feat(opentui-v2): Phase 5c — session switcher overlay (list → pick → resume)

A first-class picker (spec §2b, Ink activeSessionSwitcher): /sessions (aliases
/resume, /switch, /session) → session.list → a native <select> overlay; Enter
resumes the chosen session via the SAME resumeInto hydrate path as launch, so
tool rows + transcript hydrate correctly. Esc closes. Reuses Phase 4b resume.

- view/overlays/sessionSwitcher.tsx: <select> of sessions (title / preview /
  message count), onSelect → onPick(id); Esc cancels.
- store: switcher state + openSwitcher/closeSwitcher; SessionItem type.
- logic/resume.ts: mapSessionList(session.list result) → SessionItem[].
- logic/slash.ts: /sessions|/resume|/switch|/session client commands +
  listSessions/openSwitcher on SlashContext.
- entry: resumeInto extracted (shared by bootstrap + switcher); slashCtx wires
  listSessions (session.list → mapSessionList) + openSwitcher; onResume runs
  resumeInto via runFork. App input zone is now prompt → switcher → composer
  (overlays replace, not stack, so the composer remounts/refocuses on close).

Verified: bun run check green (43 tests / 7 files) — slash /sessions → switcher,
+ a switcher frame test (rows render, composer replaced). LIVE tmux: /sessions
listed real titled sessions w/ counts/previews; ↓+Enter resumed the picked one
(hydrate_ms=8) → transcript hydrated incl. the ⚡terminal tool row; switcher
closed, composer returned; /quit clean. 3 of 6 first-class overlays done
(prompts, pager, switcher). Smoke P5c + matrix updated.

ui-tui-opentui-v2/src/entry/main.tsx

@@ -27,7 +27,7 @@ import { liveGatewayLayer } from '../boundary/gateway/liveGateway.ts'
 import { getLog } from '../boundary/log.ts'
 import { acquireRenderer } from '../boundary/renderer.ts'
 import { makeAppLayer } from '../boundary/runtime.ts'
-import { mapResumeHistory } from '../logic/resume.ts'
+import { mapResumeHistory, mapSessionList } from '../logic/resume.ts'
 import { dispatchSlash, type SlashContext } from '../logic/slash.ts'
 import { createSessionStore, type SessionStore } from '../logic/store.ts'
 import { App } from '../view/App.tsx'
@@ -50,6 +50,28 @@ export interface TuiInput {
 const READY_POLL = Duration.millis(100)
 const READY_TIMEOUT_MS = 20_000
 
+/**
+ * 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
+ * mapResumeHistory). Shared by the launch bootstrap and the session switcher.
+ * Timed (rpc_ms / hydrate_ms) for the resume profile.
+ */
+const resumeInto = (gateway: GatewayServiceShape, store: SessionStore, sid: string, cols: number) =>
+  Effect.gen(function* () {
+    store.beginBuffer()
+    const t0 = Date.now()
+    const resumed = yield* gateway.request<{ messages?: unknown }>('session.resume', { cols, session_id: sid })
+    const t1 = Date.now()
+    const snapshot = mapResumeHistory(resumed?.messages)
+    store.commitSnapshot(snapshot)
+    getLog().info('bootstrap', 'session resumed', {
+      count: snapshot.length,
+      hydrate_ms: Date.now() - t1,
+      rpc_ms: t1 - t0,
+      sid
+    })
+  })
+
 /**
  * Live session bootstrap: wait for the unsolicited `gateway.ready` handshake,
  * then either RESUME a session (hydrate its transcript — incl. tool rows — via
@@ -82,23 +104,7 @@ const bootstrapSession = (gateway: GatewayServiceShape, store: SessionStore, inp
         log.warn('bootstrap', 'no session to resume', { resumeId: input.resumeId })
         return
       }
-      // Buffer live events across the resume RPC, then replace history + replay.
-      store.beginBuffer()
-      const t0 = Date.now()
-      const resumed = yield* gateway.request<{ messages?: unknown }>('session.resume', {
-        cols: input.cols,
-        session_id: sid
-      })
-      const t1 = Date.now()
-      const snapshot = mapResumeHistory(resumed?.messages)
-      store.commitSnapshot(snapshot)
-      // Hydration profile: rpc_ms = server load + transport; hydrate_ms = map + store write.
-      log.info('bootstrap', 'session resumed', {
-        count: snapshot.length,
-        hydrate_ms: Date.now() - t1,
-        rpc_ms: t1 - t0,
-        sid
-      })
+      yield* resumeInto(gateway, store, sid, input.cols)
     } else {
       const created = yield* gateway.request<{ session_id?: string }>('session.create', { cols: input.cols })
       sid = created?.session_id ?? gateway.sessionId()
@@ -156,11 +162,13 @@ export const run = Effect.fn('Tui.run')(function* (input: TuiInput) {
       const slashCtx: SlashContext = {
         clearTranscript: () => store.clearTranscript(),
         confirm: (message, onConfirm) => store.setConfirm(message, onConfirm),
+        listSessions: () => Effect.runPromise(gateway.request('session.list', {})).then(mapSessionList),
         logTail: () =>
           getLog()
             .tail(200)
             .map(e => `${e.scope}: ${e.msg}`),
         openPager: (title, text) => store.openPager(title, text),
+        openSwitcher: sessions => store.openSwitcher(sessions),
         pushSystem: text => store.pushSystem(text),
         quit: () => {
           if (!renderer.isDestroyed) renderer.destroy()
@@ -170,6 +178,15 @@ export const run = Effect.fn('Tui.run')(function* (input: TuiInput) {
         submit: submitPrompt
       }
 
