diff --git a/.vscode/launch.json b/.vscode/launch.json
index 1a90b38bb..72a01bd2f 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -460,6 +460,13 @@
         "TS_NODE_SKIP_IGNORE": "true"
       }
     },
+    {
+      "type": "node",
+      "request": "launch",
+      "name": "docs: openapi-generator",
+      "runtimeArgs": ["-r", "ts-node/register/transpile-only"],
+      "args": ["${workspaceFolder}/src/openapi-generator.ts"]
+    },
     {
       "type": "node",
       "request": "launch",
diff --git a/src/api/schemas/responses/responses.ts b/src/api/schemas/responses/responses.ts
index 160075d42..9903fdcc1 100644
--- a/src/api/schemas/responses/responses.ts
+++ b/src/api/schemas/responses/responses.ts
@@ -17,8 +17,9 @@ import { NakamotoBlockSchema, SignerSignatureSchema } from '../entities/block';
 export const ErrorResponseSchema = Type.Object(
   {
     error: Type.String(),
+    message: Type.Optional(Type.String()),
   },
-  { title: 'Error Response' }
+  { title: 'Error Response', additionalProperties: true }
 );
 
 export const ServerStatusResponseSchema = Type.Object(
diff --git a/src/openapi-generator.ts b/src/openapi-generator.ts
index 817e2492d..94ec7b537 100644
--- a/src/openapi-generator.ts
+++ b/src/openapi-generator.ts
@@ -1,9 +1,10 @@
 import Fastify from 'fastify';
-import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox';
+import { TSchema, TypeBoxTypeProvider } from '@fastify/type-provider-typebox';
 import FastifySwagger from '@fastify/swagger';
 import { writeFileSync } from 'fs';
 import { OpenApiSchemaOptions } from './api/schemas/openapi';
 import { StacksApiRoutes } from './api/init';
+import { ErrorResponseSchema } from './api/schemas/responses/responses';
 
 /**
  * Generates `openapi.yaml` based on current Swagger definitions.
@@ -14,6 +15,16 @@ async function generateOpenApiFiles() {
     logger: true,
   }).withTypeProvider<TypeBoxTypeProvider>();
 
+  // If a response schema is defined but lacks a '4xx' response, add it
+  fastify.addHook(
+    'onRoute',
+    (route: { schema?: { response: Record<string | number, TSchema> } }) => {
+      if (route.schema?.response && !route.schema?.response['4xx']) {
+        route.schema.response['4xx'] = ErrorResponseSchema;
+      }
+    }
+  );
+
   await fastify.register(FastifySwagger, OpenApiSchemaOptions);
   await fastify.register(StacksApiRoutes);
   await fastify.ready();