diff --git a/ui-opentui/src/boundary/proactiveGc.ts b/ui-opentui/src/boundary/proactiveGc.ts
new file mode 100644
index 00000000000..e68be42ec1a
--- /dev/null
+++ b/ui-opentui/src/boundary/proactiveGc.ts
@@ -0,0 +1,95 @@
+/**
+ * proactiveGc — opt-in, idle-gated `global.gc()` for the low-mem path (W2).
+ *
+ * GATED on the low-mem opt-in: only runs when the user set a LOW
+ * `HERMES_TUI_HEAP_MB` (the same knob W1 threads into `--max-old-space-size`).
+ * Default / unconstrained sessions do NOTHING — no proactive GC, no divergence
+ * from Ink on the default path (Ink never calls gc proactively; it only exposes
+ * it for heapdumps, so W2 is OpenTUI-only by design — spec D5).
+ *
+ * TRIGGER MODEL — idle-gated, never mid-stream:
+ *   - A low-frequency timer ticks every IDLE_MS. On each tick it calls
+ *     `global.gc()` ONLY when (a) a turn is NOT streaming (`isStreaming()` false
+ *     — so we never pause mid-render/mid-reply) and (b) at least one full idle
+ *     window has passed since the last activity (stream end / explicit touch).
+ *   - If RSS crosses RSS_EAGER_KB (>400MB) the cadence tightens (EAGER_MS) but
+ *     it STILL waits for idle — eagerness shortens the window, never bypasses it.
+ *   - `--expose-gc` (W1) makes `global.gc` real; without it this is a silent
+ *     no-op (we detect and disable). The timer is unref'd — it never keeps the
+ *     process alive — and every failure path disables silently (a GC helper must
+ *     never break the TUI).
+ *
+ * Reuses `process.memoryUsage().rss` (same read as memlog) for the >400MB check.
+ */
+import { envFlag } from '../logic/env.ts'
+
+/** Below this heap cap (MB) we treat the session as low-mem opt-in. 8192 is the
+ *  default; anyone who set a cap materially under it wants tight memory. */
+const LOW_MEM_HEAP_MB = 4096
+/** Idle window (ms): time since last activity before a GC is allowed. */
+const IDLE_MS = 8000
+/** Tightened idle window once RSS is high. */
+const EAGER_MS = 3000
+/** RSS (KB) above which GC becomes eager (still idle-gated). 400MB. */
+const RSS_EAGER_KB = 400 * 1024
+
+/** The configured heap cap in MB from the W1 knob, or null when unset/garbage.
+ *  The Python launcher reads the same env; the child inherits it, so the Node
+ *  side can read it directly to know whether low-mem mode is active. */
+function configuredHeapMb(): number | null {
+  const v = (process.env.HERMES_TUI_HEAP_MB ?? '').trim()
+  if (!/^\d+$/.test(v)) return null
+  const n = Number.parseInt(v, 10)
+  return n > 0 ? n : null
+}
+
+/** Whether proactive GC should run: a low heap cap is set AND gc is exposed.
+ *  `HERMES_TUI_PROACTIVE_GC` can force it on/off, but defaults to the low-mem
+ *  signal so the knob composes (spec D9: independent knobs). */
+export function proactiveGcEnabled(heapMb: number | null = configuredHeapMb()): boolean {
+  const lowMem = heapMb !== null && heapMb <= LOW_MEM_HEAP_MB
+  return envFlag(process.env.HERMES_TUI_PROACTIVE_GC, lowMem) && typeof global.gc === 'function'
+}
+
+/**
+ * Start the idle-gated proactive GC watcher. `isStreaming` reports whether a
+ * turn is mid-flight (read from the store's `info.running`). Returns a stop
+ * function and a `touch()` to mark fresh activity (e.g. on keypress / stream
+ * start) so the idle clock resets. No-op (returns inert handles) when disabled.
