fix(gateway): keep custom-skill rollback filesystem IO off the event loop (#5751)

* fix(gateway): keep custom-skill rollback filesystem IO off the event loop

Closes #5747. Three blocking sites remained on the Gateway event loop in
the rollback route and its shared response reader, all offloaded through
asyncio.to_thread following the #5729 pattern:

- storage.validate_skill_markdown_content() creates a temp dir and writes
  SKILL.md into it (site 1)
- the post-scan read of the file being replaced (site 2, named by #5729's
  test docstring)
- _read_custom_skill_response(): load_skills() walks every skill directory
  and read_custom_skill() opens SKILL.md — O(installed skills) loop time
  paid per accepted rollback and per GET /skills/custom/{name} (site 3)

A blocking_io regression drives the full accepted-rollback path (past the
security scan) under the strict gate; the two pre-existing cases only
covered the pre-scan branches.

* fix(gateway): keep the skill element type in the response-parts closure

Review nit on #5751: tuple[object, ...] erased the element type for the
_skill_to_response call; Skill is already imported in this module.

backend/app/gateway/routers/skills.py

@@ -505,12 +505,22 @@ async def get_custom_skill(skill_name: str, request: Request, config: AppConfig
 async def _read_custom_skill_response(skill_name: str, config: AppConfig) -> CustomSkillContentResponse:
     try:
         skill_name = skill_name.replace("\r\n", "").replace("\n", "")
-        storage = _get_user_skill_storage(config)
-        skills = storage.load_skills(enabled_only=False)
-        skill = next((s for s in skills if s.name == skill_name and s.category == SkillCategory.CUSTOM), None)
+
+        def _load_response_parts() -> tuple[Skill | None, str | None]:
+            # Worker thread: load_skills walks every skill directory and
+            # read_custom_skill opens SKILL.md — blocking filesystem IO that
+            # scales with the number of installed skills (#5747).
+            storage = _get_user_skill_storage(config)
+            skills = storage.load_skills(enabled_only=False)
+            skill = next((s for s in skills if s.name == skill_name and s.category == SkillCategory.CUSTOM), None)
+            if skill is None:
+                return None, None
+            return skill, storage.read_custom_skill(skill_name)
+
+        skill, content = await asyncio.to_thread(_load_response_parts)
         if skill is None:
             raise HTTPException(status_code=404, detail=f"Custom skill '{skill_name}' not found")
-        return CustomSkillContentResponse(**_skill_to_response(skill).model_dump(), content=storage.read_custom_skill(skill_name))
+        return CustomSkillContentResponse(**_skill_to_response(skill).model_dump(), content=content)
     except HTTPException:
         raise
     except Exception as e:
@@ -637,11 +647,17 @@ async def rollback_custom_skill(skill_name: str, body: SkillRollbackRequest, req
         target_content = record.get("prev_content")
         if target_content is None:
             raise HTTPException(status_code=400, detail="Selected history entry has no previous content to roll back to")
-        storage.validate_skill_markdown_content(skill_name, target_content)
+        await asyncio.to_thread(storage.validate_skill_markdown_content, skill_name, target_content)
         static_findings = await _scan_static_skill_markdown_or_raise(skill_name, target_content, app_config=config)
         scan = await scan_skill_content(target_content, executable=False, location=f"{skill_name}/{SKILL_MD_FILE}", app_config=config, static_findings=static_findings)
-        skill_file = storage.get_custom_skill_file(skill_name)
-        current_content = skill_file.read_text(encoding="utf-8") if skill_file.exists() else None
+
+        def _read_current_content() -> str | None:
+            # Worker thread: the post-scan read of the file being replaced is
+            # blocking filesystem IO (#5747), same rule as the history read.
+            skill_file = storage.get_custom_skill_file(skill_name)
+            return skill_file.read_text(encoding="utf-8") if skill_file.exists() else None
+
+        current_content = await asyncio.to_thread(_read_current_content)
         history_entry = {
             "action": "rollback",
             "author": "human",

backend/tests/blocking_io/test_skills_custom_router.py

@@ -19,6 +19,7 @@ import asyncio
 import json
 from pathlib import Path
 from types import SimpleNamespace
+from unittest.mock import AsyncMock
 from uuid import UUID
 
 import pytest
@@ -84,3 +85,23 @@ async def test_rollback_history_read_does_not_block_event_loop() -> None:
 
     assert excinfo.value.status_code == 400
     assert "history_index is out of range" in str(excinfo.value.detail)
+
+
+async def test_rollback_accepted_path_does_not_block_event_loop(monkeypatch) -> None:
+    """The accepted-rollback path (validate → scan → current-content read →
+    write → append → response read) must keep every filesystem operation off
+    the loop (#5747)."""
+    await asyncio.to_thread(_install_skill)
+    await asyncio.to_thread(
+        _write_history,
+        [{"action": "edit", "ts": 1, "prev_content": _SKILL_MD, "new_content": _SKILL_MD}],
+    )
+    monkeypatch.setattr(
+        "app.gateway.routers.skills.scan_skill_content",
+        AsyncMock(return_value=SimpleNamespace(decision="allow", reason="ok", static_findings=[])),
+    )
+    config = AppConfig.model_validate({"sandbox": {"use": "test"}})
+
+    response = await rollback_custom_skill(_SKILL_NAME, SkillRollbackRequest(history_index=0), _admin_request(), config)
+
+    assert response.content == _SKILL_MD