
   type Value,
@@ -20,11 +22,11 @@ import type { Interpreter } from "../interpreter/interpreter.js"
 import { compareText } from "../tool-runtime.js"
 
 const arrayLikeSource = (source: Value): { readonly length: number; readonly source: Obj } => {
-  if (source instanceof Obj && typeof get(source, "length") === "number") {
-    const length = get(source, "length") as number
-    const normalized = Number.isNaN(length) || length <= 0 ? 0 : Math.trunc(length)
-    checkArrayLength(normalized)
-    return { length: normalized, source }
+  // JS would treat a promise as an empty array-like; that would hide a missing `await`.
+  if (source instanceof Obj && !(source instanceof PromiseObj)) {
+    const length = Math.max(0, coerceToInteger(get(source, "length")))
+    checkArrayLength(length)
+    return { length, source }
   }
   throw invalidData(`Array.from expects an iterable or array-like value, received ${describeValue(source)}.`)
 }
@@ -122,13 +124,7 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
   ])
 
   const self = (thisValue: Value, name: string) => receiver(Arr, thisValue, `Array.prototype.${name}`)
-  const optNumber = (name: string, value: Value, label: string): number | undefined => {
-    if (value === undefined) return undefined
-    if (typeof value !== "number") {
-      throw typeError(`Array.${name} expects ${label} to be a number.`)
-    }
-    return value
-  }
+  const optNumber = (value: Value): number | undefined => (value === undefined ? undefined : coerceToInteger(value))
   // Callback methods fix the iteration length while reading existing elements live.
   const iterate = (
     name: string,
@@ -153,13 +149,9 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       "join",
       1,
       (thisValue, args) => {
-        const target = self(thisValue, "join").items
-        if (args.length > 1 || (args.length === 1 && typeof args[0] !== "string")) {
-          throw typeError("Array.join expects zero arguments or one string separator.")
-        }
-        const joined = target
-          .map((item) => coerceToString(item ?? ""))
-          .join(args.length === 0 ? "," : (args[0] as string))
+        const joined = self(thisValue, "join")
+          .items.map((item) => coerceToString(item ?? ""))
+          .join(args[0] === undefined ? "," : coerceToString(args[0]))
         checkStringLength(joined.length)
         return joined
       },
@@ -176,40 +168,23 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       "includes",
       1,
       (thisValue, args) => {
-        const target = self(thisValue, "includes").items
-        if (args.length === 0 || args.length > 2) {
-          throw typeError("Array.includes expects a value and optional start index.")
-        }
-        return target.includes(args[0], optNumber("includes", args[1], "start index"))
+        return self(thisValue, "includes").items.includes(args[0], optNumber(args[1]))
       },
     ],
-    [
-      "indexOf",
-      1,
-      (thisValue, args) =>
-        self(thisValue, "indexOf").items.indexOf(args[0], optNumber("indexOf", args[1], "start index")),
-    ],
+    ["indexOf", 1, (thisValue, args) => self(thisValue, "indexOf").items.indexOf(args[0], optNumber(args[1]))],
     [
       "lastIndexOf",
       1,
       (thisValue, args) => {
         const target = self(thisValue, "lastIndexOf").items
-        return args[1] === undefined
-          ? target.lastIndexOf(args[0])
-          : target.lastIndexOf(args[0], optNumber("lastIndexOf", args[1], "start index"))
+        return args[1] === undefined ? target.lastIndexOf(args[0]) : target.lastIndexOf(args[0], optNumber(args[1]))
       },
     ],
-    ["at", 1, (thisValue, args) => self(thisValue, "at").items.at(optNumber("at", args[0], "index") ?? 0)],
+    ["at", 1, (thisValue, args) => self(thisValue, "at").items.at(optNumber(args[0]) ?? 0)],
     [
       "slice",
       2,
-      (thisValue, args) =>
-        wrap(
-          self(thisValue, "slice").items.slice(
-            optNumber("slice", args[0], "start"),
-            optNumber("slice", args[1], "end"),
-          ),
-        ),
+      (thisValue, args) => wrap(self(thisValue, "slice").items.slice(optNumber(args[0]), optNumber(args[1]))),
     ],
     [
       "concat",
@@ -228,7 +203,7 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       (thisValue, args) => {
         const flatten = (items: Array<Value>, depth: number): Array<Value> =>
           items.flatMap((item) => (item instanceof Arr && depth > 0 ? flatten(item.items, depth - 1) : [item]))
-        const flattened = flatten(self(thisValue, "flat").items, optNumber("flat", args[0], "depth") ?? 1)
+        const flattened = flatten(self(thisValue, "flat").items, optNumber(args[0]) ?? 1)
         checkArrayLength(flattened.length)
         return wrap(flattened)
       },
@@ -274,7 +249,7 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       2,
       (thisValue, args) => {
         const target = self(thisValue, "with").items
-        const index = optNumber("with", args[0], "index") ?? 0
+        const index = optNumber(args[0]) ?? 0
         const resolved = index < 0 ? target.length + index : index
         if (resolved < 0 || resolved >= target.length) throw rangeError("Array.with index is out of range.")
