diff --git a/mini_agent/acp/__init__.py b/mini_agent/acp/__init__.py
index b794e451..5a2b3c68 100644
--- a/mini_agent/acp/__init__.py
+++ b/mini_agent/acp/__init__.py
@@ -108,7 +108,12 @@ async def prompt(self, params: PromptRequest) -> PromptResponse:
         if not state:
             # Auto-create session if not found (compatibility with clients that skip newSession)
             logger.warning(f"Session '{params.sessionId}' not found, auto-creating new session")
-            new_session = await self.newSession(NewSessionRequest(cwd=None))
+            new_session = await self.newSession(
+                NewSessionRequest(
+                    cwd=str(self._config.agent.workspace_dir),
+                    mcpServers=[],
+                )
+            )
             state = self._sessions.get(new_session.sessionId)
             if not state:
                 logger.error("Failed to auto-create session")
diff --git a/tests/test_acp.py b/tests/test_acp.py
index c0fd3794..608c7c6a 100644
--- a/tests/test_acp.py
+++ b/tests/test_acp.py
@@ -1,5 +1,6 @@
 """Integration tests for the MiniMax ACP adapter."""
 
+from pathlib import Path
 from types import SimpleNamespace
 
 import pytest
@@ -82,8 +83,11 @@ async def test_acp_turn_executes_tool(acp_agent):
 
 
 @pytest.mark.asyncio
-async def test_acp_invalid_session(acp_agent):
+async def test_acp_unknown_session_auto_creates_with_configured_workspace(acp_agent):
     agent, _ = acp_agent
     prompt = SimpleNamespace(sessionId="missing", prompt=[{"text": "?"}])
     response = await agent.prompt(prompt)
-    assert response.stopReason == "refusal"
+    assert response.stopReason == "end_turn"
+    assert len(agent._sessions) == 1
+    created_session = next(iter(agent._sessions.values()))
+    assert Path(created_session.agent.workspace_dir) == Path(agent._config.agent.workspace_dir)