fix(macos): name the gateway instead of addressing it, so bottles survive it moving

The shared gateway's address is DHCP-assigned and changes whenever the infra
container is recreated — a source-hash bump, an image upgrade, a crash. Every
agent-facing URL embedded that address, and the proxy URL reaches the agent
as process environment at `container exec` time. A running process's environ
cannot be rewritten from outside, so a moved gateway stranded every running
bottle permanently: not degraded, unreachable, until relaunched and its agent
session thrown away.

Give the agent a stable name instead. `GATEWAY_HOSTNAME` replaces the address
in the egress proxy URL, NO_PROXY, git-http, and supervise URLs, and resolves
through the bottle's own /etc/hosts. Unlike environ that is a file, so it can
be rewritten inside a container that is already running — which is the whole
point: a gateway that returns at a new address is picked up by live bottles.

Launch writes the entry before anything execs (every agent URL names the
gateway, so it must resolve for the first connection), and re-points every
running bottle once the gateway is up, so one stranded by an earlier restart
re-attaches instead of needing a relaunch.

The write needs root and the agent runs as `node`: the host can repoint a
bottle's gateway name, the agent cannot repoint its own. Keep that asymmetry.

Apple Container 1.0 has no container-name DNS on a user network and
`container run` has no --add-host, so the entry is written by exec after
start rather than declared at run.

Does not address the other half of #443: per-bottle egress auth tokens are
held in memory by the orchestrator and are still lost across a restart, so a
re-attached bottle resolves its policy but not its injected credentials.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/__init__.py

@@ -45,7 +45,8 @@ from typing import TYPE_CHECKING, Any, Generic, Sequence, TypeVar
 from ..agent_provider import AgentProvisionPlan, get_provider, build_agent_provision_plan
 from ..egress import EgressPlan
 from ..git_gate import GitGatePlan
-from ..log import die, info
+from ..log import die, info, warn
+from ..util import read_tty_line
 from ..manifest import Manifest, ManifestIndex
 from ..supervise import SupervisePlan
 from ..util import expand_tilde
@@ -648,19 +649,26 @@ def __getattr__(name: str) -> Any:
 
 def get_bottle_backend(
     name: str | None = None,
+    *,
+    prompt: bool = True,
 ) -> BottleBackend[Any, Any]:
     """Resolve the bottle backend.
 
     `name` precedence:
-      1. explicit arg (CLI `--backend=<name>` passes through here)
+      1. explicit arg (e.g. resume passes the recorded backend name)
       2. BOT_BOTTLE_BACKEND env var
-      3. `macos-container` on compatible macOS hosts
-      4. `firecracker` on KVM-capable Linux hosts
-      5. default `docker`
+      3. auto-selection: VM backend first, docker fallback with prompt
+
+    `prompt` controls whether auto-selection may block on an interactive
+    [i/d/q] prompt when falling back to docker. Pass `prompt=False` in
+    non-interactive contexts (headless launches, CI) so the call dies
+    with an actionable message instead of hanging.
 
     Dies with a pointer at the known backends if the chosen name
     isn't implemented."""
-    resolved = name or os.environ.get("BOT_BOTTLE_BACKEND") or _default_backend_name()
+    resolved = name or os.environ.get("BOT_BOTTLE_BACKEND")
+    if resolved is None:
+        resolved = _auto_select_backend(prompt=prompt)
     backends = _get_backends()
     if resolved not in backends:
         known = ", ".join(sorted(backends))
@@ -668,7 +676,35 @@ def get_bottle_backend(
     return backends[resolved]
 
 
-def _default_backend_name() -> str:
+def _platform_vm_suggestion() -> str:
+    """Platform-appropriate VM backend name for install suggestions."""
+    return "macos-container" if sys.platform == "darwin" else "firecracker"
+
+
+def _print_vm_install_instructions() -> None:
+    """Print platform-appropriate VM backend install instructions to stderr."""
+    vm = _platform_vm_suggestion()
+    if vm == "macos-container":
+        info("Install Apple Container: https://github.com/apple/container/releases")
+        info("Then start the service:  container system start")
+    else:
+        info("Install Firecracker: https://github.com/firecracker-microvm/firecracker/releases")
+        info("Configure the host:  ./cli.py backend setup")
+
+
+def _auto_select_backend(prompt: bool = True) -> str:
+    """Tier-1 / tier-2 backend auto-selection.
+
+    Tier 1: VM backend — macos-container on macOS when Apple Container is
+    installed; firecracker on KVM-capable Linux even before the binary is
+    present (its preflight prints an install pointer).
+
+    Tier 2: docker, with a security warning and an interactive prompt.
+    When `prompt=False` (headless / CI), dies with an actionable message
+    instead of blocking on a TTY read. When docker is also absent, prints
+    VM install instructions and exits.
+    """
+    # --- Tier 1: VM backend -----------------------------------------
     if has_backend("macos-container"):
         return "macos-container"
     # A KVM-capable Linux host defaults to firecracker even when the
@@ -678,7 +714,37 @@ def _default_backend_name() -> str:
     from .firecracker import FirecrackerBottleBackend
     if FirecrackerBottleBackend.is_host_capable():
         return "firecracker"
-    return "docker"
+
+    # --- Tier 2: docker fallback ------------------------------------
+    if not has_backend("docker"):
+        info("No backend available on this host.")
+        _print_vm_install_instructions()
+        die("no backend available; install a VM backend and re-run")
+
+    vm = _platform_vm_suggestion()
+    warn(
+        "docker is less secure than VM backends — "
+        "containers share the host kernel."
+    )
+    if not prompt:
+        die(
+            f"no VM backend available; set BOT_BOTTLE_BACKEND=docker to proceed "
+            f"with docker, or install the {vm!r} backend."
+        )
+    sys.stderr.write(
+        f"bot-bottle: For better isolation, install the {vm!r} backend.\n"
+        f"  [i] show {vm} install instructions and exit\n"
+        "  [d] use docker anyway\n"
+        "  [q] quit\n"
+        "bot-bottle: choice [i/d/q]: "
+    )
+    sys.stderr.flush()
+    reply = read_tty_line().strip().lower()
+    if reply == "d":
+        return "docker"
+    if reply == "i":
+        _print_vm_install_instructions()
+    die("not proceeding with docker; install a VM backend or set BOT_BOTTLE_BACKEND=docker")
 
