From c2f5a875bc1491bb2342e183fb393855dc0b4ca5 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 9 Oct 2026 17:01:35 +0200
Subject: [PATCH 01/15] feat: add a concurrency limit for named child runs

---
 src/apify/_actor.py                       |  16 ++
 src/apify/_child_runs.py                  | 134 +++++++++--
 tests/e2e/test_actor_child_runs.py        |  33 +++
 tests/unit/actor/test_actor_child_runs.py | 281 ++++++++++++++++++++++
 4 files changed, 448 insertions(+), 16 deletions(-)

diff --git a/src/apify/_actor.py b/src/apify/_actor.py
index 7bb01019..613a331f 100644
--- a/src/apify/_actor.py
+++ b/src/apify/_actor.py
@@ -445,6 +445,22 @@ def child_runs(self) -> dict[str, RunClientAsync]:
         """
         return self._child_run_registry.run_clients(self.apify_client)
 
+    def set_child_run_limits(self, *, max_concurrent_runs: int | None) -> None:
+        """Limit the named child runs of this Actor run.
+
+        While `max_concurrent_runs` named child runs are `READY`, `RUNNING`, `ABORTING` or `TIMING-OUT`, a named
+        `Actor.start`, `Actor.call`, `Actor.start_task` or `Actor.call_task` that would start or resurrect a run waits
+        until one of them finishes. Reattaching to a recorded run never waits. Runs started without a `run_name` are not
+        counted and never wait. A child run not awaited by `Actor.call` or `Actor.call_task` is fetched again before it
+        is counted, if its status is more than 10 seconds old.
+
+        The limit is kept in memory, so call this method again after a migration or resurrection of this Actor run.
+
+        Args:
+            max_concurrent_runs: How many named child runs may be active at once, or `None` for no limit.
+        """
+        self._child_run_registry.set_max_concurrent_runs(max_concurrent_runs)
+
     @cached_property
     def configuration(self) -> Configuration:
         """Actor configuration, uses the default instance if not explicitly set."""
diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index d91e74da..bda8a0cb 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -3,7 +3,8 @@
 import asyncio
 import hashlib
 import json
-from datetime import datetime
+from contextlib import asynccontextmanager, suppress
+from datetime import datetime, timedelta
 from logging import getLogger
 from typing import TYPE_CHECKING, Any
 from weakref import WeakValueDictionary
@@ -14,7 +15,7 @@
 from apify_client.errors import NotFoundError
 
 if TYPE_CHECKING:
-    from collections.abc import Awaitable, Callable
+    from collections.abc import AsyncGenerator, Awaitable, Callable
 
     from apify_client import ApifyClientAsync
     from apify_client._models import Run
@@ -34,6 +35,11 @@
 
 _ABORTED_STATUSES = frozenset({'ABORTING', 'ABORTED'})
 
+_ACTIVE_STATUSES = frozenset({'READY', 'RUNNING', 'ABORTING', 'TIMING-OUT'})
+
+_STATUS_MAX_AGE = timedelta(seconds=10)
+"""How long an observed active status counts toward the concurrency limit before the run is fetched again."""
+
 _NOT_FOUND_GRACE_SECS = 3
 """How long a recorded run that the API reports as missing is looked up again before it counts as gone."""
 
@@ -113,9 +119,21 @@ def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]])
         """Serializes `find_or_start` per name. A lock is dropped once no call under its name holds it."""
         self._clients: dict[str, ApifyClientAsync] = {}
         """Client the run under each name was last started or looked up with. Lost on a migration."""
+        self._max_concurrent_runs: int | None = None
+        self._slots = asyncio.Condition()
+        self._starting: set[str] = set()
+        """Names holding a slot for a start or resurrection in flight, not counted from the records yet."""
+        self._observed: dict[str, tuple[str, float]] = {}
+        """Last observed status of the run recorded under each name, with the event loop time it was observed at."""
         self._parent_aborting = False
         """Whether this Actor run is being gracefully aborted, after which no run marked `abort_with_parent` starts."""
 
+    def set_max_concurrent_runs(self, max_concurrent_runs: int | None) -> None:
+        """Set how many recorded runs may be active at once, or remove the limit with `None`."""
+        if max_concurrent_runs is not None and max_concurrent_runs < 1:
+            raise ValueError(f'`max_concurrent_runs` must be at least 1, got {max_concurrent_runs}.')
+        self._max_concurrent_runs = max_concurrent_runs
+
     async def find_or_start(
         self,
         name: str,
@@ -135,6 +153,8 @@ async def find_or_start(
         A `FAILED` run, or one the API no longer knows, is replaced by a new run under the same name, for any Actor,
         task or input.
 
+        Starting or resurrecting a run waits while the concurrency limit is reached. Reattaching never waits.
+
         Args:
             name: Name of the child run, unique within the parent run.
             actor_id: The Actor to start.
@@ -168,9 +188,10 @@ async def find_or_start(
             record = records.get(name)
 
             if record is None:
-                run = await self._start(
-                    name, checksum=checksum, start_run=start_run, history=[], abort_with_parent=abort_with_parent
-                )
+                async with self._slot(name, client):
+                    run = await self._start(
+                        name, checksum=checksum, start_run=start_run, history=[], abort_with_parent=abort_with_parent
+                    )
                 self._clients[name] = client
                 return run, True
 
@@ -186,13 +207,14 @@ async def find_or_start(
                     status=run.status if run is not None else 'LOST',
                     started_at=record.started_at,
                 )
-                run = await self._start(
-                    name,
-                    checksum=checksum,
-                    start_run=start_run,
-                    history=[*record.history, replaced],
-                    abort_with_parent=abort_with_parent,
-                )
+                async with self._slot(name, client):
+                    run = await self._start(
+                        name,
+                        checksum=checksum,
+                        start_run=start_run,
+                        history=[*record.history, replaced],
+                        abort_with_parent=abort_with_parent,
+                    )
                 self._clients[name] = client
                 return run, True
 
@@ -202,12 +224,16 @@ async def find_or_start(
                     'Use a unique `run_name` for each child run.'
                 )
 
+            self._observe(name, run)
+
             if record.abort_with_parent != abort_with_parent:
                 await self._save(name, record.model_copy(update={'abort_with_parent': abort_with_parent}))
 
             if run.status in _RESURRECTABLE_STATUSES:
-                logger.info(f'Resurrecting child run "{name}"', extra={'run_id': run.id, 'status': run.status})
-                run = await resurrect_run(run.id)
+                async with self._slot(name, client):
+                    logger.info(f'Resurrecting child run "{name}"', extra={'run_id': run.id, 'status': run.status})
+                    run = await resurrect_run(run.id)
+                    self._observe(name, run)
             else:
                 logger.info(f'Reattaching to child run "{name}"', extra={'run_id': run.id, 'status': run.status})
 
@@ -233,14 +259,20 @@ def run_clients(self, default_client: ApifyClientAsync) -> dict[str, RunClientAs
     async def update(self, name: str, run: Run) -> None:
         """Record the latest observed status of the run recorded under `name`.
 
+        A run that is no longer active releases its slot under the concurrency limit.
+
         Args:
             name: Name of the child run.
             run: The run as just returned by the API. Nothing is recorded unless it is the current run under `name`.
         """
         record = (await self._load()).get(name)
-        if record is None or record.run_id != run.id or record.status == run.status:
+        if record is None or record.run_id != run.id:
             return
-        await self._save(name, record.model_copy(update={'status': run.status}))
+        self._observe(name, run)
+        if record.status != run.status:
+            await self._save(name, record.model_copy(update={'status': run.status}))
+        async with self._slots:
+            self._slots.notify_all()
 
     async def abort_runs_with_parent(self, client: ApifyClientAsync) -> None:
         """Gracefully abort every recorded run marked `abort_with_parent`, and refuse to start any more of them.
@@ -253,6 +285,9 @@ async def abort_runs_with_parent(self, client: ApifyClientAsync) -> None:
             client: Client used for a name not started or looked up in this process, e.g. after a migration.
         """
         self._parent_aborting = True
+        async with self._slots:
+            self._slots.notify_all()
+
         records = await self._load()
         # Names with a start in flight are not recorded yet, so their locks are awaited too.
         await asyncio.gather(*(self._abort(name, client) for name in {*records, *self._name_locks}))
@@ -292,8 +327,75 @@ async def _start(
             abort_with_parent=abort_with_parent,
         )
         await self._save(name, record)
+        self._observe(name, run)
         return run
 
