diff --git a/packages/compiler/src/frontend/lowering/class-method-values.ts b/packages/compiler/src/frontend/lowering/class-method-values.ts
index 6e8256513..ab9101cf7 100644
--- a/packages/compiler/src/frontend/lowering/class-method-values.ts
+++ b/packages/compiler/src/frontend/lowering/class-method-values.ts
@@ -378,8 +378,33 @@ function reflectedMethodValue(
     !isJsSourceFile(ts.getSourceFileOfNode(declaration)) ||
     info.localClass !== undefined ||
     lowerer.errorHierarchyClassOf(info.def.name);
+  // `super.m(...)` in a prototype method reads the base class's prototype,
+  // which lowerSuperMethodCall answers over the call-site receiver when the
+  // base has a prototype object. Other uses of `super` keep the native
+  // receiver.
+  const superCalls = !staticMethod && info.base !== null && prototypeChainViewable(info.base);
+  // Over a dynamic receiver, private fields are brand-checked and read, and private methods
+  // brand-checked and called; a private accessor, or a private method read as a value, would become a
+  // lookup of its name.
+  const privateAccessors = new Set<string>();
+  const privateMethods = new Set<string>();
+  for (const other of declaration.members.nodes)
+    if (other.name !== undefined && ts.isPrivateIdentifier(other.name)) {
+      if (ts.isAccessor(other)) privateAccessors.add(other.name.text);
+      else if (ts.isMethodDeclaration(other)) privateMethods.add(other.name.text);
+    }
   ts.walkPreorder(member, (node) => {
-    if (ts.isPrivateIdentifier(node) || node.kind === ts.SyntaxKind.SuperKeyword)
+    if (node.kind === ts.SyntaxKind.SuperKeyword && !(superCalls && isSuperMethodCallee(node)))
+      needsNativeReceiver = true;
+    if (!ts.isPrivateIdentifier(node)) return;
+    // Private members on a static method's receiver (the class value) keep
+    // the native receiver; prototype methods brand-check theirs, except
+    // private accessors and private methods used other than by a call.
+    if (
+      staticMethod ||
+      privateAccessors.has(node.text) ||
+      (privateMethods.has(node.text) && !isPrivateMethodCallee(node))
+    )
       needsNativeReceiver = true;
   });
   if (needsNativeReceiver) return null;
@@ -403,7 +428,11 @@ function reflectedMethodValue(
       loc: locOf(member),
     };
   }
+  // The method may be lowered on top of an unrelated caller's frames (a
+  // `super` call in another class's method reflects this class): its `this`
+  // is its own receiver, never an enclosing function's.
   const context = newFnCtx(false, null, null, DYN);
+  context.thisBoundary = true;
   const previousClass = lowerer.currentClass;
   lowerer.currentClass = info;
   lowerer.fnStack.push(context);
@@ -441,6 +470,55 @@ function reflectedMethodValue(
   }
 }
 
+/** Whether `base` and every class above it have a complete prototype object
+ * (reflectClassPrototype): module-level program classes that neither extend
+ * Date nor a runtime-provided class. */
+function prototypeChainViewable(base: ClassInfo): boolean {
+  for (let c: ClassInfo | null = base; c; c = c.base)
+    if (
+      c.decl === null ||
+      c.localClass !== undefined ||
+      c.generic !== undefined ||
+      c.genericInstance !== undefined ||
+      c.mixinInstance !== undefined ||
+      c.callableBase !== undefined ||
+      c.extendsDate !== undefined ||
+      c.def.runtime ||
+      c.builtinEmitter ||
+      c.builtinStream ||
+      c.builtinError ||
+      c.builtinEventTarget
+    )
+      return false;
+  return true;
+}
+
+/** `super` as the receiver of a called member: `super.m(...)`. */
+/** Whether `node`, a private name, is the method a call calls (`x.#m(...)`). */
+function isPrivateMethodCallee(node: ts.Node): boolean {
+  const access = node.parent;
+  return (
+    access !== undefined &&
+    ts.isPropertyAccessExpression(access) &&
+    access.name === node &&
+    access.parent !== undefined &&
+    ts.isCallExpression(access.parent) &&
+    access.parent.expression === access
+  );
+}
+
+function isSuperMethodCallee(node: ts.Node): boolean {
+  const access = node.parent;
+  return (
+    access !== undefined &&
+    ts.isPropertyAccessExpression(access) &&
+    access.expression === node &&
+    access.parent !== undefined &&
+    ts.isCallExpression(access.parent) &&
+    access.parent.expression === access
+  );
+}
+
 /** Complete adapters after all native class bodies and instantiations exist. */
 export function finalizeClassMethodValues(lowerer: Lowerer, functions: IrFunction[]): void {
   const byName = new Map(functions.map((fn) => [fn.name, fn]));
diff --git a/packages/compiler/src/frontend/lowering/class-prototypes.ts b/packages/compiler/src/frontend/lowering/class-prototypes.ts
index d005e64d1..0c6741c63 100644
--- a/packages/compiler/src/frontend/lowering/class-prototypes.ts
+++ b/packages/compiler/src/frontend/lowering/class-prototypes.ts
@@ -423,7 +423,10 @@ export function reflectClassPrototype(
       loc,
     },
   );
+  // The members lower on top of whatever frames requested the view (a
+  // `super` call in a subclass's method); their `this` is their own receiver.
