feat(smolmachines): build agent image from repo Dockerfile (PRD 0023 chunk 4c)

Replaces the alpine:latest placeholder with a real claude-bottle
agent image, converted into a .smolmachine artifact via an
ephemeral local OCI registry.

Why the registry hop: smolvm pack create only accepts OCI registry
refs. Empirically it rejects docker-daemon://, oci-layout://,
docker-archive: tarballs, and every other transport tested — the
crane backend treats anything with a scheme prefix as a registry
hostname. To convert a locally-built docker image into a
.smolmachine we have to push it somewhere smolvm can pull from.
Smallest path: bring up registry:2.8.3 bound to 127.0.0.1:<random>,
docker tag + docker push into it, smolvm pack create --image
localhost:<port>/claude-bottle:<id>, tear down the registry.

The .smolmachine is cached under
~/.cache/claude-bottle/smolmachines/ keyed by the docker image ID
(first 16 hex chars of the sha256), so a Dockerfile change picks
up a new image ID and invalidates the cache. Unchanged rebuilds
skip the whole build → registry → pack pipeline.

This puts `docker build` in smolmachines prepare (the docker
backend defers it to launch). Necessary because pack_create needs
the image ID to derive the cache key, and prepare is the only
hook ahead of launch that runs once per slug.

Adds:
- claude_bottle/backend/docker/util.py: image_id / tag / push
  helpers (thin docker CLI wrappers).
- claude_bottle/backend/smolmachines/local_registry.py:
  ephemeral_registry() context manager; pins registry:2.8.3 by
  digest, binds 127.0.0.1::5000 (loopback-only), force-removes on
  exit.

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

claude_bottle/backend/docker/util.py

@@ -147,6 +147,39 @@ def build_image_with_cwd(derived: str, base: str, cwd: str) -> None:
     )
 
 
+def image_id(ref: str) -> str:
+    """Return the content-addressed image ID (e.g.
+    `sha256:abcd...`) for `ref`. The smolmachines backend keys its
+    `.smolmachine` artifact cache on this, so a Dockerfile change
+    that produces a new image automatically invalidates the cache."""
+    r = subprocess.run(
+        ["docker", "image", "inspect", "--format", "{{.Id}}", ref],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if r.returncode != 0:
+        die(
+            f"docker image inspect for {ref!r} failed: "
+            f"{(r.stderr or '').strip() or '<no stderr>'}"
+        )
+    return r.stdout.strip()
+
+
+def tag(src: str, dst: str) -> None:
+    """`docker tag SRC DST`. Idempotent. Used by smolmachines prepare
+    to retag the locally-built image into a localhost:<port>/... ref
+    that the ephemeral registry will accept."""
+    subprocess.run(["docker", "tag", src, dst], check=True)
+
+
+def push(ref: str) -> None:
+    """`docker push REF`. Used by smolmachines prepare to push the
+    agent image into the ephemeral local registry so smolvm's crane
+    backend can pull it."""
+    subprocess.run(["docker", "push", ref], check=True)
+
+
 def _silent_run(cmd: Iterable[str]) -> int:
     return subprocess.run(
         list(cmd),

