feat(#269): separate agent and bottle selection at launch time

- `bottle:` in agent frontmatter is now optional; agents without it
  are portable and require bottles to be selected at launch.
- Adds `filter_multiselect` to `tui.py`: multi-select picker with
  ordered selection list, Space/Enter to toggle, Ctrl-D to confirm.
- `ManifestIndex` gains `all_bottle_names` and `load_for_agent` accepts
  `bottle_names: tuple[str, ...]` to merge bottles in order at runtime.
- `merge_bottles_runtime` in `manifest_extends.py` applies the same
  field-merge rules as `extends:` to pre-resolved bottle objects.
- `BottleSpec` gains `bottle_names`; `_validate` and `write_launch_metadata`
  thread it through so `resume` replays the same bottle configuration.
- `cmd_start` shows the bottle multiselect after agent selection,
  pre-populated from the agent's `bottle:` field when present.
- Existing agents with `bottle:` declared continue to work unchanged.

bot_bottle/backend/__init__.py

@@ -72,6 +72,9 @@ class BottleSpec:
     identity: str = ""
     label: str = ""
     color: str = ""
+    # Ordered bottle names selected at launch (issue #269). When non-empty
+    # they are merged in order and replace the agent's `bottle:` field.
+    bottle_names: tuple[str, ...] = ()
 
 
 @dataclass(frozen=True)
@@ -129,7 +132,11 @@ class BottlePlan(ABC):
         info(f"provider        : {self.agent_provision.template}")
         print_multi("env             ", env_names)
         print_multi("skills          ", list(agent.skills))
-        info(f"bottle          : {agent.bottle}")
+        effective_bottles = (
+            list(spec.bottle_names) if spec.bottle_names
+            else ([agent.bottle] if agent.bottle else [])
+        )
+        print_multi("bottle          ", effective_bottles)
 
         identity = manifest.git_identity_summary()
         if identity:
@@ -363,7 +370,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         Returns the loaded Manifest for the selected agent. Subclasses with
         additional preconditions should override and call
         `super()._validate(spec)` first."""
-        manifest = spec.manifest.load_for_agent(spec.agent_name)
+        manifest = spec.manifest.load_for_agent(spec.agent_name, spec.bottle_names)
         self._validate_skills(manifest.agent.skills)
         self._validate_agent_provider_dockerfile(spec, manifest)
         return manifest
@@ -389,9 +396,12 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         if not path.is_absolute():
             path = Path(spec.user_cwd) / path
         if not path.is_file():
+            effective = (
+                ", ".join(spec.bottle_names) if spec.bottle_names else manifest.agent.bottle
+            )
             die(
                 f"agent_provider.dockerfile for bottle "
-                f"'{manifest.agent.bottle}' not found: {path}"
+                f"'{effective}' not found: {path}"
             )
 
     @abstractmethod

bot_bottle/backend/resolve_common.py

@@ -63,6 +63,7 @@ def write_launch_metadata(
         backend=backend,
         label=spec.label,
         color=spec.color,
+        bottle_names=spec.bottle_names,
     ))
 
 

bot_bottle/bottle_state.py

@@ -112,6 +112,10 @@ class BottleMetadata:
     backend: str = ""
     label: str = ""
     color: str = ""
+    # Ordered bottle names selected at launch (issue #269). Empty tuple
+    # for state dirs written before this change; resume falls back to
+    # the agent's `bottle:` field in that case.
+    bottle_names: tuple[str, ...] = ()
 
 
 def metadata_path(identity: str) -> Path:
@@ -139,6 +143,10 @@ def read_metadata(identity: str) -> BottleMetadata | None:
     if not isinstance(raw, dict):
         return None
     raw_typed = cast(dict[str, object], raw)
+    raw_bottle_names = raw_typed.get("bottle_names", [])
+    bottle_names: tuple[str, ...] = ()
+    if isinstance(raw_bottle_names, list):
+        bottle_names = tuple(str(n) for n in raw_bottle_names if isinstance(n, str))
     return BottleMetadata(
         identity=str(raw_typed.get("identity", identity)),
         agent_name=str(raw_typed.get("agent_name", "")),
@@ -149,6 +157,7 @@ def read_metadata(identity: str) -> BottleMetadata | None:
         backend=str(raw_typed.get("backend", "")),
         label=str(raw_typed.get("label", "")),
         color=str(raw_typed.get("color", "")),
+        bottle_names=bottle_names,
     )
 
 

bot_bottle/cli/resume.py

@@ -50,6 +50,7 @@ def cmd_resume(argv: list[str]) -> int:
         copy_cwd=metadata.copy_cwd,
         user_cwd=metadata.cwd or USER_CWD,
         identity=metadata.identity,
+        bottle_names=tuple(metadata.bottle_names),
     )
     backend_name = metadata.backend or None
     return _launch_bottle(

bot_bottle/cli/start.py

@@ -74,6 +74,20 @@ def cmd_start(argv: list[str]) -> int:
 
     backend_name: str | None = args.backend
 
+    # Bottle multiselect: always show after agent selection so operators
+    # can compose bottles at launch time without editing agent manifests.
+    available_bottles = manifest.all_bottle_names
+    initial_bottle = _peek_agent_bottle(manifest, agent_name)
+    initial_bottles = [initial_bottle] if initial_bottle else []
+    selected_bottles = tui.filter_multiselect(
+        available_bottles,
+        title="Select bottles",
+        initial=initial_bottles,
+    )
+    if selected_bottles is None:
+        return 0
+    bottle_names = tuple(selected_bottles)
+
     label, color = tui.name_color_modal(default_label=agent_name)
     label, color = _resolve_unique_label(label, color)
 
@@ -84,6 +98,7 @@ def cmd_start(argv: list[str]) -> int:
         user_cwd=USER_CWD,
         label=label,
         color=color,
+        bottle_names=bottle_names,
     )
     return _launch_bottle(
         spec,
@@ -190,6 +205,38 @@ def _identity_from_plan(plan: object) -> str:
     return getattr(plan, "slug", "")
 
