diff --git a/src/panopticon/sessionservice/host.py b/src/panopticon/sessionservice/host.py
index bde59aa8..4ca1c80d 100644
--- a/src/panopticon/sessionservice/host.py
+++ b/src/panopticon/sessionservice/host.py
@@ -194,9 +194,9 @@ def run_host(
     HostDaemon(client, spawner, provisioner, interval=interval, sleep=sleep).run(until=until)
 
 
-def main(
-    argv: list[str] | None = None, *, client: TaskServiceClient | None = None
-) -> None:  # pragma: no cover - thin wiring + endless loop
+def build_arg_parser() -> argparse.ArgumentParser:
+    """Build the ``host`` daemon's CLI parser — split out from :func:`main` so tests can inspect
+    argument defaults (e.g. ``--host``) without running the endless daemon loop."""
     parser = argparse.ArgumentParser(
         prog="python -m panopticon.sessionservice.host",
         description="Per-host session service: spawn tasks + provision them (ADR 0008/0011).",
@@ -226,7 +226,13 @@ def main(
         default=2.0,
         help="change-feed long-poll wait, seconds (the keepalive ceiling between blocking calls)",
     )
-    args = parser.parse_args(argv)
+    return parser
+
+
+def main(
+    argv: list[str] | None = None, *, client: TaskServiceClient | None = None
+) -> None:  # pragma: no cover - thin wiring + endless loop
+    args = build_arg_parser().parse_args(argv)
     logging.basicConfig(
         level=logging.INFO, format="%(asctime)s %(levelname)s %(name)s: %(message)s"
     )
diff --git a/tests/sessionservice/test_host.py b/tests/sessionservice/test_host.py
index f493b9a1..226e0f35 100644
--- a/tests/sessionservice/test_host.py
+++ b/tests/sessionservice/test_host.py
@@ -9,13 +9,19 @@
 from pathlib import Path
 
 import httpx
+import pytest
 from fastapi.testclient import TestClient
 
 from panopticon.client import JsonObj, TaskServiceClient
 from panopticon.core.git import GitClones
 from panopticon.core.models import Repo
 from panopticon.sessionservice.clones import CloneCache
-from panopticon.sessionservice.host import HostDaemon, hold_runner_liveness, run_host
+from panopticon.sessionservice.host import (
+    HostDaemon,
+    build_arg_parser,
+    hold_runner_liveness,
+    run_host,
+)
 from panopticon.taskservice.api import create_app
 from panopticon.taskservice.artifacts_fs import FilesystemArtifactStore
 from panopticon.taskservice.service import TaskService
@@ -356,6 +362,28 @@ def gen() -> Generator[None, None, None]:
     assert recorded == ["box.example.com"]
 
 
+def test_build_arg_parser_host_defaults_to_empty_when_unset(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    # A local runner must not report a host at all — any truthy value makes the terminal
+    # supervisor wrap tmux attach in `ssh -t <host>`, which breaks a purely local session.
+    monkeypatch.delenv("PANOPTICON_RUNNER_HOST", raising=False)
+    args = build_arg_parser().parse_args([])
+    assert args.host == ""
+
+
+def test_build_arg_parser_host_from_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("PANOPTICON_RUNNER_HOST", "box.example.com")
+    args = build_arg_parser().parse_args([])
+    assert args.host == "box.example.com"
+
+
+def test_build_arg_parser_host_flag_overrides_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("PANOPTICON_RUNNER_HOST", "box.example.com")
+    args = build_arg_parser().parse_args(["--host", "other.example.com"])
+    assert args.host == "other.example.com"
+
+
 def test_run_host_spawns_then_provisions_end_to_end(tmp_path: Path) -> None:
     service = TaskService(SqlAlchemyStore(), {"spike": Spike()}, FilesystemArtifactStore(tmp_path))
     asyncio.run(service.init())