+      // Resume a chosen session (session switcher pick) — same hydrate path as launch.
+      const onResume = (resumeSid: string) => {
+        Effect.runFork(
+          resumeInto(gateway, store, resumeSid, input.cols).pipe(
+            Effect.catchCause(cause => Effect.sync(() => getLog().warn('resume', 'failed', { cause: String(cause) })))
+          )
+        )
+      }
+
       // The composer's submit: route `/command` through the slash ladder, else a prompt.
       const submit = (text: string) => {
         if (text.startsWith('/')) void dispatchSlash(text, slashCtx)
@@ -199,7 +216,13 @@ export const run = Effect.fn('Tui.run')(function* (input: TuiInput) {
         render(
           () => (
             <ThemeProvider theme={() => store.state.theme}>
-              <App store={store} onSubmit={submit} onRespond={respond} sessionId={() => gateway.sessionId()} />
+              <App
+                store={store}
+                onSubmit={submit}
+                onRespond={respond}
+                onResume={onResume}
+                sessionId={() => gateway.sessionId()}
+              />
             </ThemeProvider>
           ),
           renderer

ui-tui-opentui-v2/src/logic/resume.ts

@@ -9,7 +9,7 @@
  * a live one. Resumed assistant text is given a single text part so it renders
  * through the native markdown path. IDs are `r*` (distinct from live `p*`).
  */
-import type { Message, Part } from './store.ts'
+import type { Message, Part, SessionItem } from './store.ts'
 
 function readStr(value: unknown, key: string): string | undefined {
   if (!value || typeof value !== 'object') return undefined
@@ -17,6 +17,31 @@ function readStr(value: unknown, key: string): string | undefined {
   return typeof v === 'string' ? v : undefined
 }
 
+function readNum(value: unknown, key: string): number {
+  if (!value || typeof value !== 'object') return 0
+  const v = (value as { [k: string]: unknown })[key]
+  return typeof v === 'number' ? v : 0
+}
+
+/** Map a `session.list` result into switcher rows (loose-typed read). */
+export function mapSessionList(result: unknown): SessionItem[] {
+  if (!result || typeof result !== 'object') return []
+  const sessions = (result as { sessions?: unknown }).sessions
+  if (!Array.isArray(sessions)) return []
+  const out: SessionItem[] = []
+  for (const s of sessions) {
+    const id = readStr(s, 'id')
+    if (!id) continue
+    out.push({
+      id,
+      messageCount: readNum(s, 'message_count'),
+      preview: readStr(s, 'preview') ?? '',
+      title: readStr(s, 'title') ?? ''
+    })
+  }
+  return out
+}
+
 export function mapResumeHistory(history: unknown): Message[] {
   if (!Array.isArray(history)) return []
   const out: Message[] = []