 
+def _peek_agent_bottle(manifest: ManifestIndex, agent_name: str) -> str:
+    """Return the `bottle:` value from the named agent's frontmatter without
+    fully parsing the agent file, or "" when absent or unreadable.
+
+    Used to pre-populate the bottle multiselect with the agent's default
+    bottle so operators who haven't removed `bottle:` from their manifests
+    don't need to re-select it every time."""
+    if manifest.home_md is None:
+        # Eager mode (from_json_obj): agent is pre-parsed.
+        if agent_name in manifest.agents:
+            return manifest.agents[agent_name].bottle
+        return ""
+
+    from ..manifest_loader import scan_agent_names
+    from ..yaml_subset import YamlSubsetError, parse_frontmatter
+
+    home_agents = scan_agent_names(manifest.home_md / "agents")
+    cwd_agents: dict[str, Path] = {}
+    if manifest.cwd_md is not None:
+        cwd_agents = scan_agent_names(manifest.cwd_md / "agents")
+    merged = {**home_agents, **cwd_agents}
+    path = merged.get(agent_name)
+    if path is None:
+        return ""
+    try:
+        fm, _ = parse_frontmatter(path.read_text())
+        bottle = fm.get("bottle", "")
+        return str(bottle) if isinstance(bottle, str) else ""
+    except (OSError, YamlSubsetError):
+        return ""
+
+
 def _resolve_unique_label(label: str, color: str) -> tuple[str, str]:
     """Re-prompt with a disclaimer until the label's slug is not already
     in use among running bottles.  Passes through unchanged when no

bot_bottle/cli/tui.py

@@ -17,6 +17,42 @@ import sys
 from typing import Any, Optional
 
 
+def filter_multiselect(
+    items: list[str],
+    *,
+    title: str = "",
+    initial: Optional[list[str]] = None,
+    tty_path: str = "/dev/tty",
+) -> Optional[list[str]]:
+    """Render a multi-select picker over *items*.
+
+    Returns the ordered list of selected items, or ``None`` if the user
+    cancelled (Esc / ``q`` / Ctrl-C / Ctrl-D with no items).
+
+    Press Space or Enter to toggle the item under the cursor.
+    Press Ctrl-D to confirm the current selection (returns even if empty).
+    Press Esc/q to cancel (returns None).
+
+    *initial* pre-populates the selection in insertion order. Items
+    added are appended; removed items leave the remaining order unchanged.
+    """
+    if not items:
+        return []
+
+    try:
+        tty_fd = open(tty_path, "r+b", buffering=0)
+    except OSError:
+        return None
+
+    try:
+        fd_dup = os.dup(tty_fd.fileno())
+        return _run_multiselect(
+            items, title=title, initial=list(initial or []), tty_fd=fd_dup
+        )
+    finally:
+        tty_fd.close()
+
+
 def filter_select(
     items: list[str],
     *,
@@ -221,6 +257,179 @@ def _addstr_safe(screen: Any, row: int, col: int, text: str, attr: int = curses.
         pass
 
 
+# ---------------------------------------------------------------------------
+# filter_multiselect internals
+# ---------------------------------------------------------------------------
+
+_KEY_SPACE = 32
+
+
+def _run_multiselect(
+    items: list[str], *, title: str, initial: list[str], tty_fd: int
+) -> Optional[list[str]]:
+    """Drive a curses multi-select session on *tty_fd*."""
+    os.environ.setdefault("TERM", "xterm-256color")
+
+    orig_stdin = sys.__stdin__
+    orig_stdout = sys.__stdout__
+
+    try:
+        import io
+        tty_text = io.TextIOWrapper(io.FileIO(tty_fd, mode='r+'), write_through=True)
+        sys.__stdin__ = tty_text   # type: ignore[assignment]
+        sys.__stdout__ = tty_text  # type: ignore[assignment]
+
+        screen = curses.initscr()
+        curses.noecho()
+        curses.cbreak()
+        screen.keypad(True)
+
+        try:
+            result = _multiselect_loop(screen, items, title=title, initial=initial)
+        finally:
+            screen.keypad(False)
+            curses.nocbreak()
+            curses.echo()
+            curses.endwin()
+    except Exception:  # noqa: W0718
+        return None
+    finally:
+        sys.__stdin__ = orig_stdin    # type: ignore[assignment]
+        sys.__stdout__ = orig_stdout  # type: ignore[assignment]
+
+    return result
+
+
+def _multiselect_loop(
+    screen: Any, items: list[str], *, title: str, initial: list[str]
+) -> Optional[list[str]]:
+    query = ""
+    cursor = 0
+    selected: list[str] = [s for s in initial if s in items]
+
+    while True:
+        filtered = _filter_items(items, query)
+
+        if not filtered:
+            cursor = 0
+        elif cursor >= len(filtered):
+            cursor = len(filtered) - 1
+
+        try:
+            _render_multiselect(screen, filtered, cursor, query=query, title=title, selected=selected)
+        except curses.error:
+            return None
+
+        try:
+            key = screen.getch()
+        except KeyboardInterrupt:
+            return None
+
+        if key in (_KEY_ESC, _KEY_CTRL_C, ord("q")):
+            return None
+
+        if key == _KEY_CTRL_D:
+            return list(selected)
+
+        if key in (curses.KEY_ENTER, _KEY_ENTER_ALT, ord("\r"), _KEY_SPACE):
+            if filtered:
+                item = filtered[cursor]
+                if item in selected:
+                    selected.remove(item)
+                else:
+                    selected.append(item)
+
+        elif key in (curses.KEY_UP, ord("k")):
+            if cursor > 0:
+                cursor -= 1
+
+        elif key in (curses.KEY_DOWN, ord("j")):
+            if cursor < len(filtered) - 1:
+                cursor += 1
+
+        elif key in (curses.KEY_BACKSPACE, _KEY_BACKSPACE_WIN, 127):
+            query = query[:-1]
+            new_filtered = _filter_items(items, query)
+            if cursor >= len(new_filtered):
+                cursor = max(0, len(new_filtered) - 1)
+
+        elif 32 <= key <= 126 and key != _KEY_SPACE:
+            query += chr(key)
+            cursor = 0
+
+
+def _render_multiselect(
+    screen: Any,
+    filtered: list[str],
+    cursor: int,
+    *,
+    query: str,
+    title: str,
+    selected: list[str],
+) -> None:
+    screen.erase()
+    rows, cols = screen.getmaxyx()
+    min_rows = 7
+
+    if rows < min_rows:
+        raise curses.error("terminal too small")
+
+    row = 0
+
+    if title and row < rows - 1:
+        _addstr_safe(screen, row, 0, title[:cols - 1], curses.A_BOLD)
+        row += 1
+
+    filter_label = f"Filter: {query}"
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, filter_label[:cols - 1])
+        row += 1
+
+    sep = "─" * min(cols - 1, 40)
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, sep)
+        row += 1
+
+    # Reserve rows for: sep + selected-line + sep + help-line = 4
+    list_start = row
+    list_rows = rows - list_start - 4
+    if list_rows < 1:
+        return
+
+    selected_set = set(selected)
+    scroll = max(0, cursor - list_rows + 1)
+    visible = filtered[scroll: scroll + list_rows]
+
+    for idx, item in enumerate(visible):
+        abs_idx = scroll + idx
+        mark = "[*]" if item in selected_set else "[ ]"
+        prefix = "> " if abs_idx == cursor else "  "
+        line = (prefix + mark + " " + item)[:cols - 1]
+        attr = curses.A_REVERSE if abs_idx == cursor else curses.A_NORMAL
+        if row < rows - 4:
+            _addstr_safe(screen, row, 0, line, attr)
+        row += 1
+
+    if row < rows - 3:
+        _addstr_safe(screen, row, 0, sep)
+        row += 1
+
+    selected_summary = "Selected (in order): " + (", ".join(selected) if selected else "(none)")
+    if row < rows - 2:
+        _addstr_safe(screen, row, 0, selected_summary[:cols - 1])
+        row += 1
+
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, sep)
+        row += 1
+
+    help_line = "[↑↓/jk] move  [Space/Enter] toggle  [Ctrl-D] done  [Esc/q] cancel"
+    if row < rows:
+        _addstr_safe(screen, min(rows - 1, row), 0, help_line[:cols - 1])
+
+    screen.refresh()
+
+
 # ---------------------------------------------------------------------------
 # name_color_modal — two-step label + color picker
 # ---------------------------------------------------------------------------

