diff --git a/ui-tui/packages/hermes-ink/src/ink/ink.tsx b/ui-tui/packages/hermes-ink/src/ink/ink.tsx
index d8c95fcc703..90febfbe153 100644
--- a/ui-tui/packages/hermes-ink/src/ink/ink.tsx
+++ b/ui-tui/packages/hermes-ink/src/ink/ink.tsx
@@ -28,6 +28,7 @@ import { dispatchClick, dispatchHover, dispatchMouse } from './hit-test.js'
 import { applyHyperlinkHoverHighlight } from './hyperlinkHover.js'
 import instances from './instances.js'
 import { LogUpdate } from './log-update.js'
+import { maybeStartMemSampler } from './memSampler.js'
 import { nodeCache } from './node-cache.js'
 import { optimize } from './optimizer.js'
 import Output from './output.js'
@@ -177,6 +178,8 @@ export default class Ink {
   }
   // Ignore last render after unmounting a tree to prevent empty output before exit
   private isUnmounted = false
+  // Bench-only node-count sampler stopper (no-op unless HERMES_TUI_MEMSAMPLE_FD)
+  private readonly stopMemSampler: () => void
   private isPaused = false
   private readonly container: FiberRoot
   private rootNode: dom.DOMElement
@@ -391,6 +394,9 @@ export default class Ink {
     }
 
     this.rootNode = dom.createNode('ink-root')
+    // Bench-only node-count sampler — inert unless HERMES_TUI_MEMSAMPLE_FD is
+    // set (see memSampler.ts). Dark by default in production.
+    this.stopMemSampler = maybeStartMemSampler(this.rootNode)
     this.focusManager = new FocusManager((target, event) => dispatcher.dispatchDiscrete(target, event))
     this.rootNode.focusManager = this.focusManager
     this.renderer = createRenderer(this.rootNode, this.stylePool)
@@ -2426,6 +2432,8 @@ export default class Ink {
 
     this.isUnmounted = true
 
+    this.stopMemSampler()
+
     // Cancel any pending throttled renders to prevent accessing freed Yoga nodes
     this.scheduleRender.cancel?.()
 