claude_bottle/backend/smolmachines/local_registry.py

@@ -0,0 +1,124 @@
+"""Ephemeral local OCI registry for the smolmachines agent-image
+conversion path (PRD 0023 chunk 4c).
+
+`smolvm pack create --image <ref>` only accepts registry refs — it
+can't read the local docker daemon's image cache, an OCI layout
+directory, or a `docker save` tarball. To convert the agent's
+Dockerfile-built image into a `.smolmachine` artifact we run a
+short-lived `registry:2.8.3` container on `127.0.0.1:<random>`,
+push the locally-tagged image into it, and let smolvm pull from
+there. The registry container is torn down as soon as the pack
+completes.
+
+Loopback-only bind + the host's docker layer cache mean the round
+trip is fast (~5s) and there's no exposed surface on the LAN."""
+
+from __future__ import annotations
+
+import os
+import socket
+import subprocess
+import time
+import uuid
+from contextlib import contextmanager
+from typing import Iterator
+
+from ...log import die
+
+
+# registry:2.8.3, pinned by digest. Same env-override pattern as the
+# pipelock image pin in claude_bottle/backend/docker/pipelock.py.
+REGISTRY_IMAGE = os.environ.get(
+    "CLAUDE_BOTTLE_REGISTRY_IMAGE",
+    "registry@sha256:a3d8aaa63ed8681a604f1dea0aa03f100d5895b6a58ace528858a7b332415373",
+)
+
+
+# How long to wait for the registry's HTTP layer to bind before
+# giving up. Two seconds is empirically enough; bumping to 10s leaves
+# headroom for slow CI runners without making the failure mode chatty.
+_READY_TIMEOUT_S = 10.0
+
+
+@contextmanager
+def ephemeral_registry() -> Iterator[int]:
+    """Bring up a `registry:2.8.3` container on a random loopback
+    port, yield the port, force-remove the container on exit.
+
+    The container is started with `--rm` so a clean exit cleans up
+    on its own; the `finally` block force-removes on abnormal exit
+    (the calling process crashes between yield and close)."""
+    name = f"claude-bottle-registry-{uuid.uuid4().hex[:12]}"
+    subprocess.run(
+        [
+            "docker", "run", "-d", "--rm",
+            "--name", name,
+            # `127.0.0.1::5000` = bind to loopback, pick a random host
+            # port. No LAN exposure; the container hangs around just
+            # long enough for one push + one pack-create.
+            "-p", "127.0.0.1::5000",
+            REGISTRY_IMAGE,
+        ],
+        check=True,
+        capture_output=True,
+    )
+    try:
+        port = _host_port(name)
+        _wait_ready(port)
+        yield port
+    finally:
+        subprocess.run(
+            ["docker", "rm", "-f", name],
+            check=False,
+            capture_output=True,
+        )
+
+
+def _host_port(name: str) -> int:
+    """Resolve the host-side port docker mapped to the registry's
+    container port 5000. `docker port <name> 5000/tcp` returns one or
+    more `host:port` lines; the loopback-only -p binding ensures we
+    get exactly `127.0.0.1:<port>`."""
+    r = subprocess.run(
+        ["docker", "port", name, "5000/tcp"],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if r.returncode != 0:
+        die(
+            f"docker port {name} 5000/tcp failed: "
+            f"{(r.stderr or '').strip() or '<no stderr>'}"
+        )
+    # `127.0.0.1:54321\n` — split on the last colon to handle the
+    # `host:port` shape without parsing IP literals.
+    line = (r.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
+
+
+def _wait_ready(port: int) -> None:
+    """Block until the registry's HTTP layer accepts a TCP connection
+    on `127.0.0.1:<port>`, or `_READY_TIMEOUT_S` elapses.
+
+    A successful TCP connect is sufficient — registry:2.8.3 binds
+    after it's ready to serve `/v2/` requests, so the push that
+    follows will land on a working server."""
+    deadline = time.monotonic() + _READY_TIMEOUT_S
+    last_err: Exception | None = None
+    while time.monotonic() < deadline:
+        try:
+            with socket.create_connection(("127.0.0.1", port), timeout=0.5):
+                return
+        except OSError as e:
+            last_err = e
+            time.sleep(0.1)
+    die(
+        f"local registry on 127.0.0.1:{port} did not accept "
+        f"connections within {_READY_TIMEOUT_S:.0f}s "
+        f"(last error: {last_err})"
+    )

claude_bottle/backend/smolmachines/prepare.py

@@ -1,16 +1,23 @@
-"""smolmachines `_resolve_plan` (PRD 0023 chunk 2d).
+"""smolmachines `_resolve_plan` (PRD 0023 chunks 2d + 4c).
 
