fix: recognize kind field for skill priming, add storage_session_id

LibreChat's batchUploadCodeEnvFiles sends kind/id/version form fields
(not entity_id) and expects storage_session_id in the response.

Two bugs in upload_files_batch caused skill priming to always fail:
- is_agent_file checked only entity_id, so kind=skill uploads were
  treated as user files and .xsd schemas were rejected by the whitelist
- response returned session_id only; LibreChat throws if
  storage_session_id is absent

Add kind=skill/agent recognition alongside the existing entity_id path.
Add storage_session_id as an alias for session_id in the response.
Both changes are backward compatible.

Author: joohan-lee

src/api/files.py

@@ -249,7 +249,13 @@ async def upload_files_batch(
     entity_id: Optional[str] = (
         entity_id_raw if isinstance(entity_id_raw, str) and entity_id_raw else None
     )
-    is_agent_file = entity_id is not None
+    # LibreChat sends kind=skill/agent (not entity_id) for skill-priming uploads.
+    # Treat these as agent files so skill bundles bypass the user-facing extension
+    # whitelist and are correctly tagged read-only in the sandbox.
+    kind_raw = form.get("kind")
+    is_agent_file = entity_id is not None or (
+        isinstance(kind_raw, str) and kind_raw in ("skill", "agent")
+    )
 
     read_only_raw = form.get("read_only")
     is_read_only = isinstance(read_only_raw, str) and read_only_raw.lower() in (
@@ -340,6 +346,7 @@ async def upload_files_batch(
     return {
         "message": message,
         "session_id": session_id,
+        "storage_session_id": session_id,  # LibreChat alias for session_id
         "files": results,
         "succeeded": succeeded,
         "failed": failed,

tests/integration/test_librechat_compat.py

@@ -2038,6 +2038,46 @@ def test_nested_filename_preserved_in_response(
         # The stored filename also preserves the path so S3/sandbox round-trip works.
         assert "skills/weather_lookup/SKILL.md" in setup_mocks["stored"]
 
+    def test_kind_skill_marks_files_as_agent(self, client, auth_headers, setup_mocks):
+        """LibreChat sends kind=skill (not entity_id) for skill-priming uploads.
+
+        appendCodeEnvFileIdentity() in LibreChat appends kind/id/version fields
+        to the multipart form — entity_id is never sent. The endpoint must
+        recognise kind=skill as an agent-file upload so that skill bundle files
+        with non-standard extensions (.xsd schemas, .toml configs, etc.) bypass
+        the user-facing extension whitelist and are tagged read-only in the sandbox.
+        """
+        files = [("file", ("schema.xsd", io.BytesIO(b"<xs:schema/>"), "application/xml"))]
+        data = {"kind": "skill", "id": "skill_abc123", "version": "3"}
+        response = client.post(
+            "/upload/batch", files=files, data=data, headers=auth_headers
+        )
+
+        assert response.status_code == 200
+        store = setup_mocks["file_service"].store_uploaded_file
+        assert store.await_count == 1
+        kwargs = store.await_args.kwargs
+        assert kwargs["is_agent_file"] is True
+
+    def test_batch_response_includes_storage_session_id(
+        self, client, auth_headers, setup_mocks
+    ):
+        """LibreChat's batchUploadCodeEnvFiles validates storage_session_id in the response.
+
+        crud.js throws if the field is absent:
+            if (!result.storage_session_id || !Array.isArray(result.files)) {
+                throw new Error(`Unexpected batch upload response: ...`)
+            }
+        The field must equal session_id (same underlying value, different name).
+        """
+        files = [("file", ("data.csv", io.BytesIO(b"a,b"), "text/csv"))]
+        response = client.post("/upload/batch", files=files, headers=auth_headers)
+
+        assert response.status_code == 200
+        body = response.json()
+        assert "storage_session_id" in body
+        assert body["storage_session_id"] == body["session_id"]
+
 
 # =============================================================================
 # GET /sessions/{session_id}/objects/{file_id} — liveness probe