diff --git a/acp_adapter/server.py b/acp_adapter/server.py
index 7b10f2c23a24..2b76163488be 100644
--- a/acp_adapter/server.py
+++ b/acp_adapter/server.py
@@ -1761,7 +1761,16 @@ class HermesACPAgent(acp.Agent):
                     clear_session_vars,
                     set_session_vars,
                 )
-                session_tokens = set_session_vars(session_key=session_id)
+                # ``cwd`` pins the logical working directory for this context,
+                # which is what the system prompt's "Current working directory"
+                # line reports (agent/prompt_builder.py -> resolve_agent_cwd).
+                # Without it the prompt advertises the global Hermes workspace
+                # while the tools are rooted at the client's project, so the
+                # model emits absolute paths under ~/.hermes/workspace and the
+                # edit silently lands outside the editor's workspace.
+                session_tokens = set_session_vars(
+                    session_key=session_id, cwd=state.cwd,
+                )
             except Exception:
                 session_tokens = None
                 clear_session_vars = None  # type: ignore[assignment]
diff --git a/tests/acp/test_server.py b/tests/acp/test_server.py
index 96101b93ef93..c7d6fda498cc 100644
--- a/tests/acp/test_server.py
+++ b/tests/acp/test_server.py
@@ -1639,6 +1639,47 @@ class TestPrompt:
             text and "[plugin appended this]" in text for text in all_texts
         ), f"expected transformed final to be delivered, got: {all_texts!r}"
 
+    @pytest.mark.asyncio
+    async def test_prompt_pins_the_session_cwd_for_the_turn(self, agent, tmp_path):
+        """The turn must resolve the ACP client's cwd, not the Hermes workspace.
+
+        ``resolve_agent_cwd()`` is what the system prompt reports as "Current
+        working directory". When it is left unpinned it falls back to
+        TERMINAL_CWD / the launch dir, so the prompt advertises one root while
+        the tools are rooted at the editor's project. The model then emits
+        absolute paths under the advertised root and the edit silently lands
+        outside the workspace the client asked for.
+        """
+        workspace = tmp_path / "project"
+        workspace.mkdir()
+
+        new_resp = await agent.new_session(cwd=str(workspace))
+        state = agent.session_manager.get_session(new_resp.session_id)
+
+        observed = {}
+
+        def capture_cwd(*args, **kwargs):
+            from agent.runtime_cwd import resolve_agent_cwd
+
+            observed["cwd"] = str(resolve_agent_cwd())
+            return {"final_response": "ok", "messages": []}
+
+        state.agent.run_conversation = capture_cwd
+
+        mock_conn = MagicMock(spec=acp.Client)
+        mock_conn.session_update = AsyncMock()
+        agent._conn = mock_conn
+
+        await agent.prompt(
+            prompt=[TextContentBlock(type="text", text="hello")],
+            session_id=new_resp.session_id,
+        )
+
+        assert observed.get("cwd") == str(workspace), (
+            "the turn resolved "
+            f"{observed.get('cwd')!r} instead of the ACP client's cwd "
+            f"{str(workspace)!r}"
+        )
 
     @pytest.mark.asyncio
     async def test_prompt_auto_titles_session(self, agent):