-Resolves the per-bottle docker subnet + bundle IP, pre-packs the
-agent's `.smolmachine` artifact (cached under
-`~/.cache/claude-bottle/smolmachines/`), and assembles the guest
-env. No VM bringup — that's `launch.launch`'s job."""
+Resolves the per-bottle docker subnet + bundle IP, builds the
+agent's docker image from the repo Dockerfile, converts it into a
+`.smolmachine` artifact via an ephemeral local registry (smolvm's
+crane backend only reads registry refs), and assembles the guest
+env. The `.smolmachine` is cached under
+`~/.cache/claude-bottle/smolmachines/` keyed by the docker image
+ID so Dockerfile changes invalidate the cache automatically.
+
+No VM bringup — that's `launch.launch`'s job."""
 
 from __future__ import annotations
 
+import os
 from datetime import datetime, timezone
 from pathlib import Path
 
 from ...backend import BottleSpec
+from ...backend.docker import util as docker_mod
 from ...backend.docker.bottle_state import (
     BottleMetadata,
     agent_state_dir,
@@ -27,9 +34,14 @@ from ...pipelock import PipelockProxy
 from ...supervise import Supervise
 from . import smolvm as _smolvm
 from .bottle_plan import SmolmachinesBottlePlan
+from .local_registry import ephemeral_registry
 from .util import smolmachines_bundle_subnet, smolmachines_preflight
 
 
+# Repo root, used as the `docker build` context for the agent image.
+_REPO_DIR = str(Path(__file__).resolve().parent.parent.parent.parent)
+
+
 # Per-host cache for `smolvm pack create` outputs. Keyed by the
 # image ref so re-prepares for the same image hit the cache
 # (pack create is idempotent on the smolvm side but takes several
@@ -132,11 +144,15 @@ def resolve_plan(
     prompt_file.chmod(0o600)
 
     machine_name = f"claude-bottle-{slug}"
-    # Chunk 2d placeholder until the agent-image work lands.
-    # alpine pulls cleanly from docker.io via smolvm's crane
-    # backend; the real claude-bottle image lives in the local
-    # docker daemon and isn't reachable that way.
-    agent_image_ref = "alpine:latest"
+    # Build the agent image from the repo Dockerfile (shared with
+    # the docker backend, layer-cached) and convert it into a
+    # `.smolmachine` artifact via an ephemeral local registry. The
+    # CLAUDE_BOTTLE_IMAGE env var match the docker backend's
+    # resolve_plan default so both backends use the same image when
+    # one is built.
+    agent_image_ref = os.environ.get(
+        "CLAUDE_BOTTLE_IMAGE", "claude-bottle:latest"
+    )
     agent_from_path = _ensure_smolmachine(agent_image_ref)
 
     return SmolmachinesBottlePlan(
@@ -158,21 +174,37 @@ def resolve_plan(
 
 
 def _ensure_smolmachine(image_ref: str) -> Path:
-    """Cache `smolvm pack create --image <image_ref>` output under
-    `~/.cache/claude-bottle/smolmachines/<slug>`. Returns the
-    `.smolmachine.smolmachine` sidecar path — that's the file
-    `machine create --from` consumes (pack create produces a
-    launcher binary at `.smolmachine` plus the sidecar alongside
+    """Build the agent docker image and convert it into a
+    `.smolmachine` artifact, caching the result under
+    `~/.cache/claude-bottle/smolmachines/` keyed by the docker image
+    ID (so a Dockerfile change automatically invalidates the cache).
+
+    Returns the `.smolmachine.smolmachine` sidecar path — that's
+    the file `machine create --from` consumes (pack create produces
+    a launcher binary at `.smolmachine` plus the sidecar alongside
     it; the sidecar is the actual artifact).
 
