core/agent: nested todo frames for hierarchical push/pop breakdown

push of one or more titles opens a new frame (siblings at one depth);
pop leaves the frame. Supports T1,T2 → T1.1,T1.2 → pop → T2.1 pattern.
Legacy flat session JSON migrates to a single root frame.

CLAUDE.md

@@ -81,7 +81,7 @@ Module-level `@tool` handlers. Call `set_workspace_root()` before use.
 - Destructive confirms: `classify_danger` + `ToolRegistry(on_confirm=…)`; CLI default deny; config `confirm_destructive` / `path_denylist`. Session YOLO: `/yolo on|off|once` and `--yes` (skip soft confirms; hard denylists unchanged; `once` expires after the next user turn).
 - **`vcs`**: read-only VCS tools (`vcs_kind` / `vcs_status` / `vcs_diff` / `vcs_log` / `vcs_branch`) via `VcsBackend` protocol; **git** implemented; detectors are pluggable for other systems.
 - **`chat`**: human prompts as tools — `ask_user_line` / `ask_user_choice` / `ask_user_form` (shared `prompting` core).
-- **`todo`**: session sub-task stack — `todo_list` / `todo_push` / `todo_pop` / `todo_update` / `todo_clear`; stored on session row; agent injects a review user message if open todos and no todo tool use in the turn.
+- **`todo`**: nested session sub-task stack (frames) — `todo_list` / `todo_push` (multi-title = new frame) / `todo_pop` (leave frame) / `todo_update` / `todo_clear`; pattern push[T1,T2]→push[T1.1…]→pop→push[T2.1…]; stored on session row; agent injects a developer review message if open todos and no todo tool use in the turn.
 - **`DEFAULT_TOOLS`**: file + process + vcs + chat + todo tool list for a `ToolRegistry`.
 
 ### Prompting (`prompting.py`)

src/plyngent/agent/todo_stack.py

@@ -11,7 +11,7 @@ _OPEN: frozenset[str] = frozenset({"pending", "in_progress"})
 
 
 class TodoItem(Struct, omit_defaults=True):
-    """One sub-task on the session todo stack."""
+    """One sub-task within a stack frame."""
 
     id: str
     title: str
@@ -19,17 +19,29 @@ class TodoItem(Struct, omit_defaults=True):
     notes: str = ""
 
 
-class TodoStackData(Struct, omit_defaults=True):
-    """Serializable stack body (session storage)."""
+class TodoFrame(Struct, omit_defaults=True):
+    """One nesting level: sibling tasks pushed together."""
 
     items: list[TodoItem] = field(default_factory=list)
+
+
+class TodoStackData(Struct, omit_defaults=True):
+    """Serializable stack body (session storage).
+
+    ``frames`` is bottom→top: root plan first, deepest breakdown last.
+    Pattern: push [T1,T2] → push [T1.1,T1.2] → pop → push [T2.1] → …
+    """
+
+    frames: list[TodoFrame] = field(default_factory=list)
     next_id: int = 1
 
 
 class TodoStack:
-    """Session-local ordered sub-tasks for breaking work into steps.
+    """Nested todo stack for hierarchical sub-task breakdown.
 
-    Maintained mainly by model tools; humans can show/push/pop/clear via slash.
+    Each ``push`` of one or more titles creates a **new frame** (deeper level).
+    ``pop`` removes the **top frame** (leave a breakdown level).
+    Model tools maintain the stack; humans use ``/todos``.
     """
 
     def __init__(self, data: TodoStackData | None = None) -> None:
@@ -37,8 +49,12 @@ class TodoStack:
         self._touched_this_turn: bool = False
 
     @property
-    def items(self) -> list[TodoItem]:
-        return self._data.items
+    def frames(self) -> list[TodoFrame]:
+        return self._data.frames
+
+    @property
+    def depth(self) -> int:
+        return len(self._data.frames)
 
