diff --git a/docs/openapi/genarrative-external-v1.openapi.json b/docs/openapi/genarrative-external-v1.openapi.json
index e4f67a56b..74a73eba0 100644
--- a/docs/openapi/genarrative-external-v1.openapi.json
+++ b/docs/openapi/genarrative-external-v1.openapi.json
@@ -4945,7 +4945,7 @@
           }
         },
         "additionalProperties": true,
-        "description": "其它生成类型或历史结果的兼容 compact result。该 fallback 明确排除 audioKind=sound-effect，避免吞掉 SFX 专用分支。"
+        "description": "其它生成类型或历史结果的兼容 compact result。背景音乐（audioKind=background-music）与不带 audioKind 的图片、视频、图标序列帧、角色动作和 UI 拆解结果都落在这里。该 fallback 明确排除 audioKind=sound-effect，避免吞掉 SFX 专用分支。"
       },
       "ExternalEditorGenerationCompletedResult": {
         "oneOf": [
@@ -4956,13 +4956,7 @@
             "$ref": "#/components/schemas/ExternalEditorGenerationGenericCompactResult"
           }
         ],
-        "discriminator": {
-          "propertyName": "audioKind",
-          "mapping": {
-            "sound-effect": "#/components/schemas/ExternalEditorSoundEffectCompactResult"
-          }
-        },
-        "description": "External v1 轮询 completed 状态的命名 compact result 联合。"
+        "description": "External v1 轮询 completed 状态的命名 compact result 联合。两个分支已由 audioKind 的 const 与 not 精确互斥，不使用 discriminator：背景音乐返回 audioKind=background-music，图片、视频、图标序列帧、角色动作、UI 拆解与历史结果根本没有 audioKind，它们都无法映射到具名分支，且 fallback 分支不可能把 audioKind 声明为必填，因此任何 discriminator 映射都不可能覆盖全部合法结果。"
       },
       "ExternalEditorGenerationSubmissionResponse": {
         "type": "object",
diff --git a/server-rs/crates/api-server/src/external_editor_api.rs b/server-rs/crates/api-server/src/external_editor_api.rs
index 83c43bf39..2deed567d 100644
--- a/server-rs/crates/api-server/src/external_editor_api.rs
+++ b/server-rs/crates/api-server/src/external_editor_api.rs
@@ -2179,13 +2179,11 @@ mod tests {
             completed_result["oneOf"][1]["$ref"],
             "#/components/schemas/ExternalEditorGenerationGenericCompactResult"
         );
-        assert_eq!(
-            completed_result["discriminator"]["propertyName"],
-            "audioKind"
-        );
-        assert_eq!(
-            completed_result["discriminator"]["mapping"]["sound-effect"],
-            "#/components/schemas/ExternalEditorSoundEffectCompactResult"
+        assert!(
+            completed_result.get("discriminator").is_none(),
+            "completed result 联合不能带 discriminator：BGM 回 audioKind=background-music，图片、\
+             视频与历史结果没有 audioKind，而 fallback 分支不可能把 audioKind 声明为必填，\
+             任何映射都覆盖不全合法结果"
         );
         let sound_effect_result =
             &parsed["components"]["schemas"]["ExternalEditorSoundEffectCompactResult"];
@@ -2230,6 +2228,19 @@ mod tests {
             generic_result["not"]["properties"]["audioKind"]["const"],
             "sound-effect"
         );
+        assert_eq!(
+            generic_result["not"]["properties"]
+                .as_object()
+                .map(serde_json::Map::len),
+            Some(1),
+            "fallback 只能排除 SFX 一种 audioKind；再排除别的值会把 BGM 等合法结果挤出联合"
+        );
+        assert!(
+            generic_result["not"]["properties"]["audioKind"]
+                .get("enum")
+                .is_none(),
+            "排除条件必须是 const sound-effect，换成 enum 会连带排掉 BGM"
+        );
         assert!(
             parsed["paths"]
                 .get("/api/external/v1/editor/audios/background-music/generations")