diff --git a/ui-tui-opentui-v2/src/entry/main.tsx b/ui-tui-opentui-v2/src/entry/main.tsx
index 8e0310e6a16..1a48c796542 100644
--- a/ui-tui-opentui-v2/src/entry/main.tsx
+++ b/ui-tui-opentui-v2/src/entry/main.tsx
@@ -27,6 +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 { dispatchSlash, type SlashContext } from '../logic/slash.ts'
 import { createSessionStore, type SessionStore } from '../logic/store.ts'
 import { App } from '../view/App.tsx'
@@ -42,6 +43,8 @@ export interface TuiInput {
   readonly cols: number
   /** Optional initial prompt submitted once the session is ready — the Phase-1 stand-in for the composer. */
   readonly initialPrompt?: string
+  /** Resume a session instead of creating one: a session id, or 'recent'/'last' (→ session.most_recent). */
+  readonly resumeId?: string
 }
 
 const READY_POLL = Duration.millis(100)
@@ -49,10 +52,11 @@ const READY_TIMEOUT_MS = 20_000
 
 /**
  * Live session bootstrap: wait for the unsolicited `gateway.ready` handshake,
- * `session.create`, then (if given) submit the initial prompt. Forked into the
- * entry scope so it runs concurrently with the render + the quit-await. Any
- * failure is logged (file/ring sink) and swallowed — a bootstrap hiccup must
- * never tear down the rendered UI.
+ * then either RESUME a session (hydrate its transcript — incl. tool rows — via
+ * the snapshot, buffering live events across the RPC) or CREATE a fresh one, and
+ * (if given) submit the initial prompt. Forked into the entry scope so it runs
+ * concurrently with the render + the quit-await. Any failure is logged and
+ * swallowed — a bootstrap hiccup must never tear down the rendered UI.
  */
 const bootstrapSession = (gateway: GatewayServiceShape, store: SessionStore, input: TuiInput) =>
   Effect.gen(function* () {
@@ -66,13 +70,45 @@ const bootstrapSession = (gateway: GatewayServiceShape, store: SessionStore, inp
       log.warn('bootstrap', 'no gateway.ready within timeout', { waited })
       return
     }
-    const created = yield* gateway.request<{ session_id?: string }>('session.create', { cols: input.cols })
-    const sid = created?.session_id ?? gateway.sessionId()
-    if (!sid) {
-      log.warn('bootstrap', 'session.create returned no session_id')
-      return
+
+    let sid: string | undefined
+    if (input.resumeId) {
+      sid = input.resumeId
+      if (sid === 'recent' || sid === 'last') {
+        const recent = yield* gateway.request<{ session_id?: string }>('session.most_recent', {})
+        sid = recent?.session_id
+      }
+      if (!sid) {
+        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
+      })
+    } else {
+      const created = yield* gateway.request<{ session_id?: string }>('session.create', { cols: input.cols })
+      sid = created?.session_id ?? gateway.sessionId()
+      if (!sid) {
+        log.warn('bootstrap', 'session.create returned no session_id')
+        return
+      }
+      log.info('bootstrap', 'session created', { sid })
     }
-    log.info('bootstrap', 'session created', { sid })
+
     const prompt = input.initialPrompt?.trim()
     if (prompt) {
       store.pushUser(prompt)
@@ -191,8 +227,10 @@ if (import.meta.main) {
   const fake = TRUE_RE.test(process.env.HERMES_TUI_FAKE?.trim() ?? '')
   const cols = process.stdout.columns || 80
   const initialPrompt = process.env.HERMES_TUI_PROMPT?.trim() || process.argv.slice(2).join(' ').trim()
+  const resumeId = process.env.HERMES_TUI_RESUME?.trim()
   const base = { mouse: false, fake, cols }
-  const input: TuiInput = initialPrompt ? { ...base, initialPrompt } : base
+  const withPrompt = initialPrompt ? { ...base, initialPrompt } : base
+  const input: TuiInput = resumeId ? { ...withPrompt, resumeId } : withPrompt
 