     @property
     def touched_this_turn(self) -> bool:
@@ -51,11 +67,14 @@ class TodoStack:
     def mark_touched(self) -> None:
         self._touched_this_turn = True
 
+    def all_items(self) -> list[TodoItem]:
+        return [item for frame in self._data.frames for item in frame.items]
+
     def open_items(self) -> list[TodoItem]:
-        return [item for item in self._data.items if item.status in _OPEN]
+        return [item for item in self.all_items() if item.status in _OPEN]
 
     def is_empty(self) -> bool:
-        return not self._data.items
+        return not self._data.frames
 
     def needs_review(self) -> bool:
         """True when open work exists but no todo tool ran this turn."""
@@ -68,6 +87,20 @@ class TodoStack:
     def from_raw(cls, raw: object | None) -> TodoStack:
         if raw is None:
             return cls()
+        # Legacy flat shape: {items: [...], next_id: N} → single root frame.
+        if isinstance(raw, dict) and "frames" not in raw and "items" in raw:
+            try:
+                flat = cast("dict[str, object]", raw)
+                items = msgspec.convert(flat.get("items"), type=list[TodoItem])
+                next_raw = flat.get("next_id", 1)
+                next_id = max(1, int(next_raw) if isinstance(next_raw, int | str) else 1)
+            except (msgspec.ValidationError, TypeError, ValueError):
+                return cls()
+            data = TodoStackData(
+                frames=[TodoFrame(items=items)] if items else [],
+                next_id=next_id,
+            )
+            return cls(data)
         try:
             data = msgspec.convert(raw, type=TodoStackData)
         except (msgspec.ValidationError, TypeError, ValueError):
@@ -79,23 +112,29 @@ class TodoStack:
     def to_raw(self) -> dict[str, object]:
         out: object = msgspec.to_builtins(self._data)
         if not isinstance(out, dict):
-            return {"items": [], "next_id": 1}
+            return {"frames": [], "next_id": 1}
         raw = cast("dict[object, object]", out)
         return {str(key): value for key, value in raw.items()}
 
     def render(self) -> str:
-        if not self._data.items:
+        if not self._data.frames:
             return "(todo stack empty)"
-        lines: list[str] = []
-        for item in self._data.items:
-            mark = {
-                "pending": "[ ]",
-                "in_progress": "[~]",
-                "done": "[x]",
-                "cancelled": "[-]",
-            }.get(item.status, "[?]")
-            note = f" — {item.notes}" if item.notes else ""
-            lines.append(f"{mark} {item.id}: {item.title}{note}")
+        lines: list[str] = [f"(depth={self.depth})"]
+        for depth, frame in enumerate(self._data.frames):
+            indent = "  " * depth
+            lines.append(f"{indent}frame {depth}:")
+            if not frame.items:
+                lines.append(f"{indent}  (empty frame)")
+                continue
+            for item in frame.items:
+                mark = {
+                    "pending": "[ ]",
+                    "in_progress": "[~]",
+                    "done": "[x]",
+                    "cancelled": "[-]",
+                }.get(item.status, "[?]")
+                note = f" — {item.notes}" if item.notes else ""
+                lines.append(f"{indent}  {mark} {item.id}: {item.title}{note}")
         return "\n".join(lines)
 
     def review_prompt(self) -> str:
@@ -103,46 +142,56 @@ class TodoStack:
         return (
             "Todo stack review (internal control — not a human message).\n"
             "Open sub-tasks remain and you did not call any todo_* tools this turn.\n"
-            "Review the stack: mark finished items done, update in_progress, "
-            "push new sub-tasks, or pop/cancel obsolete ones. Then continue the user work.\n\n"
+            "Use nested push/pop for breakdown: push [T1,T2], push [T1.1,T1.2], "
+            "finish children, pop frame, then push children of T2, etc. "
+            "Mark done/cancelled, push new frames, or pop a finished level.\n\n"
             f"Current stack:\n{self.render()}"
         )
 