 
 def known_backend_names() -> tuple[str, ...]:

bot_bottle/backend/firecracker/util.py

@@ -88,7 +88,7 @@ def require_firecracker() -> None:
     booting a VM without it."""
     if not is_linux():
         die("firecracker backend is only supported on Linux (KVM). "
-            "On macOS use --backend=macos-container.")
+            "On macOS use the macos-container backend.")
     if shutil.which("firecracker") is None:
         info("Firecracker is required but was not found on PATH.")
         info("Install: https://github.com/firecracker-microvm/firecracker/releases")

bot_bottle/backend/macos_container/bottle.py

@@ -68,9 +68,14 @@ class MacosContainerBottle(Bottle):
         # reaches the agent (PRD 0070): registration mints it *after* the
         # container exists — its source IP is the registration key and Apple
         # Container assigns that by DHCP — so it cannot be in the run-time env
-        # the way docker's compose spec does it. `container exec --env` wins
-        # over the run-time value, so the token-bearing proxy URL set here
-        # supersedes the token-less one baked in at launch.
+        # the way docker's compose spec does it.
+        #
+        # `container exec --env` does NOT override a run-time value — it
+        # appends, leaving duplicate entries in the agent's `environ` whose
+        # resolution is runtime-specific (Node last-wins, Rust first-wins). So
+        # nothing here may rely on superseding: the proxy vars are supplied
+        # *only* at exec time and are deliberately absent from the run-time
+        # env. See `launch._agent_env_entries`.
         self._exec_env = dict(exec_env or {})
         self._closed = False
 

bot_bottle/backend/macos_container/enumerate.py

@@ -8,7 +8,11 @@ from ...bottle_state import read_metadata
 from .. import ActiveAgent
 from .infra import INFRA_NAME
 
-_PREFIX = "bot-bottle-"
+# The name every agent container carries: `bot-bottle-<slug>`. Exported
+# because callers that act on a running bottle (gateway-host rewrites,
+# registry reconciliation) have to map an enumerated slug back to a
+# container name.
+CONTAINER_NAME_PREFIX = "bot-bottle-"
 # The shared per-host infra container carries the same prefix as agent
 # containers but is infrastructure, not a bottle — one control plane + gateway
 # serves every agent, so listing it as an agent would invent one per host.
@@ -26,9 +30,9 @@ def enumerate_active() -> list[ActiveAgent]:
         return []
     out: list[ActiveAgent] = []
     for name in sorted(line.strip() for line in result.stdout.splitlines()):
-        if not name.startswith(_PREFIX) or name in _INFRA_NAMES:
+        if not name.startswith(CONTAINER_NAME_PREFIX) or name in _INFRA_NAMES:
             continue
-        slug = name[len(_PREFIX):]
+        slug = name[len(CONTAINER_NAME_PREFIX):]
         metadata = read_metadata(slug)
         out.append(ActiveAgent(
             backend_name="macos-container",

