diff --git a/ui-opentui/src/entry/main.tsx b/ui-opentui/src/entry/main.tsx
index 1ad2e4f18d0..6b9d709dc4d 100644
--- a/ui-opentui/src/entry/main.tsx
+++ b/ui-opentui/src/entry/main.tsx
@@ -67,6 +67,19 @@ const READY_TIMEOUT_MS = 20_000
 /** Window after a Ctrl+C in which a second Ctrl+C quits the TUI (item 11). */
 const QUIT_WINDOW_MS = 3_000
 
+/** Recursive renderable count under a node (the /mem store-cap diagnostic —
+ *  same walk as scripts/mem-bench.tsx; cheap: one tree pass on demand). */
+function descendantCount(node: { getChildren(): unknown[] }): number {
+  let n = 0
+  for (const child of node.getChildren()) {
+    n += 1
+    if (child && typeof child === 'object' && 'getChildren' in child) {
+      n += descendantCount(child as { getChildren(): unknown[] })
+    }
+  }
+  return n
+}
+
 /**
  * Resume a session INTO the store: buffer live events across the `session.resume`
  * RPC, then replace history + replay (gotcha §8 #5 tool rows handled by
@@ -366,6 +379,17 @@ export const run = Effect.fn('Tui.run')(function* (input: TuiInput) {
       // Promise-returning `request` + the host capabilities it needs).
       const slashCtx: SlashContext = {
         clearTranscript: () => store.clearTranscript(),
+        compact: () => store.state.compact,
+        setCompact: on => store.setCompact(on),
+        details: () => store.state.details,
+        setDetails: mode => store.setDetails(mode),
+        renderableCount: () => {
+          try {
+            return descendantCount(renderer.root)
+          } catch {
+            return undefined
+          }
+        },
         confirm: (message, onConfirm) => store.setConfirm(message, onConfirm),
         copyResponse: n => {
           const text = nthAssistantResponse(store.state.messages, n)
diff --git a/ui-opentui/src/logic/details.ts b/ui-opentui/src/logic/details.ts
new file mode 100644
index 00000000000..8b96e07dd99
--- /dev/null
+++ b/ui-opentui/src/logic/details.ts
@@ -0,0 +1,84 @@
+/**
+ * Global detail-mode logic (/details — Epic 3 utility-command port; mirrors Ink
+ * `domain/details.ts`, GLOBAL mode only — per-section overrides are explicitly
+ * deferred). The mode drives how the transcript treats tool + reasoning rows:
+ *
+ *   - `collapsed` (default): today's behaviour — headers with click-to-expand.
+ *   - `expanded`: tool bodies + settled reasoning previews default-OPEN.
+ *   - `hidden`: tool/reasoning runs reduce to ONE muted `⚡ N tools hidden`-style
+ *     line per run (never silently dropped — flipping the mode back restores,
+ *     since the parts stay in the store untouched).
+ *
+ * Pure data + functions; the store carries the flag, messageLine/toolPart/
+ * reasoningPart read it via the display context.
+ */
+import type { Part } from './store.ts'
+
+export type DetailsMode = 'hidden' | 'collapsed' | 'expanded'
+
+/** Cycle order (Ink parity: hidden → collapsed → expanded → hidden). */
+export const DETAILS_MODES = ['hidden', 'collapsed', 'expanded'] as const
+
+/** Gateway `complete.slash` suggests these per-section names after `/details ` —
+ *  recognized so picking one yields an honest "not supported yet" notice instead
+ *  of the generic usage line (per-section overrides are deferred). */
+export const DETAILS_SECTIONS = ['thinking', 'tools', 'subagents', 'activity'] as const
+
+export const DETAILS_USAGE = 'usage: /details [hidden|collapsed|expanded|cycle]'
+
+/** Parse a mode word; null for anything unrecognized (non-strings included). */
+export function parseDetailsMode(v: unknown): DetailsMode | null {
+  if (typeof v !== 'string') return null
+  const norm = v.trim().toLowerCase()
+  return DETAILS_MODES.find(m => m === norm) ?? null
+}
+
+/** The next mode in the cycle (`/details cycle`). */
+export function nextDetailsMode(m: DetailsMode): DetailsMode {
+  return DETAILS_MODES[(DETAILS_MODES.indexOf(m) + 1) % DETAILS_MODES.length] ?? 'collapsed'
+}
+
+/** One collapsed RUN of consecutive tool/reasoning parts (hidden mode). */
+export interface HiddenRun {
+  type: 'hiddenRun'
+  /** Stable-ish key: the first hidden part's id. */
+  id: string
+  tools: number
+  thoughts: number
+}
+
+/** What the transcript renders per part slot: a real part, or a hidden-run marker. */
+export type DisplayPart = Part | HiddenRun
+
+/**
+ * Hidden mode: keep text parts, fold each consecutive run of tool/reasoning
+ * parts into ONE HiddenRun marker (so a 5-tool fan-out reads as a single muted
+ * line, not 5 of them). Pure — the source parts are untouched, so switching
+ * the mode back restores everything.
+ */
+export function collapseHiddenParts(parts: readonly Part[]): DisplayPart[] {
+  const out: DisplayPart[] = []
+  let run: HiddenRun | undefined
+  for (const part of parts) {
+    if (part.type === 'text') {
+      run = undefined
+      out.push(part)
+      continue
+    }
+    if (!run) {
+      run = { id: `hidden-${part.id}`, thoughts: 0, tools: 0, type: 'hiddenRun' }
+      out.push(run)
+    }
+    if (part.type === 'tool') run.tools += 1
+    else run.thoughts += 1
+  }
+  return out
+}
+
+/** Muted one-liner for a hidden run: `2 tools · 1 thought hidden`. */
+export function hiddenRunLabel(run: HiddenRun): string {
+  const segs: string[] = []
+  if (run.tools) segs.push(`${run.tools} tool${run.tools === 1 ? '' : 's'}`)
+  if (run.thoughts) segs.push(`${run.thoughts} thought${run.thoughts === 1 ? '' : 's'}`)
+  return `${segs.join(' · ')} hidden — /details collapsed to show`
+}
diff --git a/ui-opentui/src/logic/diagnostics.ts b/ui-opentui/src/logic/diagnostics.ts
new file mode 100644
index 00000000000..65af87d66cf
--- /dev/null
+++ b/ui-opentui/src/logic/diagnostics.ts
@@ -0,0 +1,82 @@
+/**
+ * Process diagnostics for the /mem + /heapdump utility commands (Epic 3 port;
+ * Ink ref `app/slash/commands/debug.ts` + `lib/memory.ts`). Pure formatters plus
+ * the one impure seam (`performHeapdump` → `v8.writeHeapSnapshot`), kept out of
+ * slash.ts so the dispatcher stays plain and tests can mock `node:v8`.
+ */
+import { mkdirSync } from 'node:fs'
+import { homedir } from 'node:os'
+import { dirname, join } from 'node:path'
+import { writeHeapSnapshot } from 'node:v8'
+
+/** `123456789` → `117.7 MB` (binary units, one decimal above bytes). */
+export function formatBytes(n: number): string {
+  if (!Number.isFinite(n) || n < 0) return '0 B'
+  if (n < 1024) return `${Math.round(n)} B`
+  const units = ['KB', 'MB', 'GB', 'TB'] as const
+  let v = n
+  let i = -1
+  do {
+    v /= 1024
+    i += 1
+  } while (v >= 1024 && i < units.length - 1)
+  return `${v.toFixed(1)} ${units[i]}`
+}
+
+/** Where heap snapshots land: `$HERMES_HOME`/`~/.hermes` + `logs/opentui-heap-<ts>.heapsnapshot`. */
+export function heapSnapshotPath(now = new Date()): string {
+  const home = process.env.HERMES_HOME?.trim() || join(homedir(), '.hermes')
+  const ts = now.toISOString().replace(/[:.]/g, '-')
+  return join(home, 'logs', `opentui-heap-${ts}.heapsnapshot`)
+}
+
+export interface HeapdumpResult {
+  path: string
+  before: { heapUsed: number; rss: number }
+  after: { heapUsed: number; rss: number }
+}
+
+/**
+ * Write a V8 heap snapshot (SYNCHRONOUS — blocks the event loop while V8 walks
+ * the heap; that's inherent to writeHeapSnapshot) and report heap/rss before
+ * vs after. Throws on I/O failure — the caller renders the error.
+ */
+export function performHeapdump(): HeapdumpResult {
+  const before = process.memoryUsage()
+  const path = heapSnapshotPath()
+  mkdirSync(dirname(path), { recursive: true })
+  const written = writeHeapSnapshot(path)
+  const after = process.memoryUsage()
+  return {
+    after: { heapUsed: after.heapUsed, rss: after.rss },
+    before: { heapUsed: before.heapUsed, rss: before.rss },
+    path: written
+  }
+}
+
+export interface MemSnapshot {
+  heapUsed: number
+  heapTotal: number
+  external: number
+  arrayBuffers: number
+  rss: number
+}
+
+/**
+ * The /mem system-line body (Ink's Memory panel as aligned rows). `renderables`
+ * is the mounted-renderable count under the live renderer root (the store-cap
+ * diagnostic) — omitted when unavailable (e.g. no renderer in tests).
+ */
+export function memReport(usage: MemSnapshot, uptimeSeconds: number, renderables?: number): string {