bot_bottle/manifest.py

@@ -215,6 +215,65 @@ def _merge_git_user(
     )
 
 
+def _resolve_effective_bottle_eager(
+    agent_name: str,
+    agent: "ManifestAgent",
+    bottle_names: "tuple[str, ...]",
+    bottles: "Mapping[str, ManifestBottle]",
+) -> "ManifestBottle":
+    """Return the effective ManifestBottle for the eager (from_json_obj) path.
+
+    When bottle_names is non-empty they are merged in order. When empty, falls
+    back to agent.bottle. Raises ManifestError when neither is set."""
+    from .manifest_extends import merge_bottles_runtime
+
+    if bottle_names:
+        resolved: list[ManifestBottle] = []
+        for bn in bottle_names:
+            if bn not in bottles:
+                available = ", ".join(sorted(bottles.keys())) or "(none)"
+                raise ManifestError(
+                    f"bottle '{bn}' not defined. Available: {available}"
+                )
+            resolved.append(bottles[bn])
+        return merge_bottles_runtime(resolved)
+
+    if not agent.bottle:
+        raise ManifestError(
+            f"agent '{agent_name}' has no 'bottle' field and no bottles were "
+            f"selected at launch. Select at least one bottle or add "
+            f"'bottle: <name>' to the agent manifest."
+        )
+    return bottles[agent.bottle]
+
+
+def _resolve_effective_bottle_lazy(
+    agent_name: str,
+    agent_bottle: str,
+    bottle_names: "tuple[str, ...]",
+    bottles_dir: "Path",
+) -> "ManifestBottle":
+    """Return the effective ManifestBottle for the lazy (from_md_dirs) path.
+
+    When bottle_names is non-empty they are resolved from disk and merged in
+    order. When empty, falls back to agent_bottle. Raises ManifestError when
+    neither is set."""
+    from .manifest_extends import merge_bottles_runtime
+    from .manifest_loader import load_bottle_chain_from_dir
+
+    if bottle_names:
+        resolved = [load_bottle_chain_from_dir(bn, bottles_dir) for bn in bottle_names]
+        return merge_bottles_runtime(resolved)
+
+    if not agent_bottle:
+        raise ManifestError(
+            f"agent '{agent_name}' has no 'bottle' field and no bottles were "
+            f"selected at launch. Select at least one bottle or add "
+            f"'bottle: <name>' to the agent manifest."
+        )
+    return load_bottle_chain_from_dir(agent_bottle, bottles_dir)
+
+
 @dataclass(frozen=True)
 class Manifest:
     """Single-agent/bottle value type. Returned by ManifestIndex.load_for_agent().
@@ -360,6 +419,18 @@ class ManifestIndex:
         }
         return cls(bottles=bottles, agents=agents)
 
+    @property
+    def all_bottle_names(self) -> list[str]:
+        """Sorted list of all discoverable bottle names.
+
+        In names-only mode (from resolve/from_md_dirs) this scans bottle
+        filenames without reading their content. In eager mode (from
+        from_json_obj) it returns the pre-parsed bottles' names."""
+        if self.home_md is not None:
+            from .manifest_loader import scan_bottle_names
+            return scan_bottle_names(self.home_md / "bottles")
+        return sorted(self.bottles.keys())
+
     @property
     def all_agent_names(self) -> list[str]:
         """Sorted list of all discoverable agent names.
@@ -376,9 +447,18 @@ class ManifestIndex:
             return sorted(home_names | cwd_names)
         return sorted(self.agents.keys())
 
-    def load_for_agent(self, agent_name: str) -> "Manifest":
+    def load_for_agent(
+        self,
+        agent_name: str,
+        bottle_names: "tuple[str, ...] | None" = None,
+    ) -> "Manifest":
         """Parse the named agent and its bottle; return a single-value Manifest.
 