bot_bottle/backend/macos_container/gateway_hosts.py

@@ -0,0 +1,96 @@
+"""Stable gateway name for macOS agents, via each bottle's `/etc/hosts`.
+
+The shared gateway's address is assigned by vmnet's DHCP and changes whenever
+the infra container is recreated — a source-hash bump, an image upgrade, a
+crash. Every agent-facing URL (egress proxy, git-http, supervise) embeds that
+address, and the proxy URL reaches the agent as **process environment** at
+`container exec` time. A running process's `environ` cannot be rewritten from
+outside, so a moved gateway used to strand every running bottle permanently:
+not degraded, unreachable, until the bottle was relaunched and its session
+thrown away.
+
+So the agent never learns the address. It is given a stable *name*
+(`GATEWAY_HOSTNAME`) in every URL, resolved through its own `/etc/hosts`.
+Unlike `environ`, that is a file — it can be rewritten inside a container that
+is already running, so a gateway that comes back at a new address is picked up
+by live bottles instead of orphaning them.
+
+Apple Container 1.0 offers no container-name DNS on a user network (the only
+nameserver an agent sees is vmnet's, which does not know container names) and
+`container run` has no `--add-host`, so the entry is written by exec after the
+container starts.
+
+Writing it needs root, and the agent runs as `node`: the agent therefore
+cannot repoint its own gateway name, while the host (which drives `container
+exec --user root`) can. That asymmetry is deliberate — keep it.
+"""
+
+from __future__ import annotations
+
+from ...log import warn
+from . import util as container_mod
+from .enumerate import CONTAINER_NAME_PREFIX, enumerate_active
+
+# The name every agent-facing gateway URL uses. Must not collide with a real
+# DNS name the agent might resolve; it is bottle-local by construction.
+GATEWAY_HOSTNAME = "bot-bottle-gateway"
+
+# Marker so the rewrite is idempotent and only ever touches our own line —
+# the rest of /etc/hosts (localhost, the container's own name) is preserved.
+_MARKER = "# bot-bottle gateway"
+
+
+def _rewrite_script(gateway_ip: str) -> str:
+    """A shell one-liner that replaces our managed line in `/etc/hosts`.
+
+    Rewrites in place via a temp file + `cat` rather than `mv`, so the file
+    keeps its original inode, ownership, and mode — a bind-mounted or
+    pre-created `/etc/hosts` must not be replaced by a root-owned 0644 copy
+    that the runtime then refuses to update.
+    """
+    return (
+        "set -e; "
+        f"grep -v '{_MARKER}' /etc/hosts > /tmp/.bb-hosts || true; "
+        f"printf '%s %s %s\\n' '{gateway_ip}' '{GATEWAY_HOSTNAME}' "
+        f"'{_MARKER}' >> /tmp/.bb-hosts; "
+        "cat /tmp/.bb-hosts > /etc/hosts; "
+        "rm -f /tmp/.bb-hosts"
+    )
+
+
+def set_gateway_host(container_name: str, gateway_ip: str) -> None:
+    """Point `GATEWAY_HOSTNAME` at `gateway_ip` inside one running container.
+
+    Must run before the agent is exec'd: the agent's proxy URL names the
+    gateway, so the entry has to exist for its first connection. Idempotent —
+    re-running with the same address is a no-op in effect.
+    """
+    container_mod.exec_container_as_root(
+        container_name, ["sh", "-c", _rewrite_script(gateway_ip)],
+    )
+
+
+def refresh_gateway_host(gateway_ip: str) -> list[str]:
+    """Re-point every running bottle at the current gateway address.
+
+    Called once the shared gateway is known to be up, so a bottle stranded by
+    an earlier gateway restart re-attaches instead of needing a relaunch.
+    Returns the containers updated.
+
+    Best-effort per bottle: one container that refuses the write (already
+    exiting, say) must not stop the others from being repaired, and must not
+    fail the launch that triggered the sweep.
+    """
+    updated: list[str] = []
+    for agent in enumerate_active():
+        name = f"{CONTAINER_NAME_PREFIX}{agent.slug}"
+        try:
+            set_gateway_host(name, gateway_ip)
+            updated.append(name)
+        # One bad bottle must not stop the sweep, so this is deliberately broad.
+        except Exception as e:  # noqa: BLE001  # pylint: disable=broad-exception-caught
+            warn(f"could not re-point {name} at the gateway: {e}")
+    return updated
+
+
+__all__ = ["GATEWAY_HOSTNAME", "set_gateway_host", "refresh_gateway_host"]

bot_bottle/backend/macos_container/launch.py

@@ -59,6 +59,11 @@ from ..docker.egress import EGRESS_PORT
 from ..util import AGENT_CA_BUNDLE, AGENT_CA_PATH
 from . import util as container_mod
 from .bottle import MacosContainerBottle
+from .gateway_hosts import (
+    GATEWAY_HOSTNAME,
+    refresh_gateway_host,
+    set_gateway_host,
+)
 from .bottle_plan import MacosContainerBottlePlan
 from .consolidated_launch import (
     GatewayEndpoint,
@@ -100,6 +105,11 @@ def launch(
         # Step 1: the per-host singletons. Must precede the agent run — its
         # proxy env needs the gateway's address at `container run` time.
         endpoint = ensure_gateway()
+        # The gateway's address may have changed since these bottles launched
+        # (any infra recreate re-runs DHCP). They name the gateway rather than
+        # address it, so re-pointing /etc/hosts re-attaches them in place
+        # instead of leaving them stranded until relaunch.
+        refresh_gateway_host(endpoint.gateway_ip)
 