+  const rows: Array<[string, string]> = [
+    ['heap used', formatBytes(usage.heapUsed)],
+    ['heap total', formatBytes(usage.heapTotal)],
+    ['external', formatBytes(usage.external)],
+    ['array buffers', formatBytes(usage.arrayBuffers)],
+    ['rss', formatBytes(usage.rss)],
+    ['uptime', `${Math.round(uptimeSeconds)}s`]
+  ]
+  if (renderables !== undefined) rows.push(['renderables', String(renderables)])
+  const pad = Math.max(...rows.map(([k]) => k.length))
+  return ['memory', ...rows.map(([k, v]) => `  ${k.padEnd(pad)}  ${v}`)].join('\n')
+}
diff --git a/ui-opentui/src/logic/replay.ts b/ui-opentui/src/logic/replay.ts
new file mode 100644
index 00000000000..af04e91c008
--- /dev/null
+++ b/ui-opentui/src/logic/replay.ts
@@ -0,0 +1,126 @@
+/**
+ * /replay — spawn-tree inspector logic (Epic 3 port; Ink ref
+ * `app/slash/commands/ops.ts` /replay + `spawnHistoryStore.ts`). The gateway
+ * archives each completed delegation fan-out as a JSON snapshot
+ * (`spawn_tree.save`); these helpers read `spawn_tree.list` / `spawn_tree.load`
+ * payloads and format them as PAGER TEXT — the native engine renders replays
+ * through the existing pager overlay instead of Ink's agents overlay.
+ *
+ * All readers are defensive (wire JSON is loose, snapshots cross versions).
+ */
+
+export interface SpawnTreeEntry {
+  path: string
+  label: string
+  count: number
+  /** Epoch SECONDS (gateway convention). */
+  finishedAt?: number
+  sessionId?: string
+}
+
+function str(v: unknown): string | undefined {
+  return typeof v === 'string' && v ? v : undefined
+}
+
+function num(v: unknown): number | undefined {
+  return typeof v === 'number' && Number.isFinite(v) ? v : undefined
+}
+
+/** Map a `spawn_tree.list` result ({entries:[…]}) into typed rows (pathless rows dropped). */
+export function readSpawnTreeEntries(result: unknown): SpawnTreeEntry[] {
+  if (!result || typeof result !== 'object') return []
+  const entries = (result as { entries?: unknown }).entries
+  if (!Array.isArray(entries)) return []
+  const out: SpawnTreeEntry[] = []
+  for (const e of entries) {
+    if (!e || typeof e !== 'object') continue
+    const o = e as { [k: string]: unknown }
+    const path = str(o['path'])
+    if (!path) continue
+    const entry: SpawnTreeEntry = {
+      count: num(o['count']) ?? 0,
+      label: str(o['label']) ?? '',
+      path
+    }
+    const finishedAt = num(o['finished_at'])
+    if (finishedAt !== undefined) entry.finishedAt = finishedAt
+    const sessionId = str(o['session_id'])
+    if (sessionId !== undefined) entry.sessionId = sessionId
+    out.push(entry)
+  }
+  return out
+}
+
+function fmtWhen(epochSeconds: number | undefined): string {
+  if (epochSeconds === undefined) return '?'
+  try {
+    return new Date(epochSeconds * 1000).toLocaleString()
+  } catch {
+    return '?'
+  }
+}
+
+/** The bare `/replay` listing: indexed rows the user replays by number. */
+export function formatSpawnTreeList(entries: readonly SpawnTreeEntry[]): string {
+  const lines: string[] = ['Archived spawn trees — /replay <n> to view, /replay <path> for any snapshot', '']
+  entries.forEach((e, i) => {
+    const label = e.label || `${e.count} subagent${e.count === 1 ? '' : 's'}`
+    lines.push(`${String(i + 1).padStart(3)}. ${fmtWhen(e.finishedAt)} · ${e.count}× — ${label}`)
+    lines.push(`     ${e.path}`)
+  })
+  return lines.join('\n')
+}
+
+/** Status glyph for an archived subagent row. */
+function statusGlyph(status: string): string {
+  if (status === 'completed') return '✓'
+  if (status === 'error' || status === 'failed' || status === 'timeout') return '✗'
+  if (status === 'interrupted') return '⏹'
+  return '●'
+}
+
+/** One archived subagent → its pager lines (indented by spawn depth). */
+function subagentLines(raw: unknown, index: number): string[] {
+  const o = (raw && typeof raw === 'object' ? raw : {}) as { [k: string]: unknown }
+  const depth = num(o['depth']) ?? 0
+  const pad = '  '.repeat(Math.max(0, depth))
+  const status = str(o['status']) ?? 'completed'
+  const goal = str(o['goal']) ?? 'subagent'
+  const lines = [`${pad}${statusGlyph(status)} [${index + 1}] ${goal}`]
+  const meta: string[] = [status]
+  const model = str(o['model'])
+  if (model) meta.push(model)
+  const duration = num(o['durationSeconds'])
+  if (duration !== undefined) meta.push(`${Math.round(duration)}s`)
+  const tools = num(o['toolCount'])
+  if (tools) meta.push(`${tools} tool${tools === 1 ? '' : 's'}`)
+  const tokIn = num(o['inputTokens'])
+  const tokOut = num(o['outputTokens'])
+  if (tokIn !== undefined || tokOut !== undefined) meta.push(`${tokIn ?? 0} in / ${tokOut ?? 0} out tok`)
+  lines.push(`${pad}    ${meta.join(' · ')}`)
+  const summary = str(o['summary'])
+  if (summary) for (const s of summary.split('\n')) lines.push(`${pad}    ${s}`)
+  const notes = o['notes']
+  if (Array.isArray(notes)) {
+    for (const note of notes) if (typeof note === 'string' && note) lines.push(`${pad}    · ${note}`)
+  }
+  return lines
+}
+
+/** A loaded snapshot (`spawn_tree.load` payload) → the full pager text. */
+export function formatSpawnTree(payload: unknown): string {
+  const o = (payload && typeof payload === 'object' ? payload : {}) as { [k: string]: unknown }
+  const subagents = Array.isArray(o['subagents']) ? (o['subagents'] as unknown[]) : []
+  const header: string[] = []
+  const label = str(o['label'])
+  header.push(label ?? 'spawn tree')
+  const meta: string[] = []
+  const sessionId = str(o['session_id'])
+  if (sessionId) meta.push(`session ${sessionId}`)
+  meta.push(`finished ${fmtWhen(num(o['finished_at']))}`)
+  meta.push(`${subagents.length} subagent${subagents.length === 1 ? '' : 's'}`)
+  header.push(meta.join(' · '))
+  if (!subagents.length) return [...header, '', '(snapshot empty or unreadable)'].join('\n')
+  const body = subagents.flatMap((s, i) => ['', ...subagentLines(s, i)])
+  return [...header, ...body].join('\n')
+}
diff --git a/ui-opentui/src/logic/slash.ts b/ui-opentui/src/logic/slash.ts
index 66c4f3bc07e..3e78c5d0126 100644
--- a/ui-opentui/src/logic/slash.ts
+++ b/ui-opentui/src/logic/slash.ts
@@ -11,6 +11,9 @@
  *      (exec/plugin → system · alias → re-dispatch · skill/send → submit a turn ·
  *       prefill → notice). Long output routes to the pager (Phase 5a).
  */
+import { DETAILS_SECTIONS, DETAILS_USAGE, type DetailsMode, nextDetailsMode, parseDetailsMode } from './details.ts'
+import { formatBytes, memReport, performHeapdump } from './diagnostics.ts'
+import { formatSpawnTree, formatSpawnTreeList, readSpawnTreeEntries } from './replay.ts'
 import type { CompletionItem, PickerItem, PickerState, SessionItem } from './store.ts'
 
 export interface ParsedSlash {
@@ -57,6 +60,15 @@ export interface SlashContext {
   readonly modelItems: () => PickerItem[] | undefined
   /** Update the cached `/model` picker rows. */
   readonly setModelItems: (items: PickerItem[]) => void
+  /** Read / set the compact-transcript display flag (/compact — Epic 3). */
+  readonly compact: () => boolean
+  readonly setCompact: (on: boolean) => void
+  /** Read / set the global tool/reasoning detail mode (/details — Epic 3). */
+  readonly details: () => DetailsMode
+  readonly setDetails: (mode: DetailsMode) => void
+  /** Mounted-renderable count under the live renderer root (a /mem diagnostic);
+   *  undefined when no renderer is reachable (tests). */
+  readonly renderableCount: () => number | undefined
 }
 
 function readStr(value: unknown, key: string): string | undefined {
@@ -143,6 +155,11 @@ const CLIENT_HELP = [
   '/skills — browse skills',
   '/sessions, /resume — switch/resume a session',
   '/clear, /new — clear the transcript (confirm)',
+  '/compact [on|off|toggle] — compact transcript spacing',
+  '/details [hidden|collapsed|expanded|cycle] — tool/reasoning detail',
+  '/replay [n|path] — inspect an archived spawn tree',
+  '/mem — live memory stats',
+  '/heapdump — write a V8 heap snapshot',
   '/logs — recent engine log lines',
   '/quit, /exit — quit',
   '(other /commands run on the gateway)'
@@ -337,6 +354,137 @@ const skillsCmd: ClientHandler = async (_arg, ctx) => {
   })
 }
 
