diff --git a/src/relay/pty-handler-output-drain-differential.test.ts b/src/relay/pty-handler-output-drain-differential.test.ts
new file mode 100644
index 00000000000..b2527e92ec4
--- /dev/null
+++ b/src/relay/pty-handler-output-drain-differential.test.ts
@@ -0,0 +1,306 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from 'vitest'
+
+const { mockPtySpawn, mockPtyInstance } = vi.hoisted(() => ({
+  mockPtySpawn: vi.fn(),
+  mockPtyInstance: {
+    pid: process.pid,
+    onData: vi.fn(),
+    onExit: vi.fn(),
+    write: vi.fn(),
+    resize: vi.fn(),
+    kill: vi.fn(),
+    clear: vi.fn()
+  }
+}))
+
+vi.mock('node-pty', () => ({ spawn: mockPtySpawn }))
+
+import { PtyHandler } from './pty-handler'
+import type { RelayDispatcher } from './dispatcher'
+
+// Mirrors the relay drain constants; kept local so a constant change fails this oracle loudly.
+const CHUNK_CHARS = 16 * 1024
+const MAX_WRITES = 2
+const BATCH_INTERVAL_MS = 8
+const DRAIN_CONTINUE_MS = 1
+
+type PendingOutput = { data: string; rawLength?: number; seq?: number }
+type DataEvent = { id: string; data: string; seq?: number; rawLength?: number }
+
+/**
+ * The pre-optimization implementation, verbatim in behavior: snapshot the whole pending map each
+ * tick, then consume up to MAX_WRITES from that frozen list. The bounded-prefix capture must match
+ * it event-for-event — including which entry leads each tick.
+ */
+function legacyFlushTick(pendingById: Map<string, PendingOutput>): {
+  events: DataEvent[]
+  writes: number
+  reschedules: boolean
+} {
+  const events: DataEvent[] = []
+  let writes = 0
+  for (const [id, pending] of Array.from(pendingById.entries())) {
+    if (writes >= MAX_WRITES) {
+      break
+    }
+    pendingById.delete(id)
+    const chunk = pending.data.slice(0, CHUNK_CHARS)
+    const remaining = pending.data.slice(CHUNK_CHARS)
+    if (remaining) {
+      pendingById.set(id, {
+        data: remaining,
+        ...(pending.rawLength === undefined ? {} : { rawLength: remaining.length }),
+        seq: pending.seq
+      })
+    }
+    events.push({
+      id,
+      data: chunk,
+      ...(pending.seq === undefined
+        ? {}
+        : { seq: pending.seq - (pending.data.length - chunk.length) }),
+      ...(pending.rawLength === undefined ? {} : { rawLength: chunk.length })
+    })
+    writes += 1
+  }
+  return { events, writes, reschedules: pendingById.size > 0 && writes > 0 }
+}
+
+function legacyTimeline(initial: Map<string, PendingOutput>): DataEvent[] {
+  const pendingById = new Map(initial)
+  const timeline: DataEvent[] = []
+  for (let tick = 0; tick < 500 && pendingById.size > 0; tick++) {
+    const result = legacyFlushTick(pendingById)
+    timeline.push(...result.events)
+    if (!result.reschedules) {
+      break
+    }
+  }
+  return timeline
+}
+
+// xorshift32 — deterministic across platforms, no Math.random.
+function createRandom(seed: number): () => number {
+  let state = seed >>> 0 || 1
+  return () => {
+    state ^= state << 13
+    state ^= state >>> 17
+    state ^= state << 5
+    state >>>= 0
+    return state / 0x1_0000_0000
+  }
+}
+
+describe('relay PTY output drain — differential vs the pre-optimization snapshot loop', () => {
+  let dispatcher: {
+    notify: ReturnType<typeof vi.fn>
+    callRequest: (method: string, params?: Record<string, unknown>) => Promise<unknown>
+    _notifications: { method: string; params?: Record<string, unknown> }[]
+  }
+  let handler: PtyHandler
+  let dataCallbacks: Map<string, (data: string) => void>
+  let exitCallbacks: Map<string, (event: { exitCode: number }) => void>
+  let onNotifyData: ((frame: { id: string; data: string }) => void) | null
+
+  beforeEach(() => {
+    vi.useFakeTimers()
+    mockPtySpawn.mockReset()
+    dataCallbacks = new Map()
+    exitCallbacks = new Map()
+    onNotifyData = null
+
+    const requestHandlers = new Map<string, (params: Record<string, unknown>) => Promise<unknown>>()
+    const notifications: { method: string; params?: Record<string, unknown> }[] = []
+    let reentering = false
+    dispatcher = {
+      notify: vi.fn((method: string, params?: Record<string, unknown>) => {
+        notifications.push({ method, params })
+        if (method !== 'pty.data' || !onNotifyData || reentering) {
+          return
+        }
+        // Why: the relay sink is a synchronous write; this hook models a sink that re-enters
+        // PTY ingress before the drain returns.
