diff --git a/README.md b/README.md
index ac61ac1fe2..d14f48396c 100644
--- a/README.md
+++ b/README.md
@@ -1101,6 +1101,10 @@ Invalid UTF-8 in `channels/store.json` follows the same recovery policy as malfo
 SQL migration warns and skips the file, leaving it unchanged so channels can start; the
 memory backend's JSON store warns and starts empty, replacing the file on its next write.
 
+On Windows, JSON runtime channel-config saves wait for a competing file-lock holder
+without the operating system's roughly ten-second cutoff. A wait reaching 50 retries
+(about ten seconds) logs the lock-file path once; permanent lock errors fail immediately.
+
 Conversation-binding, runtime channel-credential and WeChat auth-state saves preserve the original
 persistence failure when staged-file cleanup also fails. Remaining cleanup still runs, and warnings
 identify copies that could not be removed. Buzz keeps failed snapshots available for retry, even if
diff --git a/backend/app/channels/AGENTS.md b/backend/app/channels/AGENTS.md
index bf8205c9c4..8b95a6dc37 100644
--- a/backend/app/channels/AGENTS.md
+++ b/backend/app/channels/AGENTS.md
@@ -76,6 +76,7 @@ The cached value is reused for both the blocking (`runs.wait`) and streaming (`_
 - Frontend APIs: `GET /api/channels/providers`, `GET /api/channels/connections`, `POST /api/channels/{provider}/connect`, and `DELETE /api/channels/connections/{connection_id}`.
 - Browser APIs remain protected by normal Gateway auth/CSRF. Provider messages arrive through the already-configured channel workers.
 - Runtime credentials (`runtime_config_store.py`): encrypted `channel_runtime_configs` rows (JSON only for `memory`), re-read per request, never cached on `app.state`; a save is one cancellation-owned commit.
+- The JSON runtime-config sidecar uses `utils.file_lock.retry_until_locked` on Windows: retry only EACCES/EAGAIN/EDEADLK every 0.2s without a deadline; propagate other errors before entering the mutation or unlocking. Warn once per acquisition after 50 contention retries with the sidecar path; logging failures must not interrupt the wait. POSIX keeps blocking `flock`. Coverage: `tests/test_channel_runtime_config_store_lock.py`.
 - Provider-level `connection_status` reflects the user's newest connection row. With no binding it is `not_connected`, except in auth-disabled local mode where a configured running channel reports `connected` because all channel messages already route to the default user.
 - Slack replies use the configured operator bot token from `channels.slack` unless per-connection credentials are present; unreadable or corrupt stored credentials are treated as unavailable.
 - Telegram, Slack, Discord, Feishu/Lark, DingTalk, WeChat, and WeCom workers resolve incoming platform identities to connection records before reaching `ChannelManager`.
@@ -87,7 +88,16 @@ The cached value is reused for both the blocking (`runs.wait`) and streaming (`_
 - Custom agents declare a `github:` block in their `config.yaml` to bind to repos and event triggers; the webhook route is fail-closed by default (mounted only when `GITHUB_WEBHOOK_SECRET` is set) and exempt from auth/CSRF because authenticity is enforced by HMAC.
 - Registry caching is keyed by the configured agent store's opaque signature: file storage uses agent config mtimes, while database storage hashes the ordered owner/name/config/soul contents so same-timestamp writes still invalidate webhook routing.
 - Outbound is **log-only** by design: each agent posts its own reply mid-run via the `gh` CLI from its sandbox, so the manager uses `fire_and_forget=True` and `runs.create()` returns once pending.
-- **Follow-up buffering while busy** (issue #4121): outbound is log-only, so the `THREAD_BUSY_MESSAGE` reply on a `ConflictError` was invisible to the commenter — a comment posted during an active run looked silently ignored. When `ChannelRunPolicy.buffer_followups_on_busy=True` (GitHub's default), a `ConflictError` on `runs.create()` also appends the triggering message to a per-thread in-memory buffer (`ChannelManager._followup_buffers`) — deduped by GitHub delivery id, capped at `FOLLOWUP_BUFFER_MAX_PER_THREAD` (20, oldest dropped with a WARNING). The first successful `runs.create()` on a thread captures its `run_id` and spawns a watcher on that run's `StreamBridge` stream; on `END_SENTINEL` it drains up to `FOLLOWUP_DRAIN_BATCH_SIZE` (10) entries into one `<followups-while-busy>`-wrapped input and fires a follow-up `runs.create()` — itself watched, so a deeper backlog chains into further drain cycles instead of one unbounded input. If that follow-up hits `ConflictError` (a concurrent turn raced onto the thread), the batch is requeued and retried when this manager next creates and watches a run on that thread. Reactions on buffered comments are out of scope; comments coalesce silently. **Plumbing**: the watcher reaches the Gateway's `StreamBridge` through a zero-arg closure threaded from the lifespan via `start_channel_service(get_stream_bridge=...)` → `ChannelService` → `ChannelManager` (like `ScheduledTaskService.launch_run`); without it (tests) buffering works without auto-drain. **Scope limitation**: buffer and watcher are per-process and webhooks reach any instance (the leader lease does not arbitrate GitHub), so a follow-up routed to another worker misses the buffer until a shared buffer store exists (#4120 shape).
+- **Busy follow-ups** (#4121): `ChannelRunPolicy.buffer_followups_on_busy=True`
+  (GitHub default) buffers `runs.create()` conflicts per thread, deduplicated by
+  delivery ID. Cap at 20 entries, dropping the oldest with WARNING. Watch each
+  successful run's `StreamBridge`; `END_SENTINEL` drains up to 10 entries into
+  `<followups-while-busy>` and watches the successor. Requeue drain conflicts
+  until this manager next creates and watches a run. Buffers/watchers are
+  process-local; GitHub has no leader-lease arbitration. Without the zero-arg
+  lifespan → service → manager bridge closure, buffering has no auto-drain.
+  Coalesce silently without reactions. Before changing buffering or watcher
+  plumbing, read [the full contract](../../docs/GITHUB_AGENTS.md#follow-up-buffering-while-busy-4121).
 - See [backend/docs/GITHUB_AGENTS.md](../../docs/GITHUB_AGENTS.md) for the architecture diagrams: webhook → fan-out → `InboundMessage` dispatch, `preferred_thread_id = UUID5(repo, number, agent_name)` thread determinism, mention-handle precedence chain, GH token lifecycle via `GH_TOKEN`/`GITHUB_TOKEN` per-call `extra_env`, and the narrow `ConflictError` (HTTP 409) thread-create race recovery.
 