         const copied = [...target]
@@ -309,9 +284,9 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       (thisValue, args) => {
         const target = self(thisValue, "splice")
         if (args.length === 0) return wrap(target.items.splice(0, 0))
-        const start = optNumber("splice", args[0], "start") ?? 0
+        const start = optNumber(args[0]) ?? 0
         if (args.length === 1) return wrap(target.items.splice(start))
-        const deleteCount = optNumber("splice", args[1], "delete count") ?? 0
+        const deleteCount = optNumber(args[1]) ?? 0
         const inserted = args.slice(2)
         for (const item of inserted) rejectCircularInsertion(target, item, "Array.splice result")
         return wrap(target.items.splice(start, deleteCount, ...inserted))
@@ -323,9 +298,9 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       (thisValue, args) => {
         const copied = [...self(thisValue, "toSpliced").items]
         if (args.length === 0) return wrap(copied)
-        const start = optNumber("toSpliced", args[0], "start") ?? 0
+        const start = optNumber(args[0]) ?? 0
         if (args.length === 1) copied.splice(start)
-        else copied.splice(start, optNumber("toSpliced", args[1], "delete count") ?? 0, ...args.slice(2))
+        else copied.splice(start, optNumber(args[1]) ?? 0, ...args.slice(2))
         return wrap(copied)
       },
     ],
@@ -335,7 +310,7 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       (thisValue, args) => {
         const target = self(thisValue, "fill")
         rejectCircularInsertion(target, args[0], "Array.fill result")
-        target.items.fill(args[0], optNumber("fill", args[1], "start"), optNumber("fill", args[2], "end"))
+        target.items.fill(args[0], optNumber(args[1]), optNumber(args[2]))
         return target
       },
     ],
@@ -344,11 +319,7 @@ export const arrayGlobal = <R>(ctx: Interpreter<R>) => {
       2,
       (thisValue, args) => {
         const target = self(thisValue, "copyWithin")
-        target.items.copyWithin(
-          optNumber("copyWithin", args[0], "target index") ?? 0,
-          optNumber("copyWithin", args[1], "start") ?? 0,
-          optNumber("copyWithin", args[2], "end"),
-        )
+        target.items.copyWithin(optNumber(args[0]) ?? 0, optNumber(args[1]) ?? 0, optNumber(args[2]))
         return target
       },
     ],
diff --git a/packages/codemode/src/stdlib/bytes.ts b/packages/codemode/src/stdlib/bytes.ts
index a9d60ec9f53..255fa625ba4 100644
--- a/packages/codemode/src/stdlib/bytes.ts
+++ b/packages/codemode/src/stdlib/bytes.ts
@@ -11,6 +11,7 @@ import {
   Bytes,
   hostIterator,
   Obj,
+  coerceToInteger,
   coerceToNumber,
   coerceToString,
   type Value,
@@ -75,44 +76,28 @@ export const uint8ArrayGlobal = <R>(ctx: Interpreter<R>) => {
   ])
 
   const self = (thisValue: Value, name: string) => receiver(Bytes, thisValue, `Uint8Array.prototype.${name}`)
-  const optNumber = (name: string, value: Value, label: string): number | undefined => {
-    if (value === undefined) return undefined
-    if (typeof value !== "number") throw typeError(`Uint8Array.${name} expects ${label} to be a number.`)
-    return value
-  }
+  const optNumber = (value: Value): number | undefined => (value === undefined ? undefined : coerceToInteger(value))
   const wrapAll = (items: Array<Value>) => new Arr(builtins.Array, items)
   defineAccessor(proto, "length", (thisValue) => self(thisValue, "length").bytes.length)
   methods(builtins, proto, [
-    ["at", 1, (thisValue, args) => self(thisValue, "at").bytes.at(optNumber("at", args[0], "index") ?? 0)],
+    ["at", 1, (thisValue, args) => self(thisValue, "at").bytes.at(optNumber(args[0]) ?? 0)],
     [
       "slice",
       2,
-      (thisValue, args) =>
-        wrap(
-          self(thisValue, "slice").bytes.slice(
-            optNumber("slice", args[0], "start"),
-            optNumber("slice", args[1], "end"),
-          ),
-        ),
+      (thisValue, args) => wrap(self(thisValue, "slice").bytes.slice(optNumber(args[0]), optNumber(args[1]))),
     ],
     // A view on the same bytes, as in JS: writes through one are visible through the other.
