diff --git a/bot_bottle/backend/smolmachines/launch.py b/bot_bottle/backend/smolmachines/launch.py
index ff41e0f..9102d4c 100644
--- a/bot_bottle/backend/smolmachines/launch.py
+++ b/bot_bottle/backend/smolmachines/launch.py
@@ -142,6 +142,7 @@ def launch(
         # 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.ensure_bundle_image(bundle_spec.image)
         _bundle.start_bundle(bundle_spec, env={**os.environ, **token_env})
         stack.callback(_bundle.stop_bundle, plan.slug)
 
diff --git a/bot_bottle/backend/smolmachines/sidecar_bundle.py b/bot_bottle/backend/smolmachines/sidecar_bundle.py
index 9aca944..553a972 100644
--- a/bot_bottle/backend/smolmachines/sidecar_bundle.py
+++ b/bot_bottle/backend/smolmachines/sidecar_bundle.py
@@ -29,7 +29,14 @@ from pathlib import Path
 from typing import Sequence
 
 from ...log import die, warn
-from ..docker.sidecar_bundle import SIDECAR_BUNDLE_IMAGE
+from ..docker import util as docker_mod
+from ..docker.sidecar_bundle import (
+    SIDECAR_BUNDLE_DOCKERFILE,
+    SIDECAR_BUNDLE_IMAGE,
+)
+
+
+_REPO_DIR = str(Path(__file__).resolve().parent.parent.parent.parent)
 
 
 def bundle_network_name(slug: str) -> str:
@@ -85,6 +92,21 @@ class BundleLaunchSpec:
     publish_host_ip: str = "127.0.0.1"
 
 
+def ensure_bundle_image(image: str = SIDECAR_BUNDLE_IMAGE) -> None:
+    """Build the sidecar bundle image before `docker run`.
+
+    The Docker backend gets this for free from compose's `build:`
+    stanza. smolmachines starts the bundle with plain `docker run`,
+    so without an explicit build a first launch tries to pull the
+    local-only `bot-bottle-sidecars:latest` tag from a registry.
+    """
+    docker_mod.build_image(
+        image,
+        _REPO_DIR,
+        dockerfile=SIDECAR_BUNDLE_DOCKERFILE,
+    )
+
+
 def create_bundle_network(network_name: str, subnet: str, gateway: str) -> None:
     """`docker network create` with an explicit subnet + gateway
     so the bundle's `--ip` lands on the address the Smolfile's
diff --git a/tests/unit/test_smolmachines_sidecar_bundle.py b/tests/unit/test_smolmachines_sidecar_bundle.py
index f913d23..84f181f 100644
--- a/tests/unit/test_smolmachines_sidecar_bundle.py
+++ b/tests/unit/test_smolmachines_sidecar_bundle.py
@@ -9,6 +9,7 @@ from __future__ import annotations
 
 import subprocess
 import unittest
+from pathlib import Path
 from unittest.mock import patch
 
 from bot_bottle.backend.smolmachines.sidecar_bundle import (
@@ -16,6 +17,7 @@ from bot_bottle.backend.smolmachines.sidecar_bundle import (
     bundle_container_name,
     bundle_network_name,
     create_bundle_network,
+    ensure_bundle_image,
     remove_bundle_network,
     start_bundle,
     stop_bundle,
@@ -182,6 +184,21 @@ class TestStartBundle(unittest.TestCase):
         self.assertEqual({"FOO": "bar"}, m.call_args.kwargs["env"])
 
 
+class TestEnsureBundleImage(unittest.TestCase):
+    def test_builds_sidecar_dockerfile_before_plain_docker_run(self):
+        with patch(
+            "bot_bottle.backend.smolmachines.sidecar_bundle.docker_mod.build_image",
+        ) as build:
+            ensure_bundle_image()
+
+        build.assert_called_once()
+        args = build.call_args.args
+        kwargs = build.call_args.kwargs
+        self.assertEqual("bot-bottle-sidecars:latest", args[0])
+        self.assertTrue((Path(args[1]) / "Dockerfile.sidecars").is_file())
+        self.assertEqual("Dockerfile.sidecars", kwargs["dockerfile"])
+
+
 class TestStopBundle(unittest.TestCase):
     def _patch_run(self, **kwargs):
         return patch(