diff --git a/src/anthropic/lib/environments/_poller.py b/src/anthropic/lib/environments/_poller.py
index e1f27d788..d40515885 100644
--- a/src/anthropic/lib/environments/_poller.py
+++ b/src/anthropic/lib/environments/_poller.py
@@ -137,25 +137,37 @@ def iter_work(
             time.sleep(_jitter(1.0, 3.0))
             continue
         log.info("claimed work work_id=%s work_type=%s", item.id, getattr(item.data, "type", None))
-        try:
-            work.ack(
-                item.id,
-                environment_id=environment_id,
-                extra_headers=extra_headers,
-            )
-        except TRANSIENT_ERRORS as e:
-            if _is_fatal_4xx(e):
-                log.error("ack failed permanently; force-stopping work_id=%s error=%s", item.id, e)
-                _force_stop_quietly(work, item.id, environment_id=environment_id, extra_headers=extra_headers)
+
+        acked = False
+        while not acked:
+            try:
+                work.ack(
+                    item.id,
+                    environment_id=environment_id,
+                    extra_headers=extra_headers,
+                )
+            except TRANSIENT_ERRORS as e:
+                if _is_fatal_4xx(e):
+                    log.error("ack failed permanently; force-stopping work_id=%s error=%s", item.id, e)
+                    _force_stop_quietly(work, item.id, environment_id=environment_id, extra_headers=extra_headers)
+                    ack_attempt = 0
+                    break
+                ack_attempt += 1
+                wait = _backoff(ack_attempt) + _jitter(0.0, 1.0)
+                log.warning(
+                    "ack failed, backing off work_id=%s attempt=%d backoff=%.1fs error=%s",
+                    item.id,
+                    ack_attempt,
+                    wait,
+                    e,
+                )
+                time.sleep(wait)
                 continue
-            ack_attempt += 1
-            wait = _backoff(ack_attempt) + _jitter(0.0, 1.0)
-            log.warning(
-                "ack failed, backing off work_id=%s attempt=%d backoff=%.1fs error=%s", item.id, ack_attempt, wait, e
-            )
-            time.sleep(wait)
+            ack_attempt = 0
+            acked = True
+
+        if not acked:
             continue
-        ack_attempt = 0
         if not auto_stop:
             yield item
             continue
@@ -230,25 +242,42 @@ async def aiter_work(
             await anyio.sleep(_jitter(1.0, 3.0))
             continue
         log.info("claimed work work_id=%s work_type=%s", item.id, getattr(item.data, "type", None))
-        try:
-            await work.ack(
-                item.id,
-                environment_id=environment_id,
-                extra_headers=extra_headers,
-            )
-        except TRANSIENT_ERRORS as e:
-            if _is_fatal_4xx(e):
-                log.error("ack failed permanently; force-stopping work_id=%s error=%s", item.id, e)
-                await _aforce_stop_quietly(work, item.id, environment_id=environment_id, extra_headers=extra_headers)
+
+        acked = False
+        while not acked:
+            try:
+                await work.ack(
+                    item.id,
+                    environment_id=environment_id,
+                    extra_headers=extra_headers,
+                )
+            except TRANSIENT_ERRORS as e:
+                if _is_fatal_4xx(e):
+                    log.error("ack failed permanently; force-stopping work_id=%s error=%s", item.id, e)
+                    await _aforce_stop_quietly(
+                        work,
+                        item.id,
+                        environment_id=environment_id,
+                        extra_headers=extra_headers,
+                    )
+                    ack_attempt = 0
+                    break
+                ack_attempt += 1
+                wait = _backoff(ack_attempt) + _jitter(0.0, 1.0)
+                log.warning(
+                    "ack failed, backing off work_id=%s attempt=%d backoff=%.1fs error=%s",
+                    item.id,
+                    ack_attempt,
+                    wait,
+                    e,