+        `bottle_names` is an ordered list of bottles selected at launch time.
+        When non-empty they are resolved and merged in order (index 0 = base;
+        later entries override). When empty or None, falls back to the agent's
+        own `bottle:` field. Raises ManifestError when neither is set.
+
         In lazy mode (from resolve/from_md_dirs) the agent file and its
         bottle chain are read from disk for the first time here.  In eager
         mode (from_json_obj) the data is already parsed; this just filters
@@ -389,6 +469,8 @@ class ManifestIndex:
 
         Always raises ManifestError if the agent is unknown or invalid.
         Backends call this at preflight inside _validate."""
+        effective_bottle_names: tuple[str, ...] = bottle_names or ()
+
         if self.home_md is None:
             # Eager manifest (from_json_obj): data already parsed; filter to
             # the one requested agent and its bottle so the returned Manifest
@@ -399,7 +481,9 @@ class ManifestIndex:
                     f"agent '{agent_name}' not defined. Available: {available}"
                 )
             agent = self.agents[agent_name]
-            raw_bottle = self.bottles[agent.bottle]
+            raw_bottle = _resolve_effective_bottle_eager(
+                agent_name, agent, effective_bottle_names, self.bottles
+            )
             merged = _merge_git_user(agent.git_user, raw_bottle.git_user)
             bottle = raw_bottle if merged == raw_bottle.git_user else replace(raw_bottle, git_user=merged)
             return Manifest(agent=agent, bottle=bottle)
@@ -431,26 +515,31 @@ class ManifestIndex:
 
         validate_agent_frontmatter_keys(agent_path, fm.keys())
 
-        bottle_name = fm.get("bottle")
+        # Determine the effective bottle name(s).
-        if not isinstance(bottle_name, str) or not bottle_name:
+        agent_bottle = fm.get("bottle") or ""
-            raise ManifestError(
-                f"agent '{agent_name}' must declare a 'bottle' field "
-                f"naming a defined bottle"
-            )
-
-        # Load the bottle chain (may raise ManifestError).
         bottles_dir = self.home_md / "bottles"
-        raw_bottle = load_bottle_chain_from_dir(bottle_name, bottles_dir)
+        raw_bottle = _resolve_effective_bottle_lazy(
+            agent_name, str(agent_bottle), effective_bottle_names, bottles_dir
+        )
+        effective_bottle_name = (
+            effective_bottle_names[-1] if effective_bottle_names
+            else str(agent_bottle)
+        )
 
         # Build and validate the full ManifestAgent.
         agent_dict: dict[str, object] = {
-            "bottle": bottle_name,
             "skills": fm.get("skills", []),
             "prompt": body.strip(),
         }
+        if agent_bottle:
+            agent_dict["bottle"] = agent_bottle
         if "git-gate" in fm:
             agent_dict["git-gate"] = fm["git-gate"]
-        agent = ManifestAgent.from_dict(agent_name, agent_dict, {bottle_name})
+        # Pass the effective bottle name as the known-bottles set so agents
+        # that have bottle: set are validated; agents without bottle: pass {}
+        # since bottle_names were already resolved above.
+        known = {effective_bottle_name} if effective_bottle_name else set()
+        agent = ManifestAgent.from_dict(agent_name, agent_dict, known)
 
         merged_user = _merge_git_user(agent.git_user, raw_bottle.git_user)
         bottle = raw_bottle if merged_user == raw_bottle.git_user else replace(raw_bottle, git_user=merged_user)

bot_bottle/manifest_agent.py

@@ -109,7 +109,8 @@ class ManifestAgentProvider:
 
 @dataclass(frozen=True)
 class ManifestAgent:
-    bottle: str
+    # Optional: when empty the operator selects bottles at launch time.
+    bottle: str = ""
     skills: tuple[str, ...] = ()
     prompt: str = ""
     # Per-agent git identity (issue #94). Overlays the referenced
@@ -129,18 +130,20 @@ class ManifestAgent:
                 f"allowed keys are {allowed}."
             )
 
-        bottle = d.get("bottle")
+        bottle_raw = d.get("bottle")
-        if not isinstance(bottle, str) or not bottle:
+        bottle = ""
-            raise ManifestError(
+        if bottle_raw is not None:
-                f"agent '{name}' must declare a 'bottle' field naming a "
+            if not isinstance(bottle_raw, str) or not bottle_raw:
-                f"defined bottle"
+                raise ManifestError(
-            )
+                    f"agent '{name}' bottle must be a non-empty string when declared"
-        if bottle not in bottle_names:
+                )
-            available = ", ".join(sorted(bottle_names)) or "(none defined)"
+            if bottle_raw not in bottle_names:
-            raise ManifestError(
+                available = ", ".join(sorted(bottle_names)) or "(none defined)"
-                f"agent '{name}' references bottle '{bottle}', which is not defined. "
+                raise ManifestError(
-                f"Available: {available}"
+                    f"agent '{name}' references bottle '{bottle_raw}', which is not defined. "
-            )
+                    f"Available: {available}"
+                )
+            bottle = bottle_raw
 
         skills: tuple[str, ...] = ()
         skills_raw = d.get("skills")

bot_bottle/manifest_extends.py

@@ -9,6 +9,58 @@ if TYPE_CHECKING:
     from .manifest_egress import ManifestEgressConfig
 
