diff --git a/packages/compiler/src/frontend/lowering/expressions/element-access.ts b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
index 29f249282..8e60b3329 100644
--- a/packages/compiler/src/frontend/lowering/expressions/element-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
@@ -1,4 +1,5 @@
 /* Element reads and writes (`a[i]`, `a[i] = v`), compound element updates, and keyed record reads. */
+import { OBJECT_PROTOTYPE_MEMBERS } from "../../shape-inference/domain.js";
 import * as ts from "../../ts7/adapter.js";
 import {
   lowerOptionalIndex,
@@ -1319,6 +1320,21 @@ export function lowerRecordKeyRead(
       };
       return lowerer.maybeNarrow(get, expr);
     }
+    // A literal key the shape does not declare, on a shape without an index
+    // signature, names no own property of any of its values: the read
+    // answers undefined, as Node's does. The checker's key check cannot vouch
+    // for it here, since the receiver can be typed from a package's
+    // JavaScript while the checker read its declarations. Object.prototype
+    // members keep the keyed read below.
+    if (shape.indexValue === undefined && !OBJECT_PROTOTYPE_MEMBERS.has(litKey)) {
+      return {
+        kind: "seqExpr",
+        stmts: [{ kind: "exprStmt", expr: obj, loc }],
+        result: dynUndefinedExpr(loc),
+        type: DYN,
+        loc,
+      };
+    }
   }
   let key =
     litKey !== null
@@ -1489,7 +1505,7 @@ function lowerRecordPropertyKey(lowerer: Lowerer, key: IrExpr, node: ts.Expressi
  * exactly the key JS derives; negative/computed keys stay runtime
  * expressions and canonicalize through ensureString instead). Null for
  * everything else. */
-function recordKeyLiteralText(node: ts.Expression): string | null {
+export function recordKeyLiteralText(node: ts.Expression): string | null {
   if (ts.isStringLiteral(node)) return node.text;
   if (ts.isNumericLiteral(node)) return String(Number(node.text));
   return null;
diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 006975085..984775e65 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -96,7 +96,8 @@ import {
   lowerUnionKeyedRead,
 } from "./expressions/property-access.js";
 import { lowerNullishPair } from "./expressions/binary.js";
-import { lowerElementCompoundParts } from "./expressions/element-access.js";
+import { lowerElementCompoundParts, recordKeyLiteralText } from "./expressions/element-access.js";
+import { OBJECT_PROTOTYPE_MEMBERS } from "../shape-inference/domain.js";
 export {
   DEFERRED_READ_MESSAGE,
   abstractPropertyDeclOf,
@@ -1964,6 +1965,20 @@ export function lowerAbsenceProbe(lowerer: Lowerer, node: ts.Expression): IrExpr
     if (shape && !shape.tuple && shape.indexValue && shape.indexValue.kind !== "dyn") {
       return lowerer.lowerRecordKeyRead(expr, receiverT.shapeId, shape, true);
     }
+    // A literal key the shape does not declare is absent from every value
+    // (lowerRecordKeyRead answers undefined), whatever type the checker gave
+    // the read.
+    const literal = recordKeyLiteralText(expr.argumentExpression);
+    if (
+      shape &&
+      !shape.tuple &&
+      !shape.indexValue &&
+      literal !== null &&
+      !OBJECT_PROTOTYPE_MEMBERS.has(literal) &&
+      !shape.fields.some((field) => field.name === literal)
+    ) {
+      return lowerer.lowerRecordKeyRead(expr, receiverT.shapeId, shape, true);
+    }
     return null;
   }
   // `m[2]` on a JavaScript `RegExpExecArray | null` that a previous
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 25cce9cbf..c03731464 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -1318,6 +1318,7 @@ function jsDeclarationExpandsObject(
       inferred !== null &&
       jsOpenObjectType(decl, inferred, lowerer.shapes, lowerer.unions).kind === "dyn") ||
     arms.some((arm) => arm.kind === "record" && jsObjectBindingExpands(lowerer, decl, arm)) ||
+    arms.some((arm) => arm.kind === "record" && jsObjectLiteralFilledLater(lowerer, decl, arm)) ||
     (isJsSourceFile(ts.getSourceFileOfNode(decl)) &&
       !decl.type &&
       !hasJsTypeAnnotation(decl) &&
@@ -6553,6 +6554,41 @@ function lowerVariableInitializer(
   }
 }
 
+/** A JS object literal that leaves out a required field of the binding's
+ * inferred record: tsc types `var p = {}; p.id = id;` by the members later
+ * assignments add, so the object only gains them after its initializer, and
+ * until then it is not a value of that record. Keep it a checked-dynamic
+ * object, as an object that gains keys outside the record is. */
+function jsObjectLiteralFilledLater(
+  lowerer: Lowerer,
+  decl: ts.VariableDeclaration,
+  type: IrType & { kind: "record" },
+): boolean {
+  if (decl.type || !decl.initializer || !isJsSourceFile(ts.getSourceFileOfNode(decl))) return false;
+  let initializer = decl.initializer;
+  while (ts.isParenthesizedExpression(initializer)) initializer = initializer.expression;
+  if (!ts.isObjectLiteralExpression(initializer) || hasJsTypeAnnotation(decl)) return false;
+  const shape = lowerer.shapes.get(type.shapeId);
+  if (!shape || shape.tuple) return false;
+  const given = new Set<string>();
+  for (const property of initializer.properties.nodes) {
+    if (ts.isSpreadAssignment(property)) return false;
+    const name = property.name;
+    if (name === undefined) return false;
+    if (ts.isIdentifier(name) || ts.isStringLiteral(name) || ts.isNumericLiteral(name))
+      given.add(name.text);
+    else return false;
+  }
+  // A field whose type holds undefined reads as it would before the write.
