diff --git a/tests/ui/run.mjs b/tests/ui/run.mjs
index 6483777..548dade 100644
--- a/tests/ui/run.mjs
+++ b/tests/ui/run.mjs
@@ -658,6 +658,50 @@ test("B1: a reply being written draws the stream's progress, its clock moving, a
   assert.deepEqual(p.errors, [])
 })
 
+// --- C4 (ahead of the server): the subagent tree as spec 3.4.0 §12 gives it ---------------------
+//
+// ui/js/tree.js draws subagents.md §12's data; the activity panel takes it up once the server's
+// /api/v2 carries the tree. Until then it is checked here on the spec's own example.
+
+const TREE = {
+  min: 2, max: 4, size: 3, open_slots: 1,
+  agents: [
+    { id: "lead", name: "lead", position: "the main agent", parent: null, state: "working", model: "qwen-27b", mode: "edit", request: [], manager: true, context: { used: 41200, window: 131072 }, progress: { phase: "thinking", since: 0 } },
+    { id: "s1", name: "parser", position: "reviews the parser", parent: "lead", state: "working", model: "qwen-27b", mode: "manual", request: ["write"], manager: true, task: "Read src/parse.ts", context: { used: 9100, window: 131072 }, progress: { phase: "running_tool", since: 0, tool: "read", what: "src/parse.ts" }, findings: 2, messages: 3 },
+    { id: "s3", name: "lexer", position: "the lexer's tests", parent: "s1", state: "waiting", model: "qwen-27b", mode: "manual", request: [], manager: false, context: { used: 0, window: 131072 }, progress: null, findings: 0, messages: 0 },
+  ],
+  ended: [{ id: "s0", name: "scout", position: "maps the repository", parent: "lead", state: "out_of_context", ended: 0, findings: 1 }],
+}
+
+test("C4: the tree in order, each agent's position, state, mode, context and phase; slots; the ended ones apart", async () => {
+  const p = await page(b3())
+  const got = await p.evaluate(async (tree) => {
+    const m = await import("/js/tree.js")
+    const calls = []
+    const box = m.treePart(tree, { room: [{ n: 1, from: "lead", to: ["parser"], text: "Start with the grammar.\nThen the lexer." }], now: 65000, act: { open: (a) => calls.push(["open", a.id]), findings: (a) => calls.push(["findings", a.id]), stop: (a) => calls.push(["stop", a.id]) } })
+    document.body.append(box)
+    const rows = [...box.querySelectorAll(".subagents__list > .subagent:not(.subagent--slot)")].map((r) => ({ id: r.dataset.id, depth: r.style.getPropertyValue("--depth"), text: r.textContent }))
+    box.querySelector('[data-id="s1"] .subagent__actions .btn').click()
+    box.querySelector('[data-id="s0"] .subagent__actions .btn:last-child').click()
+    box.querySelector('[data-id="s1"] .agent__stop').click()
+    return { rows, slots: box.querySelectorAll(".subagent--slot").length, endedStop: box.querySelectorAll('[data-id="s0"] button.agent__stop').length, calls }
+  }, TREE)
+  assert.deepEqual(got.rows.map((r) => [r.id, r.depth]), [["lead", "0"], ["s1", "1"], ["s3", "2"], ["s0", "0"]])
+  const parser = got.rows[1].text
+  assert.match(parser, /parser \(s1\) — reviews the parser/)
+  assert.match(parser, /6%/, "context: floor(9100 × 100 / 131072)")
+  assert.match(parser, /manual \+ write/)
+  assert.match(parser, /Running read: src\/parse\.ts · 1m 05s/)
+  assert.match(parser, /Messages: 3/)
+  assert.match(parser, /lead: Start with the grammar\./)
+  assert.match(got.rows[2].text, /waiting for a slot/)
+  assert.match(got.rows[3].text, /out of context/)
+  assert.equal(got.slots, 1)
+  assert.equal(got.endedStop, 0, "an ended subagent cannot be stopped")
+  assert.deepEqual(got.calls, [["open", "s1"], ["findings", "s0"], ["stop", "s1"]])
+  assert.deepEqual(p.errors, [])
+})
+
 // --- Run ----------------------------------------------------------------------------------------
 
 const only = process.argv[2] || ""
diff --git a/ui/js/tree.js b/ui/js/tree.js
new file mode 100644
index 0000000..58e4174
--- /dev/null
+++ b/ui/js/tree.js
@@ -0,0 +1,192 @@
+// The subagent tree (harness spec 3.4.0, subagents.md §12) as the activity panel draws it: the
+// main agent at the root, every subagent under its parent in the order added, the open slots,
+// then the ended ones, dimmed, with how each ended. Per agent: its name and id, its position,
+// its state, model, mode (and request options), how full its context is, its current phase (the
+// progress line), its messages in the room, and -- for any subagent, live or ended -- its session
+// read-only and its findings; a live one can be stopped.