+    @asynccontextmanager
+    async def _slot(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[None]:
+        """Hold a slot for starting or resurrecting the run under `name`, waiting while the limit is reached."""
+        if self._max_concurrent_runs is None:
+            yield
+            return
+
+        async with self._slots:
+            while (
+                self._max_concurrent_runs is not None
+                and await self._count_active(client, exclude=name) >= self._max_concurrent_runs
+            ):
+                if self._parent_aborting:
+                    raise RuntimeError(
+                        f'Child run "{name}" was not started, since this Actor run is being aborted and the limit '
+                        f'of {self._max_concurrent_runs} concurrent child runs is reached.'
+                    )
+                logger.debug(f'Child run "{name}" is waiting for a free slot')
+                with suppress(TimeoutError):
+                    await asyncio.wait_for(self._slots.wait(), timeout=_STATUS_MAX_AGE.total_seconds())
+            self._starting.add(name)
+
+        try:
+            yield
+        finally:
+            self._starting.discard(name)
+            async with self._slots:
+                self._slots.notify_all()
+
+    async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
+        """Count active recorded runs and slots held by others, fetching runs whose active status is not fresh."""
+        records = await self._load()
+        names = [name for name in records if name != exclude and name not in self._starting]
+        now = asyncio.get_running_loop().time()
+        stale = [
+            name
+            for name in names
+            if name not in self._observed
+            or (
+                self._observed[name][0] in _ACTIVE_STATUSES
+                and now - self._observed[name][1] >= _STATUS_MAX_AGE.total_seconds()
+            )
+        ]
+        runs = await asyncio.gather(
+            *(self._clients.get(name, client).run(records[name].run_id).get() for name in stale),
+            return_exceptions=True,
+        )
+        for name, run in zip(stale, runs, strict=True):
+            if isinstance(run, BaseException):
+                logger.warning(
+                    f'Failed to fetch child run "{name}" to count it toward the concurrency limit',
+                    extra={'run_id': records[name].run_id},
+                    exc_info=run,
+                )
+            elif run is None:
+                self._observed[name] = ('MISSING', now)
+            else:
+                self._observe(name, run)
+
+        # A run that could not be fetched counts only when an earlier observation saw it active.
+        active = sum(1 for name in names if name in self._observed and self._observed[name][0] in _ACTIVE_STATUSES)
+        return active + len(self._starting)
+
+    def _observe(self, name: str, run: Run) -> None:
+        self._observed[name] = (run.status, asyncio.get_running_loop().time())
+
     async def load(self) -> dict[str, ChildRunRecord]:
         """Read the records from the default key-value store, replacing any read before."""
         async with self._lock:
diff --git a/tests/e2e/test_actor_child_runs.py b/tests/e2e/test_actor_child_runs.py
index 05b1352a..a9eefb3b 100644
--- a/tests/e2e/test_actor_child_runs.py
+++ b/tests/e2e/test_actor_child_runs.py
@@ -131,3 +131,36 @@ async def main() -> None:
     child_run = await apify_client_async.run(child_run_id).wait_for_finish(wait_duration=timedelta(seconds=120))
     assert child_run is not None
     assert child_run.status == 'ABORTED'
+
+
+async def test_named_child_runs_respect_the_concurrency_limit(
+    make_actor: MakeActorFunction,
+    run_actor: RunActorFunction,
+) -> None:
+    """Named child runs started concurrently under a limit of one run one after another."""
+
+    async def main() -> None:
+        async with Actor:
+            actor_input = (await Actor.get_input()) or {}
+            if actor_input.get('is_child') is True:
+                await asyncio.sleep(10)
+                return
+
+            actor_id = Actor.configuration.actor_id or ''
+            Actor.set_child_run_limits(max_concurrent_runs=1)
+            runs = await asyncio.gather(
+                *(
+                    Actor.call(actor_id=actor_id, run_input={'is_child': True}, run_name=f'child-{index}')
+                    for index in range(2)
+                )
+            )
+            first, second = sorted(runs, key=lambda run: run.started_at)
+            assert first.finished_at is not None, 'first.finished_at is None'
+            assert second.started_at >= first.finished_at, f'first={first}, second={second}'
+
+    actor = await make_actor(label='child-run-limit', main_func=main)
+    run_result = await run_actor(actor)
+
+    assert run_result.status == 'SUCCEEDED'
+    # The parent run and its two child runs.
+    assert (await actor.runs().list()).total == 3
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index e4bd5d2c..a6820cc8 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1171,3 +1171,284 @@ async def test_removing_all_aborting_listeners_keeps_aborting_child_runs(
         await apify_event_manager.wait_for_all_listeners_to_complete()
 
     assert len(apify_client_async_patcher.calls['run']['abort']) == 1
+
+
+def make_client(statuses: dict[str, str]) -> Mock:
+    """A client whose `run(run_id).get()` returns the run with its current status in `statuses`."""
+    client = Mock()
+
+    def run(run_id: str) -> Mock:
+        run_client = Mock()
+        run_client.get = AsyncMock(side_effect=lambda: make_run(run_id, statuses[run_id]))
+        run_client.resurrect = AsyncMock(side_effect=lambda: make_run(run_id, 'RUNNING'))
+        run_client.abort = AsyncMock()
+        return run_client
+
+    client.run.side_effect = run
+    return client
+
+
+async def start_child(
+    registry: ChildRunRegistry, client: Mock, name: str, statuses: dict[str, str], *, run_id: str | None = None
+) -> Run:
+    """Start a named child run with the registry, adding its run to `statuses` as `RUNNING`."""
+
+    async def start_run() -> Run:
+        new_run_id = run_id or f'{name}-run'
+        statuses[new_run_id] = 'RUNNING'
+        return make_run(new_run_id, 'READY')
+
+    run, _ = await registry.find_or_start(
+        name,
+        actor_id='some-actor',
+        run_input=None,
+        client=client,
+        start_run=start_run,
+        resurrect_run=lambda run_id: client.run(run_id).resurrect(),
+    )
+    return run
+
+
+async def assert_waiting(task: asyncio.Task) -> None:
+    await asyncio.sleep(0.05)
+    assert not task.done()
+
+
+@pytest.mark.parametrize(
+    'max_concurrent_runs',
+    [
+        pytest.param(0, id='zero'),
+        pytest.param(-1, id='negative'),
+    ],
+)
+async def test_set_child_run_limits_rejects_non_positive_limit(max_concurrent_runs: int) -> None:
+    """A concurrency limit below 1 is rejected."""
+    async with Actor:
+        with pytest.raises(ValueError, match='must be at least 1'):
+            Actor.set_child_run_limits(max_concurrent_runs=max_concurrent_runs)
+
+
+async def test_named_start_waits_while_the_limit_is_reached() -> None:
+    """A named start waits while the limit is reached and proceeds once an active child run finishes."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        first = await start_child(registry, client, 'first', statuses)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+
+        statuses[first.id] = 'SUCCEEDED'
+        await registry.update('first', make_run(first.id, 'SUCCEEDED'))
+        second = await second_task
+
+    assert second.id == 'second-run'
+
+
+async def test_removing_the_limit_releases_waiting_starts(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A named start waiting for a slot proceeds once the limit is removed."""
+    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE', timedelta(seconds=0.05))
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        await start_child(registry, client, 'first', statuses)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+
+        registry.set_max_concurrent_runs(None)
+        second = await asyncio.wait_for(second_task, timeout=1)
+
+    assert second.id == 'second-run'
+
+
+async def test_stale_active_status_is_refreshed_before_counting(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A child run nobody awaited is fetched again once its status is stale, so a finished one frees its slot."""
+    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE', timedelta(0))
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        first = await start_child(registry, client, 'first', statuses)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+
+        statuses[first.id] = 'SUCCEEDED'
+        second = await asyncio.wait_for(second_task, timeout=1)
+
+    assert second.id == 'second-run'
+
+
+async def test_child_run_recorded_by_an_earlier_attempt_counts_toward_the_limit() -> None:
+    """An active child run recorded before a migration holds a slot, since it is fetched before it is counted."""
+    statuses = {'old-run': 'RUNNING'}
+    client = make_client(statuses)
+
+    async with Actor:
+        await record_child_run('first', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+        second_task.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await second_task
+
+
+async def test_child_run_that_cannot_be_fetched_does_not_block_the_limit() -> None:
+    """A recorded child run whose fetch fails is not counted, so it does not fail or block a named start."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+    run_client_factory = client.run.side_effect
+
+    def run(run_id: str) -> Mock:
+        run_client = run_client_factory(run_id)
+        if run_id == 'old-run':
+            run_client.get = AsyncMock(side_effect=RuntimeError('forbidden'))
+        return run_client
+
+    client.run.side_effect = run
+
+    async with Actor:
+        await record_child_run('first', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        second = await asyncio.wait_for(start_child(registry, client, 'second', statuses), timeout=1)
+
+    assert second.id == 'second-run'
+
+
+async def test_reattach_does_not_wait_for_a_slot() -> None:
+    """Reattaching to an active recorded child run returns it even while the limit is reached."""
+    statuses = {'old-run': 'RUNNING'}
+    client = make_client(statuses)
+
+    async with Actor:
+        await record_child_run('first', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        run = await asyncio.wait_for(start_child(registry, client, 'first', statuses), timeout=1)
+
+    assert run.id == 'old-run'
+
+
+async def test_resurrection_waits_for_a_slot() -> None:
+    """Resurrecting an aborted child run waits while the limit is reached."""
+    statuses = {'old-run': 'ABORTED'}
+    client = make_client(statuses)
+
+    async with Actor:
+        await record_child_run('first', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        second = await start_child(registry, client, 'second', statuses)
+        first_task = asyncio.create_task(start_child(registry, client, 'first', statuses))
+        await assert_waiting(first_task)
+
+        statuses[second.id] = 'SUCCEEDED'
+        await registry.update('second', make_run(second.id, 'SUCCEEDED'))
+        first = await first_task
+
+    assert first.id == 'old-run'
+    assert first.status == 'RUNNING'
+
+
+async def test_concurrent_named_starts_respect_the_limit() -> None:
+    """Concurrent named starts under different names start no more runs than the limit."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        await registry.load()
+        registry.set_max_concurrent_runs(2)
+        tasks = [asyncio.create_task(start_child(registry, client, f'child-{i}', statuses)) for i in range(3)]
+        await asyncio.sleep(0.05)
+        for task in tasks:
+            task.cancel()
+        await asyncio.gather(*tasks, return_exceptions=True)
+
+    assert sum(task.cancelled() for task in tasks) == 1
+    assert len(statuses) == 2
+
+
+async def test_failed_start_releases_its_slot() -> None:
+    """A named start that raises frees its slot for the next one."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        with pytest.raises(RuntimeError, match='start failed'):
+            await registry.find_or_start(
+                'first',
+                actor_id='some-actor',
+                run_input=None,
+                client=client,
+                start_run=AsyncMock(side_effect=RuntimeError('start failed')),
+                resurrect_run=AsyncMock(),
+            )
+        second = await asyncio.wait_for(start_child(registry, client, 'second', statuses), timeout=1)
+
+    assert second.id == 'second-run'
+
+
+async def test_parent_abort_stops_starts_waiting_for_a_slot() -> None:
+    """A named start waiting for a slot raises once the parent is aborted, without starting a run."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        await start_child(registry, client, 'first', statuses)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+
+        await asyncio.wait_for(registry.abort_runs_with_parent(client), timeout=1)
+        with pytest.raises(RuntimeError, match='being aborted'):
+            await second_task
+
+    assert 'second-run' not in statuses
+
+
+async def test_named_call_frees_its_slot_when_the_run_finishes(
+    apify_client_async_patcher: ApifyClientAsyncPatcher,
+) -> None:
+    """A child run awaited by a named call to its end frees its slot without being fetched again."""
+    apify_client_async_patcher.patch('actor', 'start', return_value=make_run('new-run', 'READY'))
+    # A fetch would report the run as still running, so only the awaited status can free the slot.
+    apify_client_async_patcher.patch('run', 'get', return_value=make_run('new-run', 'RUNNING'))
+    apify_client_async_patcher.patch('run', 'wait_for_finish', return_value=make_run('new-run', 'SUCCEEDED'))
+
+    async with Actor:
+        Actor.set_child_run_limits(max_concurrent_runs=1)
+        await Actor.call('some-actor', run_name='first', logger=None)
+        await asyncio.wait_for(Actor.start('some-actor', run_name='second'), timeout=1)
+
+    assert len(apify_client_async_patcher.calls['actor']['start']) == 2
+
+
+async def test_named_call_task_frees_its_slot_when_the_run_finishes(
+    apify_client_async_patcher: ApifyClientAsyncPatcher,
+) -> None:
+    """A task run awaited by a named task call to its end frees its slot without being fetched again."""
+    apify_client_async_patcher.patch('task', 'start', return_value=make_run('task-run', 'READY'))
+    apify_client_async_patcher.patch('actor', 'start', return_value=make_run('actor-run', 'READY'))
+    # A fetch would report the run as still running, so only the awaited status can free the slot.
+    apify_client_async_patcher.patch('run', 'get', return_value=make_run('task-run', 'RUNNING'))
+    apify_client_async_patcher.patch('run', 'wait_for_finish', return_value=make_run('task-run', 'SUCCEEDED'))
+
+    async with Actor:
+        Actor.set_child_run_limits(max_concurrent_runs=1)
+        await Actor.call_task('some-task', run_name='first')
+        await asyncio.wait_for(Actor.start('some-actor', run_name='second'), timeout=1)
+
+    assert len(apify_client_async_patcher.calls['actor']['start']) == 1