+ */
+export function startProactiveGc(isStreaming: () => boolean): { stop: () => void; touch: () => void } {
+  if (!proactiveGcEnabled()) return { stop: () => {}, touch: () => {} }
+  const gc = global.gc
+  if (typeof gc !== 'function') return { stop: () => {}, touch: () => {} }
+
+  let lastActivity = Date.now()
+  const touch = () => {
+    lastActivity = Date.now()
+  }
+
+  // Tick at the eager cadence; the idle-window check (below) does the real
+  // gating, so a high-RSS session reacts within EAGER_MS while a calm one still
+  // waits the full IDLE_MS. One cheap rss read + compare per tick.
+  const timer = setInterval(() => {
+    try {
+      if (isStreaming()) {
+        // mid-stream: defer entirely and keep the clock fresh so a GC can't fire
+        // the instant the stream ends — it waits a full idle window after.
+        lastActivity = Date.now()
+        return
+      }
+      const rssKb = Math.floor(process.memoryUsage().rss / 1024)
+      const window = rssKb > RSS_EAGER_KB ? EAGER_MS : IDLE_MS
+      if (Date.now() - lastActivity < window) return
+      gc()
+      // After a collection, reset the clock so we don't GC every tick — the next
+      // one waits another full idle window.
+      lastActivity = Date.now()
+    } catch {
+      clearInterval(timer) // a failing GC helper must not retry forever
+    }
+  }, EAGER_MS)
+  timer.unref?.()
+
+  return { stop: () => clearInterval(timer), touch }
+}
diff --git a/ui-opentui/src/entry/main.tsx b/ui-opentui/src/entry/main.tsx
index d52464f3a4e..dff357dd53d 100644
--- a/ui-opentui/src/entry/main.tsx
+++ b/ui-opentui/src/entry/main.tsx
@@ -30,6 +30,7 @@ import { GatewayService, type GatewayServiceShape } from '../boundary/gateway/Ga
 import { liveGatewayLayer } from '../boundary/gateway/liveGateway.ts'
 import { getLog } from '../boundary/log.ts'
 import { startMemlog } from '../boundary/memlog.ts'
+import { startProactiveGc } from '../boundary/proactiveGc.ts'
 import { registerVendoredParsers } from '../boundary/parsers.ts'
 import { acquireRenderer } from '../boundary/renderer.ts'
 import { makeAppLayer } from '../boundary/runtime.ts'
@@ -420,6 +421,11 @@ export const run = Effect.fn('Tui.run')(function* (input: TuiInput) {
       // switch — boundary/memlog.ts). Scoped acquire→release like the renderer.
       const stopMemlog = startMemlog()
       yield* Effect.addFinalizer(() => Effect.sync(stopMemlog))
+      // Proactive idle GC (W2) — opt-in via a low HERMES_TUI_HEAP_MB (no-op on
+      // the default path). Idle-gated on the store's streaming flag so it never
+      // collects mid-reply. Scoped release like memlog.
+      const proactiveGc = startProactiveGc(() => store.state.info.running === true)
+      yield* Effect.addFinalizer(() => Effect.sync(proactiveGc.stop))
       doQuit = () => {
         if (!renderer.isDestroyed) renderer.destroy()
       }
diff --git a/ui-opentui/src/test/proactiveGc.test.ts b/ui-opentui/src/test/proactiveGc.test.ts
new file mode 100644
index 00000000000..4c082bcd614
--- /dev/null
+++ b/ui-opentui/src/test/proactiveGc.test.ts
@@ -0,0 +1,122 @@
+/**
+ * proactiveGc tests (W2) — the opt-in idle-gated GC gating + timing.
+ *
+ * Pins: enabled ONLY under a low heap cap + exposed gc; never fires mid-stream;
+ * fires after a full idle window; tightens to the eager window once RSS is high;
+ * and is a no-op (inert handles) on the default path.