+//
+// The data is the spec's, as the server passes it on in /api/v2 for a chat's own subagents and
+// for an agent chat's alike:
+//
+//   { min, max, size, open_slots,
+//     agents: [{ id, name, position, parent, state, model, mode, request, manager, task,
+//                context: { used, window }, progress, findings, messages }],
+//     ended:  [{ id, name, position, parent, state, ended, findings }] }
+//
+// Everything in it but the numbers is a model's words: set as text, never as markup.
+
+import { h, icon, ICONS } from "./dom.js"
+import { progressLine } from "./progress.js"
+
+/** The states' words (loop.json team.read.states). */
+const STATES = {
+  working: "working",
+  waiting: "waiting for a slot",
+  finished: "finished",
+  idle: "idle",
+  paused: "paused: context full",
+  done: "done",
+  out_of_context: "out of context",
+  stopped: "stopped",
+}
+export const TREE_SENTENCES = Object.values(STATES)
+
+export function treeState(state) {
+  return STATES[state] ? t(STATES[state]) : String(state || "")
+}
+
+/** floor(used × 100 / window), or null when the window is not known. */
+export function contextPercent(c) {
+  const used = Number(c?.used)
+  const window = Number(c?.window)
+  if (!(window > 0) || !(used >= 0)) return null
+  return Math.min(100, Math.floor((used * 100) / window))
+}
+
+function modeWords(a) {
+  if (!a.mode) return ""
+  const req = (a.request || []).filter(Boolean)
+  return req.length ? `${a.mode} ${req.map((r) => `+ ${r}`).join(" ")}` : a.mode
+}
+
+/** The agents in tree order (an agent, then those under it, depth first), each with its depth. */
+export function treeOrder(agents) {
+  const ids = new Set(agents.map((a) => a.id))
+  const under = new Map()
+  for (const a of agents) {
+    const parent = a.parent && ids.has(a.parent) && a.parent !== a.id ? a.parent : null
+    if (!under.has(parent)) under.set(parent, [])
+    under.get(parent).push(a)
+  }
+  const out = []
+  const seen = new Set()
+  const walk = (parent, depth) => {
+    for (const a of under.get(parent) || []) {
+      if (seen.has(a.id)) continue
+      seen.add(a.id)
+      out.push({ agent: a, depth })
+      walk(a.id, depth + 1)
+    }
+  }
+  walk(null, 0)
+  for (const a of agents) if (!seen.has(a.id)) out.push({ agent: a, depth: 0 })
+  return out
+}
+
+/** The room's messages to or from an agent (by name; `all` reaches everyone). */
+function messagesOf(room, a) {
+  return room.filter((m) => m.from === a.name || (m.to || []).includes(a.name) || (m.recipients || []).includes(a.name))
+}
+
+function firstLine(text) {
+  return String(text || "").split("\n").find((l) => l.trim()) || ""
+}
+
+/**
+ * The tree as rows. `act` holds what the rows can do: `open(a)` (its session, read-only),
+ * `findings(a)`, `stop(a)`; `isOpen(key)` / `toggled(key, open)` keep unfolded rows unfolded
+ * across redraws; `room` is the room as read so far.