+  const holdsUndefined = (t: IrType): boolean =>
+    t.kind === "dyn" ||
+    t.kind === "undefinedT" ||
+    (t.kind === "union" && lowerer.armTag(t.unionId, UNDEFINED_T) >= 0);
+  return shape.fields.some(
+    (field) => !given.has(field.name) && !field.name.startsWith("%") && !holdsUndefined(field.type),
+  );
+}
+
 /** A JS object literal's inferred fields do not close its later storage.
  * Keep the original native object when writes add keys or delete fields;
  * converting it to a fixed record would discard that behavior. */
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/var-object-filled-later.js.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/var-object-filled-later.js.json
new file mode 100644
index 000000000..26329147f
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/var-object-filled-later.js.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/var-object-filled-later.js"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-keyed-construction/main.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-keyed-construction/main.ts.json
new file mode 100644
index 000000000..5817aa040
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-keyed-construction/main.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/fixtures/npm/cases/types-keyed-construction/main.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts.json
new file mode 100644
index 000000000..8c56898f2
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/var-object-filled-later.js b/tests/corpus/var-object-filled-later.js
new file mode 100644
index 000000000..009d8b54d
--- /dev/null
+++ b/tests/corpus/var-object-filled-later.js
@@ -0,0 +1,48 @@
+// A function-scoped `var` initialized with an object literal that later
+// assignments fill in: tsc types the binding by the members those
+// assignments add, but the object only gains them after its initializer.
+"use strict";
+function load(id) {
+  var p = {};
+  console.log(p.count, "count" in p, Object.keys(p).length);
+  p.id = id;
+  p.count = id * 2;
+  p.label = "item-" + id;
+  return p;
+}
+const p = load(7);
+console.log(p.id + p.count, p.label.toUpperCase(), JSON.stringify(p), Object.keys(p).join(","));
+
+function partial(name) {
+  var entry = { name: name };
+  entry.size = name.length;
+  entry.tags = [name, name + "!"];
+  return entry;
+}
+const e = partial("abc");
+console.log(e.size * 10, e.tags.join("|"), JSON.stringify(e));
+
+function nested(depth) {
+  var node = {};
+  node.depth = depth;
+  node.child = depth > 0 ? nested(depth - 1) : null;
+  return node;
+}
+console.log(JSON.stringify(nested(2)));
+
+function rows(n) {
+  var out = [];
+  for (var i = 0; i < n; i++) {
+    var row = {};
+    row.index = i;
+    row.square = i * i;
+    out.push(row);
+  }
+  return out;
+}
+console.log(rows(4).map(function (r) { return r.index + ":" + r.square; }).join(" "));
+
+var top = {};
+top.ready = true;
+top.total = rows(3).reduce(function (s, r) { return s + r.square; }, 0);
+console.log(top.ready, top.total, JSON.stringify(top));
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/main.ts b/tests/fixtures/npm/cases/types-keyed-construction/main.ts
new file mode 100644
index 000000000..b163f3d1c
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/main.ts
@@ -0,0 +1,12 @@
+// A plain-JavaScript package typed by an @types package builds its result
+// key by key (`var p = {}; p.id = id; ...`): the object crosses the
+// declaration boundary with every field, including the declared fields
+// whose keys are not identifiers.