From a20b3c7b9c96ccf5149bd938bc7e943d940c7bb9 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:18:38 +0100
Subject: [PATCH 02/15] refactor: keep the child run status max age in seconds
 and mark missing runs as LOST

---
 src/apify/_child_runs.py                  | 13 +++++--------
 tests/unit/actor/test_actor_child_runs.py |  4 ++--
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index bda8a0cb..47e9cc03 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -4,7 +4,7 @@
 import hashlib
 import json
 from contextlib import asynccontextmanager, suppress
-from datetime import datetime, timedelta
+from datetime import datetime
 from logging import getLogger
 from typing import TYPE_CHECKING, Any
 from weakref import WeakValueDictionary
@@ -37,7 +37,7 @@
 
 _ACTIVE_STATUSES = frozenset({'READY', 'RUNNING', 'ABORTING', 'TIMING-OUT'})
 
-_STATUS_MAX_AGE = timedelta(seconds=10)
+_STATUS_MAX_AGE_SECS = 10
 """How long an observed active status counts toward the concurrency limit before the run is fetched again."""
 
 _NOT_FOUND_GRACE_SECS = 3
@@ -349,7 +349,7 @@ async def _slot(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[Non
                     )
                 logger.debug(f'Child run "{name}" is waiting for a free slot')
                 with suppress(TimeoutError):
-                    await asyncio.wait_for(self._slots.wait(), timeout=_STATUS_MAX_AGE.total_seconds())
+                    await asyncio.wait_for(self._slots.wait(), timeout=_STATUS_MAX_AGE_SECS)
             self._starting.add(name)
 
         try:
@@ -368,10 +368,7 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
             name
             for name in names
             if name not in self._observed
-            or (
-                self._observed[name][0] in _ACTIVE_STATUSES
-                and now - self._observed[name][1] >= _STATUS_MAX_AGE.total_seconds()
-            )
+            or (self._observed[name][0] in _ACTIVE_STATUSES and now - self._observed[name][1] >= _STATUS_MAX_AGE_SECS)
         ]
         runs = await asyncio.gather(
             *(self._clients.get(name, client).run(records[name].run_id).get() for name in stale),
@@ -385,7 +382,7 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
                     exc_info=run,
                 )
             elif run is None:
-                self._observed[name] = ('MISSING', now)
+                self._observed[name] = ('LOST', now)
             else:
                 self._observe(name, run)
 
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index a6820cc8..20b66427 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1249,7 +1249,7 @@ async def test_named_start_waits_while_the_limit_is_reached() -> None:
 
 async def test_removing_the_limit_releases_waiting_starts(monkeypatch: pytest.MonkeyPatch) -> None:
     """A named start waiting for a slot proceeds once the limit is removed."""
-    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE', timedelta(seconds=0.05))
+    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0.05)
     statuses: dict[str, str] = {}
     client = make_client(statuses)
 
@@ -1268,7 +1268,7 @@ async def test_removing_the_limit_releases_waiting_starts(monkeypatch: pytest.Mo
 
 async def test_stale_active_status_is_refreshed_before_counting(monkeypatch: pytest.MonkeyPatch) -> None:
     """A child run nobody awaited is fetched again once its status is stale, so a finished one frees its slot."""
-    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE', timedelta(0))
+    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0)
     statuses: dict[str, str] = {}
     client = make_client(statuses)
 

From fc48db722dc745c43e97b54e39240ddd601054ff Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:19:24 +0100
Subject: [PATCH 03/15] docs: document that a start waiting for a slot raises
 once the parent is aborting

---
 src/apify/_actor.py      | 3 ++-
 src/apify/_child_runs.py | 7 +++++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/apify/_actor.py b/src/apify/_actor.py
index 613a331f..2f6fa06b 100644
--- a/src/apify/_actor.py
+++ b/src/apify/_actor.py
@@ -452,7 +452,8 @@ def set_child_run_limits(self, *, max_concurrent_runs: int | None) -> None:
         `Actor.start`, `Actor.call`, `Actor.start_task` or `Actor.call_task` that would start or resurrect a run waits
         until one of them finishes. Reattaching to a recorded run never waits. Runs started without a `run_name` are not
         counted and never wait. A child run not awaited by `Actor.call` or `Actor.call_task` is fetched again before it
-        is counted, if its status is more than 10 seconds old.
+        is counted, if its status is more than 10 seconds old. Once this Actor run is being gracefully aborted, a call
+        that would wait raises a `RuntimeError`, with or without `abort_with_parent`.
 