+ */
+export function treePart(tree, { room = [], act = {}, now = Date.now() } = {}) {
+  const rows = []
+  for (const { agent, depth } of treeOrder(tree.agents || [])) rows.push(agentRow(agent, depth, { room, act, now, ended: false }))
+  for (let i = 0; i < (Number(tree.open_slots) || 0); i++) {
+    rows.push(h("div", { class: "subagent subagent--slot", style: `--depth: 1` }, h("span", { class: "subagent__slot", text: t("Open slot") })))
+  }
+  const ended = tree.ended || []
+  const box = h("div", { class: "subagents" })
+  box.append(
+    h("p", { class: "subagents__size", text: t("%(size)s of at most %(max)s agents", { size: tree.size ?? (tree.agents || []).length, max: tree.max ?? "?" }) }),
+    h("div", { class: "subagents__list" }, rows),
+  )
+  if (ended.length) {
+    const key = "fold:ended"
+    const fold = h(
+      "details",
+      { class: "tasks__more subagents__ended", open: act.isOpen?.(key) },
+      h("summary", null, h("span", { class: "tasks__chev", "aria-hidden": "true" }, icon(ICONS.chevron)), h("span", { text: t("Ended %(n)s", { n: ended.length }) })),
+      h(
+        "div",
+        { class: "subagents__list" },
+        ended.map((a) => agentRow(a, 0, { room, act, now, ended: true })),
+      ),
+    )
+    fold.addEventListener("toggle", () => act.toggled?.(key, fold.open))
+    box.append(fold)
+  }
+  return box
+}
+
+function agentRow(a, depth, { room, act, now, ended }) {
+  const key = `sub:${a.id}`
+  const main = !a.parent && a.id === "lead"
+  const pct = contextPercent(a.context)
+  const line = a.progress ? progressLine(a.progress, now) : ""
+  const said = messagesOf(room, a)
+  const last = said[said.length - 1]
+  const stop =
+    !ended && !main && act.stop
+      ? h(
+          "button",
+          {
+            type: "button",
+            class: "agent__stop",
+            "aria-label": t("Stop %(name)s", { name: a.name }),
+            title: t("Stop %(name)s", { name: a.name }),
+            onclick: (e) => {
+              e.preventDefault()
+              e.stopPropagation()
+              act.stop(a)
+            },
+          },
+          h("span", { class: "agent__square", "aria-hidden": "true" }),
+        )
+      : h("span", { class: "agent__stop is-none", "aria-hidden": "true" })
+  const el = h(
+    "details",
+    { class: `agent subagent is-${String(a.state || "").replace(/[^a-z_]/g, "")}${ended ? " is-ended" : ""}`, open: act.isOpen?.(key), dataset: { id: a.id } },
+    h(
+      "summary",
+      { class: "agent__row" },
+      h("span", { class: "agent__chev", "aria-hidden": "true" }, icon(ICONS.chevron)),
+      h(
+        "span",
+        { class: "agent__what", title: a.position || a.name },
+        h("bdi", { class: "agent__name", text: a.name || a.id }),
+        h("span", { class: "subagent__id", text: ` (${a.id})` }),
+        a.position ? h("span", { class: "subagent__position", text: ` — ${main ? t("the main agent") : a.position}` }) : null,
+      ),
+      h("span", { class: "subagent__pct", text: pct === null ? "" : `${pct}%`, title: pct === null ? "" : t("%(percent)s%% of the context window used", { percent: pct }).replace(/%%/g, "%") }),
+      stop,
+    ),
+    h(
+      "div",
+      { class: "agent__body" },
+      h(
+        "p",
+        { class: "agent__facts" },
+        h("span", { class: "agent__state", text: treeState(a.state) }),
+        a.model ? h("span", { class: "agent__model", text: a.model }) : null,
+        modeWords(a) ? h("span", { class: "subagent__mode", text: modeWords(a) }) : null,
+        a.findings ? h("span", { text: t("Findings: %(n)s", { n: a.findings }) }) : null,
+      ),
+      pct === null ? null : h("div", { class: "subagent__bar", role: "meter", "aria-valuemin": "0", "aria-valuemax": "100", "aria-valuenow": String(pct), "aria-label": t("Context") }, h("span", { style: `--pct: ${pct}%` })),
+      line ? h("p", { class: "subagent__phase", "data-phase": a.progress?.phase || "", text: line }) : null,
+      a.task ? h("p", { class: "agent__brief", text: a.task }) : null,
+      said.length || a.messages
+        ? h("p", { class: "subagent__room" }, h("span", { class: "chip", text: t("Messages: %(n)s", { n: a.messages ?? said.length }) }), last ? h("span", { class: "subagent__last", text: `${last.from}: ${firstLine(last.text)}` }) : null)
+        : null,
+      main
+        ? null
+        : h(
+            "div",
+            { class: "subagent__actions" },
+            act.open ? h("button", { type: "button", class: "btn btn--sm", text: t("Open its session"), onclick: () => act.open(a) }) : null,
+            act.findings ? h("button", { type: "button", class: "btn btn--sm", text: t("Findings"), onclick: () => act.findings(a) }) : null,
+          ),
+    ),
+  )
+  el.style.setProperty("--depth", String(Math.max(0, Math.min(6, depth))))
+  el.addEventListener("toggle", () => act.toggled?.(key, el.open))
+  return el
+}