+/** `on`/`off`/`toggle`/bare → the next flag value; null on garbage (Ink flagFromArg). */
+function flagFromArg(arg: string, current: boolean): boolean | null {
+  const mode = arg.trim().toLowerCase()
+  if (!mode || mode === 'toggle') return !current
+  if (mode === 'on') return true
+  if (mode === 'off') return false
+  return null
+}
+
+/** `/compact [on|off|toggle]` — compact transcript spacing. The flag flips locally
+ *  (the store drives the render); persistence mirrors Ink: a fire-and-forget
+ *  `config.set {key:'compact'}` so the Ink TUI + future launches share the pref
+ *  (the gateway does NOT send the persisted value to this TUI, so each launch
+ *  starts off — see store.ts `compact`). */
+const compactCmd: ClientHandler = (arg, ctx) => {
+  const next = flagFromArg(arg, ctx.compact())
+  if (next === null) {
+    ctx.pushSystem('usage: /compact [on|off|toggle]')
+    return
+  }
+  ctx.setCompact(next)
+  void ctx.request('config.set', { key: 'compact', value: next ? 'on' : 'off' }).catch(() => {})
+  ctx.pushSystem(`compact ${next ? 'on' : 'off'}`)
+}
+
+/**
+ * `/details [hidden|collapsed|expanded|cycle]` — GLOBAL detail mode (per-section
+ * overrides deferred; the gateway's arg completion also suggests section names,
+ * so those get an honest "not supported yet" notice). Bare `/details` reports the
+ * persisted mode (`config.get details_mode`) and syncs the local flag to it; a
+ * mode set persists via `config.set` (fire-and-forget, Ink parity).
+ */
+const detailsCmd: ClientHandler = async (arg, ctx) => {
+  const first = arg.trim().toLowerCase().split(/\s+/)[0] ?? ''
+  if (!first) {
+    try {
+      const r = await ctx.request('config.get', { key: 'details_mode' })
+      const mode = parseDetailsMode(readStr(r, 'value')) ?? ctx.details()
+      ctx.setDetails(mode)
+      ctx.pushSystem(`details: ${mode}`)
+    } catch {
+      ctx.pushSystem(`details: ${ctx.details()}`)
+    }
+    return
+  }
+  if ((DETAILS_SECTIONS as readonly string[]).includes(first)) {
+    ctx.pushSystem(`per-section detail overrides are not supported in the native engine yet — ${DETAILS_USAGE}`)
+    return
+  }
+  const next = first === 'cycle' || first === 'toggle' ? nextDetailsMode(ctx.details()) : parseDetailsMode(first)
+  if (!next) {
+    ctx.pushSystem(DETAILS_USAGE)
+    return
+  }
+  ctx.setDetails(next)
+  void ctx.request('config.set', { key: 'details_mode', value: next }).catch(() => {})
+  ctx.pushSystem(`details: ${next}`)
+}
+
+/** Fetch + map the session's archived spawn trees (`spawn_tree.list`). */
+async function listSpawnTrees(ctx: SlashContext) {
+  const r = await ctx.request('spawn_tree.list', { limit: 30, session_id: ctx.sessionId() ?? 'default' })
+  return readSpawnTreeEntries(r)
+}
+
+/**
+ * `/replay [n|path]` — spawn-tree inspector through the pager (Ink renders these
+ * in its agents overlay; the flow + RPCs are the same): bare lists the archived
+ * trees with indices, `<n>` loads the n-th listed tree, anything else is treated
+ * as a snapshot path on disk (`load <path>` accepted for Ink muscle memory).
+ */
+const replayCmd: ClientHandler = async (arg, ctx) => {
+  const raw = arg.trim()
+  const lower = raw.toLowerCase()
+  try {
+    if (!raw || lower === 'list' || lower === 'ls') {
+      const entries = await listSpawnTrees(ctx)
+      if (!entries.length) {
+        ctx.pushSystem('no archived spawn trees for this session — completed delegations are archived automatically')
+        return
+      }
+      ctx.openPager('Spawn trees', formatSpawnTreeList(entries))
+      return
+    }
+    if (/^\d+$/.test(raw)) {
+      const n = Number.parseInt(raw, 10)
+      const entries = await listSpawnTrees(ctx)
+      const entry = entries[n - 1]
+      if (!entry) {
+        ctx.pushSystem(
+          entries.length
+            ? `replay: index out of range 1..${entries.length} — /replay to list`
+            : 'no archived spawn trees for this session'
+        )
+        return
+      }
+      const tree = await ctx.request('spawn_tree.load', { path: entry.path })
+      ctx.openPager(`Replay ${n}`, formatSpawnTree(tree))
+      return
+    }
+    const path = lower.startsWith('load ') ? raw.slice(5).trim() : raw
+    const tree = await ctx.request('spawn_tree.load', { path })
+    ctx.openPager('Replay', formatSpawnTree(tree))
+  } catch (error) {
+    ctx.pushSystem(`/replay: ${error instanceof Error ? error.message : 'failed'}`)
+  }
+}
+
+/** `/heapdump` — write a V8 heap snapshot to `$HERMES_HOME|~/.hermes/logs/` and
+ *  report the path + heap/rss before vs after (Ink ref debug.ts /heapdump). */
+const heapdumpCmd: ClientHandler = (_arg, ctx) => {
+  const pre = process.memoryUsage()
+  ctx.pushSystem(`writing heap dump (heap ${formatBytes(pre.heapUsed)} · rss ${formatBytes(pre.rss)})…`)
+  try {
+    const { after, before, path } = performHeapdump()
+    ctx.pushSystem(
+      `heapdump: ${path}\n` +
+        `heap ${formatBytes(before.heapUsed)} → ${formatBytes(after.heapUsed)} · ` +
+        `rss ${formatBytes(before.rss)} → ${formatBytes(after.rss)}`
+    )
+  } catch (error) {
+    ctx.pushSystem(`heapdump failed: ${error instanceof Error ? error.message : String(error)}`)
+  }
+}
+
+/** `/mem` — live V8 heap/rss numbers + uptime + the mounted-renderable count
+ *  (the store-cap diagnostic) as one system block (Ink ref debug.ts /mem). */
+const memCmd: ClientHandler = (_arg, ctx) => {
+  ctx.pushSystem(memReport(process.memoryUsage(), process.uptime(), ctx.renderableCount()))
+}
+
 /** `/tools` — fetch the tool roster from the gateway and show it in the pager (navigable). */
 const toolsCmd: ClientHandler = async (arg, ctx) => {
   const command = arg.trim() ? `tools ${arg.trim()}` : 'tools'
@@ -352,12 +500,18 @@ const toolsCmd: ClientHandler = async (arg, ctx) => {
 const CLIENT: Record<string, ClientHandler> = {
   agents: (_arg, ctx) => ctx.openDashboard(),
   clear: (_arg, ctx) => ctx.confirm('Clear the transcript?', ctx.clearTranscript),
+  compact: compactCmd,
   copy: (arg, ctx) => {
     const n = Math.max(1, Number.parseInt(arg, 10) || 1)
     if (!ctx.copyResponse(n)) ctx.pushSystem('Nothing to copy yet.')
   },
+  detail: detailsCmd,
+  details: detailsCmd,
   exit: (_arg, ctx) => ctx.quit(),
+  heapdump: heapdumpCmd,
+  mem: memCmd,
   model: modelCmd,
+  replay: replayCmd,
   resume: openSwitcher,
   session: openSwitcher,
   sessions: openSwitcher,
@@ -379,6 +533,11 @@ const CLIENT: Record<string, ClientHandler> = {
   quit: (_arg, ctx) => ctx.quit()
 }
 