     [
       "subarray",
       2,
-      (thisValue, args) =>
-        wrap(
-          self(thisValue, "subarray").bytes.subarray(
-            optNumber("subarray", args[0], "start"),
-            optNumber("subarray", args[1], "end"),
-          ),
-        ),
+      (thisValue, args) => wrap(self(thisValue, "subarray").bytes.subarray(optNumber(args[0]), optNumber(args[1]))),
     ],
     [
       "set",
       1,
       (thisValue, args) => {
         const target = self(thisValue, "set")
-        const offset = optNumber("set", args[1], "offset") ?? 0
+        const offset = optNumber(args[1]) ?? 0
         return Effect.map(collectBytes(ctx, args[0], "Uint8Array.set"), (source) => {
           if (!Number.isInteger(offset) || offset < 0 || source.length + offset > target.bytes.length) {
             throw rangeError("Uint8Array.set: the source does not fit at that offset.")
@@ -127,11 +112,7 @@ export const uint8ArrayGlobal = <R>(ctx: Interpreter<R>) => {
       1,
       (thisValue, args) => {
         const target = self(thisValue, "fill")
-        target.bytes.fill(
-          coerceToNumber(args[0]),
-          optNumber("fill", args[1], "start"),
-          optNumber("fill", args[2], "end"),
-        )
+        target.bytes.fill(coerceToNumber(args[0]), optNumber(args[1]), optNumber(args[2]))
         return target
       },
     ],
@@ -147,8 +128,7 @@ export const uint8ArrayGlobal = <R>(ctx: Interpreter<R>) => {
     [
       "indexOf",
       1,
-      (thisValue, args) =>
-        self(thisValue, "indexOf").bytes.indexOf(coerceToNumber(args[0]), optNumber("indexOf", args[1], "start index")),
+      (thisValue, args) => self(thisValue, "indexOf").bytes.indexOf(coerceToNumber(args[0]), optNumber(args[1])),
     ],
     [
       "lastIndexOf",
@@ -157,26 +137,19 @@ export const uint8ArrayGlobal = <R>(ctx: Interpreter<R>) => {
         const target = self(thisValue, "lastIndexOf").bytes
         return args[1] === undefined
           ? target.lastIndexOf(coerceToNumber(args[0]))
-          : target.lastIndexOf(coerceToNumber(args[0]), optNumber("lastIndexOf", args[1], "start index"))
+          : target.lastIndexOf(coerceToNumber(args[0]), optNumber(args[1]))
       },
     ],
     [
       "includes",
       1,
-      (thisValue, args) =>
-        self(thisValue, "includes").bytes.includes(
-          coerceToNumber(args[0]),
-          optNumber("includes", args[1], "start index"),
-        ),
+      (thisValue, args) => self(thisValue, "includes").bytes.includes(coerceToNumber(args[0]), optNumber(args[1])),
     ],
     [
       "join",
       1,
       (thisValue, args) => {
-        if (args.length > 1 || (args.length === 1 && typeof args[0] !== "string")) {
-          throw typeError("Uint8Array.join expects zero arguments or one string separator.")
-        }
-        const joined = self(thisValue, "join").bytes.join(args.length === 0 ? "," : (args[0] as string))
+        const joined = self(thisValue, "join").bytes.join(args[0] === undefined ? "," : coerceToString(args[0]))
         checkStringLength(joined.length)
         return joined
       },
diff --git a/packages/codemode/src/stdlib/json.ts b/packages/codemode/src/stdlib/json.ts
index 8a87463e513..335e2514f98 100644
--- a/packages/codemode/src/stdlib/json.ts
+++ b/packages/codemode/src/stdlib/json.ts
@@ -3,10 +3,10 @@ import { methods } from "../interpreter/native.js"
 import { applyCollectionCallback } from "../interpreter/callback.js"
 import type { Interpreter } from "../interpreter/interpreter.js"
 import { checkStringLength } from "../interpreter/limits.js"
-import { syntaxError, typeError } from "../interpreter/model.js"
+import { syntaxError } from "../interpreter/model.js"
 import { typeofValue } from "../interpreter/references.js"
 import { fromJson, toJson } from "../data.js"
-import { get, keys, Arr, Obj, record, remove, set, type Value } from "../interpreter/objects.js"
+import { get, keys, Arr, Obj, coerceToString, record, remove, set, type Value } from "../interpreter/objects.js"
 
 export const jsonGlobal = <R>(ctx: Interpreter<R>) => {
   const json = new Obj(ctx.builtins.Object)
@@ -18,8 +18,7 @@ export const jsonGlobal = <R>(ctx: Interpreter<R>) => {
 }
 
 const parse = <R>(ctx: Interpreter<R>, args: Array<Value>): Effect.Effect<Value, unknown, R> => {
-  const text = args[0]
-  if (typeof text !== "string") throw typeError("JSON.parse expects a string.")