 
+def merge_bottles_runtime(bottles: "list[ManifestBottle]") -> "ManifestBottle":
+    """Merge an ordered list of pre-resolved ManifestBottle objects.
+
+    Index 0 is the base; each subsequent entry is applied on top using
+    the same field-merge rules as the file-based extends machinery:
+      env: dict merge, later wins; git_user: per-field overlay, later
+      wins on non-empty; git (repos): union by name, later wins; egress
+      routes: concatenate; agent_provider, supervise: later replaces.
+    """
+    if not bottles:
+        raise ValueError("merge_bottles_runtime requires at least one bottle")
+    result = bottles[0]
+    for override in bottles[1:]:
+        result = _merge_two_bottles_runtime(result, override)
+    return result
+
+
+def _merge_two_bottles_runtime(base: "ManifestBottle", override: "ManifestBottle") -> "ManifestBottle":
+    from .manifest import ManifestBottle, ManifestGitUser
+    from .manifest_egress import ManifestEgressConfig
+
+    merged_env = {**base.env, **override.env}
+
+    merged_git_user = ManifestGitUser(
+        name=override.git_user.name or base.git_user.name,
+        email=override.git_user.email or base.git_user.email,
+    )
+
+    # git repos: union keyed by Name, override wins per-name.
+    base_repos_by_name = {entry.Name: entry for entry in base.git}
+    override_repos_by_name = {entry.Name: entry for entry in override.git}
+    merged_repos_names = list(base_repos_by_name) + [
+        n for n in override_repos_by_name if n not in base_repos_by_name
+    ]
+    merged_git = tuple(
+        override_repos_by_name.get(n, base_repos_by_name[n])
+        for n in merged_repos_names
+    )
+
+    merged_routes: tuple[ManifestEgressRoute, ...] = base.egress.routes + override.egress.routes
+    merged_egress = ManifestEgressConfig(routes=merged_routes, Log=override.egress.Log)
+
+    return ManifestBottle(
+        env=merged_env,
+        agent_provider=override.agent_provider,
+        git=merged_git,
+        git_user=merged_git_user,
+        egress=merged_egress,
+        supervise=override.supervise,
+    )
+
+
 def resolve_bottles(raws: dict[str, dict[str, object]]) -> dict[str, ManifestBottle]:
     """Apply `extends:` chains and return resolved ManifestBottle objects."""
     cache: dict[str, ManifestBottle] = {}

bot_bottle/manifest_loader.py

@@ -32,6 +32,25 @@ def check_stale_json(dir_path: Path, md_dir: Path, label: str) -> None:
         )
 
 
+def scan_bottle_names(bottles_dir: Path) -> list[str]:
+    """Scan `<bottles_dir>/*.md` for valid filenames and return sorted bottle names.
+
+    No file content is read. Invalid filenames are skipped with a warning."""
+    result: list[str] = []
+    if not bottles_dir.is_dir():
+        return result
+    for path in sorted(bottles_dir.glob("*.md")):
+        name = entity_name_from_path(path)
+        if name is None:
+            warn(
+                f"skipping {path}: filename must match "
+                f"[a-z][a-z0-9-]*.md (got {path.name!r})"
+            )
+            continue
+        result.append(name)
+    return result
+
+
 def scan_agent_names(agents_dir: Path) -> dict[str, Path]:
     """Scan `<agents_dir>/*.md` for valid filenames and return `{name: path}`.
 

bot_bottle/manifest_schema.py

@@ -18,8 +18,8 @@ _FILENAME_RX = re.compile(r"^[a-z][a-z0-9-]*$")
 BOTTLE_KEYS = frozenset(
     {"env", "extends", "agent_provider", "git-gate", "egress", "supervise"}
 )
-AGENT_KEYS_REQUIRED = frozenset({"bottle"})
+AGENT_KEYS_REQUIRED: frozenset[str] = frozenset()
-AGENT_KEYS_OPTIONAL = frozenset({"skills", "git-gate"})
+AGENT_KEYS_OPTIONAL = frozenset({"bottle", "skills", "git-gate"})
 
 # Claude Code subagent fields bot-bottle ignores at launch but does
 # not reject. This lets the same file double as

tests/unit/test_cli_start_selector.py

@@ -1,7 +1,8 @@
-"""Unit: cmd_start selector dispatch (PRD 0051).
+"""Unit: cmd_start selector dispatch (PRD 0051, issue #269).
 
 Tests that cmd_start calls filter_select only when the agent name is
-absent, skips it when the agent is explicit, and returns 0 on cancel.
+absent, shows the bottle multiselect after agent selection, and skips
+pickers when both are explicitly set.
 
 All actual launch work is stubbed so no container is created.
 """
@@ -10,17 +11,23 @@ from __future__ import annotations
 
 import os
 import unittest
-from unittest.mock import MagicMock, patch
+from unittest.mock import MagicMock, call, patch
 
 import bot_bottle.cli.start as start_mod
 import bot_bottle.cli.tui as tui_mod
 from bot_bottle.backend import ActiveAgent
 
 
-def _make_manifest(agent_names: list[str]):
+def _make_manifest(
+    agent_names: list[str],
+    bottle_names: list[str] | None = None,
+    agent_bottle: str = "",
+):
     manifest = MagicMock()
-    manifest.agents = {name: MagicMock() for name in agent_names}
+    manifest.agents = {name: MagicMock(bottle=agent_bottle) for name in agent_names}
     manifest.all_agent_names = sorted(agent_names)
+    manifest.all_bottle_names = sorted(bottle_names or [])
+    manifest.home_md = None  # eager mode so _peek_agent_bottle uses agents dict
     return manifest
 
 
@@ -28,27 +35,27 @@ class TestCmdStartSelector(unittest.TestCase):
     """Drive cmd_start with a minimal set of stubs."""
 
     def setUp(self):
-        # Stub Manifest.resolve so no on-disk manifest is needed.
+        self._manifest = _make_manifest(["researcher", "implementer"], ["claude", "dev"])
-        self._manifest = _make_manifest(["researcher", "implementer"])
         self._resolve_patch = patch(
             "bot_bottle.cli.start.ManifestIndex.resolve",
             return_value=self._manifest,
         )
         self._resolve_patch.start()
 
-        # Stub _launch_bottle so no real container work happens.
         self._launch_patch = patch(
             "bot_bottle.cli.start._launch_bottle",
             return_value=0,
         )
         self._launch_mock = self._launch_patch.start()
 
-        # Stub filter_select to avoid opening /dev/tty.
+        # Stub filter_select (agent picker) and filter_multiselect (bottle picker).
-        self._tui_patch = patch.object(tui_mod, "filter_select")
+        self._agent_picker_patch = patch.object(tui_mod, "filter_select")
-        self._tui_mock = self._tui_patch.start()
+        self._agent_picker_mock = self._agent_picker_patch.start()
+
+        self._bottle_picker_patch = patch.object(tui_mod, "filter_multiselect")
+        self._bottle_picker_mock = self._bottle_picker_patch.start()
+        self._bottle_picker_mock.return_value = ["claude"]  # default: one bottle selected
 