   const onFatal = (error: unknown) => {
     getLog().error('entry', 'fatal', { error: String(error) })
diff --git a/ui-tui-opentui-v2/src/logic/resume.ts b/ui-tui-opentui-v2/src/logic/resume.ts
new file mode 100644
index 00000000000..258d526e5ee
--- /dev/null
+++ b/ui-tui-opentui-v2/src/logic/resume.ts
@@ -0,0 +1,55 @@
+/**
+ * Resume snapshot mapper (spec §1 lifecycle; gotcha §8 #5). Maps the
+ * `session.resume` response `messages` (tui_gateway `_history_to_messages`) into
+ * the store's `Message[]`. Each history entry is either `{role, text}` (user/
+ * assistant/system) or `{role:'tool', name, context}` (NO text — render it).
+ *
+ * Tool rows are folded into the PRECEDING assistant turn's ordered `parts[]`
+ * (state:'complete', summary=context) so a resumed transcript renders inline like
+ * 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'
+
+function readStr(value: unknown, key: string): string | undefined {
+  if (!value || typeof value !== 'object') return undefined
+  const v = (value as { [k: string]: unknown })[key]
+  return typeof v === 'string' ? v : undefined
+}
+
+export function mapResumeHistory(history: unknown): Message[] {
+  if (!Array.isArray(history)) return []
+  const out: Message[] = []
+  let seq = 0
+  const id = () => `r${++seq}`
+  let currentAssistant: Message | undefined
+
+  for (const raw of history) {
+    const role = readStr(raw, 'role')
+
+    if (role === 'tool') {
+      const name = readStr(raw, 'name') ?? 'tool'
+      const context = readStr(raw, 'context')
+      const tool: Part = { type: 'tool', id: id(), name, state: 'complete' }
+      if (context) tool.summary = context
+      if (!currentAssistant) {
+        currentAssistant = { role: 'assistant', text: '', parts: [] }
+        out.push(currentAssistant)
+      }
+      ;(currentAssistant.parts ??= []).push(tool)
+      continue
+    }
+
+    const text = readStr(raw, 'text') ?? ''
+    if (role === 'assistant') {
+      const parts: Part[] = text ? [{ type: 'text', id: id(), text }] : []
+      currentAssistant = { role: 'assistant', text, parts }
+      out.push(currentAssistant)
+    } else if (role === 'user' || role === 'system') {
+      out.push({ role, text })
+      currentAssistant = undefined
+    }
+  }
+
+  return out
+}
diff --git a/ui-tui-opentui-v2/src/logic/store.ts b/ui-tui-opentui-v2/src/logic/store.ts
index b2341153ba2..66b88364893 100644
--- a/ui-tui-opentui-v2/src/logic/store.ts
+++ b/ui-tui-opentui-v2/src/logic/store.ts
@@ -306,20 +306,28 @@ export function createSessionStore() {
     setState('prompt', undefined)
   }
 
-  /**
-   * Begin a resume hydrate: buffer live events, replace history with the
-   * snapshot, then replay buffered events. `loadSnapshot` maps the gateway's
-   * historical messages into the store's Message[] (Phase 4 fills the mapping).
-   */
-  function hydrate(loadSnapshot: () => Message[]): void {
-    buffering = []
-    const snapshot = loadSnapshot()
+  // ── resume hydrate (opencode sync-v2): buffer live events while the snapshot
+  // loads, then replace history + replay the buffer in order. Split into begin/
+  // commit so the buffer can span an async `session.resume` RPC.
+  /** Start buffering live events (call BEFORE the async resume RPC). Idempotent. */
+  function beginBuffer(): void {
+    if (!buffering) buffering = []
+  }
+
+  /** Replace history with the resume snapshot, then replay events buffered meanwhile. */
+  function commitSnapshot(snapshot: Message[]): void {
     setState('messages', snapshot)
-    const pending = buffering
+    const pending = buffering ?? []
     buffering = null
     for (const event of pending) applyNow(event)
   }
 