+  const text = coerceToString(args[0])
 
   const parsed = (() => {
     try {
diff --git a/packages/codemode/src/stdlib/math.ts b/packages/codemode/src/stdlib/math.ts
index 23f35d50a52..8c1d15de01e 100644
--- a/packages/codemode/src/stdlib/math.ts
+++ b/packages/codemode/src/stdlib/math.ts
@@ -1,7 +1,7 @@
 import { Effect } from "effect"
 import { constants, type Method, methods } from "../interpreter/native.js"
 import { typeError } from "../interpreter/model.js"
-import { Obj, type Value } from "../interpreter/objects.js"
+import { Obj, coerceToNumber } from "../interpreter/objects.js"
 import { preserveConsumerError } from "../interpreter/callback.js"
 import type { Interpreter } from "../interpreter/interpreter.js"
 
@@ -14,31 +14,18 @@ declare global {
 
 // Validate only the arguments a method consumes; like JS, extras are ignored
 // (so built-ins work as callbacks receiving (element, index, array)).
-const number = (name: string, args: Array<Value>, index: number): number => {
-  if (index >= args.length) return Number.NaN
-  const arg = args[index]
-  if (typeof arg !== "number") throw typeError(`Math.${name} expects number arguments.`)
-  return arg
-}
-
-const unary = (name: string, op: (a: number) => number): Method => [name, 1, (_, args) => op(number(name, args, 0))]
+const unary = (name: string, op: (a: number) => number): Method => [name, 1, (_, args) => op(coerceToNumber(args[0]))]
 
 const binary = (name: string, op: (a: number, b: number) => number): Method => [
   name,
   2,
-  (_, args) => op(number(name, args, 0), number(name, args, 1)),
+  (_, args) => op(coerceToNumber(args[0]), coerceToNumber(args[1])),
 ]
 
 const variadic = (name: string, op: (...values: Array<number>) => number): Method => [
   name,
   2,
-  (_, args) =>
-    op(
-      ...args.map((arg) => {
-        if (typeof arg !== "number") throw typeError(`Math.${name} expects number arguments.`)
-        return arg
-      }),
-    ),
+  (_, args) => op(...args.map(coerceToNumber)),
 ]
 
 export const mathGlobal = <R>(ctx: Interpreter<R>) => {
diff --git a/packages/codemode/src/stdlib/number.ts b/packages/codemode/src/stdlib/number.ts
index 369e66755de..4cb90ce151d 100644
--- a/packages/codemode/src/stdlib/number.ts
+++ b/packages/codemode/src/stdlib/number.ts
@@ -1,5 +1,5 @@
 import { constructor, constants, methods } from "../interpreter/native.js"
-import { coerceToString, type Value } from "../interpreter/objects.js"
+import { coerceToNumber, coerceToString, type Value } from "../interpreter/objects.js"
 import { rangeError, typeError } from "../interpreter/model.js"
 import type { Interpreter } from "../interpreter/interpreter.js"
 import { coercion } from "./value.js"
@@ -30,11 +30,7 @@ export const numberGlobal = <R>(ctx: Interpreter<R>) => {
       "parseInt",
       2,
       (_, args) => {
-        const radix = args[1]
-        if (radix !== undefined && typeof radix !== "number") {
-          throw typeError("Number.parseInt expects a numeric radix.")