-    Re-runs of pack create against the same image hit smolvm's
-    layer cache; we still skip the call entirely when the
-    sidecar is already on disk, since each invocation costs
-    several seconds even on a hot cache."""
+    Conversion path: `docker build` (the existing layer cache makes
+    no-change rebuilds cheap) → `docker tag` with a
+    `localhost:<port>/...` ref → bring up the ephemeral registry
+    container → `docker push` into it → `smolvm pack create --image
+    <localhost ref>` → tear down the registry. Each pack-create
+    costs several seconds even on a hot cache, so we skip the whole
+    pipeline when the cached sidecar is already on disk for this
+    image ID."""
     _SMOLMACHINE_CACHE_DIR.mkdir(parents=True, exist_ok=True)
-    slug = image_ref.replace(":", "_").replace("/", "_")
-    binary = _SMOLMACHINE_CACHE_DIR / f"{slug}.smolmachine"
-    sidecar = _SMOLMACHINE_CACHE_DIR / f"{slug}.smolmachine.smolmachine"
-    if not sidecar.is_file():
-        _smolvm.pack_create(image_ref, binary)
+    docker_mod.build_image(image_ref, _REPO_DIR)
+    # `sha256:abcd...` -> `abcd...` first 16 chars: short enough to
+    # keep filenames manageable, long enough to make collisions
+    # astronomically unlikely.
+    digest = docker_mod.image_id(image_ref).split(":", 1)[-1][:16]
+    binary = _SMOLMACHINE_CACHE_DIR / f"{digest}.smolmachine"
+    sidecar = _SMOLMACHINE_CACHE_DIR / f"{digest}.smolmachine.smolmachine"
+    if sidecar.is_file():
+        return sidecar
+    with ephemeral_registry() as port:
+        local_ref = f"localhost:{port}/claude-bottle:{digest}"
+        docker_mod.tag(image_ref, local_ref)
+        docker_mod.push(local_ref)
+        _smolvm.pack_create(local_ref, binary)
     return sidecar

docs/prds/0023-smolmachines-backend.md

@@ -393,10 +393,19 @@ Three changes vs. the Docker backend:
 2. Derive a per-bottle docker subnet from `sha256(slug) % 254`
    (skipping the docker-default 17): `192.168.X.0/24`. The bundle
    IP is always `192.168.X.2` (gateway is `.1`).
-3. Resolve the agent guest image: convert the existing
-   `Dockerfile` into a `.smolmachine` artifact via
-   `smolvm pack create --image <name> -o <stage>/agent.smolmachine`
-   (idempotent, layer-cached).
+3. Resolve the agent guest image: `docker build` the existing
+   `Dockerfile`, then convert the resulting image into a
+   `.smolmachine` artifact. Empirically `smolvm pack create` only
+   reads OCI registry refs — it rejects `docker-daemon://`,
+   `oci-layout://`, `docker-archive:` tarballs, and every other
+   transport tested. The conversion path is a registry hop: bring
+   up an ephemeral `registry:2.8.3` container bound to
+   `127.0.0.1:<random>`, `docker tag` + `docker push` into it,
+   `smolvm pack create --image localhost:<port>/claude-bottle:<id>`,
+   tear down the registry. The `.smolmachine` is cached under
+   `~/.cache/claude-bottle/smolmachines/` keyed by the docker
+   image ID, so Dockerfile changes invalidate the cache and
+   unchanged rebuilds skip the whole pipeline.
 4. Render the per-bottle Smolfile to `stage_dir/smolfile.toml`
    using smolvm 0.8.0's schema:
    - `image` / `entrypoint` / `cmd` — bundled into the

