feat: persist backend in BottleMetadata; use it in resume and dashboard reattach (PRD 0040)

BottleMetadata gains a backend field (default ""). Docker prepare writes
"docker"; smolmachines prepare writes "smolmachines". read_metadata
deserialises it with "" as the backward-compatible default.

resume now passes metadata.backend to _launch_bottle so a preserved
smolmachines bottle is resumed on the right backend without requiring
BOT_BOTTLE_BACKEND to be set manually.

_bottle_for_slug now reads metadata.backend and constructs a
SmolmachinesBottle for smolmachines slugs instead of always defaulting
to DockerBottle. No-metadata slugs still fall back to Docker.

Closes #137

bot_bottle/backend/docker/bottle_state.py

@@ -105,6 +105,10 @@ class BottleMetadata:
     # written before chunk 3 (resume / inspect should fall back to
     # deriving from identity in that case).
     compose_project: str = ""
+    # PRD 0040: backend name ("docker" or "smolmachines"). Empty string
+    # for state dirs written before PRD 0040; callers default to "docker"
+    # for backward compatibility.
+    backend: str = ""
 
 
 def metadata_path(identity: str) -> Path:
@@ -138,6 +142,7 @@ def read_metadata(identity: str) -> BottleMetadata | None:
         copy_cwd=bool(raw.get("copy_cwd", False)),
         started_at=str(raw.get("started_at", "")),
         compose_project=str(raw.get("compose_project", "")),
+        backend=str(raw.get("backend", "")),
     )
 
 

bot_bottle/backend/docker/prepare.py

@@ -79,6 +79,7 @@ def resolve_plan(
         copy_cwd=spec.copy_cwd,
         started_at=datetime.now(timezone.utc).isoformat(),
         compose_project=f"bot-bottle-{slug}",
+        backend="docker",
     ))
     # Clear any leftover preserve marker from a prior capability-block
     # so this fresh launch can be cleaned up at session-end unless

bot_bottle/backend/smolmachines/prepare.py

@@ -70,9 +70,8 @@ def resolve_plan(
         cwd=spec.user_cwd if spec.copy_cwd else "",
         copy_cwd=spec.copy_cwd,
         started_at=datetime.now(timezone.utc).isoformat(),
-        # No compose project for smolmachines bottles; chunk 4
-        # will give dashboard discovery a backend-specific path.
         compose_project="",
+        backend="smolmachines",
     ))
 
     subnet, gateway, bundle_ip = smolmachines_bundle_subnet(slug)

bot_bottle/cli/dashboard.py

@@ -640,23 +640,19 @@ def _bottle_for_slug(
 ) -> tuple["object", str]:
     """Return `(bottle_handle, prompt_path_hint)` for a re-attach.
     If the slug is in `bottles` (dashboard-owned), return the stored
-    handle directly. Otherwise synthesize a `DockerBottle` from the
-    container name `bot-bottle-<slug>`. For synthesized bottles
-    the prompt-file path comes from the manifest's agent if we can
-    resolve it via metadata.json + the loaded manifest; otherwise
-    the re-attach runs without `--append-system-prompt-file`.
+    handle directly. Otherwise synthesize a bottle from the persisted
+    metadata. The backend field in metadata (PRD 0040) selects Docker
+    or smolmachines; unknown or missing metadata defaults to Docker.
 
     Returns the empty string for prompt_path_hint when we omit the
     flag — the caller passes None to DockerBottle in that case."""
     from ..backend.docker.bottle import DockerBottle
     from ..backend.docker.bottle_state import read_metadata
+    from ..backend.smolmachines.bottle import SmolmachinesBottle
     if slug in bottles:
         _cm, bottle, _identity = bottles[slug]
         return bottle, ""
-    # The container hosting the agent's agent process is named
-    # `bot-bottle-<slug>` — set by the compose renderer
-    # (no service suffix on the agent service, by design).
-    container_name = f"bot-bottle-{slug}"
+    machine_name = f"bot-bottle-{slug}"
     prompt_path: str | None = None
     metadata = read_metadata(slug)
     if metadata is not None and manifest is not None:
@@ -666,11 +662,18 @@ def _bottle_for_slug(
                 "BOT_BOTTLE_CONTAINER_HOME", "/home/node",
             )
             prompt_path = f"{container_home}/.bot-bottle-prompt.txt"
-    synth = DockerBottle(
-        container=container_name,
-        teardown=lambda: None,
-        prompt_path_in_container=prompt_path,
-    )
+    backend = metadata.backend if metadata is not None else ""
+    if backend == "smolmachines":
+        synth: object = SmolmachinesBottle(
+            machine_name,
+            prompt_path=prompt_path,
+        )
+    else:
+        synth = DockerBottle(
+            container=machine_name,
+            teardown=lambda: None,
+            prompt_path_in_container=prompt_path,
+        )
     return synth, (prompt_path or "")
 