   const context = newFnCtx(false, null, null, DYN);
+  context.thisBoundary = true;
   lowerer.fnStack.push(context);
   try {
     for (const member of info.decl.members.nodes) {
diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 3e1ce541a..6a7aa2f65 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -1,3 +1,4 @@
+import { checkedPrivateMethodReceiver, privateNameClass } from "./private-brands.js";
 import { assertFunctionAliasOf } from "./lower-assert.js";
 import { keyedHelperName } from "./stable-names.js";
 import { perKeyOrder } from "./key-order-witness.js";
@@ -589,6 +590,17 @@ export function lowerCall(lowerer: Lowerer, expr: ts.CallExpression): IrExpr {
   ) {
     return lowerer.lowerSuperMethodCall(expr, expr.expression);
   }
+  // `value.#m(...)` on a checked-dynamic value (`this` in a JavaScript
+  // class's receiver-independent method): the private method of the
+  // brand-checked receiver.
+  if (
+    ts.isPropertyAccessExpression(expr.expression) &&
+    ts.isPrivateIdentifier(expr.expression.name) &&
+    !expr.expression.questionDotToken
+  ) {
+    const privateCall = lowerCheckedPrivateMethodCall(lowerer, expr, expr.expression);
+    if (privateCall) return privateCall;
+  }
 
   const consoleMember = lowerer.consoleCallMember(expr);
   const consoleMethod = consoleMember === null ? lowerer.consoleMethodMember(expr) : null;
@@ -11430,3 +11442,35 @@ function lowerShimIntrinsicCall(
   );
   return { kind: "libCall", fn: intrinsic.fn, args, type: intrinsic.ret, loc };
 }
+
+/** `value.#m(args)` where `value` lowers to a checked-dynamic value and the
+ * declaration-level class whose body declares `#m` declares it as a method:
+ * the method called on the brand-checked receiver
+ * (checkedPrivateMethodReceiver). Null for any other private member call. */
+function lowerCheckedPrivateMethodCall(
+  lowerer: Lowerer,
+  call: ts.CallExpression,
+  access: ts.PropertyAccessExpression,
+): IrExpr | null {
+  if (!ts.isPrivateIdentifier(access.name)) return null;
+  const name = access.name.text;
+  const owner = privateNameClass(lowerer, access.name);
+  const found = owner ? lowerer.findMethodOn(owner, name) : null;
+  if (!owner || !found || found.declarer !== owner || found.sig.abstract === true) return null;
+  const probed = tryLowerExpression(lowerer, access.expression);
+  if (!probed || !checkedDynamicOperand(probed)) return null;
+  const value = checkedDynamicOperand(lowerer.lowerExpr(access.expression));
+  if (!value) return null;
+  const loc = locOf(call);
+  const callee = `%${owner.def.name}.${name}`;
+  lowerer.noteEdge(callee);
+  const receiver = checkedPrivateMethodReceiver(lowerer, owner, name, value, loc);
+  const args = lowerer.completeArgs(call.arguments.nodes, found.sig.params, loc, call);
+  return { kind: "call", callee, args: [receiver, ...args], type: found.sig.ret, loc };
+}
+
+/** A checked-dynamic value, or the dynamic value under a checked extraction. */
+function checkedDynamicOperand(expr: IrExpr): IrExpr | null {
+  if (expr.type.kind === "dyn") return expr;
+  return expr.kind === "dynCheck" && expr.value.type.kind === "dyn" ? expr.value : null;
+}
diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts
index aff989ce1..47b7df2eb 100644
--- a/packages/compiler/src/frontend/lowering/lower-classes.ts
+++ b/packages/compiler/src/frontend/lowering/lower-classes.ts
@@ -190,7 +190,7 @@ import {
 import { lowerObjectFactoryNew } from "./object-factory-new.js";
 import { tryLowerExpression } from "./expressions/try-lower-expression.js";
 import { lowerInstanceConstructorNew } from "./class-instance-constructor.js";
-import { classPrototypeData } from "./class-prototypes.js";
+import { classPrototypeData, reflectClassPrototype } from "./class-prototypes.js";
 import { constructionEscape } from "./construction-escapes.js";
 import { checkedClassConstruction, checkedClassConstructionPacked } from "./class-construction.js";
 import { initializeRuntimeStatics, hasRuntimeStatics } from "./class-runtime-statics.js";
@@ -9430,8 +9430,16 @@ export function lowerSuperMethodCall(
   if (!thisLocal) lowerer.unsupported("SC1080", access);
   const loc = locOf(call);
   const thisRef: IrExpr = { kind: "varRef", localId: thisLocal.id, type: thisLocal.type, loc };
-  if (lowerer.prototypeMethodAccesses.has(access.name.text)) {
-    const prototype = classPrototypeData(lowerer, cls.base, loc);
+  // A method whose `this` is the call-site receiver (a JavaScript class's
+  // receiver-independent method value, reflectedClassMethodValue) looks the
+  // method up on the base class's complete prototype, as `super` reads
+  // [[HomeObject]].[[Prototype]], and calls it with that receiver.