+        reentering = true
+        try {
+          onNotifyData(params as unknown as { id: string; data: string })
+        } finally {
+          reentering = false
+        }
+      }),
+      callRequest: async (method: string, params: Record<string, unknown> = {}) => {
+        const h = requestHandlers.get(method)
+        if (!h) {
+          throw new Error(`No handler for ${method}`)
+        }
+        return h(params)
+      },
+      _notifications: notifications
+    }
+    const full = {
+      onRequest: vi.fn((method: string, h: (params: Record<string, unknown>) => Promise<unknown>) =>
+        requestHandlers.set(method, h)
+      ),
+      onNotification: vi.fn(),
+      notify: dispatcher.notify
+    }
+    handler = new PtyHandler(full as unknown as RelayDispatcher)
+  })
+
+  afterEach(async () => {
+    onNotifyData = null
+    const cleanup = handler.dispose({ waitForPhysicalExit: false })
+    await vi.runAllTimersAsync()
+    await cleanup.catch(() => {})
+    vi.useRealTimers()
+  })
+
+  async function spawnPtys(count: number): Promise<string[]> {
+    const ids: string[] = []
+    for (let i = 0; i < count; i++) {
+      mockPtySpawn.mockReturnValueOnce({
+        ...mockPtyInstance,
+        onData: vi.fn((cb: (data: string) => void) => {
+          dataCallbacks.set(`pty-${i + 1}`, cb)
+        }),
+        onExit: vi.fn((cb: (event: { exitCode: number }) => void) => {
+          exitCallbacks.set(`pty-${i + 1}`, cb)
+        })
+      })
+      const spawned = (await dispatcher.callRequest('pty.spawn', {})) as { id: string }
+      ids.push(spawned.id)
+    }
+    return ids
+  }
+
+  function recordedDataEvents(): DataEvent[] {
+    return dispatcher._notifications
+      .filter((n) => n.method === 'pty.data')
+      .map((n) => n.params as unknown as DataEvent)
+  }
+
+  it('emits the same pty.data timeline as the snapshot loop for multi-PTY multi-chunk drains', async () => {
+    const sessionCount = 4
+    const ids = await spawnPtys(sessionCount)
+
+    const payloads = new Map<string, string>()
+    const expectedPending = new Map<string, PendingOutput>()
+    ids.forEach((id, index) => {
+      // Vary chunk counts so PTYs finish on different ticks.
+      const data = `${String.fromCharCode(97 + index)}`.repeat(CHUNK_CHARS * (index + 1) + index)
+      payloads.set(id, data)
+      expectedPending.set(id, { data })
+    })
+
+    for (const id of ids) {
+      dataCallbacks.get(id)!(payloads.get(id)!)
+    }
+
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    for (let tick = 0; tick < 200; tick++) {
+      await vi.advanceTimersByTimeAsync(DRAIN_CONTINUE_MS)
+    }
+
+    expect(recordedDataEvents()).toEqual(legacyTimeline(expectedPending))
+  })
+
+  it('matches the snapshot loop across randomized session counts and payload sizes', async () => {
+    const random = createRandom(0x5eed)
+    const sessionCount = 5
+    const ids = await spawnPtys(sessionCount)
+
+    const expectedPending = new Map<string, PendingOutput>()
+    for (const id of ids) {
+      const chunks = 1 + Math.floor(random() * 3)
+      const extra = Math.floor(random() * 64)
+      const data = 'z'.repeat(CHUNK_CHARS * chunks + extra)
+      expectedPending.set(id, { data })
+      dataCallbacks.get(id)!(data)
+    }
+
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    for (let tick = 0; tick < 300; tick++) {
+      await vi.advanceTimersByTimeAsync(DRAIN_CONTINUE_MS)
+    }
+
+    expect(recordedDataEvents()).toEqual(legacyTimeline(expectedPending))
+  })
+
+  it('keeps one write per tick when a single PTY drains — pacing a lazy iterator walk breaks', async () => {
+    const [id] = await spawnPtys(1)
+    const first = 'x'.repeat(CHUNK_CHARS)
+    dataCallbacks.get(id)!(`${first}tail`)
+
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    // Iterating the live map lazily would pick the re-queued remainder up again in this same tick.
+    expect(recordedDataEvents()).toEqual([{ id, data: first }])
+
+    await vi.advanceTimersByTimeAsync(DRAIN_CONTINUE_MS)
+    expect(recordedDataEvents()).toEqual([
+      { id, data: first },
+      { id, data: 'tail' }
+    ])
+  })
+
+  it('writes at most two PTYs per tick and rotates the rest to the next tick', async () => {
+    const ids = await spawnPtys(3)
+    for (const id of ids) {
+      dataCallbacks.get(id)!('a'.repeat(CHUNK_CHARS + 4))
+    }
+
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    // First tick writes the two head PTYs only.
