The model-facing `todo_write` tool: the agent's whole task list, replaced wholesale on each call.
## What it does
Registers one tool, `todo_write(todos: [{ content, status }])`, on `ctx.tools`. The model sends the ENTIRE list every call — there are no partial updates or per-item edits. Each call appends a `todo/write` event (the full list snapshot) to the calling agent's session log via `agent.session.append('todo/write', { todos })`; the current list is the most recent such event (last-write-wins on replay).
`status` is one of `pending`, `in_progress`, `completed` — exactly the ACP `PlanEntryStatus` triple.
## Single owner
The list belongs to the ONE agent session that called the tool. There is no subagent/shared/swarm scope: a non-agent caller (no `exec.agent`) has nowhere to write the list and is rejected. This is a deliberate scope limit — see the RFC.
## Validation
Beyond the schema's type/required/enum checks, `execute` rejects an empty or duplicate `content` and more than one `in_progress` task (a coherent plan has at most one task active). Ordering and the discipline of keeping the list current are left to the model via the tool description.
The tool writes only the session event; it does not render. UIs subscribe to `session/event` and render the `todo/write` data themselves: the [stdio app's readline UI](../../ui/stdio-agent) prints a glyphed checklist, and the [ACP bridge](../../ui/acp) maps the list to a `plan` sessionUpdate (synthesizing the `priority` ACP requires).
A function/namespace plugin: it exports `name` / `inject` / `apply` and NO default. A stray `export default` would collapse the module via the Loader's `unwrapExports` and drop `inject` (see [docs/postmortem/0001](../../../docs/postmortem/0001-acp-default-export-drops-inject.md)).
| Context surface | What the model sees | Token effect |
|---|---|---|
| Tool schema | The model sees `todo_write` with the complete-list array and the three status values. | Fixed schema cost on every request where the tool is visible. |
| Tool-call history and result | Each assistant tool call retains the entire replacement list in its arguments. The tool result reports only pending, in-progress, and completed counts; the full `todo/write` session event is UI and replay state, not a second model message. | Token growth scales with every full list the model submits, and those call arguments remain until compaction. The result itself is small and fixed-shape. |
- **Single-owner scope only** — the list belongs to the one calling agent session; subagent/shared/swarm scopes are a deliberate cut (see § Single owner), and a non-agent caller is rejected.
- **The item shape is deliberately minimal** — `content` plus three-state `status`; no id, priority, or active-form fields, and the ACP bridge synthesizes the `priority` ACP requires.
- **Whole-list replacement is the only operation** — no partial updates, no read-back tool; the model must resend the entire list each call.