+import { loadProfile, type Profile } from "profilekit";
+
+const p: Profile = loadProfile(7);
+console.log(p["a;b"].toUpperCase(), p["a;b"].length);
+console.log(p["my-key"], p["my-key"].length, p["x y"].length);
+console.log(p.id, p.count, JSON.stringify(p));
+const keys: string[] = Object.keys(p);
+console.log(keys.join("|"));
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/index.d.ts b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/index.d.ts
new file mode 100644
index 000000000..2da797940
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/index.d.ts
@@ -0,0 +1,8 @@
+export interface Profile {
+  id: number;
+  "a;b": string;
+  "my-key": string;
+  "x y": string;
+  count: number;
+}
+export declare function loadProfile(id: number): Profile;
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/package.json b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/package.json
new file mode 100644
index 000000000..a7dd89b11
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/@types/profilekit/package.json
@@ -0,0 +1 @@
+{ "name": "@types/profilekit", "version": "1.0.0", "types": "index.d.ts" }
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/index.js b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/index.js
new file mode 100644
index 000000000..8d8df9336
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/index.js
@@ -0,0 +1,12 @@
+"use strict";
+// Plain JavaScript: the profile is built key by key.
+function loadProfile(id) {
+  var p = {};
+  p.id = id;
+  p["a;b"] = "semi" + id;
+  p["my-key"] = "dash";
+  p["x y"] = "space";
+  p.count = id * 2;
+  return p;
+}
+exports.loadProfile = loadProfile;
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/package.json b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/package.json
new file mode 100644
index 000000000..340a0a6c5
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/node_modules/profilekit/package.json
@@ -0,0 +1 @@
+{ "name": "profilekit", "version": "1.0.0", "main": "index.js" }
diff --git a/tests/fixtures/npm/cases/types-keyed-construction/package.json b/tests/fixtures/npm/cases/types-keyed-construction/package.json
new file mode 100644
index 000000000..24992c953
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-keyed-construction/package.json
@@ -0,0 +1 @@
+{ "name": "types-keyed-construction-case", "private": true, "type": "module" }
diff --git a/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts b/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts
new file mode 100644
index 000000000..d069face6
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-undeclared-quoted-key/main.ts
@@ -0,0 +1,20 @@
+// A package typed from its JavaScript returns profiles without a key its
+// declarations promise ("a;b"). A literal-key read of that key answers
+// undefined, as Node's does; it never reads another field.
+import { loadProfiles } from "hostilekit";
+
+const profiles = loadProfiles();
+for (const p of profiles) {
+  console.log(p.id, p["a;b"] === undefined, typeof p["a;b"], "a;b" in p);
+  try {
+    console.log(p["a;b"].toUpperCase());
+  } catch (e) {
+    console.log("upper threw", e instanceof TypeError);
+  }
+  try {
+    console.log(p["a;b"].length);
+  } catch (e) {
+    console.log("length threw", e instanceof TypeError);
+  }
+  console.log(p.id.toFixed(1), JSON.stringify(p));
+}
diff --git a/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.d.ts b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.d.ts
new file mode 100644
index 000000000..4bf22dfea
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.d.ts
@@ -0,0 +1,2 @@
+export type Profile = { id: number; "a;b": string };
+export declare function loadProfiles(): Profile[];
diff --git a/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.js b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.js
new file mode 100644
index 000000000..316501256
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/index.js
@@ -0,0 +1,5 @@
+"use strict";
+function loadProfiles() {
+  return [{ id: 1 }];
+}
+exports.loadProfiles = loadProfiles;
diff --git a/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/package.json b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/package.json
new file mode 100644
index 000000000..2fdbc5b41
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-undeclared-quoted-key/node_modules/hostilekit/package.json
@@ -0,0 +1 @@
+{ "name": "hostilekit", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/fixtures/npm/cases/types-undeclared-quoted-key/package.json b/tests/fixtures/npm/cases/types-undeclared-quoted-key/package.json
new file mode 100644
index 000000000..afdb3cf5e
--- /dev/null
+++ b/tests/fixtures/npm/cases/types-undeclared-quoted-key/package.json
@@ -0,0 +1 @@
+{ "name": "types-undeclared-quoted-key-case", "private": true, "type": "module" }
diff --git a/tests/harness/corpus-costs/var-object-filled-later.js.json b/tests/harness/corpus-costs/var-object-filled-later.js.json
new file mode 100644
index 000000000..1e74b61e7
--- /dev/null
+++ b/tests/harness/corpus-costs/var-object-filled-later.js.json
@@ -0,0 +1 @@
+{ "plain": 3400 }