diff --git a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
index 0d492714..8e42c452 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
@@ -2316,6 +2316,45 @@ describe("SchemaForm multiline strings (#2042)", () => {
       expect(onChange).toHaveBeenCalledWith({ count: undefined });
     });
 
+    it("seeds a newly pinned field when the schema changes in place", async () => {
+      // A tool refreshed in place keeps its `resetKey` and its branch while the
+      // schema changes underneath — the new read-only field must still be
+      // seeded, or a required one leaves submit disabled with no way to fix it.
+      const onChange = vi.fn();
+      const before: InspectorFormSchema = {
+        type: "object",
+        properties: { a: { type: "string", title: "A" } },
+      };
+      const { rerender } = renderWithMantine(
+        <SchemaForm
+          schema={before}
+          values={{}}
+          onChange={onChange}
+          resetKey="same-tool"
+        />,
+      );
+      await Promise.resolve();
+      onChange.mockClear();
+
+      rerender(
+        <SchemaForm
+          schema={{
+            type: "object",
+            properties: {
+              a: { type: "string", title: "A" },
+              version: { type: "string", const: "2" },
+            },
+          }}
+          values={{}}
+          onChange={onChange}
+          resetKey="same-tool"
+        />,
+      );
+      await waitFor(() =>
+        expect(onChange).toHaveBeenCalledWith({ version: "2" }),
+      );
+    });
+
     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 7cd2b174..047502bb 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
@@ -719,6 +719,11 @@ export function SchemaForm({
   // 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.
+  // The fixed values themselves, as a stable key: callers rebuild the schema
+  // object every render, so its identity says nothing, while this changes
+  // exactly when what has to be seeded changes.
+  const seedKey = serializeJson(collectSchemaDefaults(effectiveSchema));
+
   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.
@@ -737,9 +742,11 @@ export function SchemaForm({
     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]);
+    // Keyed by the entity and branch being edited, and by the fixed values
+    // themselves: a tool refreshed in place keeps its `resetKey` and its branch
+    // while its schema changes underneath, and a newly pinned field would
+    // otherwise be rendered read-only and never seeded.
+  }, [draftKey, seedKey]);
 
   // 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
diff --git a/clients/web/src/test/core/rootUnion.test.ts b/clients/web/src/test/core/rootUnion.test.ts
index 1fddfb0c..69100561 100644
--- a/clients/web/src/test/core/rootUnion.test.ts
+++ b/clients/web/src/test/core/rootUnion.test.ts
@@ -583,6 +583,20 @@ describe("resolveRootUnion", () => {
       expect(branches[0].label).toBe("email");
     });
 
+    it("labels boolean and null constants by their values", () => {
+      // `hasDiscriminator` accepts these, so the label must too — otherwise a
+      // perfectly discriminated union reads as "Option 1"/"Option 2".
+      const { branches } = resolveRootUnion({
+        type: "object",
+        required: ["v"],
+        oneOf: [
+          { type: "object", properties: { v: { const: true }, a: {} } },
+          { type: "object", properties: { v: { const: null }, b: {} } },
+        ],
+      });
+      expect(branches.map((branch) => branch.label)).toEqual(["true", "null"]);
+    });
+
     it("labels a numeric const by its value", () => {
       const { branches } = resolveRootUnion({
         type: "object",
diff --git a/core/json/rootUnion.ts b/core/json/rootUnion.ts
index 01db8290..b480f4e8 100644
--- a/core/json/rootUnion.ts
+++ b/core/json/rootUnion.ts
@@ -586,20 +586,27 @@ function branchLabel(
     return branch.title;
   }
   const properties = propertiesOf(branch);
-  const constOf = (name: string): string | null => {
+  // Every primitive constant is a usable label, not just the string and number
+  // cases: `hasDiscriminator` accepts distinct booleans and nulls, so a
+  // `true`/`false` union would otherwise be labelled "Option 1"/"Option 2"
+  // while being discriminated perfectly well. An object or array constant is
+  // not a label — `undefined` marks "nothing usable here", which lets a `null`
+  // constant be a value rather than the sentinel.
+  const constOf = (name: string): string | undefined => {
     const property = toBranch(properties[name]) as { const?: unknown } | null;
-    const value = property?.const;
-    return typeof value === "string" || typeof value === "number"
+    if (property === null || !("const" in property)) return undefined;
+    const value = property.const;
+    return value === null || typeof value !== "object"
       ? String(value)
-      : null;
+      : undefined;
   };
   if (discriminatorProperty !== undefined) {
     const value = constOf(discriminatorProperty);
-    if (value !== null) return value;
+    if (value !== undefined) return value;
   }
   const constants = Object.keys(properties)
     .map((name) => constOf(name))
-    .filter((value): value is string => value !== null);
+    .filter((value): value is string => value !== undefined);
   if (constants.length === 1) return constants[0];
   return `Option ${index + 1}`;
 }