diff --git a/backend/packages/harness/deerflow/tools/builtins/invoke_acp_agent_tool.py b/backend/packages/harness/deerflow/tools/builtins/invoke_acp_agent_tool.py
index 4d464cece..d7e32314e 100644
--- a/backend/packages/harness/deerflow/tools/builtins/invoke_acp_agent_tool.py
+++ b/backend/packages/harness/deerflow/tools/builtins/invoke_acp_agent_tool.py
@@ -215,9 +215,9 @@ def build_invoke_acp_agent_tool(agents: dict) -> BaseTool:
         client = _CollectingClient()
         cmd = agent_config.command
         args = agent_config.args or []
-        physical_cwd = _get_work_dir(thread_id)
+        physical_cwd = await asyncio.to_thread(_get_work_dir, thread_id)
         try:
-            mcp_servers = _build_acp_mcp_servers()
+            mcp_servers = await asyncio.to_thread(_build_acp_mcp_servers)
         except ValueError as exc:
             logger.warning(
                 "Invalid MCP server configuration for ACP agent '%s'; continuing without MCP servers: %s",
@@ -268,7 +268,7 @@ def build_invoke_acp_agent_tool(agents: dict) -> BaseTool:
             return result or "(no response)"
         except Exception as e:
             logger.error("ACP agent '%s' invocation failed: %s", agent, e)
-            return _format_invocation_error(agent, cmd, e)
+            return await asyncio.to_thread(_format_invocation_error, agent, cmd, e)
 
     return StructuredTool.from_function(
         name="invoke_acp_agent",
diff --git a/backend/tests/blocking_io/test_invoke_acp_agent_tool.py b/backend/tests/blocking_io/test_invoke_acp_agent_tool.py
new file mode 100644
index 000000000..97ef268cd
--- /dev/null
+++ b/backend/tests/blocking_io/test_invoke_acp_agent_tool.py
@@ -0,0 +1,68 @@
+"""Regression test for ACP invocation setup on the event loop."""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+from types import SimpleNamespace
+from typing import Any
+
+import acp
+import pytest
+
+from deerflow.config.acp_config import ACPAgentConfig
+from deerflow.tools.builtins import invoke_acp_agent_tool as acp_tool
+
+pytestmark = pytest.mark.asyncio
+
+
+async def test_invoke_acp_agent_setup_does_not_block_event_loop(monkeypatch, tmp_path) -> None:
+    from deerflow.config import paths as paths_module
+
+    configured_paths = SimpleNamespace(
+        base_dir=tmp_path,
+        acp_workspace_dir=lambda thread_id, user_id=None: tmp_path / "threads" / thread_id / "acp-workspace",
+    )
+    monkeypatch.setattr(paths_module, "get_paths", lambda: configured_paths)
+    config_path = tmp_path / "extensions_config.json"
+    await asyncio.to_thread(
+        config_path.write_text,
+        '{"mcpServers": {"test-server": {"enabled": true, "type": "http", "url": "https://example.test/mcp"}}, "skills": {}}',
+        encoding="utf-8",
+    )
+    monkeypatch.setenv("DEER_FLOW_EXTENSIONS_CONFIG_PATH", str(config_path))
+
+    captured: dict[str, Any] = {}
+
+    class _Connection:
+        async def initialize(self, **kwargs: Any) -> None:
+            captured["initialize"] = kwargs
+
+        async def new_session(self, **kwargs: Any) -> SimpleNamespace:
+            captured["new_session"] = kwargs
+            return SimpleNamespace(session_id="session-1")
+
+        async def prompt(self, **kwargs: Any) -> None:
+            captured["prompt"] = kwargs
+
+    @contextlib.asynccontextmanager
+    async def fake_spawn_agent_process(client, command, *args, env=None, cwd=None):
+        captured["cwd"] = cwd
+        yield _Connection(), SimpleNamespace()
+
+    monkeypatch.setattr(acp, "spawn_agent_process", fake_spawn_agent_process)
+
+    tool = acp_tool.build_invoke_acp_agent_tool(
+        {"test-agent": ACPAgentConfig(command="test-agent", description="Test agent")},
+    )
+    result = await tool.coroutine(
+        agent="test-agent",
+        prompt="run",
+        config={"configurable": {"thread_id": "thread-1"}},
+    )
+
+    expected_cwd = tmp_path / "threads" / "thread-1" / "acp-workspace"
+    assert result == "(no response)"
+    assert captured["cwd"] == str(expected_cwd)
+    assert captured["new_session"]["mcp_servers"] == [{"name": "test-server", "type": "http", "url": "https://example.test/mcp", "headers": []}]
+    assert expected_cwd.is_dir()