diff --git a/ui-tui/packages/hermes-ink/src/ink/memSampler.test.ts b/ui-tui/packages/hermes-ink/src/ink/memSampler.test.ts
new file mode 100644
index 00000000000..5622185a114
--- /dev/null
+++ b/ui-tui/packages/hermes-ink/src/ink/memSampler.test.ts
@@ -0,0 +1,68 @@
+import { closeSync, openSync, readFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+
+import { afterEach, describe, expect, it } from 'vitest'
+
+import { countNodes, maybeStartMemSampler } from './memSampler.js'
+
+const sleep = (ms: number) => new Promise(r => setTimeout(r, ms))
+
+const tree = {
+  yogaNode: {},
+  childNodes: [
+    { yogaNode: {}, childNodes: [{ childNodes: [] }] }, // text child without yoga
+    { yogaNode: {} },
+    {} // virtual node, no yoga, no children
+  ]
+}
+
+describe('memSampler', () => {
+  afterEach(() => {
+    delete process.env['HERMES_TUI_MEMSAMPLE_FD']
+    delete process.env['HERMES_TUI_MEMSAMPLE_MS']
+  })
+
+  it('counts DOM and yoga nodes', () => {
+    expect(countNodes(tree)).toEqual({ dom: 5, yoga: 3 })
+  })
+
+  it('is a no-op when the env gate is unset', () => {
+    const stop = maybeStartMemSampler(tree)
+
+    expect(typeof stop).toBe('function')
+    stop()
+  })
+
+  it('writes NDJSON samples to the configured fd', async () => {
+    const path = join(tmpdir(), `memsampler-test-${process.pid}-${Date.now()}.ndjson`)
+    const fd = openSync(path, 'w')
+
+    process.env['HERMES_TUI_MEMSAMPLE_FD'] = String(fd)
+
+    const stop = maybeStartMemSampler(tree, 10)
+
+    await sleep(60)
+    stop()
+    closeSync(fd)
+
+    const lines = readFileSync(path, 'utf8').trim().split('\n')
+
+    expect(lines.length).toBeGreaterThanOrEqual(2)
+
+    const sample = JSON.parse(lines[0]!) as { dom: number; t: number; yoga: number }
+
+    expect(sample.dom).toBe(5)
+    expect(sample.yoga).toBe(3)
+    expect(sample.t).toBeGreaterThan(0)
+  })
+
+  it('goes dark permanently on a bad fd instead of throwing', async () => {
+    process.env['HERMES_TUI_MEMSAMPLE_FD'] = '987'
+
+    const stop = maybeStartMemSampler(tree, 10)
+
+    await sleep(40)
+    stop()
+  })
+})
diff --git a/ui-tui/packages/hermes-ink/src/ink/memSampler.ts b/ui-tui/packages/hermes-ink/src/ink/memSampler.ts
new file mode 100644
index 00000000000..d72c7604d09
--- /dev/null
+++ b/ui-tui/packages/hermes-ink/src/ink/memSampler.ts
@@ -0,0 +1,87 @@
+// Bench-only live node-count sampler (dark by default). When
+// HERMES_TUI_MEMSAMPLE_FD is set to a writable file descriptor, periodically
+// walks the forked reconciler's root DOM tree and writes one NDJSON line per
+// sample to that fd: {"t":<epoch ms>,"dom":<DOM nodes>,"yoga":<live Yoga nodes>}.
+//
+// Used by bench/ (the instrumented node-count runs — see
+// docs/plans/opentui-bench-suite.md). RSS from instrumented runs is flagged and
+// never headlined; this sampler exists ONLY as the mechanism witness for the
+// transcript-growth claim. It writes to a dedicated fd (3 by convention), never
+// stdout/stderr, so it cannot perturb the rendered frame stream.
+//
+// Failure policy: any error (bad fd, closed pipe) disables the sampler
+// silently — production behavior must be identical with the env unset.
+
+import { writeSync } from 'node:fs'
+
+interface WalkableNode {
+  childNodes?: WalkableNode[]
+  yogaNode?: unknown
+}
+
+/** Count DOM nodes and nodes holding a live Yoga node under `root` (inclusive). */
+export function countNodes(root: WalkableNode): { dom: number; yoga: number } {
+  let dom = 0
+  let yoga = 0
+  const stack: WalkableNode[] = [root]
+  while (stack.length > 0) {
+    const node = stack.pop() as WalkableNode
+    dom++
+    if (node.yogaNode !== undefined && node.yogaNode !== null) yoga++
+    const children = node.childNodes
+    if (children) {
+      for (let i = 0; i < children.length; i++) {
+        stack.push(children[i] as WalkableNode)
+      }
+    }
+  }
+  return { dom, yoga }
+}
+
+/**
+ * Start the env-gated sampler. Returns a stop function (no-op when the gate is
+ * off). `intervalMs` falls back to HERMES_TUI_MEMSAMPLE_MS, then 1000.
+ */
+export function maybeStartMemSampler(root: WalkableNode, intervalMs?: number): () => void {
+  const rawFd = process.env['HERMES_TUI_MEMSAMPLE_FD']
+  if (!rawFd) {
+    return () => {}
+  }
+
+  const fd = Number.parseInt(rawFd, 10)
+
+  if (!Number.isInteger(fd) || fd < 0) {
+    return () => {}
+  }
+
+  const rawMs = Number.parseInt(process.env['HERMES_TUI_MEMSAMPLE_MS'] ?? '', 10)
+  const period = intervalMs ?? (Number.isFinite(rawMs) && rawMs > 0 ? rawMs : 1000)
+
+  let disabled = false
+
+  const tick = () => {
+    if (disabled) {
+      return
+    }
+
+    try {
+      const counts = countNodes(root)
+
+      writeSync(fd, `${JSON.stringify({ t: Date.now(), dom: counts.dom, yoga: counts.yoga })}\n`)
+    } catch {
+      // Bad/closed fd: go dark permanently rather than risk the render loop.
+      disabled = true
+      clearInterval(timer)
+    }
+  }
+
+  const timer = setInterval(tick, period)
+
+  // Never keep the process alive for the sampler.
+  timer.unref?.()
+
+  return () => {
+    disabled = true
+    clearInterval(timer)
+  }
+}