         The limit is kept in memory, so call this method again after a migration or resurrection of this Actor run.
 
diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 47e9cc03..933f1b2d 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -126,7 +126,8 @@ def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]])
         self._observed: dict[str, tuple[str, float]] = {}
         """Last observed status of the run recorded under each name, with the event loop time it was observed at."""
         self._parent_aborting = False
-        """Whether this Actor run is being gracefully aborted, after which no run marked `abort_with_parent` starts."""
+        """Whether this Actor run is being gracefully aborted, after which no run marked `abort_with_parent` starts,
+        nor any run that would wait for a slot under the concurrency limit."""
 
     def set_max_concurrent_runs(self, max_concurrent_runs: int | None) -> None:
         """Set how many recorded runs may be active at once, or remove the limit with `None`."""
@@ -173,7 +174,9 @@ async def find_or_start(
         Raises:
             ValueError: If the recorded run would be reused, but was started for a different Actor, task or input.
             RuntimeError: If `abort_with_parent` is set and this Actor run is being gracefully aborted. No run is
-                started, resurrected or reattached.
+                started, resurrected or reattached. Also if starting or resurrecting the run would wait for a slot
+                under the concurrency limit while this Actor run is being gracefully aborted, whatever
+                `abort_with_parent` is.
         """
         checksum = checksum_request(actor_id=actor_id, task_id=task_id, run_input=run_input)
 

From 0e84cdeffd715e5f89ab222c164f54bae3de5669 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:19:39 +0100
Subject: [PATCH 04/15] test: cover a recorded child run the API no longer
 knows under the concurrency limit

---
 tests/unit/actor/test_actor_child_runs.py | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index 20b66427..87ce55c8 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1324,6 +1324,29 @@ def run(run_id: str) -> Mock:
     assert second.id == 'second-run'
 
 
+async def test_child_run_the_api_no_longer_knows_does_not_block_the_limit() -> None:
+    """A recorded child run the API reports as missing is not counted, so it does not block a named start."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+    run_client_factory = client.run.side_effect
+
+    def run(run_id: str) -> Mock:
+        run_client = run_client_factory(run_id)
+        if run_id == 'old-run':
+            run_client.get = AsyncMock(return_value=None)
+        return run_client
+
+    client.run.side_effect = run
+
+    async with Actor:
+        await record_child_run('first', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        second = await asyncio.wait_for(start_child(registry, client, 'second', statuses), timeout=1)
+
+    assert second.id == 'second-run'
+
+
 async def test_reattach_does_not_wait_for_a_slot() -> None:
     """Reattaching to an active recorded child run returns it even while the limit is reached."""
     statuses = {'old-run': 'RUNNING'}

From 70fefc954bcdd1c5eb54ca7dafb25bd9043ee5d5 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:19:53 +0100
Subject: [PATCH 05/15] fix: count a named start already in flight when the
 concurrency limit is set

---
 src/apify/_child_runs.py                  |  4 ---
 tests/unit/actor/test_actor_child_runs.py | 38 +++++++++++++++++++++++
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 933f1b2d..afead060 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -336,10 +336,6 @@ async def _start(
     @asynccontextmanager
     async def _slot(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[None]:
         """Hold a slot for starting or resurrecting the run under `name`, waiting while the limit is reached."""
-        if self._max_concurrent_runs is None:
-            yield
-            return
-
         async with self._slots:
             while (
                 self._max_concurrent_runs is not None
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index 87ce55c8..d36a7062 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1401,6 +1401,44 @@ async def test_concurrent_named_starts_respect_the_limit() -> None:
     assert len(statuses) == 2
 
 
+async def test_start_in_flight_when_the_limit_is_set_holds_a_slot() -> None:
+    """A named start begun before the limit was set holds a slot, so a later start waits for it."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+    release_first = asyncio.Event()
+
+    async def start_first() -> Run:
+        await release_first.wait()
+        statuses['first-run'] = 'RUNNING'
+        return make_run('first-run', 'READY')
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        first_task = asyncio.create_task(
+            registry.find_or_start(
+                'first',
+                actor_id='some-actor',
+                run_input=None,
+                client=client,
+                start_run=start_first,
+                resurrect_run=AsyncMock(),
+            )
+        )
+        await asyncio.sleep(0.05)
+        registry.set_max_concurrent_runs(1)
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+
+        release_first.set()
+        await first_task
+        await assert_waiting(second_task)
+        second_task.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await second_task
+
+    assert 'second-run' not in statuses
+
+
 async def test_failed_start_releases_its_slot() -> None:
     """A named start that raises frees its slot for the next one."""
     statuses: dict[str, str] = {}

From 5d03404126ee956f3734f04c32e8d09c086034b1 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:20:08 +0100
Subject: [PATCH 06/15] fix: count a named start that finishes while the active
 child runs are counted

---
 src/apify/_child_runs.py                  |  4 +-
 tests/unit/actor/test_actor_child_runs.py | 59 +++++++++++++++++++++++
 2 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index afead060..ae318969 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -362,6 +362,8 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
         """Count active recorded runs and slots held by others, fetching runs whose active status is not fresh."""
         records = await self._load()
         names = [name for name in records if name != exclude and name not in self._starting]
+        # A start in flight can finish during the fetch below, leaving `_starting` before it is in `names`.
+        starting = len(self._starting)
         now = asyncio.get_running_loop().time()
         stale = [
             name
@@ -387,7 +389,7 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
 
         # A run that could not be fetched counts only when an earlier observation saw it active.
         active = sum(1 for name in names if name in self._observed and self._observed[name][0] in _ACTIVE_STATUSES)
-        return active + len(self._starting)
+        return active + starting
 
     def _observe(self, name: str, run: Run) -> None:
         self._observed[name] = (run.status, asyncio.get_running_loop().time())
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index d36a7062..b304a5cc 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1439,6 +1439,65 @@ async def start_first() -> Run:
     assert 'second-run' not in statuses
 
 
+async def test_start_finishing_while_a_waiting_start_counts_still_holds_a_slot(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A start that finishes while a waiting start fetches runs to count them is still counted once."""
+    monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0.01)
+    statuses = {'old-run': 'RUNNING'}
+    client = make_client(statuses)
+    run_client_factory = client.run.side_effect
+    old_run_fetchable = asyncio.Event()
+    old_run_fetchable.set()
+
+    async def get_old_run() -> Run:
+        await old_run_fetchable.wait()
+        return make_run('old-run', 'RUNNING')
+
+    def run(run_id: str) -> Mock:
+        run_client = run_client_factory(run_id)
+        if run_id == 'old-run':
+            run_client.get = AsyncMock(side_effect=get_old_run)
+        return run_client
+
+    client.run.side_effect = run
+    release_first = asyncio.Event()
+
+    async def start_first() -> Run:
+        await release_first.wait()
+        statuses['first-run'] = 'RUNNING'
+        return make_run('first-run', 'READY')
+
+    async with Actor:
+        await record_child_run('old', 'old-run')
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(2)
+        first_task = asyncio.create_task(
+            registry.find_or_start(
+                'first',
+                actor_id='some-actor',
+                run_input=None,
+                client=client,
+                start_run=start_first,
+                resurrect_run=AsyncMock(),
+            )
+        )
+        await asyncio.sleep(0.05)
+
+        old_run_fetchable.clear()
+        second_task = asyncio.create_task(start_child(registry, client, 'second', statuses))
+        await assert_waiting(second_task)
+        release_first.set()
+        await asyncio.sleep(0.05)
+        old_run_fetchable.set()
+
+        await assert_waiting(second_task)
+        await first_task
+        second_task.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await second_task
+
+    assert 'second-run' not in statuses
+
+
 async def test_failed_start_releases_its_slot() -> None:
     """A named start that raises frees its slot for the next one."""
     statuses: dict[str, str] = {}

From 158c3e6dde167f26375dd04523e48d3d7a4e243a Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:20:22 +0100
Subject: [PATCH 07/15] perf: skip fetching finished child runs when counting
 toward the concurrency limit

---
 src/apify/_child_runs.py                  |  8 ++++++--
 tests/unit/actor/test_actor_child_runs.py | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index ae318969..cf8dd9d8 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -368,8 +368,12 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
         stale = [
             name
             for name in names
-            if name not in self._observed
-            or (self._observed[name][0] in _ACTIVE_STATUSES and now - self._observed[name][1] >= _STATUS_MAX_AGE_SECS)
+            if (name not in self._observed and records[name].status in _ACTIVE_STATUSES)
+            or (
+                name in self._observed
+                and self._observed[name][0] in _ACTIVE_STATUSES
+                and now - self._observed[name][1] >= _STATUS_MAX_AGE_SECS
+            )
         ]
         runs = await asyncio.gather(
             *(self._clients.get(name, client).run(records[name].run_id).get() for name in stale),
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index b304a5cc..b1d4f69a 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1347,6 +1347,22 @@ def run(run_id: str) -> Mock:
     assert second.id == 'second-run'
 
 
+async def test_finished_recorded_child_run_is_not_fetched_for_the_limit() -> None:
+    """A recorded child run whose recorded status is final is not fetched to count it toward the limit."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+
+    async with Actor:
+        kvs = await Actor.open_key_value_store()
+        await kvs.set_value(CHILD_RUNS_KEY, {'first': stored_record('old-run', 'SUCCEEDED')})
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        second = await asyncio.wait_for(start_child(registry, client, 'second', statuses), timeout=1)
+
+    assert second.id == 'second-run'
+    assert all(call.args != ('old-run',) for call in client.run.call_args_list)
+
+
 async def test_reattach_does_not_wait_for_a_slot() -> None:
     """Reattaching to an active recorded child run returns it even while the limit is reached."""
     statuses = {'old-run': 'RUNNING'}

From ca902ab21bdc78511c3e8d24ba2b5eea01b1ad3e Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:22:01 +0100
Subject: [PATCH 08/15] docs: add docstrings to the child run module constants

---
 src/apify/_child_runs.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index cf8dd9d8..4b35e270 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -32,10 +32,13 @@
 """Statuses that end as `ABORTED` / `TIMED-OUT` shortly, and are resurrectable once they do."""
 
