diff --git a/docs/api/openapi.json b/docs/api/openapi.json
index 2913b12..f8d3df5 100644
--- a/docs/api/openapi.json
+++ b/docs/api/openapi.json
@@ -14479,7 +14479,7 @@
     "/article/{sku}/father": {
       "post": {
         "summary": "Toggle father mode for an article",
-        "description": "Enables or disables father article status. Enabling requires specifying an attributeNameId; disabling requires the article to have zero children.",
+        "description": "Enables or disables father article status. Enabling requires specifying an\nattributeNameId; disabling requires the article to have zero children.\n\n`attributeNameId` is **conditionally required**: it is optional in the schema\nbecause it is only required when `isFather` is `true`. Omitting it (or passing\n`0`/a negative value) together with `isFather: true` returns `400`\n`attributeNameId is required to become a father article`. It must not be sent\nwhen `isFather` is `false`.\n",
         "tags": [
           "Article"
         ],
@@ -14515,7 +14515,7 @@
                   },
                   "attributeNameId": {
                     "type": "integer",
-                    "description": "Required when isFather is true. The attributename id for variants."
+                    "description": "**Conditionally required when `isFather` is `true`** (required iff isFather=true).\nThe attributename id used for the article's variants. Omitting it while\nenabling father mode returns 400. Must be omitted when `isFather` is `false`.\n"
                   }
                 }
               }
@@ -14539,7 +14539,7 @@
             }
           },
           "400": {
-            "description": "Invalid input or article still has children"
+            "description": "Invalid input, `attributeNameId` missing while `isFather` is true, article still\nhas children, article has stock/suppliers/a Stückliste/options, or the attribute is\nalready used by an ancestor father article.\n"
           },
           "404": {
             "description": "Article or attribute name not found"
diff --git a/docs/api/routes/article/_sku/father.md b/docs/api/routes/article/_sku/father.md
index 27820f5..4de2268 100644
--- a/docs/api/routes/article/_sku/father.md
+++ b/docs/api/routes/article/_sku/father.md
@@ -4,7 +4,15 @@ Protected. Requires a valid session cookie (`credentials: 'include'`).
 
 Toggle father mode for an article
 
-Enables or disables father article status. Enabling requires specifying an attributeNameId; disabling requires the article to have zero children.
+Enables or disables father article status. Enabling requires specifying an
+attributeNameId; disabling requires the article to have zero children.
+
+`attributeNameId` is **conditionally required**: it is optional in the schema
+because it is only required when `isFather` is `true`. Omitting it (or passing
+`0`/a negative value) together with `isFather: true` returns `400`
+`attributeNameId is required to become a father article`. It must not be sent
+when `isFather` is `false`.
+
 
 ## Path parameters
 
@@ -17,7 +25,7 @@ Enables or disables father article status. Enabling requires specifying an attri
 | Field | Required | Type | Description |
 |-------|----------|------|-------------|
 | `isFather` | yes | boolean | True to make article a father, false to revert to regular article |
-| `attributeNameId` | no | integer | Required when isFather is true. The attributename id for variants. |
+| `attributeNameId` | no | integer | **Conditionally required when `isFather` is `true`** (required iff isFather=true). The attributename id used for the article's variants. Omitting it while enabling father mode returns 400. Must be omitted when `isFather` is `false`. |
 
 ```json
 {
@@ -46,7 +54,10 @@ Updated article
 
 | Status | Description |
 |--------|-------------|
-| 400 | Invalid input or article still has children |
+| 400 | Invalid input, `attributeNameId` missing while `isFather` is true, article still
+has children, article has stock/suppliers/a Stückliste/options, or the attribute is
+already used by an ancestor father article.
+ |
 | 404 | Article or attribute name not found |
 
 ---
diff --git a/src/api/routes/article.js b/src/api/routes/article.js
index 3059ff8..8e11565 100644
--- a/src/api/routes/article.js
+++ b/src/api/routes/article.js
@@ -1806,7 +1806,15 @@ export function register(app, basePath = '') {
      * /article/{sku}/father:
      *   post:
      *     summary: Toggle father mode for an article
-     *     description: Enables or disables father article status. Enabling requires specifying an attributeNameId; disabling requires the article to have zero children.
+     *     description: |
+     *       Enables or disables father article status. Enabling requires specifying an
+     *       attributeNameId; disabling requires the article to have zero children.
+     *
+     *       `attributeNameId` is **conditionally required**: it is optional in the schema
+     *       because it is only required when `isFather` is `true`. Omitting it (or passing
+     *       `0`/a negative value) together with `isFather: true` returns `400`
+     *       `attributeNameId is required to become a father article`. It must not be sent
+     *       when `isFather` is `false`.
      *     tags:
      *       - Article
      *     security:
@@ -1831,7 +1839,10 @@ export function register(app, basePath = '') {
      *                 description: True to make article a father, false to revert to regular article
      *               attributeNameId:
      *                 type: integer
-     *                 description: Required when isFather is true. The attributename id for variants.
+     *                 description: |
+     *                   **Conditionally required when `isFather` is `true`** (required iff isFather=true).
+     *                   The attributename id used for the article's variants. Omitting it while
+     *                   enabling father mode returns 400. Must be omitted when `isFather` is `false`.
      *     responses:
      *       200:
      *         description: Updated article
@@ -1843,7 +1854,10 @@ export function register(app, basePath = '') {
      *                 article:
      *                   $ref: '#/components/schemas/Article'
      *       400:
-     *         description: Invalid input or article still has children
+     *         description: |
+     *           Invalid input, `attributeNameId` missing while `isFather` is true, article still
+     *           has children, article has stock/suppliers/a Stückliste/options, or the attribute is
+     *           already used by an ancestor father article.
      *       404:
      *         description: Article or attribute name not found
      */