diff --git a/agent/copilot_acp_client.py b/agent/copilot_acp_client.py
index 783f94956..94d40d2d9 100644
--- a/agent/copilot_acp_client.py
+++ b/agent/copilot_acp_client.py
@@ -46,6 +46,47 @@ def _resolve_args() -> list[str]:
     return shlex.split(raw)
 
 
+def _resolve_home_dir() -> str:
+    """Return a stable HOME for child ACP processes."""
+
+    try:
+        from hermes_constants import get_subprocess_home
+
+        profile_home = get_subprocess_home()
+        if profile_home:
+            return profile_home
+    except Exception:
+        pass
+
+    home = os.environ.get("HOME", "").strip()
+    if home:
+        return home
+
+    expanded = os.path.expanduser("~")
+    if expanded and expanded != "~":
+        return expanded
+
+    try:
+        import pwd
+
+        resolved = pwd.getpwuid(os.getuid()).pw_dir.strip()
+        if resolved:
+            return resolved
+    except Exception:
+        pass
+
+    # Last resort: /tmp (writable on any POSIX system). Avoids crashing the
+    # subprocess with no HOME; callers can set HERMES_HOME explicitly if they
+    # need a different writable dir.
+    return "/tmp"
+
+
+def _build_subprocess_env() -> dict[str, str]:
+    env = os.environ.copy()
+    env["HOME"] = _resolve_home_dir()
+    return env
+
+
 def _jsonrpc_error(message_id: Any, code: int, message: str) -> dict[str, Any]:
     return {
         "jsonrpc": "2.0",
@@ -382,6 +423,7 @@ class CopilotACPClient:
                 text=True,
                 bufsize=1,
                 cwd=self._acp_cwd,
+                env=_build_subprocess_env(),
             )
         except FileNotFoundError as exc:
             raise RuntimeError(
diff --git a/tests/agent/test_copilot_acp_client.py b/tests/agent/test_copilot_acp_client.py
index 52ad20a35..63c87fdab 100644
--- a/tests/agent/test_copilot_acp_client.py
+++ b/tests/agent/test_copilot_acp_client.py
@@ -144,3 +144,60 @@ class CopilotACPClientSafetyTests(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+# ── HOME env propagation tests (from PR #11285) ─────────────────────
+
+from unittest.mock import patch as _patch
+import pytest
+
+
+def _make_home_client(tmp_path):
+    return CopilotACPClient(
+        api_key="copilot-acp",
+        base_url="acp://copilot",
+        acp_command="copilot",
+        acp_args=["--acp", "--stdio"],
+        acp_cwd=str(tmp_path),
+    )
+
+
+def _fake_popen_capture(captured):
+    def _fake(cmd, **kwargs):
+        captured["cmd"] = cmd
+        captured["kwargs"] = kwargs
+        raise FileNotFoundError("copilot not found")
+    return _fake
+
+
+def test_run_prompt_prefers_profile_home_when_available(monkeypatch, tmp_path):
+    hermes_home = tmp_path / "hermes"
+    profile_home = hermes_home / "home"
+    profile_home.mkdir(parents=True)
+
+    monkeypatch.delenv("HOME", raising=False)
+    monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+
+    captured = {}
+    client = _make_home_client(tmp_path)
+
+    with _patch("agent.copilot_acp_client.subprocess.Popen", side_effect=_fake_popen_capture(captured)):
+        with pytest.raises(RuntimeError, match="Could not start Copilot ACP command"):
+            client._run_prompt("hello", timeout_seconds=1)
+
+    assert captured["kwargs"]["env"]["HOME"] == str(profile_home)
+
+
+def test_run_prompt_passes_home_when_parent_env_is_clean(monkeypatch, tmp_path):
+    monkeypatch.delenv("HOME", raising=False)
+    monkeypatch.delenv("HERMES_HOME", raising=False)
+
+    captured = {}
+    client = _make_home_client(tmp_path)
+
+    with _patch("agent.copilot_acp_client.subprocess.Popen", side_effect=_fake_popen_capture(captured)):
+        with pytest.raises(RuntimeError, match="Could not start Copilot ACP command"):
+            client._run_prompt("hello", timeout_seconds=1)
+
+    assert "env" in captured["kwargs"]
+    assert captured["kwargs"]["env"]["HOME"]