-    def push(self, title: str, *, notes: str = "", status: TodoStatus = "pending") -> TodoItem:
-        token = title.strip()
-        if not token:
-            msg = "title must not be empty"
+    def push_titles(
+        self,
+        titles: list[str],
+        *,
+        notes: str = "",
+        status: TodoStatus = "pending",
+    ) -> list[TodoItem]:
+        """Push a new frame containing *titles* (one or more siblings at this depth)."""
+        cleaned = [t.strip() for t in titles if t and t.strip()]
+        if not cleaned:
+            msg = "at least one non-empty title is required"
             raise ValueError(msg)
-        item_id = f"t{self._data.next_id}"
-        self._data.next_id += 1
-        item = TodoItem(id=item_id, title=token, status=status, notes=notes.strip())
-        self._data.items.append(item)
+        note = notes.strip()
+        created: list[TodoItem] = []
+        for title in cleaned:
+            item_id = f"t{self._data.next_id}"
+            self._data.next_id += 1
+            created.append(TodoItem(id=item_id, title=title, status=status, notes=note))
+        self._data.frames.append(TodoFrame(items=created))
         self.mark_touched()
-        return item
+        return created
 
-    def pop(self) -> TodoItem | None:
-        """Remove and return the last open item, else the last item, else None."""
-        if not self._data.items:
+    def push(self, title: str, *, notes: str = "", status: TodoStatus = "pending") -> TodoItem:
+        """Push a new frame with a single task (convenience for one title)."""
+        items = self.push_titles([title], notes=notes, status=status)
+        return items[0]
+
+    def pop(self) -> TodoFrame | None:
+        """Pop and return the top frame (leave the current breakdown level)."""
+        if not self._data.frames:
             return None
-        open_ids = {i.id for i in self.open_items()}
-        for index in range(len(self._data.items) - 1, -1, -1):
-            if self._data.items[index].id in open_ids:
-                item = self._data.items.pop(index)
-                self.mark_touched()
-                return item
-        item = self._data.items.pop()
+        frame = self._data.frames.pop()
         self.mark_touched()
-        return item
+        return frame
 
     def clear(self) -> int:
-        n = len(self._data.items)
-        self._data.items.clear()
+        n = sum(len(frame.items) for frame in self._data.frames)
+        self._data.frames.clear()
         if n:
             self.mark_touched()
         return n
 
     def get(self, item_id: str) -> TodoItem | None:
-        for item in self._data.items:
+        for item in self.all_items():
             if item.id == item_id:
                 return item
         return None
@@ -155,18 +204,42 @@ class TodoStack:
         status: TodoStatus | None = None,
         notes: str | None = None,
     ) -> TodoItem:
-        for index, item in enumerate(self._data.items):
-            if item.id != item_id:
-                continue
-            new_title = title.strip() if title is not None else item.title
-            if not new_title:
-                msg = "title must not be empty"
-                raise ValueError(msg)
-            new_status = status if status is not None else item.status
-            new_notes = notes if notes is not None else item.notes
-            updated = TodoItem(id=item.id, title=new_title, status=new_status, notes=new_notes)
-            self._data.items[index] = updated
-            self.mark_touched()
-            return updated
+        for frame in self._data.frames:
+            for index, item in enumerate(frame.items):
+                if item.id != item_id:
+                    continue
+                new_title = title.strip() if title is not None else item.title
+                if not new_title:
+                    msg = "title must not be empty"
+                    raise ValueError(msg)
+                new_status = status if status is not None else item.status
+                new_notes = notes if notes is not None else item.notes
+                updated = TodoItem(id=item.id, title=new_title, status=new_status, notes=new_notes)
+                frame.items[index] = updated
+                self.mark_touched()
+                return updated
         msg = f"unknown todo id {item_id!r}"
         raise KeyError(msg)
+
+
+def parse_push_titles(raw: str) -> list[str]:
+    """Parse multi-title push input: JSON array, or newline/semicolon-separated."""
+    text = raw.strip()
+    if not text:
+        return []
+    if text.startswith("["):
+        try:
+            data: object = msgspec.json.decode(text.encode())
+        except (msgspec.DecodeError, UnicodeEncodeError):
+            data = None
+        if isinstance(data, list):
+            out = [item.strip() for item in cast("list[object]", data) if isinstance(item, str) and item.strip()]
+            if out:
+                return out
+    # Newlines, then ``;`` as separators.
+    parts: list[str] = []
+    for line in text.replace(";", "\n").splitlines():
+        token = line.strip()
+        if token:
+            parts.append(token)
+    return parts

