fix: remove CLI backend assumptions and add docker fallback prompt

- Remove hardcoded --backend=macos-container flag reference in
  firecracker/util.py require_firecracker() error message
- Remove --backend flag from cli.py start; backend selection now
  driven exclusively by BOT_BOTTLE_BACKEND env var or auto-selection
- Skip unavailable backends in cli.py cleanup (fixes crash on Linux
  when macos-container.prepare_cleanup calls require_container())
- Add two-tier auto-selection: VM backend first (macos-container on
  macOS, firecracker on Linux+KVM); fall back to docker with a
  security warning and interactive i/d/q prompt; exit if docker
  also unavailable and print VM install instructions

Closes #344

tests/unit/test_backend_selection.py

@@ -57,14 +57,16 @@ class TestGetBottleBackend(unittest.TestCase):
                 return self._available
 
         # No macOS container and the host can't run firecracker (no
-        # KVM / not Linux) → docker is the last resort.
+        # KVM / not Linux) → docker fallback with a prompt. Simulate
+        # the user picking "d" (use docker anyway).
         with patch.dict(os.environ, {}, clear=True), \
              patch.object(backend_mod.FirecrackerBottleBackend,
                           "is_host_capable", classmethod(lambda cls: False)), \
              patch.object(backend_mod, "_backends", {
                  "macos-container": _FakeBackend("macos-container", False),
                  "docker": _FakeBackend("docker", True),
-             }):
+             }), \
+             patch.object(backend_mod, "_read_tty_line", return_value="d"):
             b = get_bottle_backend()
         self.assertEqual("docker", b.name)
 
@@ -96,6 +98,49 @@ class TestGetBottleBackend(unittest.TestCase):
             with self.assertRaises(SystemExit):
                 get_bottle_backend("nonexistent")
 
+    def test_no_backend_available_dies(self):
+        # No VM and no docker → print install instructions and die.
+        class _FakeBackend:
+            def __init__(self, name: str, available: bool) -> None:
+                self.name = name
+                self._available = available
+
+            def is_available(self) -> bool:
+                return self._available
+
+        with patch.dict(os.environ, {}, clear=True), \
+             patch.object(backend_mod.FirecrackerBottleBackend,
+                          "is_host_capable", classmethod(lambda cls: False)), \
+             patch.object(backend_mod, "_backends", {
+                 "macos-container": _FakeBackend("macos-container", False),
+                 "docker": _FakeBackend("docker", False),
+             }), \
+             patch.object(backend_mod, "die", side_effect=SystemExit("die")):
+            with self.assertRaises(SystemExit):
+                get_bottle_backend()
+
+    def test_docker_fallback_user_quits(self):
+        # VM unavailable, docker available, user picks "q" → die.
+        class _FakeBackend:
+            def __init__(self, name: str, available: bool) -> None:
+                self.name = name
+                self._available = available
+
+            def is_available(self) -> bool:
+                return self._available
+
+        with patch.dict(os.environ, {}, clear=True), \
+             patch.object(backend_mod.FirecrackerBottleBackend,
+                          "is_host_capable", classmethod(lambda cls: False)), \
+             patch.object(backend_mod, "_backends", {
+                 "macos-container": _FakeBackend("macos-container", False),
+                 "docker": _FakeBackend("docker", True),
+             }), \
+             patch.object(backend_mod, "_read_tty_line", return_value="q"), \
+             patch.object(backend_mod, "die", side_effect=SystemExit("die")):
+            with self.assertRaises(SystemExit):
+                get_bottle_backend()
+
 
 class TestKnownBackendNames(unittest.TestCase):
     def test_returns_backends_sorted(self):

tests/unit/test_cli_cleanup_cross_backend.py

@@ -1,8 +1,10 @@
-"""Unit: `cli.py cleanup` walks every backend (issue follow-up).
+"""Unit: `cli.py cleanup` walks every available backend.
 