-        }
-        return parseInt(coerceToString(args[0]), radix)
+        return parseInt(coerceToString(args[0]), coerceToNumber(args[1]))
       },
     ],
     ["parseFloat", 1, (_, args) => parseFloat(coerceToString(args[0]))],
@@ -44,25 +40,17 @@ export const numberGlobal = <R>(ctx: Interpreter<R>) => {
     if (typeof thisValue === "number") return thisValue
     throw typeError(`Number.prototype.${name} requires that 'this' be a Number.`)
   }
-  const optNum = (name: string, arg: Value): number | undefined => {
-    if (arg === undefined) return undefined
-    if (typeof arg !== "number") throw typeError(`Number.${name} expects a number argument.`)
-    return arg
-  }
+  const optNum = (arg: Value): number | undefined => (arg === undefined ? undefined : coerceToNumber(arg))
   methods(builtins, builtins.Number, [
-    ["toFixed", 1, (thisValue, args) => self(thisValue, "toFixed").toFixed(optNum("toFixed", args[0]))],
+    ["toFixed", 1, (thisValue, args) => self(thisValue, "toFixed").toFixed(optNum(args[0]))],
     ["toLocaleString", 0, (thisValue) => self(thisValue, "toLocaleString").toLocaleString("en-US")],
-    [
-      "toExponential",
-      1,
-      (thisValue, args) => self(thisValue, "toExponential").toExponential(optNum("toExponential", args[0])),
-    ],
+    ["toExponential", 1, (thisValue, args) => self(thisValue, "toExponential").toExponential(optNum(args[0]))],
     [
       "toPrecision",
       1,
       (thisValue, args) => {
         const value = self(thisValue, "toPrecision")
-        const digits = optNum("toPrecision", args[0])
+        const digits = optNum(args[0])
         return digits === undefined ? value.toString() : value.toPrecision(digits)
       },
     ],
@@ -71,7 +59,7 @@ export const numberGlobal = <R>(ctx: Interpreter<R>) => {
       1,
       (thisValue, args) => {
         const value = self(thisValue, "toString")
-        const radix = optNum("toString", args[0])
+        const radix = optNum(args[0])
         if (radix !== undefined && (radix < 2 || radix > 36)) {
           throw rangeError("Number.toString radix must be between 2 and 36.")
         }
diff --git a/packages/codemode/src/stdlib/string.ts b/packages/codemode/src/stdlib/string.ts
index 84f69da4e12..0e27e920e1f 100644
--- a/packages/codemode/src/stdlib/string.ts
+++ b/packages/codemode/src/stdlib/string.ts
@@ -93,15 +93,7 @@ export const stringGlobal = <R>(ctx: Interpreter<R>) => {
   const codeUnits = (name: string, op: (...codes: Array<number>) => string): Method => [
     name,
     1,
-    (_, args) =>
-      op(
-        ...args.map((arg) => {
-          if (typeof arg !== "number") {
-            throw typeError(`String.${name} expects number arguments.`)
-          }
-          return arg
-        }),
-      ),
+    (_, args) => op(...args.map(coerceToNumber)),
   ]
   methods(builtins, string, [
     codeUnits("fromCharCode", String.fromCharCode),
diff --git a/packages/codemode/src/stdlib/value.ts b/packages/codemode/src/stdlib/value.ts
index 4308ad791da..0669f0b3350 100644
--- a/packages/codemode/src/stdlib/value.ts
+++ b/packages/codemode/src/stdlib/value.ts
@@ -1,5 +1,4 @@
 import { fn } from "../interpreter/native.js"
-import { typeError } from "../interpreter/model.js"
 import { coerceToNumber, coerceToString, type Native, type Value } from "../interpreter/objects.js"
 import type { Interpreter } from "../interpreter/interpreter.js"
 
@@ -20,11 +19,7 @@ const coerce = <R>(ctx: Interpreter<R>, name: Coercion, args: Array<Value>): Val
   if (name === "isFinite") return Number.isFinite(coerceToNumber(raw))
   if (name === "isNaN") return Number.isNaN(coerceToNumber(raw))
   if (name === "parseInt") {
-    const radix = args[1]
-    if (radix !== undefined && typeof radix !== "number") {
-      throw typeError("parseInt expects a numeric radix.")
-    }
-    return parseInt(coerceToString(raw), radix)
+    return parseInt(coerceToString(raw), coerceToNumber(args[1]))
   }
   if (name === "parseFloat") return parseFloat(coerceToString(raw))
   return coerceToString(raw)
diff --git a/packages/codemode/test/callbacks.test.ts b/packages/codemode/test/callbacks.test.ts
index 60bb8dacef1..9722ae19a1a 100644
--- a/packages/codemode/test/callbacks.test.ts
+++ b/packages/codemode/test/callbacks.test.ts
@@ -271,10 +271,8 @@ describe("still-rejected callables get the wrap hint", () => {
   test("built-in references work as replacers", async () => {
     // Like real JS: JSON.stringify(match, offset, string) quotes the match.
     expect(await value(`return "abc".replace(/b/, JSON.stringify)`)).toBe('a"b"c')
-    // Math methods stay strict about consumed arguments: a match string is not coerced.
-    expect((await error(`return "3.7".replace(/\\d\\.\\d/, Math.floor)`)).message).toContain(
-      "Math.floor expects number arguments",
-    )
+    // Math methods coerce the match string, as in JS.