src/plyngent/cli/slash.py

@@ -954,43 +954,51 @@ def history_cmd(
 @click.argument("action", required=False, type=str)
 @click.argument("rest", required=False, nargs=-1, type=str)
 @click.pass_obj
-def todos_cmd(  # noqa: C901, PLR0911, PLR0912
+def todos_cmd(  # noqa: C901, PLR0911, PLR0912, PLR0915
     state: ReplState, action: str | None, rest: tuple[str, ...]
 ) -> None:
-    """Show or edit the todo/task stack (sub-tasks).
+    """Show or edit the nested todo/task stack.
 
-    ``/todos`` — list
-    ``/todos push <title>`` — push pending item
-    ``/todos pop`` — remove last open item
+    ``/todos`` — list frames
+    ``/todos push <title>`` or ``/todos push T1; T2`` — new frame of siblings
+    ``/todos pop`` — pop top breakdown frame
     ``/todos done <id>`` / ``/todos cancel <id>`` — set status
     ``/todos clear`` — wipe stack
     """
+    from plyngent.agent.todo_stack import parse_push_titles
+
     stack = state.todo_stack
     act = (action or "list").strip().lower()
     if act in {"list", "show", "ls"}:
         click.echo(stack.render())
         return
     if act == "push":
-        title = " ".join(rest).strip()
-        if not title:
-            click.echo("error: usage: /todos push <title>")
+        raw = " ".join(rest).strip()
+        if not raw:
+            click.echo("error: usage: /todos push <title> | /todos push T1; T2")
+            return
+        titles = parse_push_titles(raw)
+        if not titles:
+            click.echo("error: no titles to push")
             return
         try:
-            item = stack.push(title)
+            items = stack.push_titles(titles)
         except ValueError as exc:
             click.echo(f"error: {exc}")
             return
         _await(state.persist_todo_stack())
-        click.echo(f"pushed {item.id}: {item.title}")
+        ids = ", ".join(i.id for i in items)
+        click.echo(f"pushed frame depth={stack.depth} items=[{ids}]")
         click.echo(stack.render())
         return
     if act == "pop":
-        item = stack.pop()
-        if item is None:
+        frame = stack.pop()
+        if frame is None:
             click.echo("todo stack empty")
             return
         _await(state.persist_todo_stack())
-        click.echo(f"popped {item.id}: {item.title}")
+        titles = ", ".join(f"{i.id}:{i.title}" for i in frame.items) or "(empty)"
+        click.echo(f"popped frame ({titles})")
         click.echo(stack.render())
         return
     if act in {"done", "cancel", "pending", "in_progress"}:

src/plyngent/tools/todo.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 from typing import TYPE_CHECKING, cast
 
 from plyngent.agent import tool
+from plyngent.agent.todo_stack import parse_push_titles
 
 if TYPE_CHECKING:
     from collections.abc import Callable
@@ -41,7 +42,10 @@ def _notify() -> None:
 
 @tool(name="todo_list")
 def todo_list() -> str:
-    """List the current todo/task stack (sub-tasks for multi-step work)."""
+    """List the nested todo stack (frames = breakdown levels).
+
+    Pattern: push [T1,T2] → push [T1.1,T1.2] → finish children → pop → push [T2.1]…
+    """
     stack = _require_stack()
     stack.mark_touched()
     _notify()
@@ -49,26 +53,40 @@ def todo_list() -> str:
 