+    expect(recordedDataEvents().map((event) => event.id)).toEqual([ids[0], ids[1]])
+
+    await vi.advanceTimersByTimeAsync(DRAIN_CONTINUE_MS)
+    // Third PTY leads the next tick; the two re-queued remainders sit behind it.
+    expect(recordedDataEvents().map((event) => event.id)).toEqual([ids[0], ids[1], ids[2], ids[0]])
+  })
+
+  it('freezes the tick batch before the first send, matching the snapshot loop under re-entrancy', async () => {
+    const ids = await spawnPtys(2)
+    dataCallbacks.get(ids[0])!('first')
+    dataCallbacks.get(ids[1])!('second')
+
+    // Append to the second PTY while the first is being sent. Capturing the batch up front freezes
+    // the same values the whole-map snapshot froze, so this tick's output is unchanged.
+    onNotifyData = (frame) => {
+      if (frame.id === ids[0]) {
+        dataCallbacks.get(ids[1])!('-appended')
+      }
+    }
+
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    expect(recordedDataEvents()).toEqual([
+      { id: ids[0], data: 'first' },
+      { id: ids[1], data: 'second' }
+    ])
+
+    onNotifyData = null
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    await vi.advanceTimersByTimeAsync(BATCH_INTERVAL_MS)
+    // Verbatim pre-change behavior, quirk included: the frozen entry is sent and then deleted, so
+    // bytes appended to it mid-tick are dropped. Reachable only from a sink that re-enters PTY
+    // ingress synchronously; the relay's real sinks are stdout/socket writes that cannot. Asserted
+    // here so the optimization is pinned to "no behavior change" rather than a silent improvement.
+    expect(recordedDataEvents()).toEqual([
+      { id: ids[0], data: 'first' },
+      { id: ids[1], data: 'second' }
+    ])
+  })
+
+  it('flushes pending bytes before pty.exit', async () => {
+    // PTY exit routes through flushPtyOutput (the whole-entry path), not the chunked drain.
+    const [id] = await spawnPtys(1)
+    dataCallbacks.get(id)!('tail bytes')
+    exitCallbacks.get(id)!({ exitCode: 0 })
+
+    const methods = dispatcher._notifications.map((n) => n.method)
+    // Ordering invariant: buffered output must land before the exit frame.
+    expect(methods.indexOf('pty.data')).toBeGreaterThanOrEqual(0)
+    expect(methods.indexOf('pty.data')).toBeLessThan(methods.indexOf('pty.exit'))
+
+    const dataFrames = dispatcher._notifications.filter((n) => n.method === 'pty.data')
+    expect(dataFrames).toHaveLength(1)
+    expect(dataFrames[0].params).toEqual({ id, data: 'tail bytes' })
+  })
+})
diff --git a/src/relay/pty-handler.ts b/src/relay/pty-handler.ts
index ce2059e30aa..26dd6e085ca 100644
--- a/src/relay/pty-handler.ts
+++ b/src/relay/pty-handler.ts
@@ -723,11 +723,21 @@ export class PtyHandler {
 
   private flushPendingOutput(): void {
     this.outputFlushTimer = null
-    let writes = 0
-    for (const [id, pending] of Array.from(this.pendingOutputByPty.entries())) {
-      if (writes >= PTY_OUTPUT_FLUSH_MAX_WRITES) {
+    // Why: the send loop has no skip path and stops at PTY_OUTPUT_FLUSH_MAX_WRITES, so it can never
+    // reach a third entry — `Array.from(entries())` allocated one tuple per session every tick to
+    // consume at most two. Capture only that prefix, and capture it *before* the first send so a
+    // re-entrant sink still reads the values a whole-map snapshot would have frozen.
+    // Why the explicit iterator: `for...of` would advance one tuple past the limit and discard it.
+    const pendingEntries = this.pendingOutputByPty[Symbol.iterator]()
+    const batch: [string, PendingPtyOutput][] = []
+    while (batch.length < PTY_OUTPUT_FLUSH_MAX_WRITES) {
+      const next = pendingEntries.next()
+      if (next.done === true) {
         break
       }
+      batch.push(next.value)
+    }
+    for (const [id, pending] of batch) {
       this.pendingOutputByPty.delete(id)
       const chunk = pending.transformed
         ? pending.data
@@ -754,9 +764,8 @@ export class PtyHandler {
         ...(chunkRawLength === undefined ? {} : { rawLength: chunkRawLength }),
         ...(pending.transformed ? { transformed: true } : {})
       })
-      writes++
     }
-    if (this.pendingOutputByPty.size > 0 && writes > 0) {
+    if (this.pendingOutputByPty.size > 0 && batch.length > 0) {
       // Why: yield between slices of a large chunk so client input and control frames can interleave.
       this.scheduleOutputFlush(PTY_OUTPUT_DRAIN_CONTINUE_MS)
     }