ui-tui-opentui-v2/src/logic/slash.ts

@@ -9,9 +9,10 @@
  *   2. `slash.exec {command, session_id}` → `{output, warning?}` → system line
  *   3. on reject → `command.dispatch {arg, name, session_id}` → typed action
  *      (exec/plugin → system · alias → re-dispatch · skill/send → submit a turn ·
- *       prefill → notice). Pager routing for long output lands with Phase 5a; for
- *      now long output is shown as a (multi-line) system message.
+ *       prefill → notice). Long output routes to the pager (Phase 5a).
  */
+import type { SessionItem } from './store.ts'
+
 export interface ParsedSlash {
   name: string
   arg: string
@@ -42,6 +43,10 @@ export interface SlashContext {
   readonly quit: () => void
   /** Recent log lines for `/logs` (the ring buffer). */
   readonly logTail: () => string[]
+  /** Fetch the resumable sessions (`session.list`) for the switcher. */
+  readonly listSessions: () => Promise<SessionItem[]>
+  /** Open the session switcher with the given rows. */
+  readonly openSwitcher: (sessions: SessionItem[]) => void
 }
 
 function readStr(value: unknown, key: string): string | undefined {
@@ -61,6 +66,7 @@ function present(ctx: SlashContext, title: string, text: string): void {
 
 const CLIENT_HELP = [
   '/help — list commands',
+  '/sessions, /resume — switch/resume a session',
   '/clear, /new — clear the transcript (confirm)',
   '/logs — recent engine log lines',
   '/quit, /exit — quit',
@@ -69,10 +75,21 @@ const CLIENT_HELP = [
 
 type ClientHandler = (arg: string, ctx: SlashContext) => void | Promise<void>
 
+/** Fetch sessions and open the switcher (shared by /sessions, /resume, /switch, /session). */
+const openSwitcher: ClientHandler = async (_arg, ctx) => {
+  const sessions = await ctx.listSessions()
+  if (sessions.length) ctx.openSwitcher(sessions)
+  else ctx.pushSystem('No sessions to resume.')
+}
+
 /** The TUI-only client commands (run in-process, never hit the gateway). */
 const CLIENT: Record<string, ClientHandler> = {
   clear: (_arg, ctx) => ctx.confirm('Clear the transcript?', ctx.clearTranscript),
   exit: (_arg, ctx) => ctx.quit(),
+  resume: openSwitcher,
+  session: openSwitcher,
+  sessions: openSwitcher,
+  switch: openSwitcher,
   help: async (_arg, ctx) => {
     // Prefer the live catalog; fall back to the client list if it's unavailable.
     try {

ui-tui-opentui-v2/src/logic/store.ts

@@ -64,6 +64,14 @@ export interface PagerState {
   text: string
 }
 
+/** One row in the session switcher (from `session.list`). */
+export interface SessionItem {
+  id: string
+  title: string
+  preview: string
+  messageCount: number
+}
+
 export interface StoreState {
   ready: boolean
   messages: Message[]
@@ -72,6 +80,8 @@ export interface StoreState {
   prompt: ActivePrompt | undefined
   /** The open pager overlay (replaces the transcript while set); undefined when none. */
   pager: PagerState | undefined
+  /** The open session switcher (replaces the composer while set); undefined when none. */
+  switcher: SessionItem[] | undefined
 }
 
 const LRU_LIMIT = 1000
@@ -88,7 +98,8 @@ export function createSessionStore() {
     messages: [],
     theme: DEFAULT_THEME,
     prompt: undefined,
-    pager: undefined
+    pager: undefined,
+    switcher: undefined
   })
 
   // Monotonic part id (stable `key` per part so a new tool part below a streaming
@@ -192,6 +203,16 @@ export function createSessionStore() {
     setState('pager', undefined)
   }
 
+  /** Open the session switcher with the given session rows (/sessions, /resume). */
+  function openSwitcher(sessions: SessionItem[]) {
+    setState('switcher', sessions)
+  }
+
+  /** Close the session switcher. */
+  function closeSwitcher() {
+    setState('switcher', undefined)
+  }
+
   /** Reduce a decoded gateway event into the store. The sole boundary->Solid sink. */
   function apply(event: GatewayEvent): void {
     if (buffering) {
@@ -356,6 +377,8 @@ export function createSessionStore() {
     setConfirm,
     openPager,
     closePager,
+    openSwitcher,
+    closeSwitcher,
     hydrate,
     beginBuffer,
     commitSnapshot,