         # Step 2: mint this bottle's deploy keys, then point it at the SHARED
         # gateway's CA + git-http/supervise ports.
@@ -117,6 +127,9 @@ def launch(
         # attribution key; `--cap-drop CAP_NET_RAW` at run is what makes it
         # unforgeable. Poll: `container run --detach` can return before vmnet's
         # DHCP has assigned the address.
+        # Resolve the gateway name before anything execs: every agent-facing
+        # URL uses it, so the entry must exist for the first connection.
+        set_gateway_host(plan.container_name, endpoint.gateway_ip)
         source_ip = container_mod.wait_container_ipv4_on_network(
             plan.container_name, endpoint.network,
         )
@@ -231,13 +244,20 @@ def _stamp_agent_urls(
 ) -> MacosContainerBottlePlan:
     """Point the agent's git-gate insteadOf rewrites + supervise MCP at the
     shared gateway's ports. Both bypass the egress proxy (NO_PROXY covers the
-    gateway address)."""
+    gateway name).
+
+    Addressed by `GATEWAY_HOSTNAME`, never by IP: these URLs are baked into
+    the agent's gitconfig and MCP config at provision time, so an address here
+    would strand the bottle the moment the gateway moved. The name is resolved
+    per connection through `/etc/hosts`, which stays rewritable while the
+    bottle runs."""
+    del endpoint  # addressed by name; the address reaches the bottle via /etc/hosts
     git_gate_url = (
-        f"http://{endpoint.gateway_ip}:{_GIT_HTTP_PORT}"
+        f"http://{GATEWAY_HOSTNAME}:{_GIT_HTTP_PORT}"
         if plan.git_gate_plan.upstreams else ""
     )
     supervise_url = (
-        f"http://{endpoint.gateway_ip}:{SUPERVISE_PORT}/"
+        f"http://{GATEWAY_HOSTNAME}:{SUPERVISE_PORT}/"
         if plan.supervise_plan is not None else ""
     )
     return dataclasses.replace(
@@ -247,30 +267,43 @@ def _stamp_agent_urls(
     )
 
 
-def _proxy_url(gateway_ip: str, identity_token: str = "") -> str:
+def _proxy_url(identity_token: str = "") -> str:
     """The agent's egress proxy URL. The identity token rides as proxy
     credentials — the gateway reads Proxy-Authorization, resolves the
     (source_ip, token) pair against the control plane, and strips it before
-    upstream. Without a valid pair `/resolve` denies the request (#366)."""
+    upstream. Without a valid pair `/resolve` denies the request (#366).
+
+    Names the gateway rather than addressing it: this URL reaches the agent as
+    process environment, which cannot be rewritten once the agent is running,
+    so an address baked here is unfixable if the gateway moves."""
     cred = f"bottle:{identity_token}@" if identity_token else ""
-    return f"http://{cred}{gateway_ip}:{EGRESS_PORT}"
+    return f"http://{cred}{GATEWAY_HOSTNAME}:{EGRESS_PORT}"
 
 
-def _no_proxy(gateway_ip: str) -> str:
+def _no_proxy() -> str:
     # git-http + supervise live on the gateway and must NOT go through the
-    # egress proxy — the agent reaches them directly by its address.
-    return f"localhost,127.0.0.1,{gateway_ip}"
+    # egress proxy — the agent reaches them directly by name. Deliberately
+    # address-free: NO_PROXY is baked into the run-time env and is therefore
+    # just as unfixable as the proxy URL if the gateway moves.
+    return f"localhost,127.0.0.1,{GATEWAY_HOSTNAME}"
 
 
 def _identity_proxy_env(
     endpoint: GatewayEndpoint, identity_token: str,
 ) -> dict[str, str]:
-    """The token-bearing proxy env applied at `container exec`. It supersedes
-    the token-less run-time value (exec `--env` wins), which is the only way to
-    get the token in: it does not exist until after the container runs."""
+    """The token-bearing proxy env applied at `container exec` — the only way
+    to get the token in, since it does not exist until after the container
+    runs (registration keys on the DHCP-assigned address).
+
+    This is the *sole* source of `*_PROXY` for the agent. It deliberately does
+    not rely on overriding a run-time value: `container exec --env` appends
+    rather than replaces, so a run-time `HTTPS_PROXY` would survive alongside
+    this one and first-wins runtimes would read the wrong entry. See
+    `_agent_env_entries`."""
     if not identity_token:
         return {}
-    url = _proxy_url(endpoint.gateway_ip, identity_token)
+    del endpoint  # the gateway is named, not addressed
+    url = _proxy_url(identity_token)
     return {
         "HTTPS_PROXY": url, "HTTP_PROXY": url,
         "https_proxy": url, "http_proxy": url,
@@ -317,16 +350,23 @@ def _agent_run_argv(
 def _agent_env_entries(
     plan: MacosContainerBottlePlan, endpoint: GatewayEndpoint,
 ) -> tuple[str, ...]:
-    # Token-less at run time — the token does not exist yet (see
-    # `_identity_proxy_env`). Anything egressing before the exec-time override
-    # is denied by `/resolve`, which is the safe direction.
-    proxy_url = _proxy_url(endpoint.gateway_ip)
-    no_proxy = _no_proxy(endpoint.gateway_ip)
+    # No `*_PROXY` here on purpose. The token-bearing URL is applied at
+    # `container exec` (`_identity_proxy_env`), and Apple's `container exec
+    # --env` **appends** to the run-time environment rather than replacing it:
+    # setting a token-less value here leaves two `HTTPS_PROXY` entries in the
+    # agent's `environ`, token-less first. Which one a runtime reads is then
+    # pure luck — Node takes the last (and worked), Rust's `std::env::var`
+    # takes the first, so Codex proxied without its identity token and
+    # `/resolve` fail-closed on every request.
+    #
+    # A token-less proxy URL has no legitimate consumer anyway: the init
+    # process is `sleep` and everything that egresses arrives via exec. Its
+    # only value was a tidy 403 for unattributed callers, which is not worth
+    # silently dropping attribution for. Without it a process that egresses
+    # before the exec-time env still fails closed — the agent network is
+    # host-only, so there is no route off it except the gateway.
+    no_proxy = _no_proxy()
     env = [
-        f"HTTPS_PROXY={proxy_url}",
-        f"HTTP_PROXY={proxy_url}",
-        f"https_proxy={proxy_url}",
-        f"http_proxy={proxy_url}",
         f"NO_PROXY={no_proxy}",
         f"no_proxy={no_proxy}",
         f"NODE_EXTRA_CA_CERTS={AGENT_CA_PATH}",

