diff --git a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
index a0ed9b2f..575c52d0 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
@@ -7,6 +7,7 @@ import {
   fireEvent,
   renderWithMantine,
   screen,
+  waitFor,
 } from "../../../test/renderWithMantine";
 import { SchemaForm } from "./SchemaForm";
 
@@ -2264,6 +2265,32 @@ describe("SchemaForm multiline strings (#2042)", () => {
       expect(screen.getByRole("textbox", { name: /Alpha/ })).toBeTruthy();
     });
 
+    it("reports the branch's fixed values upward when mounted with none", async () => {
+      // A read-only `const` the caller never seeded would otherwise be
+      // displayed and, if required, keep submit disabled forever.
+      const onChange = vi.fn();
+      renderWithMantine(
+        <SchemaForm schema={UNION_SCHEMA} values={{}} onChange={onChange} />,
+      );
+      await waitFor(() =>
+        expect(onChange).toHaveBeenCalledWith({ kind: "email" }),
+      );
+    });
+
+    it("reports nothing when the caller already seeded them", async () => {
+      const onChange = vi.fn();
+      renderWithMantine(
+        <SchemaForm
+          schema={UNION_SCHEMA}
+          values={{ kind: "email" }}
+          onChange={onChange}
+        />,
+      );
+      // Nothing is missing, so the form does not touch the caller's values.
+      await Promise.resolve();
+      expect(onChange).not.toHaveBeenCalled();
+    });
+
     it("renders no picker for a single-branch union but still shows its fields", () => {
       const schema: InspectorFormSchema = {
         type: "object",
diff --git a/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx b/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
index 155f5205..8dfde16f 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
@@ -706,6 +706,41 @@ export function SchemaForm({
     [],
   );
 
+  // Report the schema's own fixed values upward once per entity, for a caller
+  // that mounts the form with nothing seeded.
+  //
+  // A `const` field is rendered read-only, so the user cannot supply it — and a
+  // required one then leaves submit disabled forever, on a value that was never
+  // in doubt. Most callers seed through `collectSchemaDefaults` before mounting;
+  // this covers the ones that do not, rather than making every caller
+  // responsible for a value the form is already displaying.
+  //
+  // An effect, not a render-time update: `onChange` belongs to the parent, and
+  // calling it during our render would update another component mid-render.
+  // Only names absent from `values` are added, so a caller that has already
+  // seeded them sees no call at all, and re-running cannot loop.
+  const latestSeed = useRef({ schema: effectiveSchema, values, onChange });
+  // Written in an effect, not during render — the same shape the validity
+  // reporter below uses, and what `react-hooks/refs` requires.
+  useEffect(() => {
+    latestSeed.current = { schema: effectiveSchema, values, onChange };
+  });
+  useEffect(() => {
+    const {
+      schema: current,
+      values: held,
+      onChange: report,
+    } = latestSeed.current;
+    const missing = Object.entries(collectSchemaDefaults(current)).filter(
+      ([name]) => !Object.hasOwn(held, name),
+    );
+    if (missing.length > 0) {
+      report({ ...held, ...Object.fromEntries(missing) });
+    }
+    // Keyed by the entity and branch being edited: a different one has
+    // different fixed values, and the same one needs this only once.
+  }, [draftKey]);
+
   // Read through a ref so the callback's identity is not a dependency. It has
   // to be one or the other, and a *stable* dependency is what this needs: a
   // nested form is handed a fresh closure every render, and re-running the