diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index c7e8155ed..2efe14160 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -2112,6 +2112,51 @@ export class Lowerer {
     return fallback ?? null;
   }
 
+  /** A TypeScript object literal handed to a JavaScript checked-value
+   * parameter whose inferred type is open (`constructor({ factory, ...options
+   * } = {})` — the rest element admits any property): the literal names a
+   * property the contextual JS type does not declare, so it has no record
+   * layout there and is built as the plain object the JS side receives. */
+  private excessJsContextualProperty(
+    literal: ts.ObjectLiteralExpression,
+    contextual: readonly ts.Type[],
+  ): boolean {
+    const objects = contextual.filter(
+      (part) => !(part.flags & (ts.TypeFlags.Undefined | ts.TypeFlags.Null)),
+    );
+    if (objects.length === 0) return false;
+    // Only a destructuring pattern with a rest element is open; other
+    // inferred JS types keep their record lowering.
+    const fromJs = objects.every((part) => {
+      const sym = part.getSymbol();
+      return (
+        sym !== undefined &&
+        this.checker.declarationsOf(sym).some((d) => {
+          if (!isJsSourceFile(d.getSourceFile())) return false;
+          // The pattern itself, or the `= {}` default of a pattern parameter.
+          const pattern =
+            ts.isObjectLiteralExpression(d) &&
+            d.parent !== undefined &&
+            ts.isParameter(d.parent) &&
+            d.parent.initializer === d
+              ? d.parent.name
+              : d;
+          return (
+            ts.isObjectBindingPattern(pattern) &&
+            pattern.elements.nodes.some((element) => element.dotDotDotToken !== undefined)
+          );
+        })
+      );
+    });
+    if (!fromJs) return false;
+    return literal.properties.nodes.some((prop) => {
+      if (!prop.name || !(ts.isIdentifier(prop.name) || ts.isStringLiteral(prop.name)))
+        return false;
+      const name = prop.name.text;
+      return objects.every((part) => !this.checker.getPropertyOfType(part, name));
+    });
+  }
+
   /** The index value of a keyed read of a record's index signature that
    * can answer undefined for a missing key while its checker type does not
    * admit undefined, or null. A literal key naming a declared field is a
@@ -9615,7 +9660,8 @@ export class Lowerer {
           builtinOption("SpawnOptions") ||
           builtinOption("UrlObject") ||
           builtinOption("URLFormatOptions") ||
-          !this.mapTypeOf(this.typeOf(x))
+          !this.mapTypeOf(this.typeOf(x)) ||
+          this.excessJsContextualProperty(x, parts)
         ) {
           return lowerDynObjectLiteral(this, x);
         }
diff --git a/tests/corpus/cjs-open-options-npm/main.ts b/tests/corpus/cjs-open-options-npm/main.ts
new file mode 100644
index 000000000..9fd133155
--- /dev/null
+++ b/tests/corpus/cjs-open-options-npm/main.ts
@@ -0,0 +1,12 @@
+// TypeScript object literals passed to a JavaScript constructor whose
+// options parameter is a destructuring pattern with a rest element: the
+// literal's extra properties reach the rest object.
+import { Agent } from "optkit";
+
+console.log(new Agent().describe("a"));
+console.log(new Agent({ keepAliveTimeout: 10 }).describe("b"));
+console.log(new Agent({ maxOrigins: 2, keepAliveTimeout: 5, pipelining: 1 }).describe("c"));
+console.log(new Agent({ factory: (origin) => "client(" + origin + ")" }).describe("d"));
+
+class TracingAgent extends Agent {}
+console.log(new TracingAgent({ pipelining: 3 }).describe("e"));
diff --git a/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.d.ts b/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.d.ts
new file mode 100644
index 000000000..643b92d84
--- /dev/null
+++ b/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.d.ts
@@ -0,0 +1,11 @@
+export interface Options {
+  factory?: (origin: string) => string;
+  maxOrigins?: number;
+  keepAliveTimeout?: number;
+  pipelining?: number;
+}
+
+export declare class Agent {
+  constructor(opts?: Options);
+  describe(origin: string): string;
+}
diff --git a/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.js b/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.js
new file mode 100644
index 000000000..0034fc442
--- /dev/null
+++ b/tests/corpus/cjs-open-options-npm/node_modules/optkit/index.js
@@ -0,0 +1,21 @@
+'use strict';
+
+function defaultFactory(origin) {
+  return 'pool(' + origin + ')';
+}
+
+// undici's Agent constructor shape: named options with defaults, and a rest
+// element collecting every other option for the per-origin dispatchers.
+class Agent {
+  constructor({ factory = defaultFactory, maxOrigins = Infinity, ...options } = {}) {
+    this.factory = factory;
+    this.maxOrigins = maxOrigins;
+    this.options = options;
+  }
+
+  describe(origin) {
+    return this.factory(origin) + ' max=' + this.maxOrigins + ' ' + JSON.stringify(this.options);
+  }
+}
+
+module.exports = { Agent };
diff --git a/tests/corpus/cjs-open-options-npm/node_modules/optkit/package.json b/tests/corpus/cjs-open-options-npm/node_modules/optkit/package.json
new file mode 100644
index 000000000..9552d73ee
--- /dev/null
+++ b/tests/corpus/cjs-open-options-npm/node_modules/optkit/package.json
@@ -0,0 +1,6 @@
+{
+  "name": "optkit",
+  "version": "1.0.0",
+  "main": "index.js",
+  "types": "index.d.ts"
+}