 _RESURRECTABLE_STATUSES = frozenset({'ABORTED', 'TIMED-OUT'})
+"""Statuses of a recorded run that a call under its name resurrects."""
 
 _ABORTED_STATUSES = frozenset({'ABORTING', 'ABORTED'})
+"""Statuses that show an abort request took effect on the child run."""
 
 _ACTIVE_STATUSES = frozenset({'READY', 'RUNNING', 'ABORTING', 'TIMING-OUT'})
+"""Statuses of a child run that count toward the concurrency limit."""
 
 _STATUS_MAX_AGE_SECS = 10
 """How long an observed active status counts toward the concurrency limit before the run is fetched again."""
@@ -44,6 +47,7 @@
 """How long a recorded run that the API reports as missing is looked up again before it counts as gone."""
 
 _NOT_FOUND_RETRY_INTERVAL_SECS = 0.25
+"""How long to wait between lookups of a recorded run that the API reports as missing."""
 
 
 class ChildRunSnapshot(BaseModel):

From 3447fabb88c96a5a34e51b21b45bdb12583d0704 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Thu, 29 Oct 2026 15:22:37 +0100
Subject: [PATCH 09/15] docs: add docstrings to the child run registry
 attributes

---
 src/apify/_child_runs.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 4b35e270..1f2871df 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -116,7 +116,9 @@ class ChildRunRegistry:
 
     def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]]) -> None:
         self._open_key_value_store = open_key_value_store
+        """Opens the default key-value store, where the records are persisted."""
         self._records: dict[str, ChildRunRecord] | None = None
+        """Records of the child runs by name, `None` until they are read from the key-value store."""
         self._lock = asyncio.Lock()
         """Guards loading the records and writing them back to the key-value store."""
         self._name_locks: WeakValueDictionary[str, asyncio.Lock] = WeakValueDictionary()
@@ -124,7 +126,9 @@ def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]])
         self._clients: dict[str, ApifyClientAsync] = {}
         """Client the run under each name was last started or looked up with. Lost on a migration."""
         self._max_concurrent_runs: int | None = None
+        """How many recorded runs may be active at once, or `None` for no limit."""
         self._slots = asyncio.Condition()
+        """Serializes counting the active runs, and wakes the starts waiting for a slot when one may have freed up."""
         self._starting: set[str] = set()
         """Names holding a slot for a start or resurrection in flight, not counted from the records yet."""
         self._observed: dict[str, tuple[str, float]] = {}

From a4569a765c95ddd87898613bff667a7a6c760d34 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 09:17:25 +0100
Subject: [PATCH 10/15] refactor: rename the child run concurrency limit
 internals and method

---
 src/apify/_actor.py                       |  2 +-
 src/apify/_child_runs.py                  | 89 ++++++++++++-----------
 tests/e2e/test_actor_child_runs.py        |  2 +-
 tests/unit/actor/test_actor_child_runs.py | 42 +++++------
 4 files changed, 69 insertions(+), 66 deletions(-)

diff --git a/src/apify/_actor.py b/src/apify/_actor.py
index 2f6fa06b..425a04a9 100644
--- a/src/apify/_actor.py
+++ b/src/apify/_actor.py
@@ -445,7 +445,7 @@ def child_runs(self) -> dict[str, RunClientAsync]:
         """
         return self._child_run_registry.run_clients(self.apify_client)
 
-    def set_child_run_limits(self, *, max_concurrent_runs: int | None) -> None:
+    def set_named_child_run_limits(self, *, max_concurrent_runs: int | None) -> None:
         """Limit the named child runs of this Actor run.
 
         While `max_concurrent_runs` named child runs are `READY`, `RUNNING`, `ABORTING` or `TIMING-OUT`, a named
diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 1f2871df..31056aca 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -127,15 +127,15 @@ def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]])
         """Client the run under each name was last started or looked up with. Lost on a migration."""
         self._max_concurrent_runs: int | None = None
         """How many recorded runs may be active at once, or `None` for no limit."""
-        self._slots = asyncio.Condition()
-        """Serializes counting the active runs, and wakes the starts waiting for a slot when one may have freed up."""
-        self._starting: set[str] = set()
-        """Names holding a slot for a start or resurrection in flight, not counted from the records yet."""
-        self._observed: dict[str, tuple[str, float]] = {}
+        self._capacity_available = asyncio.Condition()
+        """Serializes counting the active runs, and wakes the waiting starts when a run may have finished."""
+        self._starting_names: set[str] = set()
+        """Names with a start or resurrection in flight, counted toward the limit but not from the records yet."""
+        self._timestamped_run_statuses: dict[str, tuple[str, float]] = {}
         """Last observed status of the run recorded under each name, with the event loop time it was observed at."""
         self._parent_aborting = False
         """Whether this Actor run is being gracefully aborted, after which no run marked `abort_with_parent` starts,
-        nor any run that would wait for a slot under the concurrency limit."""
+        nor any run that would wait under the concurrency limit."""
 
     def set_max_concurrent_runs(self, max_concurrent_runs: int | None) -> None:
         """Set how many recorded runs may be active at once, or remove the limit with `None`."""
@@ -182,9 +182,8 @@ async def find_or_start(
         Raises:
             ValueError: If the recorded run would be reused, but was started for a different Actor, task or input.
             RuntimeError: If `abort_with_parent` is set and this Actor run is being gracefully aborted. No run is
-                started, resurrected or reattached. Also if starting or resurrecting the run would wait for a slot
-                under the concurrency limit while this Actor run is being gracefully aborted, whatever
-                `abort_with_parent` is.
+                started, resurrected or reattached. Also if starting or resurrecting the run would wait under the
+                concurrency limit while this Actor run is being gracefully aborted, whatever `abort_with_parent` is.
         """
         checksum = checksum_request(actor_id=actor_id, task_id=task_id, run_input=run_input)
 
@@ -199,7 +198,7 @@ async def find_or_start(
             record = records.get(name)
 
             if record is None:
-                async with self._slot(name, client):
+                async with self._reserve_capacity(name, client):
                     run = await self._start(
                         name, checksum=checksum, start_run=start_run, history=[], abort_with_parent=abort_with_parent
                     )
@@ -218,7 +217,7 @@ async def find_or_start(
                     status=run.status if run is not None else 'LOST',
                     started_at=record.started_at,
                 )
-                async with self._slot(name, client):
+                async with self._reserve_capacity(name, client):
                     run = await self._start(
                         name,
                         checksum=checksum,
@@ -235,16 +234,16 @@ async def find_or_start(
                     'Use a unique `run_name` for each child run.'
                 )
 
-            self._observe(name, run)
+            self._timestamp_run_status(name, run)
 
             if record.abort_with_parent != abort_with_parent:
                 await self._save(name, record.model_copy(update={'abort_with_parent': abort_with_parent}))
 
             if run.status in _RESURRECTABLE_STATUSES:
-                async with self._slot(name, client):
+                async with self._reserve_capacity(name, client):
                     logger.info(f'Resurrecting child run "{name}"', extra={'run_id': run.id, 'status': run.status})
                     run = await resurrect_run(run.id)
-                    self._observe(name, run)
+                    self._timestamp_run_status(name, run)
             else:
                 logger.info(f'Reattaching to child run "{name}"', extra={'run_id': run.id, 'status': run.status})
 
@@ -270,7 +269,7 @@ def run_clients(self, default_client: ApifyClientAsync) -> dict[str, RunClientAs
     async def update(self, name: str, run: Run) -> None:
         """Record the latest observed status of the run recorded under `name`.
 
-        A run that is no longer active releases its slot under the concurrency limit.
+        A run that is no longer active stops counting toward the concurrency limit.
 