+                )
+                await anyio.sleep(wait)
                 continue
-            ack_attempt += 1
-            wait = _backoff(ack_attempt) + _jitter(0.0, 1.0)
-            log.warning(
-                "ack failed, backing off work_id=%s attempt=%d backoff=%.1fs error=%s", item.id, ack_attempt, wait, e
-            )
-            await anyio.sleep(wait)
+            ack_attempt = 0
+            acked = True
+
+        if not acked:
             continue
-        ack_attempt = 0
         if not auto_stop:
             yield item
             continue
diff --git a/tests/lib/environments/test_poller_ack_retry.py b/tests/lib/environments/test_poller_ack_retry.py
new file mode 100644
index 000000000..02716149c
--- /dev/null
+++ b/tests/lib/environments/test_poller_ack_retry.py
@@ -0,0 +1,106 @@
+from __future__ import annotations
+
+from typing import Any, cast
+
+import httpx
+import pytest
+
+from anthropic import APIStatusError
+from anthropic.lib.environments import _poller
+from anthropic.lib.environments._poller import aiter_work, iter_work
+
+
+class _WorkData:
+    type = "session"
+
+
+class _WorkItem:
+    def __init__(self, work_id: str = "work_1") -> None:
+        self.id = work_id
+        self.data = _WorkData()
+
+
+def _api_status_error(code: int) -> APIStatusError:
+    request = httpx.Request("POST", "https://api.example/work")
+    response = httpx.Response(code, request=request, content=b"{}")
+    return APIStatusError("boom", response=response, body=None)
+
+
+class _SyncWork:
+    def __init__(self) -> None:
+        self.item = _WorkItem()
+        self.events: list[str] = []
+        self.poll_calls = 0
+        self.ack_calls = 0
+
+    def poll(self, _environment_id: str, **_kwargs: Any) -> _WorkItem:
+        self.poll_calls += 1
+        self.events.append("poll")
+        if self.poll_calls > 1:
+            raise AssertionError("polled again before the claimed item was acknowledged")
+        return self.item
+
+    def ack(self, work_id: str, **_kwargs: Any) -> None:
+        assert work_id == self.item.id
+        self.ack_calls += 1
+        self.events.append("ack")
+        if self.ack_calls == 1:
+            raise _api_status_error(500)
+
+    def stop(self, _work_id: str, **_kwargs: Any) -> None:
+        raise AssertionError("auto_stop=False should not stop the item")
+
+
+class _AsyncWork:
+    def __init__(self) -> None:
+        self.item = _WorkItem()
+        self.events: list[str] = []
+        self.poll_calls = 0
+        self.ack_calls = 0
+
+    async def poll(self, _environment_id: str, **_kwargs: Any) -> _WorkItem:
+        self.poll_calls += 1
+        self.events.append("poll")
+        if self.poll_calls > 1:
+            raise AssertionError("polled again before the claimed item was acknowledged")
+        return self.item
+
+    async def ack(self, work_id: str, **_kwargs: Any) -> None:
+        assert work_id == self.item.id
+        self.ack_calls += 1
+        self.events.append("ack")
+        if self.ack_calls == 1:
+            raise _api_status_error(500)
+
+    async def stop(self, _work_id: str, **_kwargs: Any) -> None:
+        raise AssertionError("auto_stop=False should not stop the item")
+
+
+def test_sync_transient_ack_failure_retries_same_claim_before_polling(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(_poller.time, "sleep", lambda _seconds: None)
+    work = _SyncWork()
+
+    item = next(iter_work(cast(Any, work), environment_id="env_1", auto_stop=False))
+
+    assert item is work.item
+    assert work.events == ["poll", "ack", "ack"]
+    assert work.poll_calls == 1
+
+
+async def test_async_transient_ack_failure_retries_same_claim_before_polling(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def _no_sleep(_seconds: float) -> None:
+        return None
+
+    monkeypatch.setattr(_poller.anyio, "sleep", _no_sleep)
+    work = _AsyncWork()
+    iterator = aiter_work(cast(Any, work), environment_id="env_1", auto_stop=False)
+
+    item = await iterator.__anext__()
+
+    assert item is work.item
+    assert work.events == ["poll", "ack", "ack"]
+    assert work.poll_calls == 1