diff --git a/src/modules/investments/instruments/stocks/presentation/stock.dto.ts b/src/modules/investments/instruments/stocks/presentation/stock.dto.ts
index c4d47f4..14b0e60 100644
--- a/src/modules/investments/instruments/stocks/presentation/stock.dto.ts
+++ b/src/modules/investments/instruments/stocks/presentation/stock.dto.ts
@@ -93,7 +93,7 @@ export class StockScreenerItemDto {
   @ApiProperty({ nullable: true, type: String }) declare currency: null | string;
   @ApiProperty({ nullable: true, type: String }) declare realExchange: null | string;
   @ApiProperty({ nullable: true, type: String }) declare countryOfRisk: null | string;
-  @ApiProperty({ nullable: true, type: 'integer' }) declare lot: null | number;
+  @ApiProperty({ nullable: true, minimum: MIN_TICKER_LENGTH, type: 'integer' }) declare lot: null | number;
 }
 
 export class StockScreenerPageDto {
diff --git a/src/openapi.ts b/src/openapi.ts
index 46e13fb..3ea3fda 100644
--- a/src/openapi.ts
+++ b/src/openapi.ts
@@ -3,9 +3,20 @@ import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
 
 export const API_PREFIX = 'api/v1';
 
+// Nest Swagger cannot express additionalProperties on a class, so closed request
+// bodies are restored here after generation; the runtime allowlist stays in the controller.
+const CLOSED_SCHEMAS = ['StockScreenerCriteriaDto'];
+
 export function createOpenApiDocument(app: INestApplication) {
-  return SwaggerModule.createDocument(
+  const document = SwaggerModule.createDocument(
     app,
     new DocumentBuilder().setTitle('WealthLedger API').setVersion('v1').build(),
   );
+  const schemas = document.components?.schemas ?? {};
+  for (const [name, schema] of Object.entries(schemas)) {
+    if (CLOSED_SCHEMAS.includes(name) && 'properties' in schema) {
+      schema.additionalProperties = false;
+    }
+  }
+  return document;
 }
diff --git a/test/openapi-contract.e2e-spec.ts b/test/openapi-contract.e2e-spec.ts
index d95b945..f9eef12 100644
--- a/test/openapi-contract.e2e-spec.ts
+++ b/test/openapi-contract.e2e-spec.ts
@@ -122,6 +122,13 @@ describe('OpenAPI contract (integration)', () => {
   });
 
   it('generates the runtime contract without server startup or deployment configuration', async () => {
+    const schemas = createOpenApiDocument(app).components?.schemas;
+    expect(schemas?.StockScreenerCriteriaDto).toMatchObject({
+      additionalProperties: false,
+    });
+    expect(schemas?.StockScreenerItemDto).toMatchObject({
+      properties: { lot: { type: 'integer', minimum: 1, nullable: true } },
+    });
     expect(
       createOpenApiDocument(app).components?.schemas?.ProblemDetailsDto,
     ).toMatchObject({