         Args:
             name: Name of the child run.
@@ -279,11 +278,11 @@ async def update(self, name: str, run: Run) -> None:
         record = (await self._load()).get(name)
         if record is None or record.run_id != run.id:
             return
-        self._observe(name, run)
+        self._timestamp_run_status(name, run)
         if record.status != run.status:
             await self._save(name, record.model_copy(update={'status': run.status}))
-        async with self._slots:
-            self._slots.notify_all()
+        async with self._capacity_available:
+            self._capacity_available.notify_all()
 
     async def abort_runs_with_parent(self, client: ApifyClientAsync) -> None:
         """Gracefully abort every recorded run marked `abort_with_parent`, and refuse to start any more of them.
@@ -296,8 +295,8 @@ async def abort_runs_with_parent(self, client: ApifyClientAsync) -> None:
             client: Client used for a name not started or looked up in this process, e.g. after a migration.
         """
         self._parent_aborting = True
-        async with self._slots:
-            self._slots.notify_all()
+        async with self._capacity_available:
+            self._capacity_available.notify_all()
 
         records = await self._load()
         # Names with a start in flight are not recorded yet, so their locks are awaited too.
@@ -338,13 +337,13 @@ async def _start(
             abort_with_parent=abort_with_parent,
         )
         await self._save(name, record)
-        self._observe(name, run)
+        self._timestamp_run_status(name, run)
         return run
 
     @asynccontextmanager
-    async def _slot(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[None]:
-        """Hold a slot for starting or resurrecting the run under `name`, waiting while the limit is reached."""
-        async with self._slots:
+    async def _reserve_capacity(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[None]:
+        """Count starting or resurrecting the run under `name` toward the limit, waiting while the limit is reached."""
+        async with self._capacity_available:
             while (
                 self._max_concurrent_runs is not None
                 and await self._count_active(client, exclude=name) >= self._max_concurrent_runs
@@ -354,33 +353,33 @@ async def _slot(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[Non
                         f'Child run "{name}" was not started, since this Actor run is being aborted and the limit '
                         f'of {self._max_concurrent_runs} concurrent child runs is reached.'
                     )
-                logger.debug(f'Child run "{name}" is waiting for a free slot')
+                logger.debug(f'Child run "{name}" is waiting for the concurrency limit')
                 with suppress(TimeoutError):
-                    await asyncio.wait_for(self._slots.wait(), timeout=_STATUS_MAX_AGE_SECS)
-            self._starting.add(name)
+                    await asyncio.wait_for(self._capacity_available.wait(), timeout=_STATUS_MAX_AGE_SECS)
+            self._starting_names.add(name)
 
         try:
             yield
         finally:
-            self._starting.discard(name)
-            async with self._slots:
-                self._slots.notify_all()
+            self._starting_names.discard(name)
+            async with self._capacity_available:
+                self._capacity_available.notify_all()
 
     async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
-        """Count active recorded runs and slots held by others, fetching runs whose active status is not fresh."""
+        """Count active recorded runs and starts in flight under other names, refetching stale active statuses."""
         records = await self._load()
-        names = [name for name in records if name != exclude and name not in self._starting]
-        # A start in flight can finish during the fetch below, leaving `_starting` before it is in `names`.
-        starting = len(self._starting)
+        names = [name for name in records if name != exclude and name not in self._starting_names]
+        # A start in flight can finish during the fetch below, leaving `_starting_names` before it is in `names`.
+        starting = len(self._starting_names)
         now = asyncio.get_running_loop().time()
         stale = [
             name
             for name in names
-            if (name not in self._observed and records[name].status in _ACTIVE_STATUSES)
+            if (name not in self._timestamped_run_statuses and records[name].status in _ACTIVE_STATUSES)
             or (
-                name in self._observed
-                and self._observed[name][0] in _ACTIVE_STATUSES
-                and now - self._observed[name][1] >= _STATUS_MAX_AGE_SECS
+                name in self._timestamped_run_statuses
+                and self._timestamped_run_statuses[name][0] in _ACTIVE_STATUSES
+                and now - self._timestamped_run_statuses[name][1] >= _STATUS_MAX_AGE_SECS
             )
         ]
         runs = await asyncio.gather(
@@ -395,16 +394,20 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
                     exc_info=run,
                 )
             elif run is None:
-                self._observed[name] = ('LOST', now)
+                self._timestamped_run_statuses[name] = ('LOST', now)
             else:
-                self._observe(name, run)
+                self._timestamp_run_status(name, run)
 
         # A run that could not be fetched counts only when an earlier observation saw it active.
-        active = sum(1 for name in names if name in self._observed and self._observed[name][0] in _ACTIVE_STATUSES)
+        active = sum(
+            1
+            for name in names
+            if name in self._timestamped_run_statuses and self._timestamped_run_statuses[name][0] in _ACTIVE_STATUSES
+        )
         return active + starting
 
-    def _observe(self, name: str, run: Run) -> None:
-        self._observed[name] = (run.status, asyncio.get_running_loop().time())
+    def _timestamp_run_status(self, name: str, run: Run) -> None:
+        self._timestamped_run_statuses[name] = (run.status, asyncio.get_running_loop().time())
 
     async def load(self) -> dict[str, ChildRunRecord]:
         """Read the records from the default key-value store, replacing any read before."""
diff --git a/tests/e2e/test_actor_child_runs.py b/tests/e2e/test_actor_child_runs.py
index a9eefb3b..8b5920bc 100644
--- a/tests/e2e/test_actor_child_runs.py
+++ b/tests/e2e/test_actor_child_runs.py
@@ -147,7 +147,7 @@ async def main() -> None:
                 return
 
             actor_id = Actor.configuration.actor_id or ''
-            Actor.set_child_run_limits(max_concurrent_runs=1)
+            Actor.set_named_child_run_limits(max_concurrent_runs=1)
             runs = await asyncio.gather(
                 *(
                     Actor.call(actor_id=actor_id, run_input={'is_child': True}, run_name=f'child-{index}')
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index b1d4f69a..09a55917 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1225,7 +1225,7 @@ async def test_set_child_run_limits_rejects_non_positive_limit(max_concurrent_ru
     """A concurrency limit below 1 is rejected."""
     async with Actor:
         with pytest.raises(ValueError, match='must be at least 1'):
-            Actor.set_child_run_limits(max_concurrent_runs=max_concurrent_runs)
+            Actor.set_named_child_run_limits(max_concurrent_runs=max_concurrent_runs)
 
 
 async def test_named_start_waits_while_the_limit_is_reached() -> None:
@@ -1248,7 +1248,7 @@ async def test_named_start_waits_while_the_limit_is_reached() -> None:
 
 
 async def test_removing_the_limit_releases_waiting_starts(monkeypatch: pytest.MonkeyPatch) -> None:
-    """A named start waiting for a slot proceeds once the limit is removed."""
+    """A named start waiting under the limit proceeds once the limit is removed."""
     monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0.05)
     statuses: dict[str, str] = {}
     client = make_client(statuses)
@@ -1267,7 +1267,7 @@ async def test_removing_the_limit_releases_waiting_starts(monkeypatch: pytest.Mo
 
 
 async def test_stale_active_status_is_refreshed_before_counting(monkeypatch: pytest.MonkeyPatch) -> None:
-    """A child run nobody awaited is fetched again once its status is stale, so a finished one frees its slot."""
+    """A child run nobody awaited is fetched again once its status is stale, so a finished one stops counting."""
     monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0)
     statuses: dict[str, str] = {}
     client = make_client(statuses)
@@ -1286,7 +1286,7 @@ async def test_stale_active_status_is_refreshed_before_counting(monkeypatch: pyt
 
 
 async def test_child_run_recorded_by_an_earlier_attempt_counts_toward_the_limit() -> None:
-    """An active child run recorded before a migration holds a slot, since it is fetched before it is counted."""
+    """An active child run recorded before a migration counts toward the limit, since it is fetched first."""
     statuses = {'old-run': 'RUNNING'}
     client = make_client(statuses)
 
@@ -1363,7 +1363,7 @@ async def test_finished_recorded_child_run_is_not_fetched_for_the_limit() -> Non
     assert all(call.args != ('old-run',) for call in client.run.call_args_list)
 
 
-async def test_reattach_does_not_wait_for_a_slot() -> None:
+async def test_reattach_does_not_wait_under_the_limit() -> None:
     """Reattaching to an active recorded child run returns it even while the limit is reached."""
     statuses = {'old-run': 'RUNNING'}
     client = make_client(statuses)
@@ -1377,7 +1377,7 @@ async def test_reattach_does_not_wait_for_a_slot() -> None:
     assert run.id == 'old-run'
 
 
-async def test_resurrection_waits_for_a_slot() -> None:
+async def test_resurrection_waits_under_the_limit() -> None:
     """Resurrecting an aborted child run waits while the limit is reached."""
     statuses = {'old-run': 'ABORTED'}
     client = make_client(statuses)
@@ -1417,8 +1417,8 @@ async def test_concurrent_named_starts_respect_the_limit() -> None:
     assert len(statuses) == 2
 
 
-async def test_start_in_flight_when_the_limit_is_set_holds_a_slot() -> None:
-    """A named start begun before the limit was set holds a slot, so a later start waits for it."""
+async def test_start_in_flight_when_the_limit_is_set_counts_toward_it() -> None:
+    """A named start begun before the limit was set counts toward it, so a later start waits for it."""
     statuses: dict[str, str] = {}
     client = make_client(statuses)
     release_first = asyncio.Event()
@@ -1455,7 +1455,7 @@ async def start_first() -> Run:
     assert 'second-run' not in statuses
 
 
-async def test_start_finishing_while_a_waiting_start_counts_still_holds_a_slot(monkeypatch: pytest.MonkeyPatch) -> None:
+async def test_start_finishing_while_a_waiting_start_counts_is_counted_once(monkeypatch: pytest.MonkeyPatch) -> None:
     """A start that finishes while a waiting start fetches runs to count them is still counted once."""
     monkeypatch.setattr('apify._child_runs._STATUS_MAX_AGE_SECS', 0.01)
     statuses = {'old-run': 'RUNNING'}
@@ -1514,8 +1514,8 @@ async def start_first() -> Run:
     assert 'second-run' not in statuses
 