+/** The registered client-command names (catalog introspection — tests/menus). */
+export function clientCommandNames(): string[] {
+  return Object.keys(CLIENT).sort()
+}
+
 /** Render the gateway `commands.catalog` into a help block (loose-typed read).
  *  The TUI catalog shape is `{ pairs: [["/name","desc"], …], canon, categories }`
  *  (tui_gateway/server.py `commands.catalog`). */
diff --git a/ui-opentui/src/logic/store.ts b/ui-opentui/src/logic/store.ts
index f76a9214b1b..5d031abd696 100644
--- a/ui-opentui/src/logic/store.ts
+++ b/ui-opentui/src/logic/store.ts
@@ -21,6 +21,7 @@ import {
   type CatalogDecoded,
   type SessionInfoPatchDecoded
 } from '../boundary/schema/SessionInfo.ts'
+import type { DetailsMode } from './details.ts'
 import { diffStats, type DiffStats } from './diff.ts'
 import { envOutputLinesSet } from './env.ts'
 import { stripAnsi, stripOmittedNote, stripToolEnvelope } from './toolOutput.ts'
@@ -242,6 +243,14 @@ export interface StoreState {
   modelItems: PickerItem[] | undefined
   /** The current session id (shown in the home panel; updated on create/resume). */
   sessionId: string | undefined
+  // ── display flags (utility commands — Epic 3) ────────────────────────────
+  /** Compact transcript (/compact): collapses the blank line between turns/parts.
+   *  Defaults OFF — the persisted `display.tui_compact` config doesn't reach the
+   *  TUI via session.info, so the flag starts false each launch. */
+  compact: boolean
+  /** Global tool/reasoning detail mode (/details): collapsed (default) /
+   *  expanded (bodies default-open) / hidden (runs fold to one muted line). */
+  details: DetailsMode
 }
 
 const LRU_LIMIT = 1000
@@ -399,7 +408,9 @@ export function createSessionStore() {
     hint: undefined,
     catalog: undefined,
     modelItems: undefined,
-    sessionId: undefined
+    sessionId: undefined,
+    compact: false,
+    details: 'collapsed'
   })
 
   // Monotonic part id (stable `key` per part so a new tool part below a streaming
@@ -586,6 +597,16 @@ export function createSessionStore() {
     setState('hint', text)
   }
 
+  /** /compact — set the compact-transcript display flag (Epic 3). */
+  function setCompact(on: boolean): void {
+    setState('compact', on)
+  }
+
+  /** /details — set the global tool/reasoning detail mode (Epic 3). */
+  function setDetails(mode: DetailsMode): void {
+    setState('details', mode)
+  }
+
   /** Merge a session-info patch into the chrome state (status bar — item 14). */
   function applyInfo(raw: { readonly [k: string]: unknown }): void {
     const patch = readInfoPatch(raw)
@@ -974,6 +995,8 @@ export function createSessionStore() {
     clearCompletions,
     applyInfo,
     setHint,
+    setCompact,
+    setDetails,
     openDashboard,
     closeDashboard,
     hydrate,
diff --git a/ui-opentui/src/test/displayModes.test.tsx b/ui-opentui/src/test/displayModes.test.tsx
new file mode 100644
index 00000000000..f4fbf051092
--- /dev/null
+++ b/ui-opentui/src/test/displayModes.test.tsx
@@ -0,0 +1,137 @@
+/**
+ * Display-mode frame tests (Epic 3: /compact + /details store flags → render).
+ * Headless frames through the real App tree (store → Transcript →
+ * DisplayProvider → messageLine/toolPart/reasoningPart):
+ *   - details collapsed (default) vs expanded vs hidden on tool + reasoning rows,
+ *     including that flipping hidden back RESTORES the rows (nothing dropped),
+ *   - compact collapses the blank line between messages (frame line-distance).
+ */
+import { describe, expect, test } from 'vitest'
+
+import { createSessionStore } from '../logic/store.ts'
+import { App } from '../view/App.tsx'
+import { ThemeProvider } from '../view/theme.tsx'
+import { renderProbe, type RenderProbe } from './lib/render.ts'
+
+type Store = ReturnType<typeof createSessionStore>
+
+async function mountApp(store: Store, width = 80, height = 30): Promise<RenderProbe> {
+  return renderProbe(
+    () => (
+      <ThemeProvider theme={() => store.state.theme}>
+        <App store={store} />
+      </ThemeProvider>
+    ),
+    { height, width }
+  )
+}
+
+/** Seed one settled assistant turn: reasoning + a multi-line tool + answer text. */
+function seedDetailedTurn(store: Store) {
+  store.apply({ type: 'gateway.ready' })
+  store.apply({ type: 'message.start' })
+  store.apply({ payload: { text: '**Plan**\n\nthink about the steps' }, type: 'reasoning.delta' })
+  store.apply({ payload: { context: 'ls -la', name: 'terminal', tool_id: 't1' }, type: 'tool.start' })
+  store.apply({
+    payload: {
+      args: { command: 'ls -la' },
+      duration_s: 0.3,
+      name: 'terminal',
+      result_text: 'alpha.txt\nbeta.txt\ngamma.txt',
+      tool_id: 't1'
+    },
+    type: 'tool.complete'
+  })
+  store.apply({ payload: { text: 'done listing' }, type: 'message.delta' })
+  store.apply({ type: 'message.complete' })
+}
+
+describe('/details — global detail mode drives default expansion (frame)', () => {
+  test('collapsed (default) → headers only; expanded → tool body + reasoning preview open', async () => {
+    const store = createSessionStore()
+    seedDetailedTurn(store)
+    const probe = await mountApp(store)
+    try {
+      // default: collapsed — tool body lines stay hidden, Thought folded.
+      // (Markdown BODY text never paints in the headless char frame — a known
+      // harness limitation, see render.test.tsx — so assertions stick to the
+      // plain-text renderables: tool output lines + the ▶/▼ headers.)
+      const collapsed = await probe.waitForFrame(f => f.includes('terminal'))
+      expect(collapsed).toContain('▶ Thought: Plan')
+      expect(collapsed).not.toContain('beta.txt')
+
+      // /details expanded → tool body + reasoning preview default-open (no clicks)
+      store.setDetails('expanded')
+      const expanded = await probe.waitForFrame(f => f.includes('beta.txt'))
+      expect(expanded).toContain('alpha.txt')
+      expect(expanded).toContain('▼ Thought: Plan')
+
+      // back to collapsed → bodies fold again
+      store.setDetails('collapsed')
+      const back = await probe.waitForFrame(f => !f.includes('beta.txt'))
+      expect(back).toContain('terminal')
+      expect(back).toContain('▶ Thought: Plan')
+    } finally {
+      probe.destroy()
+    }
+  })
+
+  test('hidden → one muted run line replaces the tool+reasoning rows; flipping back restores', async () => {
+    const store = createSessionStore()
+    seedDetailedTurn(store)
+    const probe = await mountApp(store)
+    try {
+      await probe.waitForFrame(f => f.includes('terminal'))
+      store.setDetails('hidden')
+      // reasoning + tool fold into ONE honest run line
+      const hidden = await probe.waitForFrame(f => f.includes('hidden'))
+      expect(hidden).toContain('⚡ 1 tool · 1 thought hidden — /details collapsed to show')
+      expect(hidden).not.toContain('terminal')
+      expect(hidden).not.toContain('Thought: Plan')
+      // the parts are still in the store (folding is render-only — recoverable)
+      expect((store.state.messages.at(-1)!.parts ?? []).map(p => p.type)).toEqual(['reasoning', 'tool', 'text'])
+
+      // restore — flipping the mode back brings the rows straight back
+      store.setDetails('collapsed')
+      const restored = await probe.waitForFrame(f => f.includes('terminal'))
+      expect(restored).toContain('▶ Thought: Plan')
+      expect(restored).not.toContain('hidden — /details')
+    } finally {
+      probe.destroy()
+    }
+  })
+})
+
+describe('/compact — transcript spacing (frame line-count)', () => {
+  test('compact on collapses the blank line between messages; off restores it', async () => {
+    const store = createSessionStore()
+    store.apply({ type: 'gateway.ready' })
+    store.pushUser('alpha-line')
+    store.pushUser('beta-line')
+    const probe = await mountApp(store)
+    try {
+      const spaced = await probe.waitForFrame(f => f.includes('beta-line'))
+      const rows = spaced.split('\n')
+      const a = rows.findIndex(r => r.includes('alpha-line'))
+      const b = rows.findIndex(r => r.includes('beta-line'))
+      expect(a).toBeGreaterThanOrEqual(0)
+      expect(b - a).toBe(2) // one blank line between turns
+
+      store.setCompact(true)
+      await probe.settle()
+      const dense = probe.frame().split('\n')
+      const a2 = dense.findIndex(r => r.includes('alpha-line'))
+      const b2 = dense.findIndex(r => r.includes('beta-line'))
+      expect(b2 - a2).toBe(1) // adjacent rows — densified
+
+      store.setCompact(false)
+      await probe.settle()
+      const again = probe.frame().split('\n')
+      const a3 = again.findIndex(r => r.includes('alpha-line'))
+      const b3 = again.findIndex(r => r.includes('beta-line'))
+      expect(b3 - a3).toBe(2)
+    } finally {
+      probe.destroy()
+    }
+  })
+})
diff --git a/ui-opentui/src/test/slash.test.ts b/ui-opentui/src/test/slash.test.ts
index 8212a800508..ad9d1c0ab7f 100644
--- a/ui-opentui/src/test/slash.test.ts
+++ b/ui-opentui/src/test/slash.test.ts
@@ -4,6 +4,7 @@
  */
 import { afterEach, describe, expect, test } from 'vitest'
 