tests/unit/test_docker_util_image.py

@@ -0,0 +1,79 @@
+"""Unit: image_id / tag / push helpers in
+claude_bottle.backend.docker.util (PRD 0023 chunk 4c additions).
+
+Tests mock `subprocess.run` and assert on argv shape + parsing.
+The actual docker round-trip is covered by the chunk 4c
+integration smoke."""
+
+from __future__ import annotations
+
+import subprocess
+import unittest
+from unittest.mock import patch
+
+from claude_bottle.backend.docker import util as docker_mod
+
+
+def _ok(stdout: str = "", stderr: str = "") -> subprocess.CompletedProcess:
+    return subprocess.CompletedProcess(
+        args=[], returncode=0, stdout=stdout, stderr=stderr,
+    )
+
+
+def _fail(stderr: str = "boom") -> subprocess.CompletedProcess:
+    return subprocess.CompletedProcess(
+        args=[], returncode=1, stdout="", stderr=stderr,
+    )
+
+
+class TestImageId(unittest.TestCase):
+    def test_strips_trailing_newline(self):
+        # docker image inspect --format ... emits a trailing newline.
+        with patch.object(
+            docker_mod.subprocess, "run",
+            return_value=_ok(stdout="sha256:abcdef\n"),
+        ) as run:
+            self.assertEqual(
+                "sha256:abcdef", docker_mod.image_id("claude-bottle:latest")
+            )
+        argv = run.call_args.args[0]
+        self.assertEqual(
+            ["docker", "image", "inspect", "--format", "{{.Id}}", "claude-bottle:latest"],
+            argv,
+        )
+
+    def test_dies_on_inspect_failure(self):
+        with patch.object(
+            docker_mod.subprocess, "run", return_value=_fail("No such image"),
+        ), patch.object(
+            docker_mod, "die", side_effect=SystemExit("die"),
+        ) as die:
+            with self.assertRaises(SystemExit):
+                docker_mod.image_id("missing:tag")
+            die.assert_called_once()
+            self.assertIn("missing:tag", die.call_args.args[0])
+
+
+class TestTagPush(unittest.TestCase):
+    def test_tag_runs_docker_tag(self):
+        with patch.object(
+            docker_mod.subprocess, "run", return_value=_ok(),
+        ) as run:
+            docker_mod.tag("claude-bottle:latest", "localhost:5000/cb:abc")
+        argv = run.call_args.args[0]
+        self.assertEqual(
+            ["docker", "tag", "claude-bottle:latest", "localhost:5000/cb:abc"],
+            argv,
+        )
+
+    def test_push_runs_docker_push(self):
+        with patch.object(
+            docker_mod.subprocess, "run", return_value=_ok(),
+        ) as run:
+            docker_mod.push("localhost:5000/cb:abc")
+        argv = run.call_args.args[0]
+        self.assertEqual(["docker", "push", "localhost:5000/cb:abc"], argv)
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_smolmachines_local_registry.py