-        # Ensure BOT_BOTTLE_BACKEND is absent so omitted --backend
-        # flows through to the resolver default.
         self._env_patch = patch.dict(os.environ, {}, clear=False)
         self._env_patch.start()
         os.environ.pop("BOT_BOTTLE_BACKEND", None)
@@ -56,50 +63,108 @@ class TestCmdStartSelector(unittest.TestCase):
     def tearDown(self):
         self._resolve_patch.stop()
         self._launch_patch.stop()
-        self._tui_patch.stop()
+        self._agent_picker_patch.stop()
+        self._bottle_picker_patch.stop()
         self._env_patch.stop()
 
     # ------------------------------------------------------------------
-    # Both explicit — no picker shown
+    # Agent explicit — agent picker skipped; bottle picker always shown
     # ------------------------------------------------------------------
 
-    def test_both_explicit_skips_picker(self):
+    def test_explicit_agent_skips_agent_picker(self):
-        self._tui_mock.return_value = "researcher"
         rc = start_mod.cmd_start(["--backend=docker", "researcher"])
         self.assertEqual(0, rc)
-        self._tui_mock.assert_not_called()
+        self._agent_picker_mock.assert_not_called()
+        self._bottle_picker_mock.assert_called_once()
         self._launch_mock.assert_called_once()
-        _, kwargs = self._launch_mock.call_args
+
-        self.assertEqual("docker", kwargs["backend_name"])
+    def test_explicit_agent_bottle_picker_shows_available_bottles(self):
+        start_mod.cmd_start(["researcher"])
+        call_kwargs = self._bottle_picker_mock.call_args
+        self.assertEqual(["claude", "dev"], call_kwargs[0][0])
+        self.assertIn("bottle", call_kwargs[1]["title"].lower())
 
     # ------------------------------------------------------------------
-    # Agent absent → agent picker fires; backend explicit
+    # Agent absent → agent picker fires; bottle picker always follows
     # ------------------------------------------------------------------
 
     def test_agent_absent_shows_agent_picker(self):
-        self._tui_mock.return_value = "researcher"
+        self._agent_picker_mock.return_value = "researcher"
         rc = start_mod.cmd_start(["--backend=docker"])
         self.assertEqual(0, rc)
-        self._tui_mock.assert_called_once()
+        self._agent_picker_mock.assert_called_once()
-        call_kwargs = self._tui_mock.call_args
+        call_kwargs = self._agent_picker_mock.call_args
         self.assertEqual(["implementer", "researcher"], call_kwargs[0][0])
         self.assertIn("agent", call_kwargs[1]["title"].lower())
+        # Bottle picker must also fire after agent selection.
+        self._bottle_picker_mock.assert_called_once()
 
-    def test_agent_picker_cancel_returns_0(self):
+    def test_agent_picker_cancel_skips_bottle_picker(self):
-        self._tui_mock.return_value = None
+        self._agent_picker_mock.return_value = None
         rc = start_mod.cmd_start(["--backend=docker"])
         self.assertEqual(0, rc)
+        self._bottle_picker_mock.assert_not_called()
+        self._launch_mock.assert_not_called()
+
+    def test_bottle_picker_cancel_returns_0(self):
+        self._bottle_picker_mock.return_value = None
+        rc = start_mod.cmd_start(["researcher"])
+        self.assertEqual(0, rc)
         self._launch_mock.assert_not_called()
 
     # ------------------------------------------------------------------
-    # Agent explicit, backend absent → no picker
+    # Bottle selection is forwarded to BottleSpec
     # ------------------------------------------------------------------
 
-    def test_backend_absent_uses_default_without_picker(self):
+    def test_selected_bottles_forwarded_to_spec(self):
-        rc = start_mod.cmd_start(["researcher"])
+        self._bottle_picker_mock.return_value = ["claude", "dev"]
-        self.assertEqual(0, rc)
+        start_mod.cmd_start(["researcher"])
-        self._tui_mock.assert_not_called()
         self._launch_mock.assert_called_once()
+        spec = self._launch_mock.call_args[0][0]
+        self.assertEqual(("claude", "dev"), spec.bottle_names)
+
+    def test_empty_bottle_selection_forwarded(self):
+        self._bottle_picker_mock.return_value = []
+        start_mod.cmd_start(["researcher"])
+        self._launch_mock.assert_called_once()
+        spec = self._launch_mock.call_args[0][0]
+        self.assertEqual((), spec.bottle_names)
+
+    # ------------------------------------------------------------------
+    # Agent default bottle pre-populates the picker
+    # ------------------------------------------------------------------
+
+    def test_agent_bottle_prepopulates_bottle_picker(self):
+        manifest = _make_manifest(
+            ["implementer"], ["claude", "dev"], agent_bottle="claude"
+        )
+        with patch(
+            "bot_bottle.cli.start.ManifestIndex.resolve", return_value=manifest
+        ):
+            start_mod.cmd_start(["implementer"])
+        call_kwargs = self._bottle_picker_mock.call_args
+        self.assertEqual(["claude"], call_kwargs[1]["initial"])
+
+    def test_no_agent_bottle_empty_initial(self):
+        manifest = _make_manifest(["researcher"], ["claude", "dev"], agent_bottle="")
+        with patch(
+            "bot_bottle.cli.start.ManifestIndex.resolve", return_value=manifest
+        ):
+            start_mod.cmd_start(["researcher"])
+        call_kwargs = self._bottle_picker_mock.call_args
+        self.assertEqual([], call_kwargs[1]["initial"])
+
+    # ------------------------------------------------------------------
+    # 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"])
 
@@ -110,28 +175,21 @@ class TestCmdStartSelector(unittest.TestCase):
         finally:
             os.environ.pop("BOT_BOTTLE_BACKEND", None)
         self.assertEqual(0, rc)
-        self._tui_mock.assert_not_called()
 
