diff --git a/packages/compiler/src/frontend/lowering/lower-builtin-values.ts b/packages/compiler/src/frontend/lowering/lower-builtin-values.ts
index 5e04ee83c..e5ebc6d8e 100644
--- a/packages/compiler/src/frontend/lowering/lower-builtin-values.ts
+++ b/packages/compiler/src/frontend/lowering/lower-builtin-values.ts
@@ -116,6 +116,7 @@ export function isNativeBuiltinValueInitializer(
       ["parseInt", "parseFloat"].includes(lowerer.stdlibGlobalMember(expr, "Number") ?? "") ||
       lowerer.stdlibGlobalMember(expr, "Buffer") === "isBuffer" ||
       lowerer.stdlibGlobalMember(expr, "Array") === "prototype" ||
+      lowerer.stdlibGlobalMember(expr, "Promise") === "prototype" ||
       (ts.isPropertyAccessExpression(expr.expression) &&
         lowerer.stdlibGlobalMember(expr.expression, "Array") === "prototype") ||
       lowerer.stdlibGlobalMember(expr, "Object") === "assign" ||
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index e2236f592..a9759b82f 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -2073,6 +2073,9 @@ export class Lowerer {
    * setEncoding call whose receiver is a socket or an untyped value.
    * Computed once on first use (null until then). */
   socketEncodingReachable: boolean | null = null;
+  /** Whether some program source can define a thenable
+   * (programMayDefineThenables). Null until first asked. */
+  thenablesMayBeDefined: boolean | null = null;
   /** Whether some server in this program may listen on a Unix socket: a
    * listen call whose first argument is a string. Computed once on first
    * use (null until then); address() answers the path only when true. */
diff --git a/packages/compiler/src/frontend/lowering/thenables.ts b/packages/compiler/src/frontend/lowering/thenables.ts
index 7c0072ae3..54854ae75 100644
--- a/packages/compiler/src/frontend/lowering/thenables.ts
+++ b/packages/compiler/src/frontend/lowering/thenables.ts
@@ -15,22 +15,19 @@ import { thenReader } from "./checked-promise-all.js";
 import { builtinGlobalValue } from "./expressions/property-access.js";
 import type { Lowerer } from "./lowerer.js";
 
-const mayDefineThenables = new WeakMap<Lowerer, boolean>();
-
 /** Whether any program source can define a thenable: a `then` method or
- * function, a `then:` key, an assignment to `.then`, or a quoted `'then'`
- * key. `.then(` calls alone do not count. Programs without one keep the
- * plain checked-dynamic await. */
+ * function (generic ones too), a `then:` key, a quoted `'then'` key, or an
+ * assignment to `then` (a class field such as `then = (resolve) => ...` or
+ * a `.then` property). `.then(` calls alone do not count. Programs without
+ * one keep the plain checked-dynamic await. */
 export function programMayDefineThenables(lowerer: Lowerer): boolean {
-  let known = mayDefineThenables.get(lowerer);
-  if (known === undefined) {
-    const pattern = /(^|[^.\w$])then\s*\(|\bthen\s*:|['"`]then['"`]|\.then\s*=(?!=)/m;
-    known = lowerer.program
+  if (lowerer.thenablesMayBeDefined === null) {
+    const pattern = /(^|[^.\w$])then\s*[(<]|\bthen\s*:|['"`]then['"`]|(^|[^\w$])then\s*=(?![=>])/m;
+    lowerer.thenablesMayBeDefined = lowerer.program
       .getImplementationSourceFiles()
       .some((sf) => !sf.isDeclarationFile && pattern.test(sf.text));
-    mayDefineThenables.set(lowerer, known);
   }
-  return known;
+  return lowerer.thenablesMayBeDefined;
 }
 
 /** `await v` over a checked-dynamic value: PromiseResolve(v), then the await
diff --git a/tests/harness/errors.test.ts b/tests/harness/errors.test.ts
index c35022f51..7290bc362 100644
--- a/tests/harness/errors.test.ts
+++ b/tests/harness/errors.test.ts
@@ -1514,7 +1514,6 @@ describe("builtin constructor values (JS lane)", () => {
       ["Date.prototype", "Date.prototype"],
       ["RegExp.prototype", "RegExp.prototype"],
       ["Set.prototype", "Set.prototype"],
-      ["Promise.prototype", "Promise.prototype"],
       ["WeakMap.prototype", "WeakMap.prototype"],
       ["WeakSet.prototype", "WeakSet.prototype"],
       ["Date.prototype.getYear", "Date.prototype.getYear"],
@@ -1547,6 +1546,8 @@ function report(label, e) {
 ${reads.map(([label, read]) => `try { const value = ${read}; console.log(${JSON.stringify(label)}, "read", typeof value); } catch (e) { report(${JSON.stringify(label)}, e); }`).join("\n")}
 console.log(Date.length, RegExp.length, Promise.length, Object.length, Map.length, Set.length);
 console.log(Date.name, Map.name, Map.constructor === Function, Object.constructor === Function);
+const promisePrototype = Promise.prototype;
+console.log(typeof promisePrototype, typeof promisePrototype.then, promisePrototype.constructor === Promise);
 `;
     const r = await compileAndRun("builtin-constructor-members", source, "cjs");
     expect(r.stderr).toBe("");
@@ -1561,7 +1562,12 @@ console.log(Date.name, Map.name, Map.constructor === Function, Object.constructo
         ),
       ),
     );
-    expect(lines.slice(reads.length)).toEqual(["7 2 1 1 0 0", "Date Map true true"]);
+    // Promise.prototype reads as an object holding the runtime's methods.
+    expect(lines.slice(reads.length)).toEqual([
+      "7 2 1 1 0 0",
+      "Date Map true true",
+      "object function true",
+    ]);
   });
 });