ui-tui-opentui-v2/src/test/render.test.tsx

@@ -125,4 +125,27 @@ describe('App render (Phase 1, themed)', () => {
     expect(frame).not.toContain('a previous message') // transcript replaced by the pager
     expect(frame).not.toContain('Type your message') // composer hidden while the pager is open
   })
+
+  test('the session switcher renders session rows and replaces the composer', async () => {
+    const store = createSessionStore()
+    store.apply({ type: 'gateway.ready' })
+    store.openSwitcher([
+      { id: 's1', title: 'First chat', preview: 'hi', messageCount: 5 },
+      { id: 's2', title: 'Second chat', preview: 'yo', messageCount: 12 }
+    ])
+
+    const frame = await captureFrame(
+      () => (
+        <ThemeProvider theme={() => store.state.theme}>
+          <App store={store} />
+        </ThemeProvider>
+      ),
+      { until: 'Resume a session', width: 72, height: 18 }
+    )
+
+    expect(frame).toContain('Resume a session') // switcher header
+    expect(frame).toContain('First chat') // session row
+    expect(frame).toContain('Second chat')
+    expect(frame).not.toContain('Type your message') // composer hidden while switcher open
+  })
 })

ui-tui-opentui-v2/src/test/slash.test.ts

@@ -5,6 +5,9 @@
 import { describe, expect, test } from 'bun:test'
 
 import { dispatchSlash, parseSlash, type SlashContext } from '../logic/slash.ts'
+import type { SessionItem } from '../logic/store.ts'
+
+const FAKE_SESSIONS: SessionItem[] = [{ id: 's1', messageCount: 5, preview: 'hello there', title: 'First chat' }]
 
 describe('parseSlash', () => {
   test('splits name + arg; rejects non-slash / empty', () => {
@@ -22,6 +25,7 @@ interface Probe {
   submitted: string[]
   confirmed: Array<{ message: string; onConfirm: () => void }>
   paged: Array<{ title: string; text: string }>
+  switched: SessionItem[][]
   quit: { value: boolean }
   cleared: { value: boolean }
 }
@@ -32,13 +36,16 @@ function makeCtx(request: (method: string, params: Record<string, unknown>) => P
   const submitted: string[] = []
   const confirmed: Probe['confirmed'] = []
   const paged: Probe['paged'] = []
+  const switched: Probe['switched'] = []
   const quit = { value: false }
   const cleared = { value: false }
   const ctx: SlashContext = {
     clearTranscript: () => (cleared.value = true),
     confirm: (message, onConfirm) => confirmed.push({ message, onConfirm }),
+    listSessions: () => Promise.resolve(FAKE_SESSIONS),
     logTail: () => ['gateway: spawned', 'bootstrap: session created'],
     openPager: (title, text) => paged.push({ text, title }),
+    openSwitcher: sessions => switched.push(sessions),
     pushSystem: text => system.push(text),
     quit: () => (quit.value = true),
     request: (method, params) => {
@@ -48,7 +55,7 @@ function makeCtx(request: (method: string, params: Record<string, unknown>) => P
     sessionId: () => 'sid-1',
     submit: text => submitted.push(text)
   }
-  return { calls, cleared, confirmed, ctx, paged, quit, submitted, system }
+  return { calls, cleared, confirmed, ctx, paged, quit, submitted, switched, system }
 }
 
 describe('dispatchSlash — client commands', () => {
@@ -75,6 +82,16 @@ describe('dispatchSlash — client commands', () => {
     expect(p.paged[0]?.text).toContain('session created')
   })
 
+  test('/sessions (and /resume) open the switcher with session.list rows', async () => {
+    const p = makeCtx(async () => ({}))
+    await dispatchSlash('/sessions', p.ctx)
+    expect(p.switched).toHaveLength(1)
+    expect(p.switched[0]).toEqual(FAKE_SESSIONS)
+    const p2 = makeCtx(async () => ({}))
+    await dispatchSlash('/resume', p2.ctx)
+    expect(p2.switched).toHaveLength(1)
+  })
+
   test('/help renders the gateway catalog', async () => {
     const p = makeCtx(async method =>
       method === 'commands.catalog' ? { pairs: [['/model', 'switch model']], canon: {} } : {}

ui-tui-opentui-v2/src/view/App.tsx

@@ -20,6 +20,7 @@ import type { SessionStore } from '../logic/store.ts'
 import { Composer } from './composer.tsx'
 import { Header } from './header.tsx'
 import { Pager } from './overlays/pager.tsx'
+import { SessionSwitcher } from './overlays/sessionSwitcher.tsx'
 import { PromptOverlay } from './prompts/promptOverlay.tsx'
 import { Transcript } from './transcript.tsx'
 