-    # ------------------------------------------------------------------
+    def test_both_absent_shows_agent_picker_then_bottle_picker(self):
-    # Both absent → only agent picker
+        self._agent_picker_mock.return_value = "researcher"
-    # ------------------------------------------------------------------
-
-    def test_both_absent_shows_only_agent_picker(self):
-        self._tui_mock.return_value = "researcher"
         rc = start_mod.cmd_start([])
         self.assertEqual(0, rc)
-        self._tui_mock.assert_called_once()
+        self._agent_picker_mock.assert_called_once()
-        title = self._tui_mock.call_args[1]["title"].lower()
+        self._bottle_picker_mock.assert_called_once()
-        self.assertIn("agent", title)
         self._launch_mock.assert_called_once()
-        _, kwargs = self._launch_mock.call_args
-        self.assertIsNone(kwargs["backend_name"])
 
-    def test_both_absent_agent_cancel_skips_backend_picker(self):
+    def test_both_absent_agent_cancel_skips_bottle_and_launch(self):
-        self._tui_mock.side_effect = [None]
+        self._agent_picker_mock.return_value = None
         rc = start_mod.cmd_start([])
         self.assertEqual(0, rc)
-        self.assertEqual(1, self._tui_mock.call_count)
+        self._agent_picker_mock.assert_called_once()
+        self._bottle_picker_mock.assert_not_called()
         self._launch_mock.assert_not_called()
 
 
@@ -149,11 +207,13 @@ class TestCmdStartLabelCollision(unittest.TestCase):
     """cmd_start re-prompts when the label's slug is already running."""
 
     def setUp(self):
-        self._manifest = _make_manifest(["researcher"])
+        self._manifest = _make_manifest(["researcher"], ["claude"])
         patch("bot_bottle.cli.start.ManifestIndex.resolve", return_value=self._manifest).start()
         self._launch_mock = patch(
             "bot_bottle.cli.start._launch_bottle", return_value=0,
         ).start()
+        # Stub the bottle picker to always return a selection.
+        patch.object(tui_mod, "filter_multiselect", return_value=["claude"]).start()
         self.addCleanup(patch.stopall)
 
     def test_no_collision_proceeds_without_reprompt(self):

tests/unit/test_cli_tui.py

@@ -1,4 +1,4 @@
-"""Unit tests for bot_bottle.cli.tui — filter_select internals.
+"""Unit tests for bot_bottle.cli.tui — filter_select and filter_multiselect.
 
 We test the pure-Python logic (_filter_items, cursor movement, confirm,
 cancel) by exercising the internal helpers directly, without spinning up
@@ -9,7 +9,7 @@ from __future__ import annotations
 
 import unittest
 
-from bot_bottle.cli.tui import _filter_items, filter_select
+from bot_bottle.cli.tui import _filter_items, filter_multiselect, filter_select
 
 
 class TestFilterItems(unittest.TestCase):
@@ -46,5 +46,16 @@ class TestFilterSelectEmptyItems(unittest.TestCase):
         self.assertIsNone(result)
 
 
+class TestFilterMultiselectEmptyItems(unittest.TestCase):
+    def test_returns_empty_list_for_empty_items(self):
+        # No TTY needed — short-circuits before opening tty.
+        result = filter_multiselect([], title="Select", tty_path="/dev/null")
+        self.assertEqual([], result)
+
+    def test_returns_none_when_tty_unavailable(self):
+        result = filter_multiselect(["a", "b"], tty_path="/nonexistent/tty")
+        self.assertIsNone(result)
+
+
 if __name__ == "__main__":
     unittest.main()