bot_bottle/backend/macos_container/util.py

@@ -360,6 +360,21 @@ def exec_container(name: str, argv: list[str]) -> None:
         )
 
 
+def exec_container_as_root(name: str, argv: list[str]) -> None:
+    """`exec_container`, but as uid 0 inside the container.
+
+    For host-driven maintenance the agent itself must not be able to perform —
+    rewriting `/etc/hosts` to point the gateway name at an address. The agent
+    runs as `node`, so it cannot repoint its own gateway; the host can.
+    """
+    result = _run_container_op([_CONTAINER, "exec", "--user", "root", name, *argv])
+    if result.returncode != 0:
+        die(
+            f"container exec (root) in {name} failed: "
+            f"{(result.stderr or '').strip() or '<no stderr>'}"
+        )
+
+
 def _run_container_op(cmd: list[str]) -> subprocess.CompletedProcess[str]:
     result = subprocess.run(
         cmd,

bot_bottle/cli/_common.py

@@ -3,18 +3,10 @@
 from __future__ import annotations
 
 import os
-import sys
 from pathlib import Path
 
+from ..util import read_tty_line as read_tty_line
+
 PROG = "cli.py"
 USER_CWD = os.getcwd()
 REPO_DIR = str(Path(__file__).resolve().parent.parent.parent)
-
-
-def read_tty_line() -> str:
-    """Mirror `IFS= read -r REPLY </dev/tty`. Falls back to stdin."""
-    try:
-        with open("/dev/tty", "r", encoding="utf-8") as tty:
-            return tty.readline().rstrip("\n")
-    except OSError:
-        return sys.stdin.readline().rstrip("\n")

bot_bottle/cli/cleanup.py

@@ -21,16 +21,20 @@ from __future__ import annotations
 
 import sys
 
-from ..backend import get_bottle_backend, known_backend_names
+from ..backend import get_bottle_backend, has_backend, known_backend_names
 from ..log import info
 from ._common import read_tty_line
 
 
 def cmd_cleanup(_argv: list[str]) -> int:
     # Order: stable backend iteration so the y/N output is
-    # deterministic across runs.
+    # deterministic across runs. Skip backends whose runtime
+    # isn't available on this host so e.g. macos-container
+    # doesn't error on Linux.
     plans = [
-        (name, get_bottle_backend(name)) for name in known_backend_names()
+        (name, get_bottle_backend(name))
+        for name in known_backend_names()
+        if has_backend(name)
     ]
     prepared = [(name, b, b.prepare_cleanup()) for name, b in plans]
 

bot_bottle/cli/start.py

@@ -27,7 +27,6 @@ from ..backend import (
     BottleSpec,
     enumerate_active_agents,
     get_bottle_backend,
-    known_backend_names,
 )
 from ..backend.docker import util as docker_mod
 from ..backend.docker.bottle_plan import DockerBottlePlan
@@ -57,15 +56,6 @@ def cmd_start(argv: list[str]) -> int:
             "into a cached layer."
         ),
     )
-    parser.add_argument(
-        "--backend",
-        choices=known_backend_names(),
-        default=None,
-        help=(
-            "backend to launch the bottle on (default: $BOT_BOTTLE_BACKEND "
-            "or host auto-selection). Overrides the env var when set."
-        ),
-    )
     parser.add_argument(
         "--headless",
         action="store_true",
@@ -115,11 +105,10 @@ def cmd_start(argv: list[str]) -> int:
         os.environ["BOT_BOTTLE_NO_CACHE"] = "1"
 
     manifest = ManifestIndex.resolve(USER_CWD)
-    backend_name: str | None = args.backend
 
     if args.headless:
         return _start_headless(
-            manifest, args, dry_run=dry_run, backend_name=backend_name
+            manifest, args, dry_run=dry_run
         )
 
     agent_name: str | None = args.name
@@ -170,7 +159,6 @@ def cmd_start(argv: list[str]) -> int:
     return _launch_bottle(
         spec,
         dry_run=dry_run,
-        backend_name=backend_name,
     )
 
 
@@ -182,7 +170,6 @@ def _start_headless(
     args: argparse.Namespace,
     *,
     dry_run: bool,
-    backend_name: str | None,
 ) -> int:
     """Non-interactive launch path for orchestrators / CI / webhooks.
 
@@ -230,7 +217,6 @@ def _start_headless(
     return _launch_bottle(
         spec,
         dry_run=dry_run,
-        backend_name=backend_name,
         assume_yes=True,
         headless_prompt_text=prompt,
     )
@@ -268,15 +254,18 @@ def prepare_with_preflight(
     injected callable, prompt y/N via the injected callable.
 
