diff --git a/app/config/openapi.js b/app/config/openapi.js
index 3fcd408..05006d9 100644
--- a/app/config/openapi.js
+++ b/app/config/openapi.js
@@ -405,7 +405,28 @@ const spec = {
                             },
                         },
                     },
-                    503: { description: 'Degraded — DB unreachable' },
+                    503: {
+                        description: 'Degraded — DB unreachable',
+                        content: {
+                            'application/json': {
+                                schema: {
+                                    type: 'object',
+                                    properties: {
+                                        status: { type: 'string', enum: ['degraded'] },
+                                        db: { type: 'string', enum: ['down'] },
+                                        uptime_s: { type: 'integer' },
+                                        version: { type: 'string' },
+                                        elapsed_ms: { type: 'number' },
+                                        migration: { type: 'string', nullable: true },
+                                        db_error: {
+                                            type: 'string',
+                                            description: 'The underlying DB connection error message (e.g. "ECONNREFUSED 127.0.0.1:5432"). Useful for operator debugging; never includes credentials. Present only on the 503 path.',
+                                        },
+                                    },
+                                },
+                            },
+                        },
+                    },
                 },
             },
         },
diff --git a/tests/api/openapi.test.js b/tests/api/openapi.test.js
index d0ccb38..4c97365 100644
--- a/tests/api/openapi.test.js
+++ b/tests/api/openapi.test.js
@@ -160,6 +160,25 @@ describe('OpenAPI spec', () => {
         expect(m.get.responses['200']).toBeDefined();
     });
 
+    test('/healthz 503 response documents the db_error field', async () => {
+        // The healthz controller (app/controllers/healthcontroller.js)
+        // appends a `db_error` string field to the 503 body when the
+        // SELECT 1 probe throws. Operators rely on this for debugging;
+        // SDK generators read it from the spec. Pin the schema so a
+        // future controller refactor that drops the field also fails
+        // here and the docs stay in sync with reality.
+        const res = await request(app).get('/openapi.json');
+        const h = res.body.paths['/healthz'];
+        const r503 = h.get.responses['503'];
+        expect(r503).toBeDefined();
+        const schema = r503.content
+            && r503.content['application/json']
+            && r503.content['application/json'].schema;
+        expect(schema, '/healthz 503 should declare a body schema').toBeDefined();
+        expect(schema.properties.db_error, 'db_error should appear in the 503 body').toBeDefined();
+        expect(schema.properties.db_error.type).toBe('string');
+    });
+
     test('GET /docs serves Swagger UI HTML', async () => {
         const res = await request(app).get('/docs/');
         // swagger-ui-express serves HTML; we don't pin the exact body