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 Agent Note.

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.

Rendering

The canonical result is { todos, counts: { pending, inProgress, completed } }; its Native renderer returns the compact update acknowledgement. The tool also writes the full todo/write session event. UIs subscribe to session/event and render that durable list themselves: the TUI app shows a persistent plan, and the ACP bridge maps the list to a plan sessionUpdate (synthesizing the priority ACP requires).

Export shape

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).

Model Experience

Tool schema

What the model sees

The model sees the generated todo_write schema.

Token effect

Fixed schema cost on every request where the tool is visible.

KV Cache effect

Prefix-stable while the definition and visibility are unchanged. Plugin lifecycle or scoped restrictions may invalidate reuse from this schema.

Tool-call history and result

What the model sees

Each assistant tool call retains the entire replacement list in its arguments. Success returns exactly Updated todo list: <pending> pending, <inProgress> in progress, <completed> completed. Stable failures are Error: invalid todo: `content` must be a non-empty string, Error: invalid todos: duplicate content "<content>", Error: invalid todos: at most one task may be in_progress, got <count>, and Error: todo_write requires an owning agent session. The full todo/write session event is UI and replay state, not a second model message.

Token effect

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.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Known Limitations and Deferred Work

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.