     `backend_name` selects which backend prepares the plan
-    (`None` → `$BOT_BOTTLE_BACKEND` → host auto-selection). The CLI
-    passes whatever `--backend` resolved to.
+    (`None` → `$BOT_BOTTLE_BACKEND` → host auto-selection).
+
+    When `spec.headless` is True the docker-fallback prompt is suppressed:
+    auto-selection dies with an actionable message rather than blocking
+    on a TTY read (which would hang CI, webhook dispatch, and orchestrators).
 
     Returns `(plan, identity)`. `plan` is None on dry-run or
     operator-N, but `identity` is set as soon as `backend.prepare`
     returns so callers can reap the prepare-time state dir via
     `settle_state(identity)` in their finally — exactly the existing
     semantics."""
-    backend = get_bottle_backend(backend_name)
+    backend = get_bottle_backend(backend_name, prompt=not spec.headless)
     plan = backend.prepare(spec, stage_dir=stage_dir)
     identity = _identity_from_plan(plan)
 

bot_bottle/util.py

@@ -7,6 +7,7 @@ from __future__ import annotations
 
 import ipaddress
 import os
+import sys
 
 
 def is_ip_literal(value: str) -> bool:
@@ -17,6 +18,15 @@ def is_ip_literal(value: str) -> bool:
     return True
 
 
+def read_tty_line() -> str:
+    """Mirror `IFS= read -r REPLY </dev/tty`. Falls back to stdin."""
+    try:
+        with open("/dev/tty", "r", encoding="utf-8") as tty:
+            return tty.readline().rstrip("\n")
+    except OSError:
+        return sys.stdin.readline().rstrip("\n")
+
+
 def expand_tilde(path: str) -> str:
     """Expand a leading '~' to $HOME. Leaves paths without a leading
     tilde unchanged. Falls back to the empty string if $HOME is unset

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 +98,145 @@ 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()
+
+
+    def test_docker_fallback_non_interactive_dies(self):
+        # prompt=False: headless/CI contexts must not block on a TTY read.
+        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, "die", side_effect=SystemExit("die")):
+            with self.assertRaises(SystemExit):
+                get_bottle_backend(prompt=False)
+
+    def test_docker_fallback_user_picks_install(self):
+        # User picks [i] → print install instructions then 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="i"), \
+             patch.object(backend_mod, "_print_vm_install_instructions") as mock_inst, \
+             patch.object(backend_mod, "die", side_effect=SystemExit("die")):
+            with self.assertRaises(SystemExit):
+                get_bottle_backend()
+        mock_inst.assert_called_once()
+
+
+class TestReadTtyLine(unittest.TestCase):
+    """Unit tests for the shared read_tty_line helper in bot_bottle.util."""
+
+    def test_reads_from_dev_tty(self):
+        from unittest.mock import mock_open
+        from bot_bottle.util import read_tty_line
+
+        m = mock_open(read_data="hello\n")
+        with patch("builtins.open", m):
+            result = read_tty_line()
+        self.assertEqual("hello", result)
+        m.assert_called_once_with("/dev/tty", "r", encoding="utf-8")
+
+    def test_falls_back_to_stdin_on_oserror(self):
+        import io
+        from bot_bottle.util import read_tty_line
+        import sys as _sys
+
+        with patch("builtins.open", side_effect=OSError("no tty")), \
+             patch.object(_sys, "stdin", io.StringIO("world\n")):
+            result = read_tty_line()
+        self.assertEqual("world", result)
+
+
+class TestPrintVmInstallInstructions(unittest.TestCase):
+    """Unit tests for _print_vm_install_instructions platform branches."""
+
+    def test_linux_prints_firecracker_instructions(self):
+        from bot_bottle.backend import _print_vm_install_instructions
+
+        with patch.object(backend_mod, "_platform_vm_suggestion",
+                          return_value="firecracker"), \
+             patch.object(backend_mod, "info") as mock_info:
+            _print_vm_install_instructions()
+
+        messages = [str(c[0][0]) for c in mock_info.call_args_list]
+        self.assertTrue(any("Firecracker" in m for m in messages))
+
+    def test_macos_prints_apple_container_instructions(self):
+        from bot_bottle.backend import _print_vm_install_instructions
+
+        with patch.object(backend_mod, "_platform_vm_suggestion",
+                          return_value="macos-container"), \
+             patch.object(backend_mod, "info") as mock_info:
+            _print_vm_install_instructions()
+
+        messages = [str(c[0][0]) for c in mock_info.call_args_list]
+        self.assertTrue(any("Apple Container" in m for m in messages))
+
 
 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:

tests/unit/test_macos_container_launch_wiring.py

@@ -17,10 +17,10 @@ from unittest.mock import patch
 from bot_bottle.backend.macos_container.bottle import MacosContainerBottle
 from bot_bottle.backend.macos_container.bottle_plan import MacosContainerBottlePlan
 from bot_bottle.backend.macos_container.consolidated_launch import GatewayEndpoint
+from bot_bottle.backend.macos_container.gateway_hosts import GATEWAY_HOSTNAME
 from bot_bottle.backend.macos_container.launch import (
     _agent_run_argv,
     _identity_proxy_env,
-    _proxy_url,
 )
 from bot_bottle.manifest import ManifestIndex
 