+  const dynamicThis = thisRef.type.kind === "dyn";
+  if (dynamicThis || lowerer.prototypeMethodAccesses.has(access.name.text)) {
+    const prototype =
+      dynamicThis && cls.base.decl
+        ? reflectClassPrototype(lowerer, cls.base, loc, classValueRef(lowerer, cls.base, access))
+        : classPrototypeData(lowerer, cls.base, loc);
     if (prototype) {
       const callee: IrExpr = {
         kind: "dynKeyGet",
@@ -9440,12 +9448,21 @@ export function lowerSuperMethodCall(
         type: DYN,
         loc,
       };
+      // Spread arguments are spread into the dynamic call, as in a dynamic method call.
+      const spreads: { arg: number; what: string }[] = [];
+      const args = call.arguments.nodes.map((arg, index) => {
+        if (!ts.isSpreadElement(arg)) return lowerer.lowerExprExpecting(arg, DYN);
+        const spelling = ts.getTextOfNode(arg.expression);
+        spreads.push({ arg: index, what: spelling });
+        return checkedIterableSpread(lowerer, lowerer.lowerExpr(arg.expression), spelling, loc);
+      });
       const value: IrExpr = {
         kind: "dynCall",
         callee,
         receiver: lowerer.coerceToExpected(thisRef, DYN),
         calleeName: ts.getTextOfNode(access),
-        args: call.arguments.nodes.map((arg) => lowerer.lowerExprExpecting(arg, DYN)),
+        args,
+        ...(spreads.length > 0 ? { spreads } : {}),
         type: DYN,
         loc,
       };
diff --git a/packages/compiler/src/frontend/lowering/private-brands.ts b/packages/compiler/src/frontend/lowering/private-brands.ts
index c7881bce1..92bee4fdd 100644
--- a/packages/compiler/src/frontend/lowering/private-brands.ts
+++ b/packages/compiler/src/frontend/lowering/private-brands.ts
@@ -17,8 +17,9 @@ import type { Lowerer } from "./lowerer.js";
 import type { ClassInfo } from "./lower-classes.js";
 
 /** How a brand helper uses its receiver: `in` answers `#x in v`, `read` and
- * `write` return the receiver as the declaring class for a field access. */
-type BrandUse = "in" | "read" | "write";
+ * `write` return the receiver as the declaring class for a field access, and
+ * `call` for a private method call. */
+type BrandUse = "in" | "read" | "write" | "call";
 
 const PREFIX = "%private.";
 
@@ -214,6 +215,35 @@ export function checkedPrivateValue(
   return { kind: "call", callee: helper, args: [receiver], type: resultType, loc };
 }
 
+/** `value.#m(...)` on a checked-dynamic `value` in a declaration-level class
+ * `info` that declares the private method `#m`: the receiver as an instance
+ * of `info` when the value is one, else Node's TypeError ("Receiver must be
+ * an instance of class C"). */
+export function checkedPrivateMethodReceiver(
+  lowerer: Lowerer,
+  info: ClassInfo,
+  name: string,
+  receiver: IrExpr,
+  loc: SrcLoc,
+): IrExpr {
+  const className = info.def.name;
+  const resultType: IrType = { kind: "object", className };
+  const helper = `${PREFIX}call:${className}:${name}`;
+  if (!lowerer.valueHelpers.has(helper)) {
+    lowerer.liftedFns.push({
+      name: helper,
+      params: [{ localId: "value.0", name: "value", type: DYN }],
+      returnType: resultType,
+      locals: [{ id: "value.0", name: "value", type: DYN, mutable: false }],
+      // finalizePrivateBrands fills the body.