 
-async def test_failed_start_releases_its_slot() -> None:
-    """A named start that raises frees its slot for the next one."""
+async def test_failed_start_stops_counting_toward_the_limit() -> None:
+    """A named start that raises stops counting toward the limit, so the next one proceeds."""
     statuses: dict[str, str] = {}
     client = make_client(statuses)
 
@@ -1536,8 +1536,8 @@ async def test_failed_start_releases_its_slot() -> None:
     assert second.id == 'second-run'
 
 
-async def test_parent_abort_stops_starts_waiting_for_a_slot() -> None:
-    """A named start waiting for a slot raises once the parent is aborted, without starting a run."""
+async def test_parent_abort_stops_starts_waiting_under_the_limit() -> None:
+    """A named start waiting under the limit raises once the parent is aborted, without starting a run."""
     statuses: dict[str, str] = {}
     client = make_client(statuses)
 
@@ -1555,35 +1555,35 @@ async def test_parent_abort_stops_starts_waiting_for_a_slot() -> None:
     assert 'second-run' not in statuses
 
 
-async def test_named_call_frees_its_slot_when_the_run_finishes(
+async def test_named_call_stops_counting_when_the_run_finishes(
     apify_client_async_patcher: ApifyClientAsyncPatcher,
 ) -> None:
-    """A child run awaited by a named call to its end frees its slot without being fetched again."""
+    """A child run awaited by a named call to its end stops counting without being fetched again."""
     apify_client_async_patcher.patch('actor', 'start', return_value=make_run('new-run', 'READY'))
-    # A fetch would report the run as still running, so only the awaited status can free the slot.
+    # A fetch would report the run as still running, so only the awaited status can stop it counting.
     apify_client_async_patcher.patch('run', 'get', return_value=make_run('new-run', 'RUNNING'))
     apify_client_async_patcher.patch('run', 'wait_for_finish', return_value=make_run('new-run', 'SUCCEEDED'))
 
     async with Actor:
-        Actor.set_child_run_limits(max_concurrent_runs=1)
+        Actor.set_named_child_run_limits(max_concurrent_runs=1)
         await Actor.call('some-actor', run_name='first', logger=None)
         await asyncio.wait_for(Actor.start('some-actor', run_name='second'), timeout=1)
 
     assert len(apify_client_async_patcher.calls['actor']['start']) == 2
 
 
-async def test_named_call_task_frees_its_slot_when_the_run_finishes(
+async def test_named_call_task_stops_counting_when_the_run_finishes(
     apify_client_async_patcher: ApifyClientAsyncPatcher,
 ) -> None:
-    """A task run awaited by a named task call to its end frees its slot without being fetched again."""
+    """A task run awaited by a named task call to its end stops counting without being fetched again."""
     apify_client_async_patcher.patch('task', 'start', return_value=make_run('task-run', 'READY'))
     apify_client_async_patcher.patch('actor', 'start', return_value=make_run('actor-run', 'READY'))
-    # A fetch would report the run as still running, so only the awaited status can free the slot.
+    # A fetch would report the run as still running, so only the awaited status can stop it counting.
     apify_client_async_patcher.patch('run', 'get', return_value=make_run('task-run', 'RUNNING'))
     apify_client_async_patcher.patch('run', 'wait_for_finish', return_value=make_run('task-run', 'SUCCEEDED'))
 
     async with Actor:
-        Actor.set_child_run_limits(max_concurrent_runs=1)
+        Actor.set_named_child_run_limits(max_concurrent_runs=1)
         await Actor.call_task('some-task', run_name='first')
         await asyncio.wait_for(Actor.start('some-actor', run_name='second'), timeout=1)
 

From 206db986aee9c8b31603d2c6a1c86f34c714ddb9 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 10:03:49 +0100
Subject: [PATCH 11/15] refactor: use asyncio.timeout when waiting under the
 child run concurrency limit

---
 src/apify/_child_runs.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 31056aca..b8750564 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -355,7 +355,8 @@ async def _reserve_capacity(self, name: str, client: ApifyClientAsync) -> AsyncG
                     )
                 logger.debug(f'Child run "{name}" is waiting for the concurrency limit')
                 with suppress(TimeoutError):
-                    await asyncio.wait_for(self._capacity_available.wait(), timeout=_STATUS_MAX_AGE_SECS)
+                    async with asyncio.timeout(_STATUS_MAX_AGE_SECS):
+                        await self._capacity_available.wait()
             self._starting_names.add(name)
 
         try:

From 56f2cfecd0cbfdcaab11db98e3dc5f8ffea4cf3b Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 10:04:04 +0100
Subject: [PATCH 12/15] refactor: cache child run statuses in a named tuple

---
 src/apify/_child_runs.py | 63 +++++++++++++++++++++++-----------------
 1 file changed, 37 insertions(+), 26 deletions(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index b8750564..7a656455 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -6,7 +6,7 @@
 from contextlib import asynccontextmanager, suppress
 from datetime import datetime
 from logging import getLogger
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, NamedTuple
 from weakref import WeakValueDictionary
 
 from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, ValidationError
@@ -78,6 +78,16 @@ class ChildRunRecord(ChildRunSnapshot):
     """Whether the current run is aborted when this Actor run is gracefully aborted."""
 
 
+class _CachedRunStatus(NamedTuple):
+    """Status of a child run as this Actor run last saw it, used to count the run toward the concurrency limit."""
+
+    status: str
+    """Status of the run, or `LOST` once the platform no longer returned it."""
+
+    cached_at: float
+    """Event loop time the status was seen at."""
+
+
 def checksum_request(*, actor_id: str | None, task_id: str | None, run_input: Any) -> str:
     """Hash the Actor or task and the input of a named start, in the same JSON shape as the JS SDK."""
     request: dict[str, Any] = (
@@ -131,8 +141,8 @@ def __init__(self, open_key_value_store: Callable[[], Awaitable[KeyValueStore]])
         """Serializes counting the active runs, and wakes the waiting starts when a run may have finished."""
         self._starting_names: set[str] = set()
         """Names with a start or resurrection in flight, counted toward the limit but not from the records yet."""
-        self._timestamped_run_statuses: dict[str, tuple[str, float]] = {}
-        """Last observed status of the run recorded under each name, with the event loop time it was observed at."""
+        self._run_status_cache: dict[str, _CachedRunStatus] = {}
+        """Last seen status of the run recorded under each name. An active one is fetched again once it gets stale."""
         self._parent_aborting = False
         """Whether this Actor run is being gracefully aborted, after which no run marked `abort_with_parent` starts,
         nor any run that would wait under the concurrency limit."""
@@ -234,7 +244,7 @@ async def find_or_start(
                     'Use a unique `run_name` for each child run.'
                 )
 
-            self._timestamp_run_status(name, run)
+            self._cache_run_status(name, run)
 
             if record.abort_with_parent != abort_with_parent:
                 await self._save(name, record.model_copy(update={'abort_with_parent': abort_with_parent}))
@@ -243,7 +253,7 @@ async def find_or_start(
                 async with self._reserve_capacity(name, client):
                     logger.info(f'Resurrecting child run "{name}"', extra={'run_id': run.id, 'status': run.status})
                     run = await resurrect_run(run.id)
-                    self._timestamp_run_status(name, run)
+                    self._cache_run_status(name, run)
             else:
                 logger.info(f'Reattaching to child run "{name}"', extra={'run_id': run.id, 'status': run.status})
 
@@ -278,7 +288,7 @@ async def update(self, name: str, run: Run) -> None:
         record = (await self._load()).get(name)
         if record is None or record.run_id != run.id:
             return
-        self._timestamp_run_status(name, run)
+        self._cache_run_status(name, run)
         if record.status != run.status:
             await self._save(name, record.model_copy(update={'status': run.status}))
         async with self._capacity_available:
@@ -337,7 +347,7 @@ async def _start(
             abort_with_parent=abort_with_parent,
         )
         await self._save(name, record)
-        self._timestamp_run_status(name, run)
+        self._cache_run_status(name, run)
         return run
 
     @asynccontextmanager
@@ -373,16 +383,17 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
         # A start in flight can finish during the fetch below, leaving `_starting_names` before it is in `names`.
         starting = len(self._starting_names)
         now = asyncio.get_running_loop().time()
-        stale = [
-            name
-            for name in names
-            if (name not in self._timestamped_run_statuses and records[name].status in _ACTIVE_STATUSES)
-            or (
-                name in self._timestamped_run_statuses
-                and self._timestamped_run_statuses[name][0] in _ACTIVE_STATUSES
-                and now - self._timestamped_run_statuses[name][1] >= _STATUS_MAX_AGE_SECS
-            )
-        ]
+        stale = []
+        for name in names:
+            cached_run_status = self._run_status_cache.get(name)
+            if cached_run_status is None:
+                if records[name].status in _ACTIVE_STATUSES:
+                    stale.append(name)
+            elif (
+                cached_run_status.status in _ACTIVE_STATUSES
+                and now - cached_run_status.cached_at >= _STATUS_MAX_AGE_SECS
+            ):
+                stale.append(name)
         runs = await asyncio.gather(
             *(self._clients.get(name, client).run(records[name].run_id).get() for name in stale),
             return_exceptions=True,
@@ -395,20 +406,20 @@ async def _count_active(self, client: ApifyClientAsync, *, exclude: str) -> int:
                     exc_info=run,
                 )
             elif run is None:
-                self._timestamped_run_statuses[name] = ('LOST', now)
+                self._run_status_cache[name] = _CachedRunStatus('LOST', now)
             else:
-                self._timestamp_run_status(name, run)
+                self._cache_run_status(name, run)
 