@@ -104,19 +104,38 @@ class TestAgentRunArgv(unittest.TestCase):
         read back after start."""
         self.assertNotIn("--ip", self.argv)
 
-    def test_run_time_proxy_carries_no_identity_token(self) -> None:
-        """The token is minted by registration, which happens after this run —
-        so it cannot be here. `/resolve` denies the token-less pair (#366),
-        which is the safe direction; the real value arrives at exec time."""
-        joined = " ".join(self.argv)
-        self.assertIn(f"HTTP_PROXY={_proxy_url('192.168.128.3')}", joined)
-        self.assertNotIn("bottle:", joined)
+    def test_run_time_env_sets_no_proxy_vars_at_all(self) -> None:
+        """Regression: the proxy vars must exist *only* in the exec-time env.
+
+        `container exec --env` appends rather than replaces, so a token-less
+        `HTTPS_PROXY` here would survive next to the token-bearing one and
+        leave two entries in the agent's `environ`. Resolution is then
+        runtime-specific — Node reads the last, Rust's `std::env::var` reads
+        the first — so Codex proxied without its identity token and every
+        request fail-closed at `/resolve`.
+        """
+        for entry in self.argv:
+            self.assertFalse(
+                entry.upper().startswith(("HTTP_PROXY=", "HTTPS_PROXY=")),
+                f"run-time env must not set a proxy var, got {entry!r}",
+            )
+
+    def test_run_time_env_carries_no_identity_token(self) -> None:
+        """The token is minted by registration, which happens after this run,
+        so it cannot be here under any name."""
+        self.assertNotIn("bottle:", " ".join(self.argv))
 
     def test_gateway_bypasses_the_proxy(self) -> None:
         """git-http + supervise live on the gateway and must be reached
         directly, not through its own egress proxy."""
         entry = next(a for a in self.argv if a.startswith("NO_PROXY="))
-        self.assertIn("192.168.128.3", entry)
+        self.assertIn(GATEWAY_HOSTNAME, entry)
+
+    def test_no_proxy_names_the_gateway_and_never_addresses_it(self) -> None:
+        """NO_PROXY is baked into the run-time env, so an address here is as
+        unfixable as the proxy URL if the gateway moves."""
+        entry = next(a for a in self.argv if a.startswith("NO_PROXY="))
+        self.assertNotIn("192.168.128.3", entry)
 
     def test_forwarded_secrets_stay_off_argv(self) -> None:
         """Bare name → inherited from the run process env, so the value never
@@ -134,7 +153,7 @@ class TestIdentityTokenDelivery(unittest.TestCase):
     def test_exec_env_carries_the_token_as_proxy_credentials(self) -> None:
         env = _identity_proxy_env(_endpoint(), "s3cret")
         self.assertEqual(
-            "http://bottle:s3cret@192.168.128.3:9099", env["HTTP_PROXY"],
+            f"http://bottle:s3cret@{GATEWAY_HOSTNAME}:9099", env["HTTP_PROXY"],
         )
         self.assertEqual(env["HTTP_PROXY"], env["https_proxy"])
 
@@ -202,7 +221,7 @@ class TestPlanIdentityToken(unittest.TestCase):
         self.assertIn("HTTP_PROXY", argv)
         self.assertNotIn("s3cret", " ".join(argv))
         self.assertEqual(
-            "http://bottle:s3cret@192.168.128.3:9099", kwargs["env"]["HTTP_PROXY"],
+            f"http://bottle:s3cret@{GATEWAY_HOSTNAME}:9099", kwargs["env"]["HTTP_PROXY"],
         )
 
 