+  /** Synchronous convenience: buffer → load → commit (used by tests). */
+  function hydrate(loadSnapshot: () => Message[]): void {
+    beginBuffer()
+    commitSnapshot(loadSnapshot())
+  }
+
   return {
     state,
     apply,
@@ -328,6 +336,8 @@ export function createSessionStore() {
     clearTranscript,
     setConfirm,
     hydrate,
+    beginBuffer,
+    commitSnapshot,
     duplicate,
     clearPrompt
   } as const
diff --git a/ui-tui-opentui-v2/src/test/resume.test.ts b/ui-tui-opentui-v2/src/test/resume.test.ts
new file mode 100644
index 00000000000..df76f93b04f
--- /dev/null
+++ b/ui-tui-opentui-v2/src/test/resume.test.ts
@@ -0,0 +1,43 @@
+/**
+ * Resume mapper test (spec §1 lifecycle; gotcha §8 #5). The `session.resume`
+ * history maps into the store's Message[], folding tool rows ({name,context},
+ * NO text) into the preceding assistant turn's ordered parts so they render.
+ */
+import { describe, expect, test } from 'bun:test'
+
+import { mapResumeHistory } from '../logic/resume.ts'
+
+describe('mapResumeHistory (Phase 4b)', () => {
+  test('maps user/assistant text + folds tool rows into the preceding assistant parts', () => {
+    const msgs = mapResumeHistory([
+      { role: 'user', text: 'list files' },
+      { role: 'assistant', text: 'Listing.' },
+      { role: 'tool', name: 'terminal', context: 'ls -la' },
+      { role: 'assistant', text: 'Done.' }
+    ])
+    expect(msgs.map(m => m.role)).toEqual(['user', 'assistant', 'assistant'])
+    expect(msgs[0]).toMatchObject({ role: 'user', text: 'list files' })
+
+    const a1 = msgs[1]!
+    expect(a1.parts?.map(p => p.type)).toEqual(['text', 'tool']) // text + folded tool, inline
+    const tool = a1.parts![1]!
+    if (tool.type === 'tool') {
+      expect(tool).toMatchObject({ name: 'terminal', state: 'complete', summary: 'ls -la' })
+    } else {
+      throw new Error('expected a folded tool part')
+    }
+    expect(msgs[2]).toMatchObject({ role: 'assistant', text: 'Done.' })
+  })
+
+  test('a tool row with no preceding assistant gets a standalone assistant holder', () => {
+    const msgs = mapResumeHistory([{ role: 'tool', name: 'read_file', context: 'foo.ts' }])
+    expect(msgs).toHaveLength(1)
+    expect(msgs[0]!.role).toBe('assistant')
+    expect(msgs[0]!.parts?.[0]).toMatchObject({ type: 'tool', name: 'read_file', summary: 'foo.ts' })
+  })
+
+  test('ignores non-arrays and unknown roles', () => {
+    expect(mapResumeHistory(null)).toEqual([])
+    expect(mapResumeHistory([{ role: 'weird', text: 'x' }])).toEqual([])
+  })
+})
diff --git a/ui-tui-opentui-v2/src/test/store.test.ts b/ui-tui-opentui-v2/src/test/store.test.ts
index 22277a12e28..0452a9ae19d 100644
--- a/ui-tui-opentui-v2/src/test/store.test.ts
+++ b/ui-tui-opentui-v2/src/test/store.test.ts
@@ -153,3 +153,18 @@ describe('session store — blocking prompts (Phase 3)', () => {
     expect(store.state.prompt).toMatchObject({ kind: 'secret', envVar: 'API_KEY', requestId: 's2' })
   })
 })
+
+describe('session store — resume hydrate (Phase 4b)', () => {
+  test('beginBuffer + commitSnapshot replaces history then replays events buffered across the resume', () => {
+    const store = createSessionStore()
+    store.beginBuffer()
+    // a live event arrives DURING the (async) session.resume RPC
+    store.apply({ type: 'message.start' })
+    store.apply({ type: 'message.delta', payload: { text: 'live during resume' } })
+    // the snapshot commits afterwards
+    store.commitSnapshot([{ role: 'user', text: 'old question' }])
+    expect(store.state.messages).toHaveLength(2) // snapshot(1) + the replayed assistant turn(1)
+    expect(store.state.messages[0]).toMatchObject({ role: 'user', text: 'old question' })
+    expect(store.state.messages[1]!.parts?.[0]).toMatchObject({ type: 'text', text: 'live during resume' })
+  })
+})