 

bot_bottle/cli/resume.py

@@ -52,8 +52,10 @@ def cmd_resume(argv: list[str]) -> int:
         user_cwd=metadata.cwd or USER_CWD,
         identity=metadata.identity,
     )
+    backend_name = metadata.backend or None
     return _launch_bottle(
         spec,
         dry_run=args.dry_run,
         remote_control=args.remote_control,
+        backend_name=backend_name,
     )

tests/unit/test_bottle_state.py

@@ -216,5 +216,112 @@ class TestBottleMetadata(_FakeHomeMixin, unittest.TestCase):
         self.assertEqual("t2", loaded.started_at)
 
 
+class TestBottleMetadataBackend(_FakeHomeMixin, unittest.TestCase):
+    """PRD 0040: backend field is persisted and read back."""
+
+    def setUp(self):
+        self._setup_fake_home()
+
+    def tearDown(self):
+        self._teardown_fake_home()
+
+    def test_backend_field_roundtrips_docker(self):
+        meta = BottleMetadata(
+            identity="dev-b1",
+            agent_name="dev",
+            cwd="",
+            copy_cwd=False,
+            started_at="2026-06-02T00:00:00+00:00",
+            compose_project="bot-bottle-dev-b1",
+            backend="docker",
+        )
+        write_metadata(meta)
+        loaded = read_metadata("dev-b1")
+        self.assertIsNotNone(loaded)
+        assert loaded is not None
+        self.assertEqual("docker", loaded.backend)
+
+    def test_backend_field_roundtrips_smolmachines(self):
+        meta = BottleMetadata(
+            identity="dev-b2",
+            agent_name="dev",
+            cwd="",
+            copy_cwd=False,
+            started_at="2026-06-02T00:00:00+00:00",
+            compose_project="",
+            backend="smolmachines",
+        )
+        write_metadata(meta)
+        loaded = read_metadata("dev-b2")
+        self.assertIsNotNone(loaded)
+        assert loaded is not None
+        self.assertEqual("smolmachines", loaded.backend)
+
+    def test_missing_backend_field_defaults_to_empty(self):
+        # Old state dirs written before PRD 0040 have no backend key.
+        import json
+        from bot_bottle.backend.docker import bottle_state as bs
+        path = bs.metadata_path("dev-b3")
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(json.dumps({
+            "identity": "dev-b3",
+            "agent_name": "dev",
+            "cwd": "",
+            "copy_cwd": False,
+            "started_at": "2026-06-02T00:00:00+00:00",
+            "compose_project": "bot-bottle-dev-b3",
+        }))
+        loaded = read_metadata("dev-b3")
+        self.assertIsNotNone(loaded)
+        assert loaded is not None
+        self.assertEqual("", loaded.backend)
+
+
+class TestBottleForSlugBackend(_FakeHomeMixin, unittest.TestCase):
+    """PRD 0040: _bottle_for_slug constructs the right bottle type."""
+
+    def setUp(self):
+        self._setup_fake_home()
+
+    def tearDown(self):
+        self._teardown_fake_home()
+
+    def test_docker_metadata_returns_docker_bottle(self):
+        from bot_bottle.backend.docker.bottle import DockerBottle
+        from bot_bottle.cli.dashboard import _bottle_for_slug
+        write_metadata(BottleMetadata(
+            identity="dev-d1",
+            agent_name="dev",
+            cwd="",
+            copy_cwd=False,
+            started_at="2026-06-02T00:00:00+00:00",
+            compose_project="bot-bottle-dev-d1",
+            backend="docker",
+        ))
+        bottle, _ = _bottle_for_slug("dev-d1", {}, None)
+        self.assertIsInstance(bottle, DockerBottle)
+
+    def test_smolmachines_metadata_returns_smolmachines_bottle(self):
+        from bot_bottle.backend.smolmachines.bottle import SmolmachinesBottle
+        from bot_bottle.cli.dashboard import _bottle_for_slug
+        write_metadata(BottleMetadata(
+            identity="dev-s1",
+            agent_name="dev",
+            cwd="",
+            copy_cwd=False,
+            started_at="2026-06-02T00:00:00+00:00",
+            compose_project="",
+            backend="smolmachines",
+        ))
+        bottle, _ = _bottle_for_slug("dev-s1", {}, None)
+        self.assertIsInstance(bottle, SmolmachinesBottle)
+
+    def test_no_metadata_defaults_to_docker_bottle(self):
+        from bot_bottle.backend.docker.bottle import DockerBottle
+        from bot_bottle.cli.dashboard import _bottle_for_slug
+        bottle, _ = _bottle_for_slug("unknown-slug", {}, None)
+        self.assertIsInstance(bottle, DockerBottle)
+
+
 if __name__ == "__main__":
     unittest.main()