 
 @tool(name="todo_push")
-def todo_push(title: str, notes: str = "") -> str:
-    """Push a new sub-task onto the todo stack (pending)."""
+def todo_push(titles: str, notes: str = "") -> str:
+    """Push a new breakdown frame with one or more sibling tasks.
+
+    ``titles``: single title, newline-separated list, ``;``-separated, or JSON
+    string array. Creates a **new nesting level** (does not append to the current
+    frame). Example sequence: push ``T1\\nT2`` then push ``T1.1\\nT1.2``.
+    """
     stack = _require_stack()
+    parsed = parse_push_titles(titles)
+    if not parsed:
+        return "error: titles must contain at least one non-empty title"
     try:
-        item = stack.push(title, notes=notes)
+        items = stack.push_titles(parsed, notes=notes)
     except ValueError as exc:
         return f"error: {exc}"
     _notify()
-    return f"pushed {item.id}: {item.title}\n{stack.render()}"
+    ids = ", ".join(i.id for i in items)
+    return f"pushed frame depth={stack.depth} items=[{ids}]\n{stack.render()}"
 
 
 @tool(name="todo_pop")
 def todo_pop() -> str:
-    """Pop the last open sub-task (or last item if all closed)."""
+    """Pop the top breakdown frame (leave the current nesting level).
+
+    Use after finishing children of a task (e.g. after T1.1/T1.2, pop back to
+    the T1/T2 frame). Does not delete sibling frames below.
+    """
     stack = _require_stack()
-    item = stack.pop()
-    if item is None:
+    frame = stack.pop()
+    if frame is None:
         return "todo stack empty"
     _notify()
-    return f"popped {item.id}: {item.title} ({item.status})\n{stack.render()}"
+    titles = ", ".join(f"{i.id}:{i.title}" for i in frame.items) or "(empty)"
+    return f"popped frame ({len(frame.items)} item(s): {titles})\n{stack.render()}"
 
 
 @tool(name="todo_update")
@@ -101,7 +119,7 @@ def todo_update(
 
 @tool(name="todo_clear")
 def todo_clear() -> str:
-    """Clear the entire todo stack."""
+    """Clear the entire todo stack (all frames)."""
     stack = _require_stack()
     n = stack.clear()
     _notify()

tests/test_agent/test_todo_stack.py

@@ -5,7 +5,7 @@ from typing import TYPE_CHECKING, Literal, overload
 import pytest
 
 from plyngent.agent import ChatAgent, ToolRegistry
-from plyngent.agent.todo_stack import TodoStack
+from plyngent.agent.todo_stack import TodoStack, parse_push_titles
 from plyngent.config.models import DatabaseConfig
 from plyngent.lmproto.openai_compatible.model import (
     AssistantChatMessage,
@@ -23,38 +23,68 @@ if TYPE_CHECKING:
     from collections.abc import AsyncIterator
 
 
-def test_todo_stack_push_pop_update() -> None:
+def test_parse_push_titles() -> None:
+    assert parse_push_titles("only") == ["only"]
+    assert parse_push_titles("T1\nT2") == ["T1", "T2"]
+    assert parse_push_titles("T1; T2; T3") == ["T1", "T2", "T3"]
+    assert parse_push_titles('["A", "B"]') == ["A", "B"]
+
+
+def test_nested_push_pop_pattern() -> None:
+    """(push)[T1,T2] (push)[T1.1,T1.2] (pop) (push)[T2.1] …"""
     stack = TodoStack()
-    a = stack.push("one")
-    b = stack.push("two")
-    assert a.id == "t1"
-    assert b.id == "t2"
-    assert "one" in stack.render()
-    stack.update("t1", status="done")
+    root = stack.push_titles(["T1", "T2"])
+    assert [i.title for i in root] == ["T1", "T2"]
+    assert stack.depth == 1
+
+    children = stack.push_titles(["T1.1", "T1.2"])
+    assert stack.depth == 2
+    assert [i.title for i in children] == ["T1.1", "T1.2"]
+    stack.update(children[0].id, status="done")
+    stack.update(children[1].id, status="done")
+
     popped = stack.pop()
     assert popped is not None
-    assert popped.id == "t2"
-    assert stack.open_items() == []
+    assert [i.title for i in popped.items] == ["T1.1", "T1.2"]
+    assert stack.depth == 1
+    assert [i.title for i in stack.frames[0].items] == ["T1", "T2"]
+
+    stack.update(root[0].id, status="done")
+    _ = stack.push_titles(["T2.1"])
+    assert stack.depth == 2
+    assert stack.frames[-1].items[0].title == "T2.1"
 