+      body: [],
+      loc: helperLoc(loc),
+    });
+    lowerer.valueHelpers.set(helper, helper);
+  }
+  return { kind: "call", callee: helper, args: [receiver], type: resultType, loc };
+}
+
 /** A plain assignment, a destructuring target, or a for-in/of target writes
  * the field. Compound assignments and updates read it first, and Node
  * reports the failed read. */
@@ -358,6 +388,22 @@ export function finalizePrivateBrands(lowerer: Lowerer, functions: IrFunction[])
         },
         { kind: "return", value: { kind: "boolLit", value: false, type: BOOL, loc }, loc },
       );
+    } else if (use === "call") {
+      // V8 names the class by its own identifier (not a name inferred from
+      // a binding), "anonymous" without one, also for a nullish receiver.
+      const owner = lowerer.classes.get(className);
+      const identifier = owner?.decl?.name;
+      const shown = identifier !== undefined ? identifier.text : "anonymous";
+      for (const unit of ["undefined", "null"] as const) {
+        body.push({
+          kind: "if",
+          cond: { kind: "dynTest", test: unit, value, type: BOOL, loc },
+          then: [typeError(strLit(`Cannot read properties of ${unit} (reading '${shown}')`, loc))],
+          else_: null,
+          loc,
+        });
+      }
+      body.push(typeError(strLit(`Receiver must be an instance of class ${shown}`, loc)));
     } else {
       const verb = use === "read" ? "reading" : "setting";
       const action = use === "read" ? "Cannot read properties" : "Cannot set properties";
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7621-class-methods-foreign-receivers.mjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7621-class-methods-foreign-receivers.mjs.json
new file mode 100644
index 000000000..8f50b09a3
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7621-class-methods-foreign-receivers.mjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7621-class-methods-foreign-receivers.mjs"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/7621-class-methods-foreign-receivers.mjs b/tests/corpus/7621-class-methods-foreign-receivers.mjs
new file mode 100644
index 000000000..70826a192
--- /dev/null
+++ b/tests/corpus/7621-class-methods-foreign-receivers.mjs
@@ -0,0 +1,218 @@
+// Prototype methods called with a receiver that is not an instance of
+// their class (`C.prototype.m.call(other)`): `#private` members throw
+// Node's TypeErrors for such a receiver, and `super.m()` looks the method up
+// on the base class's prototype and calls it with the receiver.
+function report(label, run) {
+  try {
+    console.log(label, run());
+  } catch (error) {
+    console.log(label, error.constructor.name, error.message);
+  }
+}
+
+class Counter {
+  #count = 0;
+  #step = 1;
+  constructor(name) {
+    this.name = name;
+  }
+  read() {
+    return this.#count;
+  }
+  bump() {
+    this.#count += this.#step;
+    return this.#count;
+  }
+  reset(value) {
+    this.#count = value;
+    return this.name;
+  }
+  #label() {
+    return `${this.name}#${this.#count}`;
+  }
+  label() {
+    return this.#label();
+  }
+  owns(value) {
+    return #count in value;
+  }
+  ownsSelf() {
+    return #count in this;
+  }
+}
+
+const counter = new Counter("c");
+console.log(counter.bump(), counter.bump(), counter.read(), counter.label());
+console.log(counter.owns(counter), counter.owns({}), counter.ownsSelf(), counter.reset(7), counter.read());
+
+const plain = { name: "plain" };
+report("read", () => Counter.prototype.read.call(plain));
+report("bump", () => Counter.prototype.bump.call(plain));
+report("reset", () => Counter.prototype.reset.call(plain, 3));
+report("label", () => Counter.prototype.label.call(plain));
+report("ownsSelf", () => Counter.prototype.ownsSelf.call(plain));
+report("owns primitive", () => Counter.prototype.owns.call(counter, 5));
+report("label undefined", () => Counter.prototype.label.call(undefined));
+report("label null", () => Counter.prototype.label.call(null));
+report("label number", () => Counter.prototype.label.call(4));
+report("read other instance", () => Counter.prototype.read.call(new Counter("d")));