+import type { DetailsMode } from '../logic/details.ts'
 import {
   dispatchSlash,
   mapCompletions,
@@ -137,6 +138,9 @@ interface Probe {
   copyN: { value: (n: number) => boolean }
   /** The cached /model rows (Epic 7) — seed to simulate a prefetched catalog. */
   modelCache: { value: PickerItem[] | undefined }
+  /** Display flags (/compact, /details — Epic 3). */
+  compactFlag: { value: boolean }
+  detailsFlag: { value: DetailsMode }
 }
 
 function makeCtx(request: (method: string, params: Record<string, unknown>) => Promise<unknown>): Probe {
@@ -153,8 +157,15 @@ function makeCtx(request: (method: string, params: Record<string, unknown>) => P
   const copied: number[] = []
   const copyN: Probe['copyN'] = { value: () => false }
   const modelCache: Probe['modelCache'] = { value: undefined }
+  const compactFlag: Probe['compactFlag'] = { value: false }
+  const detailsFlag: Probe['detailsFlag'] = { value: 'collapsed' }
   const ctx: SlashContext = {
     clearTranscript: () => (cleared.value = true),
+    compact: () => compactFlag.value,
+    setCompact: on => (compactFlag.value = on),
+    details: () => detailsFlag.value,
+    setDetails: mode => (detailsFlag.value = mode),
+    renderableCount: () => undefined,
     confirm: (message, onConfirm) => confirmed.push({ message, onConfirm }),
     copyResponse: n => {
       copied.push(n)
@@ -180,11 +191,13 @@ function makeCtx(request: (method: string, params: Record<string, unknown>) => P
   return {
     calls,
     cleared,
+    compactFlag,
     confirmed,
     copied,
     copyN,
     ctx,
     dashboard,
+    detailsFlag,
     modelCache,
     paged,
     pickers,
diff --git a/ui-opentui/src/test/utilityCommands.test.ts b/ui-opentui/src/test/utilityCommands.test.ts
new file mode 100644
index 00000000000..ce0d3fab370
--- /dev/null
+++ b/ui-opentui/src/test/utilityCommands.test.ts
@@ -0,0 +1,427 @@
+/**
+ * Utility slash commands (Epic 3 port: /compact /details /replay /heapdump /mem).
+ * Pure logic + dispatch tests against a fake SlashContext: catalog registration,
+ * arg parsing (incl. garbage → usage lines), the store display-flag effects,
+ * replay RPC call shapes against a fake gateway, and the mem/heapdump system
+ * lines with node:v8 / process.memoryUsage mocked.
+ */
+import { mkdtempSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
+
+import {
+  collapseHiddenParts,
+  hiddenRunLabel,
+  nextDetailsMode,
+  parseDetailsMode,
+  type DetailsMode
+} from '../logic/details.ts'
+import { formatBytes, heapSnapshotPath, memReport } from '../logic/diagnostics.ts'
+import { formatSpawnTree, formatSpawnTreeList, readSpawnTreeEntries } from '../logic/replay.ts'
+import { clientCommandNames, dispatchSlash, type SlashContext } from '../logic/slash.ts'
+import type { Part } from '../logic/store.ts'
+
+// /heapdump must not write a REAL multi-MB snapshot per test run — stub the V8
+// seam; the path/mkdir plumbing still runs for real (under a temp HERMES_HOME).
+vi.mock('node:v8', () => ({ writeHeapSnapshot: vi.fn((path?: string) => path ?? 'unnamed.heapsnapshot') }))
+
+interface Probe {
+  ctx: SlashContext
+  calls: Array<{ method: string; params: Record<string, unknown> }>
+  system: string[]
+  paged: Array<{ title: string; text: string }>
+  compactFlag: { value: boolean }
+  detailsFlag: { value: DetailsMode }
+  renderables: { value: number | undefined }
+}
+
+function makeCtx(request: (method: string, params: Record<string, unknown>) => Promise<unknown>): Probe {
+  const calls: Probe['calls'] = []
+  const system: string[] = []
+  const paged: Probe['paged'] = []
+  const compactFlag = { value: false }
+  const detailsFlag: Probe['detailsFlag'] = { value: 'collapsed' }
+  const renderables: Probe['renderables'] = { value: undefined }
+  const ctx: SlashContext = {
+    clearTranscript: () => {},
+    compact: () => compactFlag.value,
+    setCompact: on => (compactFlag.value = on),
+    details: () => detailsFlag.value,
+    setDetails: mode => (detailsFlag.value = mode),
+    renderableCount: () => renderables.value,
+    confirm: () => {},
+    copyResponse: () => false,
+    listSessions: () => Promise.resolve([]),
+    logTail: () => [],
+    modelItems: () => undefined,
+    setModelItems: () => {},
+    openDashboard: () => {},
+    openPager: (title, text) => paged.push({ text, title }),
+    openPicker: () => {},
+    openSwitcher: () => {},
+    pushSystem: text => system.push(text),
+    quit: () => {},
+    request: (method, params) => {
+      calls.push({ method, params })
+      return request(method, params)
+    },
+    sessionId: () => 'sid-1',
+    submit: () => {}
+  }
+  return { calls, compactFlag, ctx, detailsFlag, paged, renderables, system }
+}
+
+/** Let the fire-and-forget config.set promise settle (it's detached). */
+const tick = () => new Promise(r => setTimeout(r, 0))
+
+describe('client command catalog (registration)', () => {
+  test('all five utility commands (and the /detail alias) are registered', () => {
+    const names = clientCommandNames()
+    for (const name of ['compact', 'details', 'detail', 'replay', 'heapdump', 'mem']) {
+      expect(names).toContain(name)
+    }
+  })
+})
+
+describe('/compact', () => {
+  test('bare /compact toggles on, persists via config.set, reports', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/compact', p.ctx)
+    expect(p.compactFlag.value).toBe(true)
+    expect(p.system).toEqual(['compact on'])
+    expect(p.calls).toEqual([{ method: 'config.set', params: { key: 'compact', value: 'on' } }])
+  })
+
+  test('/compact on|off|toggle set explicitly', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/compact on', p.ctx)
+    expect(p.compactFlag.value).toBe(true)
+    await dispatchSlash('/compact off', p.ctx)
+    expect(p.compactFlag.value).toBe(false)
+    expect(p.calls.at(-1)).toEqual({ method: 'config.set', params: { key: 'compact', value: 'off' } })
+    await dispatchSlash('/compact toggle', p.ctx)
+    expect(p.compactFlag.value).toBe(true)
+    expect(p.system).toEqual(['compact on', 'compact off', 'compact on'])
+  })
+
+  test('/compact garbage → usage line, no flag change, no RPC', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/compact sideways', p.ctx)
+    expect(p.system).toEqual(['usage: /compact [on|off|toggle]'])
+    expect(p.compactFlag.value).toBe(false)
+    expect(p.calls).toHaveLength(0)
+  })
+
+  test('a failing config.set never breaks the local toggle', async () => {
+    const p = makeCtx(async () => {
+      throw new Error('gateway down')
+    })
+    await dispatchSlash('/compact on', p.ctx)
+    await tick()
+    expect(p.compactFlag.value).toBe(true)
+    expect(p.system).toEqual(['compact on'])
+  })
+})
+
+describe('/details', () => {
+  test('sets each explicit mode + persists via config.set details_mode', async () => {
+    const p = makeCtx(async () => ({}))
+    for (const mode of ['expanded', 'hidden', 'collapsed'] as const) {
+      await dispatchSlash(`/details ${mode}`, p.ctx)
+      expect(p.detailsFlag.value).toBe(mode)
+      expect(p.calls.at(-1)).toEqual({ method: 'config.set', params: { key: 'details_mode', value: mode } })
+    }
+    expect(p.system).toEqual(['details: expanded', 'details: hidden', 'details: collapsed'])
+  })
+
+  test('/details cycle advances hidden → collapsed → expanded → hidden', async () => {
+    const p = makeCtx(async () => ({}))
+    expect(p.detailsFlag.value).toBe('collapsed')
+    await dispatchSlash('/details cycle', p.ctx)
+    expect(p.detailsFlag.value).toBe('expanded')
+    await dispatchSlash('/details cycle', p.ctx)
+    expect(p.detailsFlag.value).toBe('hidden')
+    await dispatchSlash('/details cycle', p.ctx)
+    expect(p.detailsFlag.value).toBe('collapsed')
+  })
+
+  test('/details garbage → usage line, nothing set', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/details loud', p.ctx)
+    expect(p.system).toEqual(['usage: /details [hidden|collapsed|expanded|cycle]'])
+    expect(p.detailsFlag.value).toBe('collapsed')
+    expect(p.calls).toHaveLength(0)
+  })
+
+  test('a gateway-suggested SECTION arg gets the honest deferred notice', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/details thinking expanded', p.ctx)
+    expect(p.system[0]).toContain('per-section detail overrides are not supported')
+    expect(p.detailsFlag.value).toBe('collapsed')
+    expect(p.calls).toHaveLength(0)
+  })
+
+  test('bare /details reads config.get and syncs the local flag', async () => {
+    const p = makeCtx(async method => (method === 'config.get' ? { value: 'expanded' } : {}))
+    await dispatchSlash('/details', p.ctx)
+    expect(p.calls[0]).toEqual({ method: 'config.get', params: { key: 'details_mode' } })
+    expect(p.detailsFlag.value).toBe('expanded')
+    expect(p.system).toEqual(['details: expanded'])
+  })
+
+  test('bare /details with config.get failing falls back to the live flag', async () => {
+    const p = makeCtx(async () => {
+      throw new Error('no config.get')
+    })
+    p.detailsFlag.value = 'hidden'
+    await dispatchSlash('/details', p.ctx)
+    expect(p.system).toEqual(['details: hidden'])
+  })
+
+  test('/detail (Ink alias) dispatches the same handler', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/detail expanded', p.ctx)
+    expect(p.detailsFlag.value).toBe('expanded')
+  })
+})
+
+const TREE_ENTRIES = {
+  entries: [
+    { count: 3, finished_at: 1_760_000_000, label: 'fanout A', path: '/trees/sid-1/a.json', session_id: 'sid-1' },
+    { count: 1, finished_at: 1_760_000_100, label: '', path: '/trees/sid-1/b.json', session_id: 'sid-1' }
+  ]
+}
+
+const TREE_PAYLOAD = {
+  finished_at: 1_760_000_000,
+  label: 'fanout A',
+  session_id: 'sid-1',
+  subagents: [
+    {
+      depth: 0,
+      durationSeconds: 12.4,
+      goal: 'crunch the data',
+      inputTokens: 1200,
+      model: 'hermes-4-405b',
+      outputTokens: 300,
+      status: 'completed',
+      summary: 'crunched it',
+      toolCount: 3
+    },
+    { depth: 1, goal: 'child probe', status: 'failed' }
+  ]
+}
+
+describe('/replay (spawn-tree inspector)', () => {
+  test('bare /replay lists via spawn_tree.list and pages indexed rows', async () => {
+    const p = makeCtx(async method => (method === 'spawn_tree.list' ? TREE_ENTRIES : {}))
+    await dispatchSlash('/replay', p.ctx)
+    expect(p.calls).toEqual([{ method: 'spawn_tree.list', params: { limit: 30, session_id: 'sid-1' } }])
+    expect(p.paged).toHaveLength(1)
+    expect(p.paged[0]!.title).toBe('Spawn trees')
