diff --git a/clients/web/src/test/core/rootUnion.test.ts b/clients/web/src/test/core/rootUnion.test.ts
index 384703ee..05e838cb 100644
--- a/clients/web/src/test/core/rootUnion.test.ts
+++ b/clients/web/src/test/core/rootUnion.test.ts
@@ -147,6 +147,24 @@ describe("resolveRootUnion", () => {
     expect(base.properties).toBeUndefined();
   });
 
+  it("leaves an allOf that adds names a restrictive additionalProperties forbids", () => {
+    // The root rejects `x` as an additional property; folding the member in
+    // would move it beside the keyword, where it reads as allowed.
+    const { base } = resolveRootUnion({
+      type: "object",
+      additionalProperties: false,
+      allOf: [
+        {
+          type: "object",
+          properties: { x: { type: "string" } },
+          required: ["x"],
+        },
+      ],
+    });
+    expect(base.allOf).toHaveLength(1);
+    expect(base.properties).toBeUndefined();
+  });
+
   it("merges allOf branches unconditionally", () => {
     const { base, branches } = resolveRootUnion({
       type: "object",
@@ -754,6 +772,31 @@ describe("resolveRootUnion", () => {
       expect(selectBranchIndex(objectPinned, { tag: { a: 2 } })).toBe(1);
     });
 
+    it("does not read an inherited property as a supplied constant", () => {
+      // `constructor` is a legal field name; reading the inherited one would
+      // rule out the very branch that pins it.
+      const pinnedOnInherited = resolveRootUnion({
+        type: "object",
+        anyOf: [
+          {
+            type: "object",
+            properties: Object.fromEntries([
+              ["constructor", { const: "a" }],
+              ["x", { type: "string" }],
+            ]),
+          },
+          {
+            type: "object",
+            properties: Object.fromEntries([
+              ["constructor", { const: "b" }],
+              ["y", { type: "string" }],
+            ]),
+          },
+        ] as unknown[],
+      }).branches;
+      expect(selectBranchIndex(pinnedOnInherited, { x: "supplied" })).toBe(0);
+    });
+
     it("reports none when the values identify nothing", () => {
       expect(selectBranchIndex(branches, {})).toBeNull();
       expect(selectBranchIndex(branches, { kind: "other" })).toBeNull();
diff --git a/core/json/rootUnion.ts b/core/json/rootUnion.ts
index fbb8d92e..07b5786e 100644
--- a/core/json/rootUnion.ts
+++ b/core/json/rootUnion.ts
@@ -367,6 +367,46 @@ function hasDiscriminator(
   });
 }
 
+/**
+ * Whether a schema's `additionalProperties` rejects names its own `properties`
+ * does not list.
+ *
+ * A schema that constrains nothing is the equivalent of `true`, and that is not
+ * only the empty object: `{ title: "Extra value" }` is annotation and no more.
+ * Treating either as restrictive would decline a legal permissive schema.
+ */
+function restrictsAdditional(schema: RootUnionSchema): boolean {
+  const additional = schema.additionalProperties;
+  return (
+    additional === false ||
+    (typeof additional === "object" &&
+      additional !== null &&
+      Object.keys(additional).some(
+        (keyword) => !ANNOTATION_KEYWORDS.has(keyword),
+      ))
+  );
+}
+
+/**
+ * Whether folding a member into a base would make names the base forbids look
+ * allowed.
+ *
+ * `additionalProperties` constrains whatever its **sibling** `properties` does
+ * not name, so under a restrictive one the base rejects every field the member
+ * adds. Merging moves those fields beside the keyword, where they read as
+ * permitted — and a form built from that submits what the schema forbids.
+ */
+function addsForbiddenNames(
+  base: RootUnionSchema,
+  member: RootUnionSchema,
+): boolean {
+  if (!restrictsAdditional(base)) return false;
+  const baseNames = propertiesOf(base) ?? {};
+  return Object.keys(propertiesOf(member) ?? {}).some(
+    (name) => !Object.hasOwn(baseNames, name),
+  );
+}
+
 /**
  * Every property name the schema's composition members declare, whether or not
  * the composition could be flattened.
@@ -526,7 +566,8 @@ export function resolveRootUnion<T extends RootUnionSchema>(
     if (
       branch === null ||
       !isFlattenable(member) ||
-      conflictsWithBase(merged, branch)
+      conflictsWithBase(merged, branch) ||
+      addsForbiddenNames(merged, branch)
     ) {
       return { base: schema as ResolvedSchema<T>, branches: [] };
     }
@@ -562,25 +603,9 @@ export function resolveRootUnion<T extends RootUnionSchema>(
   // branches add — the original schema admits none of them. Flattening moves
   // those fields *beside* the keyword, where they would read as allowed, so a
   // form built from it would submit what the schema forbids.
-  const additional = base.additionalProperties;
-  const restrictsAdditional =
-    additional === false ||
-    // A schema that constrains nothing is the equivalent of `true`, and that is
-    // not only the empty object: `{ title: "Extra value" }` is annotation and
-    // no more. Declining on key count alone would recreate the empty form for a
-    // legal permissive schema, so what counts is whether an assertion is there.
-    (typeof additional === "object" &&
-      additional !== null &&
-      Object.keys(additional).some(
-        (keyword) => !ANNOTATION_KEYWORDS.has(keyword),
-      ));
-  const baseNames = propertiesOf(base) ?? {};
   if (
-    restrictsAdditional &&
     branches.some((branch) =>
-      Object.keys(propertiesOf(branch as RootUnionSchema) ?? {}).some(
-        (name) => !Object.hasOwn(baseNames, name),
-      ),
+      addsForbiddenNames(base, branch as RootUnionSchema),
     )
   ) {
     return { base, branches: [] };
@@ -646,7 +671,11 @@ export function selectBranchIndex<T extends RootUnionSchema>(
     // constant it did not supply is one this identification exists to *seed* —
     // requiring it would mean a deep link naming `kind` alone matched no branch
     // whenever the branches also pin, say, a `version`.
-    const supplied = pinned.filter(([name]) => values[name] !== undefined);
+    // `hasOwn`: a pinned field legally named `constructor` would otherwise read
+    // the inherited one as a supplied value and rule its own branch out.
+    const supplied = pinned.filter(
+      ([name]) => Object.hasOwn(values, name) && values[name] !== undefined,
+    );
     if (
       supplied.length > 0 &&
       // Structural, not reference: a `const` may be an object or an array, and