 
 def test_todo_stack_needs_review() -> None:
     stack = TodoStack()
     assert not stack.needs_review()
-    stack.push("work")
+    _ = stack.push("work")
     stack.begin_turn()
     assert stack.needs_review()
     stack.mark_touched()
     assert not stack.needs_review()
 
 
+def test_todo_stack_legacy_flat_raw() -> None:
+    stack = TodoStack.from_raw(
+        {
+            "items": [{"id": "t1", "title": "old", "status": "pending", "notes": ""}],
+            "next_id": 2,
+        }
+    )
+    assert stack.depth == 1
+    assert stack.all_items()[0].title == "old"
+    raw = stack.to_raw()
+    assert "frames" in raw
+
+
 def test_todo_stack_roundtrip_raw() -> None:
     stack = TodoStack()
-    _ = stack.push("x", notes="n")
+    _ = stack.push_titles(["x", "y"], notes="n")
     raw = stack.to_raw()
     restored = TodoStack.from_raw(raw)
-    assert len(restored.items) == 1
-    assert restored.items[0].title == "x"
-    assert restored.items[0].notes == "n"
+    assert restored.depth == 1
+    assert [i.title for i in restored.frames[0].items] == ["x", "y"]
 
 
 async def test_todo_tools_and_persist(tmp_path: object) -> None:
@@ -65,16 +95,21 @@ async def test_todo_tools_and_persist(tmp_path: object) -> None:
         stack = TodoStack()
         set_todo_stack(stack, on_change=None)
         registry = ToolRegistry(list(TODO_TOOLS))
-        assert "pushed t1" in await registry.execute("todo_push", '{"title": "ship it"}')
-        assert "t1" in await registry.execute("todo_list", "{}")
-        assert "done" in await registry.execute(
-            "todo_update", '{"item_id": "t1", "status": "done"}'
-        )
+        out = await registry.execute("todo_push", '{"titles": "T1\\nT2"}')
+        assert "pushed frame" in out
+        assert stack.depth == 1
+        out2 = await registry.execute("todo_push", '{"titles": "T1.1; T1.2"}')
+        assert stack.depth == 2
+        assert "T1.1" in out2
+        out3 = await registry.execute("todo_pop", "{}")
+        assert "popped frame" in out3
+        assert stack.depth == 1
         _ = await memory.update_session_todo_stack(session.sid, stack.to_raw())
         loaded = await memory.get_session_todo_stack(session.sid)
         assert loaded is not None
         again = TodoStack.from_raw(loaded)
-        assert again.items[0].status == "done"
+        assert again.depth == 1
+        assert [i.title for i in again.frames[0].items] == ["T1", "T2"]
     finally:
         set_todo_stack(None)
         await memory.close()
@@ -99,9 +134,7 @@ class ScriptedClient:
     ) -> ChatCompletionResponse | AsyncIterator[ChatCompletionChunk]:
         del stream
         self.calls += 1
-        # First call: finish without tools; second: after review inject.
         text = "ok" if self.calls > 1 else "done without todos"
-        # Detect review message in history
         for msg in param.messages:
             if isinstance(msg, DeveloperChatMessage) and "Todo stack review" in msg.content:
                 text = "reviewed stack"