diff --git a/orchestrator/orchestrator/runner.py b/orchestrator/orchestrator/runner.py
index 42ca802..b4b0857 100644
--- a/orchestrator/orchestrator/runner.py
+++ b/orchestrator/orchestrator/runner.py
@@ -204,7 +204,9 @@ def _job_cmd(host: str, job: dict, cfg: "Config") -> list[str]:
     matching lightweight CLI subcommand — a single TC quarantineWorker call, using
     this runner's existing TC creds. "tart-host-update" runs the tart VM host maintenance
     flow (orchestrator/tart_host.py); "builder-upgrade" the Intel builder macOS 15 upgrade
-    (orchestrator/builder_upgrade.py). Anything else is the full reprovision workflow.
+    (orchestrator/builder_upgrade.py). "reprovision" (or no action: old rows) is the full EACS
+    workflow. Anything else raises ValueError: a newer Hangar can queue an action this runner
+    predates, and falling through to `run` would turn it into a wipe.
     """
     exe = _reprovision_exe()
     action = job.get("action") or "reprovision"
@@ -234,6 +236,8 @@ def _job_cmd(host: str, job: dict, cfg: "Config") -> list[str]:
         if params.get("puppet_only"):
             cmd.append("--puppet-only")
         return cmd
+    if action != "reprovision":
+        raise ValueError(f"unknown action {action!r} -- this runner predates it; update relops-bootstrap")
     cmd = [exe, "run", host]
     if cfg.unquarantine:
         cmd.append("--unquarantine")
@@ -250,7 +254,11 @@ def _run_job(client: httpx.Client, cfg: Config, job: dict) -> None:
     except ValueError as e:
         _complete(client, cfg, job_id, False, f"rejected: {e}")
         return
-    cmd = _job_cmd(host, job, cfg)
+    try:
+        cmd = _job_cmd(host, job, cfg)
+    except ValueError as e:
+        _complete(client, cfg, job_id, False, f"rejected: {e}")
+        return
     _event(client, cfg, job_id, f"runner {cfg.runner_id} starting: {' '.join(cmd[1:])}")
     try:
         proc = subprocess.Popen(
diff --git a/orchestrator/tests/test_runner.py b/orchestrator/tests/test_runner.py
index 82219fa..2a910a1 100644
--- a/orchestrator/tests/test_runner.py
+++ b/orchestrator/tests/test_runner.py
@@ -7,6 +7,7 @@
 from unittest.mock import MagicMock, patch
 
 import httpx
+import pytest
 
 from orchestrator import runner
 
@@ -95,6 +96,27 @@ def test_job_cmd_unquarantine():
         assert runner._job_cmd("macmini-m4-80", job, _Cfg()) == ["reprovision", "unquarantine", "macmini-m4-80"]
 
 
+def test_job_cmd_explicit_reprovision_action_runs_the_workflow():
+    job = {"action": "reprovision", "params": {}}
+    assert runner._job_cmd("macmini-m4-80", job, _Cfg())[1:] == ["run", "macmini-m4-80"]
+
+
+def test_job_cmd_unknown_action_is_rejected_not_run():
+    # A Hangar newer than this runner can queue an action it doesn't know. Falling through
+    # to `run` would turn that into an EACS wipe.
+    with pytest.raises(ValueError, match="unknown action"):
+        runner._job_cmd("macmini-r8-124", {"action": "some-future-action", "params": {}}, _Cfg())
+
+
+def test_run_job_fails_unknown_action_without_starting_anything():
+    client = MagicMock()
+    with patch("orchestrator.runner.subprocess.Popen") as popen:
+        runner._run_job(client, _Cfg(), {"id": 11, "short": "macmini-r8-124", "action": "some-future-action"})
+    popen.assert_not_called()
+    body = client.post.call_args.kwargs["json"]
+    assert body["success"] is False and "unknown action" in body["detail"]
+
+
 def test_complete_retries_until_success():
     client = MagicMock()
     ok = MagicMock()