         # A run that could not be fetched counts only when an earlier observation saw it active.
-        active = sum(
-            1
-            for name in names
-            if name in self._timestamped_run_statuses and self._timestamped_run_statuses[name][0] in _ACTIVE_STATUSES
-        )
+        active = 0
+        for name in names:
+            cached_run_status = self._run_status_cache.get(name)
+            if cached_run_status is not None and cached_run_status.status in _ACTIVE_STATUSES:
+                active += 1
         return active + starting
 
-    def _timestamp_run_status(self, name: str, run: Run) -> None:
-        self._timestamped_run_statuses[name] = (run.status, asyncio.get_running_loop().time())
+    def _cache_run_status(self, name: str, run: Run) -> None:
+        self._run_status_cache[name] = _CachedRunStatus(run.status, asyncio.get_running_loop().time())
 
     async def load(self) -> dict[str, ChildRunRecord]:
         """Read the records from the default key-value store, replacing any read before."""

From 10fc99c6219a11998d42903b3f22d66266dafe94 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 10:04:19 +0100
Subject: [PATCH 13/15] fix: don't start a waiting abort_with_parent child run
 once the parent is aborting

---
 src/apify/_child_runs.py                  | 16 +++++++++---
 tests/unit/actor/test_actor_child_runs.py | 31 +++++++++++++++++++++++
 2 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/src/apify/_child_runs.py b/src/apify/_child_runs.py
index 7a656455..89d58f8b 100644
--- a/src/apify/_child_runs.py
+++ b/src/apify/_child_runs.py
@@ -208,7 +208,7 @@ async def find_or_start(
             record = records.get(name)
 
             if record is None:
-                async with self._reserve_capacity(name, client):
+                async with self._reserve_capacity(name, client, abort_with_parent=abort_with_parent):
                     run = await self._start(
                         name, checksum=checksum, start_run=start_run, history=[], abort_with_parent=abort_with_parent
                     )
@@ -227,7 +227,7 @@ async def find_or_start(
                     status=run.status if run is not None else 'LOST',
                     started_at=record.started_at,
                 )
-                async with self._reserve_capacity(name, client):
+                async with self._reserve_capacity(name, client, abort_with_parent=abort_with_parent):
                     run = await self._start(
                         name,
                         checksum=checksum,
@@ -250,7 +250,7 @@ async def find_or_start(
                 await self._save(name, record.model_copy(update={'abort_with_parent': abort_with_parent}))
 
             if run.status in _RESURRECTABLE_STATUSES:
-                async with self._reserve_capacity(name, client):
+                async with self._reserve_capacity(name, client, abort_with_parent=abort_with_parent):
                     logger.info(f'Resurrecting child run "{name}"', extra={'run_id': run.id, 'status': run.status})
                     run = await resurrect_run(run.id)
                     self._cache_run_status(name, run)
@@ -351,7 +351,9 @@ async def _start(
         return run
 
     @asynccontextmanager
-    async def _reserve_capacity(self, name: str, client: ApifyClientAsync) -> AsyncGenerator[None]:
+    async def _reserve_capacity(
+        self, name: str, client: ApifyClientAsync, *, abort_with_parent: bool
+    ) -> AsyncGenerator[None]:
         """Count starting or resurrecting the run under `name` toward the limit, waiting while the limit is reached."""
         async with self._capacity_available:
             while (
@@ -367,6 +369,12 @@ async def _reserve_capacity(self, name: str, client: ApifyClientAsync) -> AsyncG
                 with suppress(TimeoutError):
                     async with asyncio.timeout(_STATUS_MAX_AGE_SECS):
                         await self._capacity_available.wait()
+            # The parent can start aborting while this start waits.
+            if abort_with_parent and self._parent_aborting:
+                raise RuntimeError(
+                    f'Child run "{name}" with `abort_with_parent` was not started, since this Actor run is being '
+                    'aborted.'
+                )
             self._starting_names.add(name)
 
         try:
diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index 09a55917..d18b6453 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1555,6 +1555,37 @@ async def test_parent_abort_stops_starts_waiting_under_the_limit() -> None:
     assert 'second-run' not in statuses
 
 
+async def test_abort_with_parent_start_freed_after_parent_abort_does_not_start() -> None:
+    """A waiting `abort_with_parent` start raises when capacity frees up after the parent began aborting."""
+    statuses: dict[str, str] = {}
+    client = make_client(statuses)
+    start_run = AsyncMock(return_value=make_run('second-run', 'READY'))
+
+    async with Actor:
+        registry = ChildRunRegistry(Actor.open_key_value_store)
+        registry.set_max_concurrent_runs(1)
+        first = await start_child(registry, client, 'first', statuses)
+        second_task = asyncio.create_task(
+            registry.find_or_start(
+                'second',
+                actor_id='some-actor',
+                run_input=None,
+                client=client,
+                start_run=start_run,
+                resurrect_run=AsyncMock(),
+                abort_with_parent=True,
+            )
+        )
+        await assert_waiting(second_task)
+
+        registry._parent_aborting = True
+        await registry.update('first', make_run(first.id, 'SUCCEEDED'))
+        with pytest.raises(RuntimeError, match='being aborted'):
+            await second_task
+
+    start_run.assert_not_called()
+
+
 async def test_named_call_stops_counting_when_the_run_finishes(
     apify_client_async_patcher: ApifyClientAsyncPatcher,
 ) -> None:

From c81ae14cca7d502a146327aac7edb870b057374e Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 10:04:34 +0100
Subject: [PATCH 14/15] test: rename the child run limit test and document its
 helper

---
 tests/unit/actor/test_actor_child_runs.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/unit/actor/test_actor_child_runs.py b/tests/unit/actor/test_actor_child_runs.py
index d18b6453..438fba0f 100644
--- a/tests/unit/actor/test_actor_child_runs.py
+++ b/tests/unit/actor/test_actor_child_runs.py
@@ -1210,6 +1210,7 @@ async def start_run() -> Run:
 
 
 async def assert_waiting(task: asyncio.Task) -> None:
+    """Assert that `task` is still pending after giving it a moment to run."""
     await asyncio.sleep(0.05)
     assert not task.done()
 
@@ -1221,7 +1222,7 @@ async def assert_waiting(task: asyncio.Task) -> None:
         pytest.param(-1, id='negative'),
     ],
 )
-async def test_set_child_run_limits_rejects_non_positive_limit(max_concurrent_runs: int) -> None:
+async def test_set_named_child_run_limits_rejects_non_positive_limit(max_concurrent_runs: int) -> None:
     """A concurrency limit below 1 is rejected."""
     async with Actor:
         with pytest.raises(ValueError, match='must be at least 1'):

From 6d957ec894c76fe95b3143bdf1b884ae96e97092 Mon Sep 17 00:00:00 2001
From: Vlada Dusek <v.dusek96@gmail.com>
Date: Fri, 30 Oct 2026 10:18:50 +0100
Subject: [PATCH 15/15] docs: clarify when the child run concurrency limit
 refetches statuses and applies a new limit

---
 src/apify/_actor.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/apify/_actor.py b/src/apify/_actor.py
index a25eeaa5..b919dcad 100644
--- a/src/apify/_actor.py
+++ b/src/apify/_actor.py
@@ -451,11 +451,12 @@ def set_named_child_run_limits(self, *, max_concurrent_runs: int | None) -> None
         While `max_concurrent_runs` named child runs are `READY`, `RUNNING`, `ABORTING` or `TIMING-OUT`, a named
         `Actor.start`, `Actor.call`, `Actor.start_task` or `Actor.call_task` that would start or resurrect a run waits
         until one of them finishes. Reattaching to a recorded run never waits. Runs started without a `run_name` are not
-        counted and never wait. A child run not awaited by `Actor.call` or `Actor.call_task` is fetched again before it
-        is counted, if its status is more than 10 seconds old. Once this Actor run is being gracefully aborted, a call
-        that would wait raises a `RuntimeError`, with or without `abort_with_parent`.
+        counted and never wait. A child run's status is fetched again before it is counted, if it is more than 10
+        seconds old. Once this Actor run is being gracefully aborted, a call that would wait raises a `RuntimeError`,
+        with or without `abort_with_parent`.
 
-        The limit is kept in memory, so call this method again after a migration or resurrection of this Actor run.
+        A call already waiting picks up a changed limit within 10 seconds. The limit is kept in memory, so call this
+        method again after a migration or resurrection of this Actor run.
 
         Args:
             max_concurrent_runs: How many named child runs may be active at once, or `None` for no limit.