@@ -0,0 +1,141 @@
+"""Unit: ephemeral local-registry helper (PRD 0023 chunk 4c).
+
+The helper brings up a `registry:2.8.3` container on a random
+loopback port, yields the port, and tears the container down on
+exit. Tests mock `subprocess.run` + `socket.create_connection` so
+they run without docker."""
+
+from __future__ import annotations
+
+import subprocess
+import unittest
+from unittest.mock import call, patch
+
+from claude_bottle.backend.smolmachines import local_registry
+
+
+def _ok(stdout: str = "", stderr: str = "") -> subprocess.CompletedProcess:
+    return subprocess.CompletedProcess(
+        args=[], returncode=0, stdout=stdout, stderr=stderr,
+    )
+
+
+class TestEphemeralRegistry(unittest.TestCase):
+    def test_yields_host_port_parsed_from_docker_port(self):
+        # docker run + docker port + docker rm in that order; the
+        # port command returns `127.0.0.1:54321` for the loopback
+        # binding.
+        with patch.object(
+            local_registry.subprocess, "run",
+            side_effect=[
+                _ok(stdout="<container-id>\n"),
+                _ok(stdout="127.0.0.1:54321\n"),
+                _ok(),
+            ],
+        ) as run, patch.object(
+            local_registry.socket, "create_connection",
+            return_value=_FakeSocket(),
+        ):
+            with local_registry.ephemeral_registry() as port:
+                self.assertEqual(54321, port)
+
+        # docker run, docker port, docker rm -f
+        self.assertEqual(3, run.call_count)
+        run_argv = run.call_args_list[0].args[0]
+        self.assertEqual(["docker", "run"], run_argv[:2])
+        self.assertIn("--rm", run_argv)
+        # Loopback-only port binding so the registry isn't exposed
+        # on the LAN even briefly.
+        self.assertIn("127.0.0.1::5000", run_argv)
+
+    def test_force_removes_container_on_clean_exit(self):
+        with patch.object(
+            local_registry.subprocess, "run",
+            side_effect=[_ok(stdout="cid\n"), _ok(stdout="127.0.0.1:1234\n"), _ok()],
+        ) as run, patch.object(
+            local_registry.socket, "create_connection",
+            return_value=_FakeSocket(),
+        ):
+            with local_registry.ephemeral_registry():
+                pass
+
+        # Last call is `docker rm -f <name>`.
+        last_argv = run.call_args_list[-1].args[0]
+        self.assertEqual(["docker", "rm", "-f"], last_argv[:3])
+
+    def test_force_removes_container_on_exception_inside_with(self):
+        with patch.object(
+            local_registry.subprocess, "run",
+            side_effect=[_ok(stdout="cid\n"), _ok(stdout="127.0.0.1:1234\n"), _ok()],
+        ) as run, patch.object(
+            local_registry.socket, "create_connection",
+            return_value=_FakeSocket(),
+        ):
+            with self.assertRaises(RuntimeError):
+                with local_registry.ephemeral_registry():
+                    raise RuntimeError("inside with")
+
+        # rm -f still ran on exception.
+        last_argv = run.call_args_list[-1].args[0]
+        self.assertEqual(["docker", "rm", "-f"], last_argv[:3])
+
+    def test_wait_ready_times_out_when_socket_never_connects(self):
+        # Drop the timeout to a value that fits the test budget.
+        with patch.object(local_registry, "_READY_TIMEOUT_S", 0.1), patch.object(
+            local_registry.subprocess, "run",
+            side_effect=[_ok(stdout="cid\n"), _ok(stdout="127.0.0.1:1234\n"), _ok()],
+        ) as run, patch.object(
+            local_registry.socket, "create_connection",
+            side_effect=OSError("conn refused"),
+        ), patch.object(
+            local_registry, "die",
+            side_effect=SystemExit("die called"),
+        ) as die:
+            with self.assertRaises(SystemExit):
+                with local_registry.ephemeral_registry():
+                    self.fail("yield reached despite unreachable registry")
+            die.assert_called_once()
+        # rm -f still ran (cleanup goes through the finally block).
+        last_argv = run.call_args_list[-1].args[0]
+        self.assertEqual(["docker", "rm", "-f"], last_argv[:3])
+
+    def test_unique_container_name_per_call(self):
+        names: list[str] = []
+
+        def capture(argv, *a, **kw):
+            if argv[:2] == ["docker", "run"]:
+                names.append(argv[argv.index("--name") + 1])
+            return _ok(stdout="cid\n" if argv[:2] == ["docker", "run"]
+                       else "127.0.0.1:1\n")
+
+        with patch.object(
+            local_registry.subprocess, "run", side_effect=capture,
+        ), patch.object(
+            local_registry.socket, "create_connection",
+            return_value=_FakeSocket(),
+        ):
+            with local_registry.ephemeral_registry():
+                pass
+            with local_registry.ephemeral_registry():
+                pass
+
+        self.assertEqual(2, len(names))
+        self.assertNotEqual(names[0], names[1])
+        for n in names:
+            self.assertTrue(n.startswith("claude-bottle-registry-"))
+
+
+class _FakeSocket:
+    """Minimal context-manager stand-in for the socket
+    `create_connection` returns. The helper only uses `with` on it
+    and discards the value, so we don't need any real network."""
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_smolmachines_prepare_image.py

