diff --git a/packages/cli/src/types.ts b/packages/cli/src/types.ts
index e99dc177..2456f924 100644
--- a/packages/cli/src/types.ts
+++ b/packages/cli/src/types.ts
@@ -4,7 +4,7 @@ import { buildRouter, createRouteManifest, nameRoutes } from "@wrnexus/router";
 // Keep the CLI checker sourced from the package contract so typecheck fixes are
 // included in each published CLI bundle.
 import { checkWrnFile, type WrnTypeDiagnostic } from "@wrnexus/typecheck";
-import { parse } from "@wrnexus/syntax";
+import { parse, type PageAst } from "@wrnexus/syntax";
 import { generate, generateTargets } from "@wrnexus/compiler";
 import { regenerateRoutes } from "./routes.ts";
 import { loadAppConfig } from "@wrnexus/styles";
@@ -99,6 +99,42 @@ function writePluginArtifacts(root: string, contributions?: PluginContributions)
   );
 }
 
+/**
+ * Type assertions for sectioned api blocks.
+ *
+ * Enforcement lives here rather than in the compiler because this file is under
+ * `app/` and is therefore compiled by the project's own tsc, while generated
+ * build artifacts are not type-checked at all.
+ */
+function apiBlockAssertions(pages: { path: string; ast: PageAst }[], apiContracts: string): string {
+  const lines: string[] = [];
+
+  for (const page of pages) {
+    for (const block of page.ast.dataApis) {
+      if (!block.sections) continue;
+
+      if (!apiContracts.includes(JSON.stringify(block.path))) {
+        console.warn(
+          `[wrnexus] api block "${block.name}" targets ${block.path}, which has no defineEndpoint contract — its declared types are not checked.`,
+        );
+      }
+
+      const fields = [...block.sections.parameters, ...block.sections.body];
+      const shape = fields.length
+        ? `{ ${fields.map((f) => `${f.name}${f.optional ? "?" : ""}: ${f.type}`).join("; ")} }`
+        : "Record<string, never>";
+
+      lines.push(
+        `  type __wrn_api_check_${block.name} = __wrn_expect_true<AssertAssignable<${shape}, ApiInput<${JSON.stringify(
+          block.path,
+        )}, ${JSON.stringify(block.method)}>>>;`,
+      );
+    }
+  }
+
+  return lines.join("\n");
+}
+
 export function generateApplicationTypes(
   appRoot: string,
   pluginContributions?: PluginContributions,
@@ -145,6 +181,10 @@ export function generateApplicationTypes(
       return `    ${JSON.stringify(component.name)}: { props: ${props ? `{ ${props} }` : "Record<string, never>"}; outputs: ${outputs ? `{ ${outputs} }` : "Record<string, never>"} };`;
     })
     .join("\n");
+  const pageAsts = files(app, (path) => extname(path) === ".wrn").map((file) => ({
+    path: file,
+    ast: parse(readFileSync(file, "utf8")),
+  }));
   const typeDir = join(app, "types");
   const apiContracts = router.api
     .map((route) => {
@@ -218,6 +258,11 @@ declare namespace WRNexusGenerated {
   ${generatedContractMap("RealtimeMessages", realtimeContracts)}
   ${generatedContractMap("QueuePayloads", queueContracts)}
   type ApplicationConfig = ${configFile ? `(typeof import(${typeImport(typeDir, configFile)}))["default"]` : "Record<string, never>"};
+  type AssertAssignable<Actual, Expected> = [Actual] extends [Expected] ? true : false;
+  type __wrn_expect_true<T extends true> = T;
+  type ApiInput<P extends ApiRoute, M> = ApiContracts[P][M]["input"];
+  type ApiOutput<P extends ApiRoute, M> = ApiContracts[P][M]["output"];
+${apiBlockAssertions(pageAsts, apiContracts)}
 }
 `;
   mkdirSync(typeDir, { recursive: true });
diff --git a/packages/cli/test/api-block-types.test.ts b/packages/cli/test/api-block-types.test.ts
new file mode 100644
index 00000000..5bb2d712
--- /dev/null
+++ b/packages/cli/test/api-block-types.test.ts
@@ -0,0 +1,73 @@
+import { afterEach, expect, test } from "bun:test";
+import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { generateApplicationTypes } from "../src/types.ts";
+
+const roots: string[] = [];
+afterEach(() => {
+  for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+});
+
+/** Minimal app with one typed endpoint and one page that calls it. */
+function fixture(block: string): string {
+  const root = mkdtempSync(join(tmpdir(), "wrnexus-api-types-"));
+  roots.push(root);
+  mkdirSync(join(root, "app/pages"), { recursive: true });
+  mkdirSync(join(root, "app/api"), { recursive: true });
+  writeFileSync(
+    join(root, "app/api/users.ts"),
+    `export const POST = async () => Response.json({ users: [] });\n`,
+  );
+  writeFileSync(
+    join(root, "app/pages/search.wrn"),
+    `page Search {\n  client {\n${block}\n  }\n\n  view { <main>x</main> }\n}\n`,
+  );
+  return root;
+}
+
+const BLOCK = `    api searchUsers POST /api/users {
+      request {
+        body {
+          name?: string
+          age?: number
+        }
+      }
+
+      response {
+        return data.users
+      }
+    }`;
+
+test("emits the ApiInput, ApiOutput and AssertAssignable helpers", () => {
+  const root = fixture(BLOCK);
+  generateApplicationTypes(root);
+  const generated = readFileSync(join(root, "app/types/wrnexus.generated.d.ts"), "utf8");
+
+  expect(generated).toContain("type AssertAssignable<");
+  expect(generated).toContain('type ApiInput<P extends ApiRoute, M> = ApiContracts[P][M]["input"]');
+  expect(generated).toContain(
+    'type ApiOutput<P extends ApiRoute, M> = ApiContracts[P][M]["output"]',
+  );
+});
+
+test("emits one assertion per sectioned block, naming its route and method", () => {
+  const root = fixture(BLOCK);
+  generateApplicationTypes(root);
+  const generated = readFileSync(join(root, "app/types/wrnexus.generated.d.ts"), "utf8");
+
+  expect(generated).toContain("__wrn_api_check_searchUsers");
+  expect(generated).toContain('ApiInput<"/api/users", "POST">');
+  expect(generated).toContain("name?: string");
+  expect(generated).toContain("age?: number");
+});
+
+test("a legacy bare-body block produces no assertion", () => {
+  const root = fixture(`    api legacyUsers GET /api/users {
+      return users.length
+    }`);
+  generateApplicationTypes(root);
+  const generated = readFileSync(join(root, "app/types/wrnexus.generated.d.ts"), "utf8");
+
+  expect(generated).not.toContain("__wrn_api_check_legacyUsers");
+});