diff --git a/.project/concepts/app-contract/polyfill-surface/contract_polyfill-surface.md b/.project/concepts/app-contract/polyfill-surface/contract_polyfill-surface.md
index 45fa9dd..ea3b659 100644
--- a/.project/concepts/app-contract/polyfill-surface/contract_polyfill-surface.md
+++ b/.project/concepts/app-contract/polyfill-surface/contract_polyfill-surface.md
@@ -103,6 +103,9 @@ export async function getObjects<T extends BaseType>(
 export type RdfOrmConfig<T> = Scope & { where?: ...; orderBy?: ... };  // the SDK's own
 export function subscribeDoc(nuri: NuriLike, onChange: (r: DocChange, t: DocChangeType) => void): Unsubscribe;
 export function subscribeDocs(nuris: NuriLike[], onChange: (r: DocChange, t: DocChangeType) => void): Unsubscribe;
+// Extracts the variant key from a DocChange - "State" / "Patch" / "TabInfo" / ... -
+// a convenience over the verified payload shape, not an upstream call.
+export function docChangeType(resp: DocChange): DocChangeType;
 
 // ── low-level document / SPARQL primitives ───────────────────────────────
 export const docs: {
diff --git a/packages/polyfill/test/vocabulary.test.ts b/packages/polyfill/test/vocabulary.test.ts
index db1558e..4c1d170 100644
--- a/packages/polyfill/test/vocabulary.test.ts
+++ b/packages/polyfill/test/vocabulary.test.ts
@@ -324,3 +324,47 @@ test("the api-contract appendix lists exactly what the entry exports", () => {
   expect({ missing, extra }).toEqual({ missing: [], extra: [] });
 });
 
+// --- the concept contract's Surface block must match the code -------------
+
+/**
+ * `.project/concepts/app-contract/polyfill-surface/contract_polyfill-surface.md` is the
+ * doc a consumer's `usage_` leaf pulls against — separate from `docs/api-contract.md`
+ * above, and with no check of its own: `docChangeType` was published and missing from its
+ * `## Surface` block, and nothing here or elsewhere caught it (T01.p). Unlike the other
+ * doc's per-subject "### Today" blocks, this one is a SINGLE fenced block under one
+ * heading, so the check is simpler: every `export` it shows must actually be exported.
+ */
+test("every `export` shown in the concept contract's Surface block is actually exported", () => {
+  const md = fs.readFileSync(
+    path.join(
+      import.meta.dir,
+      "..",
+      "..",
+      "..",
+      ".project",
+      "concepts",
+      "app-contract",
+      "polyfill-surface",
+      "contract_polyfill-surface.md",
+    ),
+    "utf8",
+  );
+  const start = md.indexOf("\n## Surface");
+  const end = md.indexOf("\n## Guarantees", start);
+  const section = md.slice(start, end);
+  const published = new Set(publishedNames());
+  const stale: string[] = [];
+  for (const block of section.matchAll(/```ts\n([\s\S]*?)```/g)) {
+    for (const line of block[1]!.split("\n")) {
+      const decl = line.match(
+        /^export (?:declare )?(?:async )?(?:const|function|class|interface|type) (\w+)/,
+      );
+      const name = decl?.[1];
+      if (name && !published.has(name) && !isNamespaceMember(name)) stale.push(name);
+    }
+  }
+  // A failure is a question, not a rename: has the symbol been removed (then say so and
+  // rule on why), or has the entry lost something this contract still promises?
+  expect([...new Set(stale)]).toEqual([]);
+});
+