tests/unit/test_macos_gateway_hosts.py

@@ -0,0 +1,140 @@
+"""Unit: stable gateway name via each bottle's /etc/hosts (issue #443).
+
+The gateway's address moves whenever the infra container is recreated. Agents
+name it instead of addressing it, and the name resolves through `/etc/hosts` —
+a file, so it stays rewritable while the bottle runs, unlike the `environ` the
+proxy URL is delivered in.
+"""
+
+from __future__ import annotations
+
+import unittest
+from types import SimpleNamespace
+from unittest.mock import patch
+
+from bot_bottle.backend.macos_container.gateway_hosts import (
+    GATEWAY_HOSTNAME,
+    refresh_gateway_host,
+    set_gateway_host,
+)
+
+_MOD = "bot_bottle.backend.macos_container.gateway_hosts"
+
+
+class TestSetGatewayHost(unittest.TestCase):
+    def _script(self, exec_root: object) -> str:
+        argv = exec_root.call_args.args[1]  # type: ignore[attr-defined]
+        self.assertEqual(["sh", "-c"], argv[:2])
+        return argv[2]
+
+    def test_writes_the_address_against_the_stable_name(self) -> None:
+        with patch(f"{_MOD}.container_mod.exec_container_as_root") as ex:
+            set_gateway_host("bot-bottle-demo", "192.168.128.19")
+        self.assertEqual("bot-bottle-demo", ex.call_args.args[0])
+        script = self._script(ex)
+        self.assertIn("192.168.128.19", script)
+        self.assertIn(GATEWAY_HOSTNAME, script)
+
+    def test_runs_as_root_so_the_agent_cannot_repoint_itself(self) -> None:
+        """The agent runs as `node`. If it could rewrite /etc/hosts it could
+        aim its own gateway name elsewhere, so the write must go through the
+        root-only helper."""
+        with patch(f"{_MOD}.container_mod.exec_container_as_root") as ex:
+            set_gateway_host("bot-bottle-demo", "10.0.0.1")
+        ex.assert_called_once()
+
+    def test_is_idempotent_by_removing_its_own_line_first(self) -> None:
+        """Re-pointing must replace the managed entry, not append a second one
+        — two entries for the same name would resolve by luck of ordering."""
+        with patch(f"{_MOD}.container_mod.exec_container_as_root") as ex:
+            set_gateway_host("bot-bottle-demo", "10.0.0.1")
+        self.assertIn("grep -v", self._script(ex))
+
+    def test_preserves_the_rest_of_the_hosts_file(self) -> None:
+        """localhost and the container's own name must survive the rewrite."""
+        with patch(f"{_MOD}.container_mod.exec_container_as_root") as ex:
+            set_gateway_host("bot-bottle-demo", "10.0.0.1")
+        script = self._script(ex)
+        # Filter-and-append, never a truncating write of just our line.
+        self.assertIn("/etc/hosts >", script)
+        self.assertIn(">> /tmp/.bb-hosts", script)
+
+    def test_keeps_the_original_inode(self) -> None:
+        """`cat >` rather than `mv`: a pre-created /etc/hosts must keep its
+        ownership and mode, not be replaced by a root-owned copy."""
+        script = None
+        with patch(f"{_MOD}.container_mod.exec_container_as_root") as ex:
+            set_gateway_host("bot-bottle-demo", "10.0.0.1")
+            script = self._script(ex)
+        self.assertIn("cat /tmp/.bb-hosts > /etc/hosts", script)
+        self.assertNotIn("mv ", script)
+
+
+class TestRefreshGatewayHost(unittest.TestCase):
+    """The re-attach sweep: bottles stranded by an earlier gateway restart get
+    re-pointed in place instead of needing a relaunch."""
+
+    def _agents(self, *slugs: str) -> list[SimpleNamespace]:
+        return [SimpleNamespace(slug=s) for s in slugs]
+
+    def test_repoints_every_running_bottle(self) -> None:
+        with patch(f"{_MOD}.enumerate_active", return_value=self._agents("a", "b")), \
+                patch(f"{_MOD}.set_gateway_host") as setter:
+            updated = refresh_gateway_host("192.168.128.19")
+        self.assertEqual(["bot-bottle-a", "bot-bottle-b"], updated)
+        self.assertEqual(
+            [("bot-bottle-a", "192.168.128.19"), ("bot-bottle-b", "192.168.128.19")],
+            [c.args for c in setter.call_args_list],
+        )
+
+    def test_one_failing_bottle_does_not_stop_the_sweep(self) -> None:
+        """A container that is already exiting must not block the repair of
+        its neighbours, nor fail the launch that triggered the sweep."""
+        def _flaky(name: str, _ip: str) -> None:
+            if name == "bot-bottle-a":
+                raise RuntimeError("container is exiting")
+
+        with patch(f"{_MOD}.enumerate_active", return_value=self._agents("a", "b")), \
+                patch(f"{_MOD}.set_gateway_host", side_effect=_flaky), \
+                patch(f"{_MOD}.warn") as warn:
+            updated = refresh_gateway_host("10.0.0.1")
+        self.assertEqual(["bot-bottle-b"], updated)
+        warn.assert_called_once()
+
+    def test_no_running_bottles_is_a_clean_no_op(self) -> None:
+        with patch(f"{_MOD}.enumerate_active", return_value=[]), \
+                patch(f"{_MOD}.set_gateway_host") as setter:
+            self.assertEqual([], refresh_gateway_host("10.0.0.1"))
+        setter.assert_not_called()
+
+
+
+class TestLaunchWiring(unittest.TestCase):
+    """Ordering matters: the name must resolve before anything execs, and the
+    stranded-bottle sweep must run once the gateway is known to be up."""
+
+    def test_launch_sets_the_host_entry_before_reading_the_source_ip(self) -> None:
+        """The agent's every URL names the gateway, so the entry has to exist
+        before the first connection — which means before the agent execs."""
+        import inspect
+
+        from bot_bottle.backend.macos_container import launch
+
+        src = inspect.getsource(launch)
+        set_at = src.index("set_gateway_host(plan.container_name")
+        exec_at = src.index("wait_container_ipv4_on_network")
+        self.assertLess(set_at, exec_at)
+
+    def test_launch_refreshes_stranded_bottles_after_ensure_gateway(self) -> None:
+        import inspect
+
+        from bot_bottle.backend.macos_container import launch
+
+        src = inspect.getsource(launch)
+        ensure_at = src.index("endpoint = ensure_gateway()")
+        refresh_at = src.index("refresh_gateway_host(endpoint.gateway_ip)")
+        self.assertLess(ensure_at, refresh_at)
+
+
+if __name__ == "__main__":
+    unittest.main()