+    expect(p.paged[0]!.text).toContain('1. ')
+    expect(p.paged[0]!.text).toContain('fanout A')
+    expect(p.paged[0]!.text).toContain('/trees/sid-1/a.json')
+    // label-less rows fall back to the subagent count
+    expect(p.paged[0]!.text).toContain('1 subagent')
+  })
+
+  test('/replay <n> lists then loads the n-th entry by path and pages the tree', async () => {
+    const p = makeCtx(async method =>
+      method === 'spawn_tree.list' ? TREE_ENTRIES : method === 'spawn_tree.load' ? TREE_PAYLOAD : {}
+    )
+    await dispatchSlash('/replay 1', p.ctx)
+    expect(p.calls.map(c => c.method)).toEqual(['spawn_tree.list', 'spawn_tree.load'])
+    expect(p.calls[1]!.params).toEqual({ path: '/trees/sid-1/a.json' })
+    expect(p.paged[0]!.title).toBe('Replay 1')
+    expect(p.paged[0]!.text).toContain('✓ [1] crunch the data')
+    expect(p.paged[0]!.text).toContain('completed · hermes-4-405b · 12s · 3 tools · 1200 in / 300 out tok')
+    expect(p.paged[0]!.text).toContain('crunched it')
+    // depth-1 child is indented under its parent and flags the failure
+    expect(p.paged[0]!.text).toContain('  ✗ [2] child probe')
+  })
+
+  test('/replay with an out-of-range index reports the valid range', async () => {
+    const p = makeCtx(async method => (method === 'spawn_tree.list' ? TREE_ENTRIES : {}))
+    await dispatchSlash('/replay 99', p.ctx)
+    expect(p.system[0]).toContain('index out of range 1..2')
+    expect(p.calls.map(c => c.method)).toEqual(['spawn_tree.list'])
+  })
+
+  test('/replay <path> loads straight from disk (no list RPC)', async () => {
+    const p = makeCtx(async method => (method === 'spawn_tree.load' ? TREE_PAYLOAD : {}))
+    await dispatchSlash('/replay /trees/sid-1/a.json', p.ctx)
+    expect(p.calls).toEqual([{ method: 'spawn_tree.load', params: { path: '/trees/sid-1/a.json' } }])
+    expect(p.paged[0]!.title).toBe('Replay')
+    expect(p.paged[0]!.text).toContain('fanout A')
+  })
+
+  test('empty archive and RPC failures land as system notices', async () => {
+    const p = makeCtx(async () => ({ entries: [] }))
+    await dispatchSlash('/replay', p.ctx)
+    expect(p.system[0]).toContain('no archived spawn trees')
+
+    const p2 = makeCtx(async () => {
+      throw new Error('boom')
+    })
+    await dispatchSlash('/replay', p2.ctx)
+    expect(p2.system).toEqual(['/replay: boom'])
+  })
+})
+
+describe('/mem', () => {
+  afterEach(() => vi.restoreAllMocks())
+
+  test('prints heap/external/rss/uptime + the renderable count, no gateway RPC', async () => {
+    vi.spyOn(process, 'memoryUsage').mockReturnValue({
+      arrayBuffers: 1024,
+      external: 2 * 1024 * 1024,
+      heapTotal: 200 * 1024 * 1024,
+      heapUsed: 123_456_789,
+      rss: 456 * 1024 * 1024
+    } as NodeJS.MemoryUsage)
+    vi.spyOn(process, 'uptime').mockReturnValue(42.4)
+    const p = makeCtx(async () => ({}))
+    p.renderables.value = 321
+    await dispatchSlash('/mem', p.ctx)
+    expect(p.calls).toHaveLength(0)
+    const out = p.system[0]!
+    expect(out).toContain('heap used')
+    expect(out).toContain('117.7 MB')
+    expect(out).toContain('heap total')
+    expect(out).toContain('200.0 MB')
+    expect(out).toContain('external')
+    expect(out).toContain('array buffers')
+    expect(out).toContain('rss')
+    expect(out).toContain('456.0 MB')
+    expect(out).toContain('uptime')
+    expect(out).toContain('42s')
+    expect(out).toContain('renderables')
+    expect(out).toContain('321')
+  })
+
+  test('omits the renderables row when no renderer is reachable', async () => {
+    vi.spyOn(process, 'uptime').mockReturnValue(5)
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/mem', p.ctx)
+    expect(p.system[0]).not.toContain('renderables')
+  })
+})
+
+describe('/heapdump', () => {
+  let home: string
+  beforeEach(() => {
+    home = mkdtempSync(join(tmpdir(), 'hermes-heap-'))
+    process.env.HERMES_HOME = home
+  })
+  afterEach(() => {
+    delete process.env.HERMES_HOME
+    rmSync(home, { force: true, recursive: true })
+    vi.restoreAllMocks()
+  })
+
+  test('writes the snapshot under $HERMES_HOME/logs and reports before/after', async () => {
+    const v8 = await import('node:v8')
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/heapdump', p.ctx)
+    expect(p.calls).toHaveLength(0)
+    expect(p.system).toHaveLength(2)
+    expect(p.system[0]).toContain('writing heap dump (heap ')
+    expect(p.system[1]).toContain(`heapdump: ${join(home, 'logs')}`)
+    expect(p.system[1]).toContain('.heapsnapshot')
+    expect(p.system[1]).toMatch(/heap .+ → .+ · rss .+ → .+/)
+    expect(vi.mocked(v8.writeHeapSnapshot)).toHaveBeenCalledTimes(1)
+    const arg = vi.mocked(v8.writeHeapSnapshot).mock.calls[0]![0] as string
+    expect(arg.startsWith(join(home, 'logs', 'opentui-heap-'))).toBe(true)
+  })
+
+  test('a write failure lands as a system error, not a crash', async () => {
+    const v8 = await import('node:v8')
+    vi.mocked(v8.writeHeapSnapshot).mockImplementationOnce(() => {
+      throw new Error('disk full')
+    })
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/heapdump', p.ctx)
+    expect(p.system[1]).toBe('heapdump failed: disk full')
+  })
+})
+
+describe('details logic (pure)', () => {
+  test('parseDetailsMode + nextDetailsMode', () => {
+    expect(parseDetailsMode(' Expanded ')).toBe('expanded')
+    expect(parseDetailsMode('nope')).toBeNull()
+    expect(nextDetailsMode('hidden')).toBe('collapsed')
+    expect(nextDetailsMode('collapsed')).toBe('expanded')
+    expect(nextDetailsMode('expanded')).toBe('hidden')
+  })
+
+  test('collapseHiddenParts folds consecutive tool/reasoning runs; text passes through by reference', () => {
+    const parts: Part[] = [
+      { id: 'p1', text: 'intro', type: 'text' },
+      { id: 'p2', name: 'bash', state: 'complete', type: 'tool' },
+      { id: 'p3', text: 'mull', type: 'reasoning' },
+      { id: 'p4', name: 'read', state: 'complete', type: 'tool' },
+      { id: 'p5', text: 'middle', type: 'text' },
+      { id: 'p6', name: 'grep', state: 'running', type: 'tool' }
+    ]
+    const out = collapseHiddenParts(parts)
+    expect(out.map(p => p.type)).toEqual(['text', 'hiddenRun', 'text', 'hiddenRun'])
+    expect(out[0]).toBe(parts[0]) // identity preserved → no remount of text parts
+    expect(out[1]).toMatchObject({ id: 'hidden-p2', thoughts: 1, tools: 2 })
+    expect(out[3]).toMatchObject({ thoughts: 0, tools: 1 })
+  })
+
+  test('hiddenRunLabel pluralizes honestly and points back to /details', () => {
+    expect(hiddenRunLabel({ id: 'h', thoughts: 0, tools: 3, type: 'hiddenRun' })).toBe(
+      '3 tools hidden — /details collapsed to show'
+    )
+    expect(hiddenRunLabel({ id: 'h', thoughts: 1, tools: 1, type: 'hiddenRun' })).toBe(
+      '1 tool · 1 thought hidden — /details collapsed to show'
+    )
+  })
+})
+
+describe('diagnostics + replay formatters (pure)', () => {
+  test('formatBytes', () => {
+    expect(formatBytes(512)).toBe('512 B')
+    expect(formatBytes(2048)).toBe('2.0 KB')
+    expect(formatBytes(123_456_789)).toBe('117.7 MB')
+    expect(formatBytes(-1)).toBe('0 B')
+  })
+
+  test('heapSnapshotPath prefers HERMES_HOME', () => {
+    process.env.HERMES_HOME = '/custom/home'
+    try {
+      const p = heapSnapshotPath(new Date('2026-06-10T12:00:00Z'))
+      expect(p).toBe('/custom/home/logs/opentui-heap-2026-06-10T12-00-00-000Z.heapsnapshot')
+    } finally {
+      delete process.env.HERMES_HOME
+    }
+  })
+
+  test('memReport without a renderable count omits the row', () => {
+    const text = memReport({ arrayBuffers: 0, external: 0, heapTotal: 1024, heapUsed: 512, rss: 2048 }, 9.6)
+    expect(text.split('\n')[0]).toBe('memory')
+    expect(text).toContain('uptime')
+    expect(text).toContain('10s')
+    expect(text).not.toContain('renderables')
+  })
+
+  test('readSpawnTreeEntries tolerates garbage', () => {
+    expect(readSpawnTreeEntries(null)).toEqual([])
+    expect(readSpawnTreeEntries({ entries: 'nope' })).toEqual([])
+    expect(readSpawnTreeEntries({ entries: [{ label: 'no path' }, 7, null] })).toEqual([])
+    expect(readSpawnTreeEntries({ entries: [{ count: 2, path: '/a.json' }] })).toEqual([
+      { count: 2, label: '', path: '/a.json' }
+    ])
+  })
+
+  test('formatSpawnTreeList indexes rows; formatSpawnTree handles an empty snapshot', () => {
+    const list = formatSpawnTreeList([{ count: 2, label: 'x', path: '/a.json' }])
+    expect(list).toContain('  1. ')
+    expect(list).toContain('/a.json')
+    expect(formatSpawnTree({ subagents: [] })).toContain('(snapshot empty or unreadable)')
+    expect(formatSpawnTree('garbage')).toContain('(snapshot empty or unreadable)')
+  })
+})
diff --git a/ui-opentui/src/view/display.tsx b/ui-opentui/src/view/display.tsx
new file mode 100644
index 00000000000..3933d7942f5
--- /dev/null
+++ b/ui-opentui/src/view/display.tsx
@@ -0,0 +1,31 @@
+/**
+ * DisplayProvider — exposes the transcript display flags (store `compact` +
+ * `details`, set by the /compact and /details utility commands) to deep view
+ * nodes without threading the store through every layer (same pattern as
+ * sessionInfo.tsx). Consumers: messageLine (compact spacing + hidden-mode part
+ * folding), toolPart + reasoningPart (expanded-mode default-open). The fallback
+ * accessor (no provider, e.g. a bare component test) is today's defaults:
+ * compact off, details collapsed.
+ */
+import { type Accessor, createContext, type JSX, useContext } from 'solid-js'
+
+import type { DetailsMode } from '../logic/details.ts'
+
+export interface DisplayFlags {
+  compact: boolean
+  details: DetailsMode
+}
+
+const DEFAULTS: DisplayFlags = { compact: false, details: 'collapsed' }
+const DEFAULT_FLAGS: Accessor<DisplayFlags> = () => DEFAULTS
+
+const Ctx = createContext<Accessor<DisplayFlags>>()
+
+export function DisplayProvider(props: { flags: Accessor<DisplayFlags>; children: JSX.Element }) {
+  return <Ctx.Provider value={props.flags}>{props.children}</Ctx.Provider>
+}
+
+/** The live display flags accessor (compact off + collapsed when no provider). */
+export function useDisplay(): Accessor<DisplayFlags> {
+  return useContext(Ctx) ?? DEFAULT_FLAGS
+}
diff --git a/ui-opentui/src/view/messageLine.tsx b/ui-opentui/src/view/messageLine.tsx
index 612f2b92706..1034b63184a 100644
--- a/ui-opentui/src/view/messageLine.tsx
+++ b/ui-opentui/src/view/messageLine.tsx
@@ -10,7 +10,9 @@
  */
 import { For, Match, Show, Switch } from 'solid-js'
 