tests/unit/test_manifest_bottle_merge.py

@@ -0,0 +1,200 @@
+"""Unit: runtime bottle composition (issue #269).
+
+Tests for merge_bottles_runtime and ManifestIndex.load_for_agent with
+the new bottle_names parameter.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import tempfile
+import textwrap
+import unittest
+from pathlib import Path
+
+from bot_bottle.manifest import ManifestBottle, ManifestError, ManifestIndex
+from bot_bottle.manifest_extends import merge_bottles_runtime
+
+
+def _index(bottles: dict, agents: dict) -> ManifestIndex:
+    return ManifestIndex.from_json_obj({"bottles": bottles, "agents": agents})
+
+
+def _bottle(**kwargs) -> ManifestBottle:
+    return ManifestBottle.from_dict("test", kwargs)
+
+
+class TestMergeBottlesRuntime(unittest.TestCase):
+    def test_single_bottle_returns_as_is(self):
+        b = _bottle(env={"FOO": "1"})
+        result = merge_bottles_runtime([b])
+        self.assertEqual({"FOO": "1"}, dict(result.env))
+
+    def test_env_later_wins(self):
+        base = _bottle(env={"FOO": "base", "ONLY_BASE": "x"})
+        override = _bottle(env={"FOO": "override", "ONLY_OVERRIDE": "y"})
+        result = merge_bottles_runtime([base, override])
+        self.assertEqual("override", result.env["FOO"])
+        self.assertEqual("x", result.env["ONLY_BASE"])
+        self.assertEqual("y", result.env["ONLY_OVERRIDE"])
+
+    def test_egress_routes_concatenated(self):
+        from bot_bottle.manifest_egress import ManifestEgressConfig, ManifestEgressRoute
+        r1 = ManifestEgressRoute(Host="api.a.com")
+        r2 = ManifestEgressRoute(Host="api.b.com")
+        base = ManifestBottle(egress=ManifestEgressConfig(routes=(r1,)))
+        override = ManifestBottle(egress=ManifestEgressConfig(routes=(r2,)))
+        result = merge_bottles_runtime([base, override])
+        hosts = [r.Host for r in result.egress.routes]
+        self.assertIn("api.a.com", hosts)
+        self.assertIn("api.b.com", hosts)
+
+    def test_supervise_later_wins(self):
+        base = _bottle(supervise=True)
+        override = _bottle(supervise=False)
+        result = merge_bottles_runtime([base, override])
+        self.assertFalse(result.supervise)
+
+    def test_three_bottles_merged_left_to_right(self):
+        b1 = _bottle(env={"A": "1", "B": "1", "C": "1"})
+        b2 = _bottle(env={"B": "2", "C": "2"})
+        b3 = _bottle(env={"C": "3"})
+        result = merge_bottles_runtime([b1, b2, b3])
+        self.assertEqual("1", result.env["A"])
+        self.assertEqual("2", result.env["B"])
+        self.assertEqual("3", result.env["C"])
+
+    def test_empty_list_raises(self):
+        with self.assertRaises(ValueError):
+            merge_bottles_runtime([])
+
+
+class TestLoadForAgentWithBottleNames(unittest.TestCase):
+    def test_bottle_names_override_agent_bottle(self):
+        idx = _index(
+            bottles={
+                "base": {"env": {"X": "base"}},
+                "override": {"env": {"X": "override"}},
+            },
+            agents={"impl": {"bottle": "base", "skills": [], "prompt": ""}},
+        )
+        m = idx.load_for_agent("impl", ("override",))
+        self.assertEqual("override", m.bottle.env["X"])
+
+    def test_bottle_names_merged_in_order(self):
+        idx = _index(
+            bottles={
+                "a": {"env": {"X": "a", "A": "only-a"}},
+                "b": {"env": {"X": "b", "B": "only-b"}},
+            },
+            agents={"impl": {"bottle": "a", "skills": [], "prompt": ""}},
+        )
+        m = idx.load_for_agent("impl", ("a", "b"))
+        self.assertEqual("b", m.bottle.env["X"])
+        self.assertEqual("only-a", m.bottle.env["A"])
+        self.assertEqual("only-b", m.bottle.env["B"])
+
+    def test_empty_bottle_names_uses_agent_bottle(self):
+        idx = _index(
+            bottles={"base": {"env": {"X": "base"}}},
+            agents={"impl": {"bottle": "base", "skills": [], "prompt": ""}},
+        )
+        m = idx.load_for_agent("impl", ())
+        self.assertEqual("base", m.bottle.env["X"])
+
+    def test_no_bottle_and_no_bottle_names_raises(self):
+        idx = _index(
+            bottles={"base": {}},
+            agents={"impl": {"skills": [], "prompt": ""}},
+        )
+        with self.assertRaises(ManifestError) as ctx:
+            idx.load_for_agent("impl", ())
+        self.assertIn("no 'bottle' field", str(ctx.exception))
+
+    def test_unknown_bottle_name_raises(self):
+        idx = _index(
+            bottles={"base": {}},
+            agents={"impl": {"bottle": "base", "skills": [], "prompt": ""}},
+        )
+        with self.assertRaises(ManifestError) as ctx:
+            idx.load_for_agent("impl", ("nonexistent",))
+        self.assertIn("nonexistent", str(ctx.exception))
+
+    def test_agent_without_bottle_works_with_bottle_names(self):
+        idx = _index(
+            bottles={"base": {"env": {"X": "base"}}},
+            agents={"impl": {"skills": [], "prompt": ""}},
+        )
+        m = idx.load_for_agent("impl", ("base",))
+        self.assertEqual("base", m.bottle.env["X"])
+
+
+class TestAllBottleNames(unittest.TestCase):
+    def test_eager_mode_returns_bottle_names(self):
+        idx = _index(
+            bottles={"alpha": {}, "beta": {}, "gamma": {}},
+            agents={"impl": {"bottle": "alpha", "skills": [], "prompt": ""}},
+        )
+        self.assertEqual(["alpha", "beta", "gamma"], idx.all_bottle_names)
+
+    def test_lazy_mode_scans_files(self):
+        home = Path(tempfile.mkdtemp(prefix="cb-home-"))
+        orig_home = os.environ.get("HOME")
+        os.environ["HOME"] = str(home)
+        try:
+            bottles_dir = home / ".bot-bottle" / "bottles"
+            agents_dir = home / ".bot-bottle" / "agents"
+            bottles_dir.mkdir(parents=True)
+            agents_dir.mkdir(parents=True)
+            (bottles_dir / "claude.md").write_text("---\n---\n")
+            (bottles_dir / "dev.md").write_text("---\n---\n")
+            (agents_dir / "impl.md").write_text("---\nbottle: claude\n---\n")
+            idx = ManifestIndex.resolve(str(home))
+            self.assertEqual(["claude", "dev"], idx.all_bottle_names)
+        finally:
+            if orig_home is None:
+                os.environ.pop("HOME", None)
+            else:
+                os.environ["HOME"] = orig_home
+            shutil.rmtree(home, ignore_errors=True)
+
+
+class TestAgentOptionalBottleMd(unittest.TestCase):
+    """Agent file without bottle: works when bottle_names are provided at launch."""
+
+    def setUp(self) -> None:
+        self.home = Path(tempfile.mkdtemp(prefix="cb-home-"))
+        self._orig_home = os.environ.get("HOME")
+        os.environ["HOME"] = str(self.home)
+
+    def tearDown(self) -> None:
+        if self._orig_home is None:
+            os.environ.pop("HOME", None)
+        else:
+            os.environ["HOME"] = self._orig_home
+        shutil.rmtree(self.home, ignore_errors=True)
+
+    def _write(self, rel: str, text: str) -> None:
+        p = self.home / ".bot-bottle" / rel
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_text(textwrap.dedent(text).lstrip("\n"))
+
+    def test_agent_without_bottle_resolves_with_bottle_names(self):
+        self._write("bottles/dev.md", "---\nenv:\n  X: dev\n---\n")
+        self._write("agents/impl.md", "---\n---\nimpl agent.\n")
+        idx = ManifestIndex.resolve(str(self.home))
+        m = idx.load_for_agent("impl", ("dev",))
+        self.assertEqual("dev", m.bottle.env["X"])
+
+    def test_agent_without_bottle_fails_without_bottle_names(self):
+        self._write("bottles/dev.md", "---\n---\n")
+        self._write("agents/impl.md", "---\n---\nimpl agent.\n")
+        idx = ManifestIndex.resolve(str(self.home))
+        with self.assertRaises(ManifestError) as ctx:
+            idx.load_for_agent("impl", ())
+        self.assertIn("no 'bottle' field", str(ctx.exception))
+
+
+if __name__ == "__main__":
+    unittest.main()