@@ -0,0 +1,121 @@
+"""Unit: smolmachines `_ensure_smolmachine` agent-image pipeline
+(PRD 0023 chunk 4c).
+
+Asserts that the cache-hit path returns without touching the
+registry / pack pipeline, and that the cache-miss path runs
+build → tag → push → pack in order against a registry port the
+helper yields."""
+
+from __future__ import annotations
+
+import tempfile
+import unittest
+from pathlib import Path
+from unittest.mock import patch
+
+from claude_bottle.backend.smolmachines import prepare as _prepare
+
+
+class TestEnsureSmolmachine(unittest.TestCase):
+    def setUp(self):
+        self._tmp = tempfile.TemporaryDirectory(prefix="cb-cache.")
+        self._cache_patch = patch.object(
+            _prepare, "_SMOLMACHINE_CACHE_DIR", Path(self._tmp.name),
+        )
+        self._cache_patch.start()
+
+    def tearDown(self):
+        self._cache_patch.stop()
+        self._tmp.cleanup()
+
+    def test_cache_hit_skips_registry_and_pack(self):
+        # Pre-populate the cache for image id `sha256:abcdef0123456789...`.
+        digest = "abcdef0123456789"
+        sidecar = Path(self._tmp.name) / f"{digest}.smolmachine.smolmachine"
+        sidecar.write_text("")
+
+        with patch.object(
+            _prepare.docker_mod, "build_image",
+        ) as build, patch.object(
+            _prepare.docker_mod, "image_id",
+            return_value=f"sha256:{digest}fffffffffffffffff",
+        ), patch.object(
+            _prepare, "ephemeral_registry",
+        ) as registry, patch.object(
+            _prepare.docker_mod, "tag",
+        ) as tag, patch.object(
+            _prepare.docker_mod, "push",
+        ) as push, patch.object(
+            _prepare._smolvm, "pack_create",
+        ) as pack:
+            result = _prepare._ensure_smolmachine("claude-bottle:latest")
+
+        self.assertEqual(sidecar, result)
+        # build still runs (Dockerfile edits land without manual rmi)
+        build.assert_called_once()
+        # No registry, no tag, no push, no pack on cache hit.
+        registry.assert_not_called()
+        tag.assert_not_called()
+        push.assert_not_called()
+        pack.assert_not_called()
+
+    def test_cache_miss_runs_build_tag_push_pack_in_order(self):
+        digest = "0123456789abcdef"
+
+        # ephemeral_registry is a context manager yielding the port.
+        class _Reg:
+            def __enter__(self_inner):
+                return 54321
+            def __exit__(self_inner, *exc):
+                return False
+
+        calls: list[str] = []
+
+        def record(name):
+            def _f(*a, **kw):
+                calls.append(name)
+            return _f
+
+        with patch.object(
+            _prepare.docker_mod, "build_image",
+            side_effect=record("build"),
+        ), patch.object(
+            _prepare.docker_mod, "image_id",
+            return_value=f"sha256:{digest}fffffffffffffffff",
+        ), patch.object(
+            _prepare, "ephemeral_registry",
+            return_value=_Reg(),
+        ), patch.object(
+            _prepare.docker_mod, "tag",
+            side_effect=record("tag"),
+        ) as tag, patch.object(
+            _prepare.docker_mod, "push",
+            side_effect=record("push"),
+        ) as push, patch.object(
+            _prepare._smolvm, "pack_create",
+            side_effect=record("pack"),
+        ) as pack:
+            _prepare._ensure_smolmachine("claude-bottle:latest")
+
+        # build first (no point pushing if the build fails), then
+        # tag → push → pack against the registry port.
+        self.assertEqual(["build", "tag", "push", "pack"], calls)
+
+        # tag goes from the source ref to a localhost:<port> ref
+        # with the digest as the tag suffix (so different builds
+        # land on different tags in the registry).
+        tag_args = tag.call_args.args
+        self.assertEqual("claude-bottle:latest", tag_args[0])
+        self.assertEqual(f"localhost:54321/claude-bottle:{digest}", tag_args[1])
+        # push targets the same localhost ref tag picks.
+        push_args = push.call_args.args
+        self.assertEqual(f"localhost:54321/claude-bottle:{digest}", push_args[0])
+        # pack_create reads from the registry ref, writes the
+        # binary alongside the cached sidecar.
+        pack_args = pack.call_args.args
+        self.assertEqual(f"localhost:54321/claude-bottle:{digest}", pack_args[0])
+        self.assertTrue(str(pack_args[1]).endswith(f"{digest}.smolmachine"))
+
+
+if __name__ == "__main__":
+    unittest.main()