+import { collapseHiddenParts, hiddenRunLabel } from '../logic/details.ts'
 import type { Message } from '../logic/store.ts'
+import { useDisplay } from './display.tsx'
 import { Markdown } from './markdown.tsx'
 import { ReasoningPart } from './reasoningPart.tsx'
 import { useTheme } from './theme.tsx'
@@ -20,6 +22,7 @@ const GUTTER = 2
 
 export function MessageLine(props: { message: Message }) {
   const theme = useTheme()
+  const display = useDisplay()
   const m = () => props.message
   const glyph = () => (m().role === 'assistant' ? theme().brand.icon : m().role === 'user' ? theme().brand.prompt : '·')
   // Role-distinct color IS the hierarchy (Ink model): the human's turn is tinted
@@ -29,11 +32,15 @@ export function MessageLine(props: { message: Message }) {
   const bodyFg = () =>
     m().role === 'user' ? theme().color.label : m().role === 'system' ? theme().color.muted : theme().color.text
   const hasParts = () => (m().parts?.length ?? 0) > 0
+  // /details hidden: fold each run of tool/reasoning parts into ONE muted line
+  // (the parts stay in the store — flipping the mode back restores them).
+  const displayParts = () => (display().details === 'hidden' ? collapseHiddenParts(m().parts ?? []) : (m().parts ?? []))
 