+    expect(await value(`return "3.7".replace(/\\d\\.\\d/, Math.floor)`)).toBe("3")
   })
 
   test("non-callables still get the plain callback error", async () => {
diff --git a/packages/codemode/test/parity.test.ts b/packages/codemode/test/parity.test.ts
index 4b467dee7b0..cd8eff62ce4 100644
--- a/packages/codemode/test/parity.test.ts
+++ b/packages/codemode/test/parity.test.ts
@@ -839,6 +839,36 @@ describe("coercion parity: global isFinite and isNaN", () => {
   })
 })
 
+describe("coercion parity: built-in arguments coerce as in JS", () => {
+  test("numeric arguments apply ToIntegerOrInfinity", async () => {
+    expect(
+      await value(`
+        return [
+          [1, 2, 3].indexOf(2, "1"), [1, 2, 3].lastIndexOf(3, "5"), [1, 2, 3].includes(1, "1"),
+          [1, 2, 3, 4].slice("1", "3"), [1, 2, 3].at(null), [1, 2, 3].at(1.7),
+          [1, [2, [3]]].flat(1.9), [1, 2, 3].with(1.5, 9), [1, 2, 3, 4].splice("1", "2"),
+          Math.max("3", "2"), Math.floor(null), Math.hypot("3", "4"),
+          parseInt("11", "2"), Number.parseInt("ff", "16"), (1.5).toFixed("2"), (255).toString("16"),
+          String.fromCharCode("65", 66.9), new Uint8Array([1, 2, 3]).indexOf(2, "1"),
+        ]
+      `),
+    ).toEqual([1, 2, false, [2, 3], 1, 2, [1, 2, [3]], [1, 9, 3], [2, 3], 3, 0, 5, 3, 255, "1.50", "ff", "AB", 1])
+  })
+
+  test("join separators, JSON.parse text, and Array.from length coerce", async () => {
+    expect(
+      await value(`
+        return [
+          [1, 2].join(null), [1, 2].join(0), [1, 2].join(undefined), new Uint8Array([1, 2]).join(null),
+          JSON.parse(123), JSON.parse(true),
+          Array.from({ length: "2" }), Array.from({ length: 2.5 }), Array.from({ length: -1 }), Array.from({}),
+        ]
+      `),
+    ).toEqual(["1null2", "102", "1,2", "1null2", 123, true, [null, null], [null, null], [], []])
+    expect((await error(`return JSON.parse(undefined)`)).message).toContain("JSON")
+  })
+})
+
 describe("coercion parity: arrays coerce to numbers through their string form", () => {
   test("arrays with objects become NaN instead of crashing on host ToPrimitive", async () => {
     expect(await value(`let x = [{}]; x++; return Number.isNaN(x)`)).toBe(true)
diff --git a/packages/codemode/test/test262/skipped.txt b/packages/codemode/test/test262/skipped.txt
index 1d2f633c9df..8da1051ea2f 100644
--- a/packages/codemode/test/test262/skipped.txt
+++ b/packages/codemode/test/test262/skipped.txt
@@ -1,15 +1,11 @@
 built-ins/Array/prototype/at/index-argument-tointeger.js  # Array.at expects index to be a number.
-built-ins/Array/prototype/at/index-non-numeric-argument-tointeger.js  # Array.at expects index to be a number.
 built-ins/Array/prototype/concat/S15.4.4.4_A2_T1.js  # Array.prototype.concat called on incompatible receiver a data object.
 built-ins/Array/prototype/concat/S15.4.4.4_A2_T2.js  # Array.prototype.concat called on incompatible receiver a data object.
 built-ins/Array/prototype/concat/S15.4.4.4_A3_T1.js  # arr.hasOwnProperty("1") must return true Expected SameValue(«false», «true») to be true
 built-ins/Array/prototype/concat/S15.4.4.4_A3_T2.js  # b.hasOwnProperty("2") must return true Expected SameValue(«false», «true») to be true
 built-ins/Array/prototype/concat/S15.4.4.4_A3_T3.js  # b.hasOwnProperty("2") must return true Expected SameValue(«false», «true») to be true
 built-ins/Array/prototype/concat/create-ctor-non-object.js  # a.concat() throws a TypeError exception Expected a TypeError to be thrown but no exception was throw
-built-ins/Array/prototype/copyWithin/coerced-values-end.js  # Array.copyWithin expects end to be a number.
 built-ins/Array/prototype/copyWithin/coerced-values-start-change-target.js  # Array.copyWithin expects start to be a number.
-built-ins/Array/prototype/copyWithin/coerced-values-start.js  # Array.copyWithin expects start to be a number.