+report("label other instance", () => Counter.prototype.label.call(new Counter("e")));
+
+const Anonymous = class {
+  #m() {
+    return 1;
+  }
+  call() {
+    return this.#m();
+  }
+};
+const Named = class Inner {
+  #m() {
+    return 2;
+  }
+  call() {
+    return this.#m();
+  }
+};
+report("anonymous", () => Anonymous.prototype.call.call({}));
+report("named", () => Named.prototype.call.call({}));
+console.log(new Anonymous().call(), new Named().call());
+
+class Shape {
+  constructor(name) {
+    this.name = name;
+  }
+  describe() {
+    return `shape ${this.name}`;
+  }
+  area() {
+    return 0;
+  }
+}
+class Square extends Shape {
+  describe() {
+    return `square: ${super.describe()} area ${this.area()}`;
+  }
+  area() {
+    return this.size * this.size;
+  }
+  both() {
+    return [super.describe(), super.area()].join("|");
+  }
+}
+class Tiny extends Square {
+  #tag = "t";
+  describe() {
+    return `tiny<${super.describe()}>`;
+  }
+  tag() {
+    return `${this.#tag}:${super.both()}`;
+  }
+}
+
+const square = new Square("s");
+square.size = 3;
+console.log(square.describe(), square.both());
+const tiny = new Tiny("t");
+tiny.size = 1;
+console.log(tiny.describe(), tiny.tag());
+const foreign = {
+  name: "f",
+  size: 2,
+  area() {
+    return 40;
+  },
+};
+report("square describe", () => Square.prototype.describe.call(foreign));
+report("square both", () => Square.prototype.both.call(foreign));
+report("tiny describe", () => Tiny.prototype.describe.call(foreign));
+report("tiny tag", () => Tiny.prototype.tag.call(foreign));
+report("square describe on tiny", () => Square.prototype.describe.call(tiny));
+
+// A base method that builds `new this.constructor()` on a subclass instance
+// builds the subclass, also once the base prototype is complete (a super call
+// above reads it) and `copy` is read through its prototype slot.
+class Node3 {
+  constructor() {
+    this.a = 0;
+  }
+  copy(source) {
+    this.a = source.a;
+    return this;
+  }
+  clone() {
+    return new this.constructor().copy(this);
+  }
+}
+class Mesh3 extends Node3 {
+  constructor(geometry) {
+    super();
+    this.geometry = geometry;
+  }
+  copy(source) {
+    super.copy(source);
+    this.geometry = source.geometry;
+    return this;
+  }
+}
+const copyValue = Node3.prototype.copy;
+const mesh = new Mesh3({ id: 1 });
+mesh.a = 5;
+const meshClone = mesh.clone();
+console.log(
+  meshClone instanceof Mesh3,
+  meshClone.constructor === Mesh3,
+  mesh.constructor === Mesh3,
+  meshClone.a,
+  JSON.stringify(meshClone.geometry),
+  typeof copyValue,
+);
+
+// Methods that use a private accessor or return a private method keep their own receiver, as values too.
+class Vault {
+  #v = 1;
+  get #value() {
+    return this.#v * 10;
+  }
+  set #value(x) {
+    this.#v = x;
+  }
+  #hidden() {
+    return "hidden " + this.#v;
+  }
+  read() {
+    return this.#value;
+  }
+  bump() {
+    this.#value = this.#v + 1;
+    return this.#v;
+  }
+  hiddenValue() {
+    return this.#hidden;
+  }
+}
+const vault = new Vault();
+const readVault = Vault.prototype.read;
+const bumpVault = Vault.prototype.bump;
+const hiddenOf = Vault.prototype.hiddenValue;
+console.log(readVault.call(vault), bumpVault.call(vault), readVault.call(vault), typeof hiddenOf.call(vault));
+console.log(hiddenOf.call(vault).call(vault));
+try {
+  readVault.call({});
+} catch (error) {
+  console.log(error.constructor.name);
+}
+
+// A method value whose body calls super with spread arguments.
+class Summer {
+  add(...xs) {
+    return xs.reduce((sum, x) => sum + x, 0);
+  }
+}
+class Doubler extends Summer {
+  add(...xs) {
+    return super.add(...xs) * 2;
+  }
+}
+const addValue = Doubler.prototype.add;
+console.log(addValue.call(new Doubler(), 1, 2, 3), addValue.apply(new Doubler(), [4, 5]));
diff --git a/tests/harness/corpus-costs/7621-class-methods-foreign-receivers.mjs.json b/tests/harness/corpus-costs/7621-class-methods-foreign-receivers.mjs.json
new file mode 100644
index 000000000..bb2ade1c2
--- /dev/null
+++ b/tests/harness/corpus-costs/7621-class-methods-foreign-receivers.mjs.json
@@ -0,0 +1 @@
+{ "plain": 6415, "san": 10107 }