   return (
     // One blank line above every turn so user / assistant / tool blocks read as
-    // distinct turns (item: spacing). The gold-vs-bright color split does the rest.
-    <box style={{ flexDirection: 'row', flexShrink: 0, marginTop: 1 }}>
+    // distinct turns (item: spacing); /compact collapses it so long sessions
+    // read denser. The gold-vs-bright color split does the rest.
+    <box style={{ flexDirection: 'row', flexShrink: 0, marginTop: display().compact ? 0 : 1 }}>
       <box style={{ flexShrink: 0, width: GUTTER }}>
         {/* the role glyph is decorative — exclude it from mouse selection (item 4).
             Bold so the user `❯` / assistant `⚕` turn boundaries pop (item 8). */}
@@ -45,8 +52,9 @@ export function MessageLine(props: { message: Message }) {
       </box>
       {/* gap owns ALL inter-part spacing (item 5) — uniform 1 line between text /
           reasoning / tool regardless of order or stream timing, so blank lines
-          don't pop in and out as parts are created/merged mid-stream. */}
-      <box style={{ flexDirection: 'column', flexGrow: 1, minWidth: 0, gap: 1 }}>
+          don't pop in and out as parts are created/merged mid-stream. /compact
+          drops the gap along with the per-turn margin above. */}
+      <box style={{ flexDirection: 'column', flexGrow: 1, minWidth: 0, gap: display().compact ? 0 : 1 }}>
         <Show
           when={m().role === 'assistant' && hasParts()}
           fallback={
@@ -71,13 +79,22 @@ export function MessageLine(props: { message: Message }) {
             </Show>
           }
         >
-          <For each={m().parts ?? []}>
+          <For each={displayParts()}>
             {part => (
               <Switch>
                 <Match when={part.type === 'tool' && part}>{tool => <ToolPart part={tool()} />}</Match>
                 <Match when={part.type === 'reasoning' && part}>
                   {r => <ReasoningPart text={r().text} streaming={m().streaming ?? false} />}
                 </Match>
+                <Match when={part.type === 'hiddenRun' && part}>
+                  {/* /details hidden — the honest minimal render for a folded
+                      tool/reasoning run; chrome, not copyable content. */}
+                  {run => (
+                    <text selectable={false}>
+                      <span style={{ fg: theme().color.muted }}>{`⚡ ${hiddenRunLabel(run())}`}</span>
+                    </text>
+                  )}
+                </Match>
                 <Match when={part.type === 'text' && part}>
                   {/* ONE stable native <markdown> fed the growing text in place (no
                       per-delta remount → no scrollbar flicker, #2); it renders GFM
diff --git a/ui-opentui/src/view/reasoningPart.tsx b/ui-opentui/src/view/reasoningPart.tsx
index 790e0d4b1d3..c4f91d3de45 100644
--- a/ui-opentui/src/view/reasoningPart.tsx
+++ b/ui-opentui/src/view/reasoningPart.tsx
@@ -14,6 +14,7 @@
 import { createMemo, createSignal, Show } from 'solid-js'
 
 import type { ThemeColors } from '../logic/theme.ts'
+import { useDisplay } from './display.tsx'
 import { Markdown } from './markdown.tsx'
 import { useScrollAnchor } from './scrollAnchor.tsx'
 import { useTheme } from './theme.tsx'
@@ -44,10 +45,12 @@ function reasoningSummary(text: string): { title?: string; body: string } {
 export function ReasoningPart(props: { text: string; streaming?: boolean }) {
   const theme = useTheme()
   const anchor = useScrollAnchor()
+  const display = useDisplay()
   const [override, setOverride] = createSignal<boolean | undefined>(undefined)
-  // live → expanded so you see it think; settled → collapsed. Click overrides.
-  const expanded = () => override() ?? !!props.streaming
-  const toggle = () => anchor(() => setOverride(e => !(e ?? !!props.streaming)))
+  // live → expanded so you see it think; settled → collapsed, unless the global
+  // /details mode is `expanded` (previews default-open). Click overrides.
+  const expanded = () => override() ?? (!!props.streaming || display().details === 'expanded')
+  const toggle = () => anchor(() => setOverride(!expanded()))
   const summary = createMemo(() => reasoningSummary(props.text))
   const label = () => (props.streaming ? 'Thinking' : 'Thought')
 
diff --git a/ui-opentui/src/view/toolPart.tsx b/ui-opentui/src/view/toolPart.tsx
index f830438c725..2e088c996cd 100644
--- a/ui-opentui/src/view/toolPart.tsx
+++ b/ui-opentui/src/view/toolPart.tsx
@@ -22,6 +22,7 @@
 import { type ToolPartState } from '../logic/store.ts'
 import type { ThemeColors } from '../logic/theme.ts'
 import { useDimensions } from './dimensions.tsx'
+import { useDisplay } from './display.tsx'
 import { createSignal, Show } from 'solid-js'
 
 import { truncate } from '../logic/toolOutput.ts'
@@ -90,8 +91,12 @@ export function ToolPart(props: { part: ToolPartState }) {
   const dims = useDimensions()
   const info = useSessionInfo() // session cwd for path-relativizing renderers
   const anchor = useScrollAnchor()
-  const [expanded, setExpanded] = createSignal(false)
-  const toggle = () => anchor(() => setExpanded(e => !e))
+  const display = useDisplay()
+  // /details expanded → settled bodies default-OPEN; a manual click overrides
+  // either way (and a later global flip applies again to un-overridden parts).
+  const [override, setOverride] = createSignal<boolean | undefined>(undefined)
+  const expanded = () => override() ?? display().details === 'expanded'
+  const toggle = () => anchor(() => setOverride(!expanded()))
 
   // Per-tool renderer (re-dispatches if the name settles on tool.complete).
   const renderer = () => rendererFor(props.part.name)
diff --git a/ui-opentui/src/view/transcript.tsx b/ui-opentui/src/view/transcript.tsx
index 6a48ed6bc1b..9e8c526368a 100644
--- a/ui-opentui/src/view/transcript.tsx
+++ b/ui-opentui/src/view/transcript.tsx
@@ -18,6 +18,7 @@ import type { ScrollBoxRenderable } from '@opentui/core'
 import { createSignal, For, Show } from 'solid-js'
 
 import type { SessionStore } from '../logic/store.ts'
+import { DisplayProvider } from './display.tsx'
 import { HomeHint } from './homeHint.tsx'
 import { MessageLine } from './messageLine.tsx'
 import { ScrollAnchorProvider } from './scrollAnchor.tsx'
@@ -32,20 +33,23 @@ export function Transcript(props: { store: SessionStore }) {
     <box style={{ flexGrow: 1, minHeight: 0 }}>
       <scrollbox ref={setScroll} style={{ flexGrow: 1, minHeight: 0 }} stickyScroll stickyStart="bottom">
         <ScrollAnchorProvider scroll={scroll}>
-          {/* empty-transcript home screen (item 12); replaced by messages on the first turn */}
-          <Show when={props.store.state.messages.length === 0}>
-            <HomeHint store={props.store} />
-          </Show>
-          {/* Honest truncation notice: the rolling cap hides the OLDEST rows from the
+          {/* display flags (/compact, /details — Epic 3) for the rows below */}
+          <DisplayProvider flags={() => ({ compact: props.store.state.compact, details: props.store.state.details })}>
+            {/* empty-transcript home screen (item 12); replaced by messages on the first turn */}
+            <Show when={props.store.state.messages.length === 0}>
+              <HomeHint store={props.store} />
+            </Show>
+            {/* Honest truncation notice: the rolling cap hides the OLDEST rows from the
               DISPLAY (never the model's context — that lives on the gateway). Point to
               the dashboard for the full transcript. selectable=false → it's chrome,
               excluded from copy/selection. */}
-          <Show when={dropped() > 0}>
-            <text selectable={false} style={{ fg: theme().color.muted }}>
-              {`⤒ ${dropped()} earlier message${dropped() === 1 ? '' : 's'} — scroll-back capped; full transcript on the dashboard${sid() ? ` · session ${sid()}` : ''}`}
-            </text>
-          </Show>
-          <For each={props.store.state.messages}>{message => <MessageLine message={message} />}</For>
+            <Show when={dropped() > 0}>
+              <text selectable={false} style={{ fg: theme().color.muted }}>
+                {`⤒ ${dropped()} earlier message${dropped() === 1 ? '' : 's'} — scroll-back capped; full transcript on the dashboard${sid() ? ` · session ${sid()}` : ''}`}
+              </text>
+            </Show>
+            <For each={props.store.state.messages}>{message => <MessageLine message={message} />}</For>
+          </DisplayProvider>
         </ScrollAnchorProvider>
       </scrollbox>
     </box>