-Asserts cmd_cleanup queries each backend's `prepare_cleanup`,
-combines the y/N output, and runs each backend's `cleanup` when
-the operator confirms.  Mocks the backends and stdin."""
+Asserts cmd_cleanup queries each available backend's `prepare_cleanup`,
+combines the y/N output, and runs each backend's `cleanup` when the
+operator confirms. Unavailable backends (e.g. macos-container on Linux)
+are skipped so that `cleanup` never errors on a platform where a backend
+is not installed. Mocks the backends and stdin."""
 
 from __future__ import annotations
 
@@ -21,7 +23,7 @@ def _make_backend(empty: bool = True):
 
 
 class TestCmdCleanup(unittest.TestCase):
-    def test_iterates_every_backend(self):
+    def test_iterates_every_available_backend(self):
         docker, docker_plan = _make_backend(empty=False)
         fc, fc_plan = _make_backend(empty=False)
         backends_by_name = {"docker": docker, "firecracker": fc}
@@ -32,6 +34,8 @@ class TestCmdCleanup(unittest.TestCase):
         ), patch.object(
             cmd, "get_bottle_backend",
             side_effect=lambda name: backends_by_name[name],  # type: ignore
+        ), patch.object(
+            cmd, "has_backend", return_value=True,
         ), patch.object(
             cmd, "_prompt_yes", return_value=True,
         ):
@@ -42,6 +46,32 @@ class TestCmdCleanup(unittest.TestCase):
         docker.cleanup.assert_called_once_with(docker_plan)
         fc.cleanup.assert_called_once_with(fc_plan)
 
+    def test_skips_unavailable_backends(self):
+        # macos-container is not available on Linux — must be silently skipped.
+        docker, docker_plan = _make_backend(empty=False)
+        macos = MagicMock()
+        backends_by_name = {"docker": docker}
+
+        def _has(name: str) -> bool:
+            return name == "docker"
+
+        with patch.object(
+            cmd, "known_backend_names",
+            return_value=("docker", "macos-container"),
+        ), patch.object(
+            cmd, "get_bottle_backend",
+            side_effect=lambda name: backends_by_name[name],  # type: ignore
+        ), patch.object(
+            cmd, "has_backend", side_effect=_has,
+        ), patch.object(
+            cmd, "_prompt_yes", return_value=True,
+        ):
+            self.assertEqual(0, cmd.cmd_cleanup([]))
+
+        docker.prepare_cleanup.assert_called_once()
+        docker.cleanup.assert_called_once_with(docker_plan)
+        macos.prepare_cleanup.assert_not_called()
+
     def test_short_circuits_when_all_empty(self):
         docker, _ = _make_backend(empty=True)
         fc, _ = _make_backend(empty=True)
@@ -53,6 +83,8 @@ class TestCmdCleanup(unittest.TestCase):
         ), patch.object(
             cmd, "get_bottle_backend",
             side_effect=lambda name: backends_by_name[name],  # type: ignore
+        ), patch.object(
+            cmd, "has_backend", return_value=True,
         ), patch.object(
             cmd, "_prompt_yes",
         ) as prompt:
@@ -72,6 +104,8 @@ class TestCmdCleanup(unittest.TestCase):
         ), patch.object(
             cmd, "get_bottle_backend",
             side_effect=lambda name: backends_by_name[name],  # type: ignore
+        ), patch.object(
+            cmd, "has_backend", return_value=True,
         ), patch.object(
             cmd, "_prompt_yes", return_value=False,
         ):
@@ -92,6 +126,8 @@ class TestCmdCleanup(unittest.TestCase):
         ), patch.object(
             cmd, "get_bottle_backend",
             side_effect=lambda name: backends_by_name[name],  # type: ignore
+        ), patch.object(
+            cmd, "has_backend", return_value=True,
         ), patch.object(
             cmd, "_prompt_yes", return_value=True,
         ):

tests/unit/test_cli_start_backend_flag.py

@@ -1,61 +1,29 @@
-"""Unit: `cli.py start --backend=<name>` flag (issue #77).
+"""Unit: backend resolution priority — explicit name > env var.
 
-Asserts that the flag wins over the env var, that the env var is
-the fallback, and that the choices are pulled from the backend
-registry (so adding a backend lights up in argparse without code
-edits)."""
+The `--backend` flag has been removed from `cli.py start`; backend
+selection is driven by BOT_BOTTLE_BACKEND or auto-selection only.
+`get_bottle_backend` still accepts an explicit name so that `resume`
+(which records the backend from a prior session) and the `backend`
+sub-command can pass one directly."""
 