-built-ins/Array/prototype/copyWithin/coerced-values-target.js  # Array.copyWithin expects target index to be a number.
 built-ins/Array/prototype/copyWithin/return-abrupt-from-end.js  # Expected a Test262Error but got a TypeError
 built-ins/Array/prototype/copyWithin/return-abrupt-from-start.js  # Expected a Test262Error but got a TypeError
 built-ins/Array/prototype/copyWithin/return-abrupt-from-target.js  # Expected a Test262Error but got a TypeError
@@ -23,7 +19,6 @@ built-ins/Array/prototype/every/15.4.4.16-8-5.js  # … cannot be constructed: u
 built-ins/Array/prototype/every/15.4.4.16-8-6.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
 built-ins/Array/prototype/every/15.4.4.16-8-7.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
 built-ins/Array/prototype/every/15.4.4.16-8-8.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
-built-ins/Array/prototype/fill/coerced-indexes.js  # Array.fill expects start to be a number.
 built-ins/Array/prototype/fill/return-abrupt-from-end.js  # Expected a Test262Error but got a TypeError
 built-ins/Array/prototype/fill/return-abrupt-from-start.js  # Expected a Test262Error but got a TypeError
 built-ins/Array/prototype/filter/15.4.4.20-10-3.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
@@ -37,7 +32,6 @@ built-ins/Array/prototype/filter/15.4.4.20-6-8.js  # … cannot be constructed:
 built-ins/Array/prototype/filter/15.4.4.20-9-6.js  # resArr.length Expected SameValue(«3», «4») to be true
 built-ins/Array/prototype/filter/15.4.4.20-9-c-i-8.js  # newArr.length Expected SameValue(«0», «1») to be true
 built-ins/Array/prototype/filter/create-ctor-non-object.js  # null value Expected a TypeError to be thrown but no exception was thrown at all
-built-ins/Array/prototype/flat/non-numeric-depth-should-not-throw.js  # Array.flat expects depth to be a number.
 built-ins/Array/prototype/flat/non-object-ctor-throws.js  # null value Expected a TypeError to be thrown but no exception was thrown at all
 built-ins/Array/prototype/forEach/15.4.4.18-7-5.js  # callCnt Expected SameValue(«4», «5») to be true
 built-ins/Array/prototype/forEach/15.4.4.18-7-c-i-8.js  # testResult !== true
@@ -50,57 +44,30 @@ built-ins/Array/prototype/forEach/15.4.4.18-8-6.js  # … cannot be constructed:
 built-ins/Array/prototype/forEach/15.4.4.18-8-7.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
 built-ins/Array/prototype/forEach/15.4.4.18-8-8.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
 built-ins/Array/prototype/forEach/15.4.4.18-8-9.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
-built-ins/Array/prototype/includes/length-zero-returns-false.js  # Array.includes expects a value and optional start index.
-built-ins/Array/prototype/includes/no-arg.js  # Array.includes expects a value and optional start index.
 built-ins/Array/prototype/includes/return-abrupt-tointeger-fromindex.js  # Expected a Test262Error but got a TypeError
 built-ins/Array/prototype/includes/tointeger-fromindex.js  # Array.includes expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-1.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-15.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-16.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-17.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-18.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-19.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-20.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-21.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-22.js  # Array.indexOf expects start index to be a number.
 built-ins/Array/prototype/indexOf/15.4.4.14-5-23.js  # Array.indexOf expects start index to be a number.
 built-ins/Array/prototype/indexOf/15.4.4.14-5-24.js  # toStringAccessed
 built-ins/Array/prototype/indexOf/15.4.4.14-5-25.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
-built-ins/Array/prototype/indexOf/15.4.4.14-5-3.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/indexOf/15.4.4.14-5-5.js  # Array.indexOf expects start index to be a number.
 built-ins/Array/prototype/indexOf/15.4.4.14-9-7.js  # Array assignment result contains a circular value.
 built-ins/Array/prototype/indexOf/15.4.4.14-9-a-3.js  # Array.indexOf expects start index to be a number.
 built-ins/Array/prototype/indexOf/15.4.4.14-9-a-6.js  # Array.indexOf expects start index to be a number.
 built-ins/Array/prototype/indexOf/15.4.4.14-9-b-i-7.js  # [, , , ].indexOf(true) Expected SameValue(«-1», «0») to be true
-built-ins/Array/prototype/indexOf/length-zero-returns-minus-one.js  # Array.indexOf expects start index to be a number.
-built-ins/Array/prototype/join/S15.4.4.5_A1.2_T2.js  # Array.join expects zero arguments or one string separator.
 built-ins/Array/prototype/join/S15.4.4.5_A2_T1.js  # Array.prototype.join called on incompatible receiver a data object.
 built-ins/Array/prototype/join/S15.4.4.5_A2_T4.js  # Array.prototype.join called on incompatible receiver a data object.
-built-ins/Array/prototype/join/S15.4.4.5_A3.1_T1.js  # Array.join expects zero arguments or one string separator.
 built-ins/Array/prototype/join/S15.4.4.5_A3.1_T2.js  # Array.join expects zero arguments or one string separator.
 built-ins/Array/prototype/join/S15.4.4.5_A3.2_T2.js  # x.join() must return "*" Expected SameValue(«"[object Object]"», «"*"») to be true
 built-ins/Array/prototype/join/S15.4.4.5_A4_T3.js  # Array.prototype.join called on incompatible receiver a data object.
 built-ins/Array/prototype/join/S15.4.4.5_A5_T1.js  # #1: Array.prototype[1] = 1; x = [0]; x.length = 2; x.join() === "0,1". Actual: 0,
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-1.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-15.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-16.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-17.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-18.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-19.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-20.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-21.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-22.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-23.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-24.js  # toStringAccessed
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-25.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-3.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-4.js  # a.lastIndexOf(2,undefined) Expected SameValue(«1», «-1») to be true
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-5-5.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-8-7.js  # Array assignment result contains a circular value.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-8-a-3.js  # Array.lastIndexOf expects start index to be a number.
-built-ins/Array/prototype/lastIndexOf/15.4.4.15-8-a-6.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/lastIndexOf/15.4.4.15-8-b-i-7.js  # [, , , ].lastIndexOf(true) Expected SameValue(«-1», «0») to be true
-built-ins/Array/prototype/lastIndexOf/length-zero-returns-minus-one.js  # Array.lastIndexOf expects start index to be a number.
 built-ins/Array/prototype/map/15.4.4.19-8-6.js  # resArr[4] Expected SameValue(«undefined», «1») to be true
 built-ins/Array/prototype/map/15.4.4.19-8-c-i-8.js  # newArr[1] Expected SameValue(«13», «true») to be true
 built-ins/Array/prototype/map/15.4.4.19-9-3.js  # … cannot be constructed: user-defined constructors and classes are not supported. Call it as a funct
@@ -175,7 +142,6 @@ built-ins/Array/prototype/shift/S15.4.4.9_A2_T5.js  # Array.prototype.shift call
 built-ins/Array/prototype/shift/S15.4.4.9_A3_T3.js  # Array.prototype.shift called on incompatible receiver a data object.
 built-ins/Array/prototype/shift/S15.4.4.9_A4_T1.js  # #2: Array.prototype[1] = 1; x = [0]; x.length = 2; x.shift(); x[0] === 1. Actual: undefined
 built-ins/Array/prototype/shift/S15.4.4.9_A4_T2.js  # Array.prototype.shift called on incompatible receiver a data object.
-built-ins/Array/prototype/slice/S15.4.4.10_A2.1_T5.js  # Array.slice expects start to be a number.
 built-ins/Array/prototype/slice/S15.4.4.10_A2.2_T5.js  # Array.slice expects end to be a number.
 built-ins/Array/prototype/slice/S15.4.4.10_A2_T1.js  # Array.prototype.slice called on incompatible receiver a data object.
 built-ins/Array/prototype/slice/S15.4.4.10_A2_T2.js  # Array.prototype.slice called on incompatible receiver a data object.
@@ -206,7 +172,6 @@ built-ins/Array/prototype/sort/S15.4.4.11_A4_T3.js  # Array.prototype.sort calle
 built-ins/Array/prototype/sort/S15.4.4.11_A6_T2.js  # Array.prototype.sort called on incompatible receiver a data object.
 built-ins/Array/prototype/sort/bug_596_1.js  # #1: [object, object].sort(); counter < 2. Actual: 0
 built-ins/Array/prototype/sort/precise-prototype-element.js  # Expected SameValue(«undefined», «4») to be true
-built-ins/Array/prototype/splice/S15.4.4.12_A2.1_T5.js  # Array.splice expects start to be a number.
 built-ins/Array/prototype/splice/S15.4.4.12_A2.2_T5.js  # Array.splice expects delete count to be a number.
 built-ins/Array/prototype/splice/S15.4.4.12_A2_T1.js  # Array.prototype.splice called on incompatible receiver a data object.
 built-ins/Array/prototype/splice/S15.4.4.12_A2_T2.js  # Array.prototype.splice called on incompatible receiver a data object.