@@ -27,19 +28,27 @@ export interface AppProps {
   readonly store: SessionStore
   readonly onSubmit?: (text: string) => void
   readonly onRespond?: (method: string, params: Record<string, unknown>) => void
+  readonly onResume?: (sessionId: string) => void
   readonly sessionId?: () => string | undefined
 }
 
 const NOOP = () => {}
 const NOOP_RESPOND = () => {}
+const NOOP_RESUME = () => {}
 const NO_SESSION = () => undefined
 
 export function App(props: AppProps) {
   const blocked = () => props.store.state.prompt !== undefined
   const pager = () => props.store.state.pager
-  // Defer the close so the key that closed the overlay (Esc/q) can't land in the
-  // freshly-remounted composer.
+  const switcher = () => props.store.state.switcher
+  // Defer the close so the key that closed an overlay (Esc/q/Enter) can't land in
+  // the freshly-remounted composer.
   const closePager = () => setTimeout(() => props.store.closePager(), 0)
+  const closeSwitcher = () => setTimeout(() => props.store.closeSwitcher(), 0)
+  const resume = (id: string) => {
+    ;(props.onResume ?? NOOP_RESUME)(id)
+    closeSwitcher()
+  }
 
   return (
     <box style={{ flexDirection: 'column', flexGrow: 1, padding: 1 }}>
@@ -49,7 +58,14 @@ export function App(props: AppProps) {
         fallback={
           <>
             <Transcript store={props.store} />
-            <Show when={blocked()} fallback={<Composer onSubmit={props.onSubmit ?? NOOP} />}>
+            <Show
+              when={blocked()}
+              fallback={
+                <Show when={switcher()} fallback={<Composer onSubmit={props.onSubmit ?? NOOP} />}>
+                  {sessions => <SessionSwitcher sessions={sessions()} onPick={resume} onClose={closeSwitcher} />}
+                </Show>
+              }
+            >
               <PromptOverlay
                 store={props.store}
                 onRespond={props.onRespond ?? NOOP_RESPOND}

ui-tui-opentui-v2/src/view/overlays/sessionSwitcher.tsx

@@ -0,0 +1,55 @@
+/**
+ * SessionSwitcher — pick a session to resume (spec §2b; Ink
+ * `activeSessionSwitcher.tsx`). A native `<select>` over `session.list` rows;
+ * Enter resumes the chosen session (the entry runs the same resume-hydrate path
+ * as launch), Esc/Ctrl+C closes. Replaces the composer while open.
+ */
+import { useKeyboard } from '@opentui/solid'
+import { createMemo } from 'solid-js'
+
+import type { SessionItem } from '../../logic/store.ts'
+import { useTheme } from '../theme.tsx'
+
+export function SessionSwitcher(props: {
+  sessions: SessionItem[]
+  onPick: (sessionId: string) => void
+  onClose: () => void
+}) {
+  const theme = useTheme()
+  useKeyboard(key => {
+    if (key.name === 'escape' || (key.ctrl && key.name === 'c')) props.onClose()
+  })
+
+  const options = createMemo(() =>
+    props.sessions.map(s => ({
+      description: `${s.messageCount} msgs${s.preview ? ` · ${s.preview.slice(0, 60)}` : ''}`,
+      name: s.title || s.preview.slice(0, 48) || s.id,
+      value: s.id
+    }))
+  )
+
+  return (
+    <box
+      style={{ borderColor: theme().color.border, flexDirection: 'column', flexShrink: 0, marginTop: 1, padding: 1 }}
+      border
+    >
+      <text fg={theme().color.accent}>
+        <b>⟲ Resume a session</b>
+      </text>
+      <select
+        focused
+        options={options()}
+        onSelect={(_index, option) => {
+          if (option) props.onPick(String(option.value))
+        }}
+        backgroundColor={theme().color.statusBg}
+        selectedBackgroundColor={theme().color.selectionBg}
+        textColor={theme().color.text}
+        selectedTextColor={theme().color.text}
+        descriptionColor={theme().color.muted}
+        style={{ height: Math.min(16, Math.max(2, options().length * 2)), marginTop: 1 }}
+      />
+      <text fg={theme().color.muted}>↑↓ select · Enter resume · Esc cancel</text>
+    </box>
+  )
+}