 from __future__ import annotations
 
-import argparse
 import os
 import unittest
 from unittest.mock import patch
 
 
-from bot_bottle.backend import known_backend_names
-
-
-class TestStartBackendFlag(unittest.TestCase):
-    """The flag is wired by `cmd_start`'s argparse and threaded
-    through `prepare_with_preflight(backend_name=...)`. Rather than
-    drive the whole start flow (which builds containers), we test
-    the argparse shape and the resolution function separately."""
-
-    def _build_parser(self):
-        # Mirror the parser definition from `cmd_start` so this
-        # test doesn't have to invoke the full command.
-        parser = argparse.ArgumentParser(prog="cb start")
-        parser.add_argument(
-            "--backend",
-            choices=known_backend_names(),
-            default=None,
-        )
-        parser.add_argument("name")
-        return parser
-
-    def test_flag_recognized(self):
-        args = self._build_parser().parse_args(["--backend=firecracker", "researcher"])
-        self.assertEqual("firecracker", args.backend)
-        self.assertEqual("researcher", args.name)
-
-    def test_flag_default_none_means_env_or_default_backend(self):
-        args = self._build_parser().parse_args(["researcher"])
-        self.assertIsNone(args.backend)
-
-    def test_invalid_backend_rejected_by_argparse(self):
-        parser = self._build_parser()
-        with self.assertRaises(SystemExit):
-            parser.parse_args(["--backend=garbage", "researcher"])
-
-    def test_resolution_priority_explicit_over_env(self):
-        # Independent assertion that get_bottle_backend (where
-        # `--backend` ultimately threads to) prefers the explicit
-        # name over BOT_BOTTLE_BACKEND.
+class TestBackendResolutionPriority(unittest.TestCase):
+    def test_explicit_name_overrides_env_var(self):
         from bot_bottle.backend import get_bottle_backend
         with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}):
             self.assertEqual("docker", get_bottle_backend("docker").name)
 
+    def test_env_var_used_when_no_explicit_name(self):
+        from bot_bottle.backend import get_bottle_backend
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}):
+            self.assertEqual("firecracker", get_bottle_backend().name)
+
 
 if __name__ == "__main__":
     unittest.main()

tests/unit/test_cli_start_headless.py

@@ -175,14 +175,6 @@ class TestCmdStartHeadless(unittest.TestCase):
             )
         self.assertEqual("researcher-2", self._spec().label)
 
-    # -- backend wiring ------------------------------------------------
-
-    def test_backend_flag_forwarded(self):
-        start_mod.cmd_start(
-            ["--headless", "--backend=docker", "researcher", "--bottle", "claude",
-             "--prompt", "Do it"]
-        )
-        self.assertEqual("docker", self._launch_mock.call_args[1]["backend_name"])
 
 
 class TestPrepareWithPreflight(unittest.TestCase):

tests/unit/test_cli_start_selector.py

@@ -82,7 +82,7 @@ class TestCmdStartSelector(unittest.TestCase):
     # ------------------------------------------------------------------
 
     def test_explicit_agent_skips_agent_picker(self):
-        rc = start_mod.cmd_start(["--backend=docker", "researcher"])
+        rc = start_mod.cmd_start(["researcher"])
         self.assertEqual(0, rc)
         self._agent_picker_mock.assert_not_called()
         self._bottle_picker_mock.assert_called_once()
@@ -100,7 +100,7 @@ class TestCmdStartSelector(unittest.TestCase):
 
     def test_agent_absent_shows_agent_picker(self):
         self._agent_picker_mock.return_value = "researcher"
-        rc = start_mod.cmd_start(["--backend=docker"])
+        rc = start_mod.cmd_start([])
         self.assertEqual(0, rc)
         self._agent_picker_mock.assert_called_once()
         call_kwargs = self._agent_picker_mock.call_args
@@ -111,7 +111,7 @@ class TestCmdStartSelector(unittest.TestCase):
 
     def test_agent_picker_cancel_skips_bottle_picker(self):
         self._agent_picker_mock.return_value = None
-        rc = start_mod.cmd_start(["--backend=docker"])
+        rc = start_mod.cmd_start([])
         self.assertEqual(0, rc)
         self._bottle_picker_mock.assert_not_called()
         self._launch_mock.assert_not_called()
@@ -168,16 +168,6 @@ class TestCmdStartSelector(unittest.TestCase):
     # Backend wiring
     # ------------------------------------------------------------------
 
-    def test_explicit_backend_forwarded(self):
-        start_mod.cmd_start(["--backend=docker", "researcher"])
-        _, kwargs = self._launch_mock.call_args
-        self.assertEqual("docker", kwargs["backend_name"])
-
-    def test_absent_backend_uses_default(self):
-        start_mod.cmd_start(["researcher"])
-        _, kwargs = self._launch_mock.call_args
-        self.assertIsNone(kwargs["backend_name"])
-
     def test_bot_bottle_backend_env_skips_backend_picker(self):
         os.environ["BOT_BOTTLE_BACKEND"] = "docker"
         try: