feat(smolmachines): per-bottle loopback alias scopes TSI to single /32

PR #74's Docker-Desktop fix routed the agent through
`127.0.0.1:<random>` loopback forwards, but TSI filters by IP
only — so the allowlist `127.0.0.1/32` let the agent VM reach
**any** host service on macOS loopback (postgres, dev servers,
other bottles' published ports, mDNSResponder, ...). Real
downgrade vs the docker backend's `--internal` network.

Resolution: per-bottle loopback alias.

- New `loopback_alias` module manages a pool of
  `127.0.0.16` .. `127.0.0.31` on `lo0`. macOS only routes
  `127.0.0.1` by default; the extras need `sudo ifconfig lo0
  alias`. `ensure_pool()` lazily adds the missing entries via
  one sudo prompt on first launch per reboot — aliases persist
  on `lo0` until reboot, so subsequent launches skip the
  prompt entirely.
- `allocate(slug)` picks the lowest-numbered unused alias by
  inspecting running bundle containers' port-binding HostIps.
  No on-disk reservation — docker is the source of truth.
- Bundle bringup binds published ports to the allocated alias
  (`docker run -p <alias>::<port>`) instead of `127.0.0.1`.
- TSI allowlist becomes the alias's /32 — narrows reachability
  to this bottle's bundle only.
- Linux native daemons share the host's network namespace;
  `127.0.0.0/8` works without aliases, so the module no-ops on
  non-Darwin and returns `127.0.0.1` from `allocate`.

Tracking issue closed: gitea/issues/75.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -200,19 +200,14 @@ sidecar bundle still in Docker. Selected via
 The integration tests run against whichever backend the env var
 selects and skip cleanly when its prerequisites are missing.
 
-**Known limitation, v1:** smolvm's TSI uses macOS networking, and
-Docker Desktop's container IPs aren't reachable from macOS, so the
-smolmachines bottle dials the sidecar bundle through host loopback
-port-forwards (`127.0.0.1:<random>`). TSI filters by IP only, so the
-allowlist is `127.0.0.1/32` — meaning the agent VM can reach **any
-service bound to macOS's loopback**, not just the bundle's published
-ports. Practical implication: while a smolmachines bottle is running,
-host-local dev services (postgres on 5432, dev servers, etc.) are
-reachable from inside the agent even if you intended them to be
-host-private. The docker backend keeps the bottle on a `--internal`
-docker network and doesn't have this issue. A future revision will
-narrow this via a per-bottle loopback alias + host-side proxy (see
-PRD 0023's "loopback scoping" section).
+**One-time sudo on first launch (macOS):** smolmachines needs
+per-bottle loopback aliases (`127.0.0.16` .. `127.0.0.31`) on `lo0`
+so each bottle's TSI allowlist scopes to its own /32. The first
+`./cli.py start` after each reboot prompts for sudo to add the pool
+via `ifconfig lo0 alias`. Aliases persist until reboot; subsequent
+launches don't prompt. Without this, every bottle would share
+`127.0.0.1` and be able to reach unrelated host services on the
+loopback.
 
 ## Manifest
 

claude_bottle/backend/smolmachines/launch.py

@@ -50,6 +50,7 @@ from ..docker.pipelock import (
     PIPELOCK_PORT as _PIPELOCK_PORT_STR,
     pipelock_tls_init,
 )
+from . import loopback_alias as _loopback
 from . import sidecar_bundle as _bundle
 from . import smolvm as _smolvm
 from .bottle import SmolmachinesBottle
@@ -76,7 +77,16 @@ def launch(
     via the ExitStack."""
     stack = ExitStack()
     try:
-        # 1. Per-bottle docker bridge.
+        # 1. Reserve a loopback alias for this bottle. macOS only
+        # routes 127.0.0.1 by default; the per-bottle alias is
+        # what bundles the docker port-publishes and TSI allowlist
+        # against, so this bottle can't reach other bottles' (or
+        # other host services') ports on the loopback. Lazy
+        # sudo-driven on first use per boot. No-op on Linux.
+        _loopback.ensure_pool()
+        loopback_ip = _loopback.allocate(plan.slug)
+
+        # 2. Per-bottle docker bridge.
         network = _bundle.bundle_network_name(plan.slug)
         _bundle.create_bundle_network(network, plan.bundle_subnet, plan.bundle_gateway)
         stack.callback(_bundle.remove_bundle_network, network)
@@ -112,21 +122,22 @@ def launch(
         )
 
         # 3. Build the BundleLaunchSpec from the (now-resolved)
-        # inner Plans: daemon subset, env, bind-mounts. The spec's
-        # ports_to_publish list expands depending on which daemons
-        # the agent needs to reach from the smolvm guest.
-        bundle_spec = _bundle_launch_spec(plan, network)
+        # inner Plans: daemon subset, env, bind-mounts, and the
+        # loopback alias to bind published ports against. The
+        # spec's ports_to_publish list expands depending on which
+        # daemons the agent needs to reach from the smolvm guest.
+        bundle_spec = _bundle_launch_spec(plan, network, loopback_ip)
         token_env = _resolve_token_env(plan, os.environ)
         _bundle.start_bundle(bundle_spec, env={**os.environ, **token_env})
         stack.callback(_bundle.stop_bundle, plan.slug)
 
         # 4. Discover the host-side ports docker assigned for the
         # bundle's published container ports, and bind the
-        # agent's URLs to `127.0.0.1:<host port>`. Docker container
-        # IPs (192.168.x.x in the daemon's bridge) aren't
-        # reachable from the smolvm guest on macOS — TSI uses
-        # macOS networking, and macOS sees the daemon's bridge
-        # via the published-port loopback forward only.
+        # agent's URLs to `<loopback_ip>:<host port>`. Docker
+        # container IPs (192.168.x.x in the daemon's bridge)
+        # aren't reachable from the smolvm guest on macOS — TSI
+        # uses macOS networking, and macOS sees the daemon's
+        # bridge via the published-port loopback forward only.
         #
         # Proxy hop order matches the docker backend: when the
         # bottle declares egress routes, the agent's first hop is
@@ -140,21 +151,21 @@ def launch(
         else:
             agent_facing_port = _PIPELOCK_PORT
         agent_facing_host_port = _bundle.bundle_host_port(
-            plan.slug, agent_facing_port,
+            plan.slug, agent_facing_port, host_ip=loopback_ip,
         )
-        agent_proxy_url = f"http://127.0.0.1:{agent_facing_host_port}"
+        agent_proxy_url = f"http://{loopback_ip}:{agent_facing_host_port}"
         agent_git_gate_host = ""
         if plan.git_gate_plan.upstreams:
             git_gate_host_port = _bundle.bundle_host_port(
-                plan.slug, _GIT_GATE_PORT,
+                plan.slug, _GIT_GATE_PORT, host_ip=loopback_ip,
             )
-            agent_git_gate_host = f"127.0.0.1:{git_gate_host_port}"
+            agent_git_gate_host = f"{loopback_ip}:{git_gate_host_port}"
         agent_supervise_url = ""
         if plan.supervise_plan is not None:
             supervise_host_port = _bundle.bundle_host_port(
-                plan.slug, _SUPERVISE_PORT,
+                plan.slug, _SUPERVISE_PORT, host_ip=loopback_ip,
             )
-            agent_supervise_url = f"http://127.0.0.1:{supervise_host_port}/"
+            agent_supervise_url = f"http://{loopback_ip}:{supervise_host_port}/"
 
         # Stamp the URLs onto the plan + guest_env. provision_git
         # and provision_supervise read the plan fields; the agent
@@ -178,15 +189,16 @@ def launch(
 
         # 5. smolvm VM. --from carries the pre-packed .smolmachine
         # artifact (built by prepare); --allow-cidr + -e carry the
-        # per-bottle TSI allowlist + env. The allowlist is
-        # `127.0.0.1/32` because every bundle daemon the agent
-        # reaches is fronted by a host loopback port-forward.
-        # Smolfile isn't usable here — smolvm 0.8.0 makes `--from`
-        # and `--smolfile` mutually exclusive.
+        # per-bottle TSI allowlist + env. The allowlist is the
+        # per-bottle loopback alias — narrowing it to one /32 keeps
+        # the agent from reaching other host loopback services or
+        # other bottles' published ports. Smolfile isn't usable
+        # here — smolvm 0.8.0 makes `--from` and `--smolfile`
+        # mutually exclusive.
         _smolvm.machine_create(
             plan.machine_name,
             from_path=plan.agent_from_path,
-            allow_cidrs=["127.0.0.1/32"],
+            allow_cidrs=[f"{loopback_ip}/32"],
             env=plan.guest_env,
         )
         stack.callback(_smolvm.machine_delete, plan.machine_name)
@@ -240,7 +252,7 @@ def launch(
 
 
 def _bundle_launch_spec(
-    plan: SmolmachinesBottlePlan, network: str
+    plan: SmolmachinesBottlePlan, network: str, loopback_ip: str,
 ) -> _bundle.BundleLaunchSpec:
     """Build a BundleLaunchSpec from the resolved inner Plans.
 
@@ -345,6 +357,7 @@ def _bundle_launch_spec(
         environment=tuple(env),
         volumes=tuple(volumes),
         ports_to_publish=tuple(ports_to_publish),
+        publish_host_ip=loopback_ip,
     )
 
 

claude_bottle/backend/smolmachines/loopback_alias.py

@@ -0,0 +1,176 @@
+"""Per-bottle loopback alias allocation (PRD 0023, follow-up to
+the Docker-Desktop fix in PR #74).
+
+After the pivot to host-loopback port-forwards, the smolmachines
+TSI allowlist was `127.0.0.1/32` — which meant the agent VM could
+reach **any** service bound to macOS's loopback, not just the
+bundle's published ports. That's a real downgrade from the
+docker backend's `--internal` network isolation.
+
+This module narrows the allowlist by allocating each bottle a
+unique loopback alias (`127.0.0.16` .. `127.0.0.31` by default).
+The bundle's port-forwards bind to that alias, TSI's allowlist is
+the alias /32, and other host loopback services stay invisible to
+the bottle.
+
+macOS only configures `127.0.0.1` on `lo0` by default; the
+additional aliases require `sudo ifconfig lo0 alias`. We lazily
+sudo-add the missing pool on first use per boot — the aliases
+persist on `lo0` until reboot, so subsequent launches don't
+prompt.
+
+Linux native daemons share the host's network namespace; the
+whole `127.0.0.0/8` is reachable by default and aliases are
+unnecessary. The pool logic detects native-Linux and skips sudo
+entirely.
+
+Allocation is coordinated by inspecting running bundle
+containers' published host IPs — each bottle's bundle owns the
+alias appearing in its port bindings. The lowest-numbered free
+alias gets handed to a new bottle."""
+
+from __future__ import annotations
+
+import json
+import os
+import platform
+import re
+import subprocess
+from typing import Iterable
+
+from ...log import die, info
+
+
+# Sixteen aliases by default. Tunable for hosts that want more
+# concurrent bottles (each bottle reserves one alias for its
+# bundle bringup). The range is chosen to avoid the reserved
+# 127.0.0.1/2/3 ports (1 is the default, 2 is sometimes used by
+# CUPS, 3 by other macOS services) and stay well clear of
+# 127.0.0.53 (systemd-resolved) and 127.0.0.54 (libvirt).
+_POOL_START = 16
+_POOL_END = 31  # inclusive
+
+
+# Loopback aliases pool: 127.0.0.<start>..127.0.0.<end>.
+def _pool_addresses() -> list[str]:
+    return [f"127.0.0.{i}" for i in range(_POOL_START, _POOL_END + 1)]
+
+
+def _is_macos() -> bool:
+    return platform.system() == "Darwin"
+
+
+def ensure_pool() -> None:
+    """Make sure each address in the pool is up on `lo0`. Lazily
+    runs `sudo ifconfig lo0 alias <ip>/32 up` for missing entries
+    (sudo prompts once, then the aliases persist on lo0 until
+    reboot). No-op on non-macOS hosts."""
+    if not _is_macos():
+        return
+    missing = [ip for ip in _pool_addresses() if not _alias_present(ip)]
+    if not missing:
+        return
+    info(
+        f"smolmachines needs {len(missing)} loopback alias(es) on lo0 "
+        f"({', '.join(missing[:3])}{', ...' if len(missing) > 3 else ''}) "
+        f"to scope per-bottle TSI allowlists. sudo will prompt once; "
+        f"aliases persist until reboot."
+    )
+    for ip in missing:
+        result = subprocess.run(
+            ["sudo", "-p", "claude-bottle (loopback alias): ",
+             "ifconfig", "lo0", "alias", f"{ip}/32", "up"],
+            check=False,
+        )
+        if result.returncode != 0:
+            die(
+                f"sudo ifconfig lo0 alias {ip} failed (exit "
+                f"{result.returncode}). Re-run with sudo available, "
+                f"or add manually: sudo ifconfig lo0 alias {ip}/32 up"
+            )
+
+
+def allocate(slug: str) -> str:
+    """Pick the lowest-numbered alias from the pool not already
+    in use by a running smolmachines bundle. Bails when the pool
+    is exhausted — the caller should report the limit to the
+    operator. `slug` is logged for traceability; not otherwise
+    used (no on-disk reservation, allocation is purely
+    docker-state-driven).
+
+    On non-macOS the whole `127.0.0.0/8` is loopback by default;
+    `127.0.0.1` is fine to share and we skip the alias dance.
+    This still returns a deterministic address so launch.py's
+    callers don't have to branch on platform."""
+    if not _is_macos():
+        return "127.0.0.1"
+    in_use = _aliases_in_use()
+    for ip in _pool_addresses():
+        if ip not in in_use:
+            return ip
+    die(
+        f"smolmachines loopback alias pool exhausted "
+        f"({_POOL_END - _POOL_START + 1} aliases, all in use). "
+        f"Stop a running bottle (`smolvm machine ls --json`) or "
+        f"raise _POOL_END in loopback_alias.py."
+    )
+    return ""  # unreachable; die() never returns
+
+
+def _alias_present(ip: str) -> bool:
+    """True iff `ifconfig lo0` shows `<ip>` as an inet address.
+    Exact-match — `127.0.0.1` shouldn't match `127.0.0.16`."""
+    result = subprocess.run(
+        ["/sbin/ifconfig", "lo0"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return False
+    pattern = re.compile(rf"\binet {re.escape(ip)}\b")
+    return bool(pattern.search(result.stdout or ""))
+
+
+def _aliases_in_use() -> set[str]:
+    """Aliases already bound by another smolmachines bundle's
+    published-port mappings. We inspect every container whose
+    name matches the smolmachines bundle prefix and pull the
+    `HostIp` out of its port bindings."""
+    result = subprocess.run(
+        ["docker", "ps", "--format", "{{.Names}}",
+         "--filter", "name=claude-bottle-sidecars-"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return set()
+    names = [n.strip() for n in (result.stdout or "").splitlines() if n.strip()]
+    in_use: set[str] = set()
+    for name in names:
+        in_use.update(_host_ips_for_container(name))
+    return in_use
+
+
+def _host_ips_for_container(name: str) -> Iterable[str]:
+    """Yield the `HostIp` values across all port bindings on
+    container `name`. A bundle binds three or four ports and
+    they all share the same HostIp, so callers can take any."""
+    result = subprocess.run(
+        ["docker", "inspect", name,
+         "--format", "{{json .HostConfig.PortBindings}}"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return ()
+    try:
+        bindings = json.loads(result.stdout or "{}")
+    except json.JSONDecodeError:
+        return ()
+    seen: set[str] = set()
+    for _port, mappings in (bindings or {}).items():
+        for m in mappings or []:
+            host_ip = m.get("HostIp") or ""
+            if host_ip:
+                seen.add(host_ip)
+    return seen
+
+
+__all__ = ["allocate", "ensure_pool"]

claude_bottle/backend/smolmachines/sidecar_bundle.py

@@ -70,13 +70,19 @@ class BundleLaunchSpec:
     environment: Sequence[str] = field(default_factory=tuple)
     # (host_path, container_path, read_only) bind mounts.
     volumes: Sequence[tuple[str, str, bool]] = field(default_factory=tuple)
-    # Container ports to publish on the host's 127.0.0.1, random
+    # Container ports to publish on `publish_host_ip`, random
     # host-side port per entry. The smolvm guest's TSI talks via
     # macOS networking, so docker container IPs (192.168.x.x in
     # the daemon's bridge) aren't directly reachable from the
     # guest — host-loopback port-forwards are. Egress's port
     # is bundle-internal and never published.
     ports_to_publish: Sequence[int] = field(default_factory=tuple)
+    # Loopback IP to bind published ports against. Per-bottle
+    # loopback aliases (`127.0.0.16` etc., added via sudo
+    # ifconfig lo0 alias) narrow the TSI allowlist so a bottle
+    # can't reach other bottles' (or other host services') ports
+    # via 127.0.0.1.
+    publish_host_ip: str = "127.0.0.1"
 
 
 def create_bundle_network(network_name: str, subnet: str, gateway: str) -> None:
@@ -145,8 +151,10 @@ def start_bundle(spec: BundleLaunchSpec, *,
     # Loopback-only host port-forwards — the smolvm guest's TSI
     # uses macOS networking, and macOS loopback is the only host
     # surface that round-trips into Docker Desktop's daemon VM.
+    # Binds to the per-bottle alias so TSI's IP-only allowlist
+    # narrows reachability to this bottle's bundle only.
     for port in spec.ports_to_publish:
-        argv += ["-p", f"127.0.0.1::{port}"]
+        argv += ["-p", f"{spec.publish_host_ip}::{port}"]
     argv.append(spec.image)
     result = subprocess.run(
         argv, capture_output=True, text=True,
@@ -159,13 +167,15 @@ def start_bundle(spec: BundleLaunchSpec, *,
         )
 
 
-def bundle_host_port(slug: str, container_port: int) -> int:
+def bundle_host_port(
+    slug: str, container_port: int, *, host_ip: str = "127.0.0.1",
+) -> int:
     """`docker port <bundle> <container_port>/tcp` → the random
-    host-side port docker assigned. Called after `start_bundle`
-    on each container port listed in `BundleLaunchSpec
-    .ports_to_publish` so the launch step can build the agent's
-    HTTPS_PROXY / GIT_GATE / SUPERVISE URLs in
-    `127.0.0.1:<host port>` form."""
+    host-side port docker assigned for the binding on `host_ip`.
+    Called after `start_bundle` on each container port listed in
+    `BundleLaunchSpec.ports_to_publish` so the launch step can
+    build the agent's HTTPS_PROXY / GIT_GATE / SUPERVISE URLs in
+    `<host_ip>:<host port>` form."""
     container = bundle_container_name(slug)
     result = subprocess.run(
         ["docker", "port", container, f"{container_port}/tcp"],
@@ -176,14 +186,22 @@ def bundle_host_port(slug: str, container_port: int) -> int:
             f"docker port {container} {container_port}/tcp failed: "
             f"{(result.stderr or '').strip() or '<no stderr>'}"
         )
-    # `127.0.0.1:54321\n` — rpartition on last colon gives the port.
-    line = (result.stdout or "").splitlines()[0].strip()
-    _, _, port_str = line.rpartition(":")
-    try:
-        return int(port_str)
-    except ValueError:
-        die(f"unexpected `docker port` output: {line!r}")
-        return -1  # unreachable; die() never returns
+    # Each line looks like `127.0.0.16:54321` — one per address
+    # family / host IP. Match on the expected host_ip prefix so
+    # bottles bound to per-bottle aliases pick the right line.
+    for raw in (result.stdout or "").splitlines():
+        line = raw.strip()
+        if line.startswith(f"{host_ip}:"):
+            _, _, port_str = line.rpartition(":")
+            try:
+                return int(port_str)
+            except ValueError:
+                die(f"unexpected `docker port` output: {line!r}")
+    die(
+        f"no port mapping on {host_ip} for {container} "
+        f"{container_port}/tcp; got: {(result.stdout or '').strip()!r}"
+    )
+    return -1  # unreachable; die() never returns
 
 
 def stop_bundle(slug: str) -> None:

docs/prds/0023-smolmachines-backend.md

@@ -600,22 +600,18 @@ PRD 0024's bundle image is a prerequisite — this PRD assumes
    the plan is to filter on a deterministic name prefix
    `claude-bottle-<slug>` + cross-reference with on-disk metadata
    under `state/<slug>/`.
-8. **Loopback scoping (Docker Desktop pivot).** The original
-   design pinned the bundle at a docker bridge IP and set TSI's
-   allowlist to `<bundle-ip>/32`. On Docker Desktop / macOS the
-   daemon runs inside its own Linux VM, so bridge IPs aren't
-   reachable from macOS networking — TSI's syscall impersonation
-   can't reach them. Resolution: publish each agent-facing bundle
-   port on host loopback (`-p 127.0.0.1::<port>`) and set TSI to
-   `127.0.0.1/32`. **This widens the TSI allowlist to anything
-   bound to macOS's loopback** — postgres, dev servers, other
-   bottles' published ports, mDNSResponder, etc. The agent can't
-   reach them by intent, but TSI can't filter by port. Follow-up
-   to scope back: bind each bottle's bundle ports on a per-bottle
-   loopback alias (e.g. `127.0.0.2` for bottle A, `127.0.0.3` for
-   B) added via `ifconfig lo0 alias`, set TSI to that single /32.
-   Needs sudo for alias setup; a small daemon-or-script we ship
-   alongside the launcher could handle it.
+8. **~~Loopback scoping (Docker Desktop pivot).~~ Resolved.**
+   Each bottle now allocates a per-bottle loopback alias from a
+   pool of `127.0.0.16` .. `127.0.0.31`, binds the bundle's
+   port-forwards to that alias, and sets TSI's allowlist to the
+   alias's /32. So a smolmachines bottle can only reach its own
+   bundle's published ports — not other bottles' ports, and not
+   unrelated host services on `127.0.0.1`. macOS loopback
+   aliases need `sudo ifconfig lo0 alias`; the launcher lazily
+   adds missing pool entries on first launch per reboot (sudo
+   prompts once, aliases persist until reboot). Linux native
+   daemons share the host's network namespace and skip the
+   alias dance.
 
 ## References
 

tests/unit/test_smolmachines_loopback_alias.py

@@ -0,0 +1,191 @@
+"""Unit: per-bottle loopback alias pool (follow-up to the
+Docker-Desktop fix in PR #74).
+
+`ensure_pool` lazily sudo-adds missing aliases on macOS; no-ops
+on Linux. `allocate` picks the lowest-numbered unused alias by
+inspecting running bundle containers' port bindings."""
+
+from __future__ import annotations
+
+import subprocess
+import unittest
+from unittest.mock import patch
+
+from claude_bottle.backend.smolmachines import loopback_alias
+
+
+def _ok(stdout: str = "") -> subprocess.CompletedProcess:
+    return subprocess.CompletedProcess(
+        args=[], returncode=0, stdout=stdout, stderr="",
+    )
+
+
+def _fail(stderr: str = "boom") -> subprocess.CompletedProcess:
+    return subprocess.CompletedProcess(
+        args=[], returncode=1, stdout="", stderr=stderr,
+    )
+
+
+# `ifconfig lo0` on macOS with the default lo0 config: just
+# 127.0.0.1. We craft fixtures around this shape.
+_LO0_DEFAULT = (
+    "lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384\n"
+    "\tinet 127.0.0.1 netmask 0xff000000\n"
+    "\tinet6 ::1 prefixlen 128\n"
+)
+
+_LO0_PARTIAL = (
+    _LO0_DEFAULT
+    + "\tinet 127.0.0.16 netmask 0xffffffff\n"
+    + "\tinet 127.0.0.17 netmask 0xffffffff\n"
+)
+
+
+def _lo0_full() -> str:
+    """All 16 pool addresses already aliased."""
+    aliases = "".join(
+        f"\tinet 127.0.0.{i} netmask 0xffffffff\n"
+        for i in range(16, 32)
+    )
+    return _LO0_DEFAULT + aliases
+
+
+class TestEnsurePool(unittest.TestCase):
+    def test_noop_on_linux(self):
+        # `_is_macos` returns False on Linux; ensure_pool should
+        # never shell out to sudo.
+        with patch.object(loopback_alias, "_is_macos", return_value=False), \
+             patch.object(loopback_alias.subprocess, "run") as run:
+            loopback_alias.ensure_pool()
+        run.assert_not_called()
+
+    def test_all_present_skips_sudo(self):
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(
+                 loopback_alias.subprocess, "run",
+                 return_value=_ok(stdout=_lo0_full()),
+             ) as run:
+            loopback_alias.ensure_pool()
+        # Just the ifconfig probe per pool address; no sudo at all.
+        for call in run.call_args_list:
+            self.assertNotIn("sudo", call.args[0])
+
+    def test_missing_aliases_dispatch_sudo(self):
+        # lo0 only has 16+17 already; sudo runs for 18..31 (14 missing).
+        runs: list[list[str]] = []
+
+        def fake_run(argv, *a, **kw):
+            runs.append(argv)
+            if argv[:2] == ["/sbin/ifconfig", "lo0"]:
+                return _ok(stdout=_LO0_PARTIAL)
+            return _ok()
+
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(loopback_alias.subprocess, "run", side_effect=fake_run):
+            loopback_alias.ensure_pool()
+
+        sudo_calls = [r for r in runs if r and r[0] == "sudo"]
+        self.assertEqual(14, len(sudo_calls))
+        sudo_ips = {call[call.index("alias") + 1].split("/")[0] for call in sudo_calls}
+        self.assertEqual(
+            {f"127.0.0.{i}" for i in range(18, 32)},
+            sudo_ips,
+        )
+
+    def test_sudo_failure_dies(self):
+        def fake_run(argv, *a, **kw):
+            if argv[:2] == ["/sbin/ifconfig", "lo0"]:
+                return _ok(stdout=_LO0_DEFAULT)
+            if argv[:1] == ["sudo"]:
+                return _fail()
+            return _ok()
+
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(loopback_alias.subprocess, "run", side_effect=fake_run), \
+             patch.object(loopback_alias, "die", side_effect=SystemExit("die")):
+            with self.assertRaises(SystemExit):
+                loopback_alias.ensure_pool()
+
+
+class TestAllocate(unittest.TestCase):
+    def test_returns_loopback_on_linux(self):
+        with patch.object(loopback_alias, "_is_macos", return_value=False):
+            self.assertEqual("127.0.0.1", loopback_alias.allocate("demo"))
+
+    def test_picks_lowest_unused_on_macos(self):
+        # No bundles running -> first pool entry.
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(loopback_alias, "_aliases_in_use", return_value=set()):
+            self.assertEqual("127.0.0.16", loopback_alias.allocate("demo-1"))
+
+    def test_skips_in_use_aliases(self):
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(
+                 loopback_alias, "_aliases_in_use",
+                 return_value={"127.0.0.16", "127.0.0.17", "127.0.0.19"},
+             ):
+            # First unused = 127.0.0.18.
+            self.assertEqual("127.0.0.18", loopback_alias.allocate("demo-3"))
+
+    def test_dies_when_pool_exhausted(self):
+        all_in_use = {f"127.0.0.{i}" for i in range(16, 32)}
+        with patch.object(loopback_alias, "_is_macos", return_value=True), \
+             patch.object(
+                 loopback_alias, "_aliases_in_use",
+                 return_value=all_in_use,
+             ), patch.object(
+                 loopback_alias, "die", side_effect=SystemExit("die"),
+             ):
+            with self.assertRaises(SystemExit):
+                loopback_alias.allocate("demo-overflow")
+
+
+class TestAliasInUseDetection(unittest.TestCase):
+    """`_aliases_in_use` inspects every running bundle and pulls
+    each container's port-binding `HostIp` out. The detection has
+    to survive: no running bundles, multiple bundles, docker
+    inspect failures."""
+
+    def test_no_bundles_returns_empty(self):
+        with patch.object(
+            loopback_alias.subprocess, "run",
+            return_value=_ok(stdout=""),
+        ):
+            self.assertEqual(set(), loopback_alias._aliases_in_use())
+
+    def test_walks_bundles_and_pulls_host_ips(self):
+        # First call: docker ps -> two bundle names.
+        # Then docker inspect each, returning a port-bindings JSON
+        # blob with a HostIp on the per-bottle alias.
+        ps_out = "claude-bottle-sidecars-a\nclaude-bottle-sidecars-b\n"
+        inspect_a = (
+            '{"8888/tcp":[{"HostIp":"127.0.0.16","HostPort":"54000"}]}'
+        )
+        inspect_b = (
+            '{"9099/tcp":[{"HostIp":"127.0.0.17","HostPort":"54001"}]}'
+        )
+
+        seq = [
+            _ok(stdout=ps_out),
+            _ok(stdout=inspect_a),
+            _ok(stdout=inspect_b),
+        ]
+        with patch.object(
+            loopback_alias.subprocess, "run", side_effect=seq,
+        ):
+            self.assertEqual(
+                {"127.0.0.16", "127.0.0.17"},
+                loopback_alias._aliases_in_use(),
+            )
+
+    def test_inspect_failures_are_skipped(self):
+        ps_out = "claude-bottle-sidecars-c\n"
+        with patch.object(
+            loopback_alias.subprocess, "run",
+            side_effect=[_ok(stdout=ps_out), _fail("inspect failed")],
+        ):
+            self.assertEqual(set(), loopback_alias._aliases_in_use())
+
+
+if __name__ == "__main__":
+    unittest.main()