+ */
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
+
+import { proactiveGcEnabled, startProactiveGc } from '../boundary/proactiveGc.ts'
+
+const ENV_KEYS = ['HERMES_TUI_HEAP_MB', 'HERMES_TUI_PROACTIVE_GC'] as const
+const saved: Record<string, string | undefined> = {}
+
+beforeEach(() => {
+  for (const k of ENV_KEYS) saved[k] = process.env[k]
+})
+afterEach(() => {
+  for (const k of ENV_KEYS) {
+    if (saved[k] === undefined) delete process.env[k]
+    else process.env[k] = saved[k]
+  }
+  vi.restoreAllMocks()
+  vi.useRealTimers()
+})
+
+/** Stub a callable global.gc and return the spy. */
+function stubGc() {
+  const spy = vi.fn()
+  // global.gc is optional in the type — assign through a loosened view.
+  ;(globalThis as { gc?: () => void }).gc = spy
+  return spy
+}
+
+describe('proactiveGcEnabled — gating', () => {
+  test('OFF on the default path (no heap cap), even with gc exposed', () => {
+    delete process.env.HERMES_TUI_HEAP_MB
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    stubGc()
+    expect(proactiveGcEnabled()).toBe(false)
+  })
+
+  test('ON under a low heap cap with gc exposed', () => {
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    stubGc()
+    expect(proactiveGcEnabled(256)).toBe(true)
+  })
+
+  test('OFF under a high heap cap (not the low-mem signal)', () => {
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    stubGc()
+    expect(proactiveGcEnabled(8192)).toBe(false)
+  })
+
+  test('OFF when gc is not exposed, even under a low cap', () => {
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    delete (globalThis as { gc?: () => void }).gc
+    expect(proactiveGcEnabled(256)).toBe(false)
+  })
+
+  test('explicit HERMES_TUI_PROACTIVE_GC=off overrides the low-cap default', () => {
+    process.env.HERMES_TUI_PROACTIVE_GC = 'off'
+    stubGc()
+    expect(proactiveGcEnabled(256)).toBe(false)
+  })
+
+  test('explicit HERMES_TUI_PROACTIVE_GC=on forces it on under a high cap', () => {
+    process.env.HERMES_TUI_PROACTIVE_GC = 'on'
+    stubGc()
+    expect(proactiveGcEnabled(8192)).toBe(true)
+  })
+})
+
+describe('startProactiveGc — idle gating', () => {
+  test('no-op inert handles on the default path', () => {
+    delete process.env.HERMES_TUI_HEAP_MB
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    const gc = stubGc()
+    vi.useFakeTimers()
+    const h = startProactiveGc(() => false)
+    vi.advanceTimersByTime(60_000)
+    expect(gc).not.toHaveBeenCalled()
+    h.stop()
+  })
+
+  test('fires after a full idle window when not streaming', () => {
+    process.env.HERMES_TUI_HEAP_MB = '256'
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    const gc = stubGc()
+    vi.useFakeTimers()
+    const h = startProactiveGc(() => false)
+    // before the idle window elapses: no GC
+    vi.advanceTimersByTime(3000)
+    expect(gc).not.toHaveBeenCalled()
+    // past the 8s idle window: GC fires
+    vi.advanceTimersByTime(9000)
+    expect(gc).toHaveBeenCalled()
+    h.stop()
+  })
+
+  test('never fires while streaming, no matter how long', () => {
+    process.env.HERMES_TUI_HEAP_MB = '256'
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    const gc = stubGc()
+    vi.useFakeTimers()
+    const h = startProactiveGc(() => true) // always streaming
+    vi.advanceTimersByTime(60_000)
+    expect(gc).not.toHaveBeenCalled()
+    h.stop()
+  })
+
+  test('stop() halts the watcher', () => {
+    process.env.HERMES_TUI_HEAP_MB = '256'
+    delete process.env.HERMES_TUI_PROACTIVE_GC
+    const gc = stubGc()
+    vi.useFakeTimers()
+    const h = startProactiveGc(() => false)
+    h.stop()
+    vi.advanceTimersByTime(60_000)
+    expect(gc).not.toHaveBeenCalled()
+  })
+})