diff --git a/distributed/client.py b/distributed/client.py
index 31bc53ae37..242cbdd76e 100644
--- a/distributed/client.py
+++ b/distributed/client.py
@@ -3916,8 +3916,19 @@ async def _restart_workers(
             timeout = self._timeout * 4
         timeout = parse_timedelta(cast("str|int|float", timeout), "s")
 
-        info = self.scheduler_info()
-        name_to_addr = {meta["name"]: addr for addr, meta in info["workers"].items()}
+        # Fetch info for all workers (not the truncated cache from
+        # ``scheduler_info``) so that name resolution and the nanny check below
+        # are correct on clusters with many workers.
+        workers_info = (await self.scheduler.identity(n_workers=-1))["workers"]
+
+        for addr, meta in workers_info.items():
+            if (addr in workers or meta["name"] in workers) and meta["nanny"] is None:
+                raise ValueError(
+                    f"Restarting workers requires a nanny to be used. Worker "
+                    f"{addr} has type {meta['type']}."
+                )
+
+        name_to_addr = {meta["name"]: addr for addr, meta in workers_info.items()}
         worker_addrs = [name_to_addr.get(w, w) for w in workers]
 
         out: dict[
@@ -3989,14 +4000,6 @@ def restart_workers(
         --------
         Client.restart
         """
-        info = self.scheduler_info()
-
-        for worker, meta in info["workers"].items():
-            if (worker in workers or meta["name"] in workers) and meta["nanny"] is None:
-                raise ValueError(
-                    f"Restarting workers requires a nanny to be used. Worker "
-                    f"{worker} has type {info['workers'][worker]['type']}."
-                )
         return self.sync(
             self._restart_workers,
             workers=workers,
@@ -4475,14 +4478,21 @@ async def _profile(
         else:
             return state
 
-    def scheduler_info(self, n_workers: int = 5, **kwargs: Any) -> SchedulerInfo:
+    def scheduler_info(self, n_workers: int = -1, **kwargs: Any) -> SchedulerInfo:
         """Basic information about the workers in the cluster
 
         Parameters
         ----------
         n_workers: int
-            The number of workers for which to fetch information. To fetch all,
-            use -1.
+            The number of workers for which to fetch information. Defaults to
+            ``-1``, which fetches all workers. Pass a positive integer to limit
+            the number of workers returned.
+
+            Note: this argument is only honored for synchronous clients. For
+            asynchronous clients this method returns the most recently cached
+            value (refreshed periodically) without a fresh fetch; use
+            ``await client.scheduler.identity(n_workers=-1)`` to fetch all
+            workers on demand.
         **kwargs : dict
             Optional keyword arguments for the remote function
 
diff --git a/distributed/tests/test_client.py b/distributed/tests/test_client.py
index d2123d43ad..d0dffeedbc 100644
--- a/distributed/tests/test_client.py
+++ b/distributed/tests/test_client.py
@@ -2112,10 +2112,12 @@ def test_repr(loop, func):
         *workers,
     ):
         with Client(s["address"], loop=loop) as c:
-            # NOTE: Intentionally testing when we have more workers than the default
-            # in `client.scheduler_info()` (xref https://github.com/dask/distributed/issues/9065)
+            # NOTE: Intentionally testing the repr with more workers than the repr
+            # itself displays (the HTML view caps the worker table at 5).
+            # ``scheduler_info()`` returns all workers by default
+            # (xref https://github.com/dask/distributed/issues/9065).
             info = c.scheduler_info()
-            assert len(info["workers"]) < nworkers
+            assert len(info["workers"]) == nworkers
 
             text = func(c)
             assert c.scheduler.address in text
@@ -3948,12 +3950,15 @@ async def test_idempotence(s, a, b):
 
 
 def test_scheduler_info(c):
+    # By default all workers are returned
     info = c.scheduler_info()
     assert isinstance(info, dict)
     assert len(info["workers"]) == 2
     assert isinstance(info["started"], float)
+    # A positive ``n_workers`` truncates the worker list
     info = c.scheduler_info(n_workers=1)
     assert len(info["workers"]) == 1
+    # ``-1`` is equivalent to the default and returns all workers
     info = c.scheduler_info(n_workers=-1)
     assert len(info["workers"]) == 2
 
@@ -4768,6 +4773,21 @@ async def test_restart_workers_no_nanny_raises(c, s, a, b):
     assert a.address in msg
 
 
+@gen_cluster(client=True, nthreads=[("", 1)] * 7)
+async def test_restart_workers_no_nanny_raises_many_workers(c, s, *workers):
+    # Regression test: the nanny check must consider all workers, not just the
+    # truncated set that ``scheduler_info`` used to return by default
+    # (xref https://github.com/dask/distributed/issues/9065). Target a worker
+    # beyond the historical 5-worker truncation cap.
+    assert len(s.workers) == 7
+    target = list(s.workers)[-1]
+    with pytest.raises(ValueError) as excinfo:
+        await c.restart_workers(workers=[target])
+    msg = str(excinfo.value).lower()
+    assert "restarting workers requires a nanny" in msg
+    assert target in msg
+
+
 @pytest.mark.slow
 @pytest.mark.parametrize("raise_for_error", (True, False))
 @gen_cluster(client=True, nthreads=[("", 1)], Worker=BlockedKillNanny)