 
diff --git a/backend/app/channels/runtime_config_store.py b/backend/app/channels/runtime_config_store.py
index a5c38089fc..0d58521485 100644
--- a/backend/app/channels/runtime_config_store.py
+++ b/backend/app/channels/runtime_config_store.py
@@ -36,15 +36,25 @@ from pathlib import Path
 from typing import TYPE_CHECKING, Any, Protocol
 
 from app.channels.credential_staging import cleanup_stale_credential_staging, credential_staging_prefix
+from deerflow.utils.file_lock import retry_until_locked
 
 if TYPE_CHECKING:
     from deerflow.config.credentials_key import CredentialsCipher
     from deerflow.persistence.channel_runtime_configs import SqlChannelRuntimeConfigRepository, StoredChannelRuntimeConfig
 
+try:
+    import fcntl
+except ImportError:  # pragma: no cover - Windows fallback
+    fcntl = None  # type: ignore[assignment]
+    import msvcrt
+
 logger = logging.getLogger(__name__)
 
 RUNTIME_CHANNEL_DISABLED_FLAG = "_runtime_disabled"
 
+# About ten seconds at the shared helper's 0.2-second retry cadence.
+_LOCK_WAIT_WARNING_ATTEMPTS = 50
+
 
 class ChannelRuntimeConfigEncryptionUnavailable(RuntimeError):
     """The database store has no credentials key, so it refuses to persist credentials."""
@@ -83,28 +93,40 @@ def _runtime_config_file_lock(path: Path) -> Iterator[None]:
     target_path = path.resolve(strict=False) if path.is_symlink() else path.absolute()
     lock_path = target_path.parent / f".{target_path.name}.lock"
     with open(lock_path, "a+b") as lock_file:
-        if os.name == "nt":
-            import msvcrt
-
+        if fcntl is not None:
+            fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX)
+        else:  # pragma: no cover - Windows fallback
             lock_file.seek(0, os.SEEK_END)
             if lock_file.tell() == 0:
                 lock_file.write(b"\0")
                 lock_file.flush()
+            # LK_LOCK gives up after roughly ten seconds. Share the
+            # contention-only retry policy, retaining an indefinite wait.
             lock_file.seek(0)
-            msvcrt.locking(lock_file.fileno(), msvcrt.LK_LOCK, 1)
-        else:
-            import fcntl
+            retries = 0
 
-            fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX)
+            def wait_for_lock(delay: float) -> None:
+                nonlocal retries
+                retries += 1
+                if retries == _LOCK_WAIT_WARNING_ATTEMPTS:
+                    # Diagnostics must not abort acquisition or flood the log.
+                    with suppress(Exception):
+                        logger.warning("Still waiting for channel runtime config lock %s after %d contention retries", lock_path, retries)
+                time.sleep(delay)
+
+            retry_until_locked(
+                lambda: msvcrt.locking(lock_file.fileno(), msvcrt.LK_NBLCK, 1),
+                sleep=wait_for_lock,
+            )
 
         try:
             yield
         finally:
-            if os.name == "nt":
+            if fcntl is not None:
+                fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
+            else:  # pragma: no cover - Windows fallback
                 lock_file.seek(0)
                 msvcrt.locking(lock_file.fileno(), msvcrt.LK_UNLCK, 1)
-            else:
-                fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
 
 
 class ChannelRuntimeConfigStore:
diff --git a/backend/packages/harness/deerflow/utils/AGENTS.md b/backend/packages/harness/deerflow/utils/AGENTS.md
index 3d06189f1f..1adf565ab5 100644
--- a/backend/packages/harness/deerflow/utils/AGENTS.md
+++ b/backend/packages/harness/deerflow/utils/AGENTS.md
@@ -173,12 +173,16 @@ retain their existing portability exemptions.
 ### Advisory File Lock Retries
 
 `file_lock.retry_until_locked` owns contention-only acquisition retries for the
-Windows Lark and extension-manager locks and bounded skill-projection reads.
+Windows Lark, extension-manager and channel runtime-config locks and bounded
+skill-projection reads.
 Retry only EACCES/EAGAIN/EDEADLK; other errors propagate without sleeping.
 Keep per-caller policies: Lark has a 15-minute Windows file-lock deadline,
-extension management waits indefinitely, and skill export retains its existing
-deadline, 0.02-second cadence and cancellation check. Clocks and sleeps are
-injectable; tests must not patch the shared stdlib time module.
+extension management and channel runtime config wait indefinitely, and skill
+export retains its existing deadline, 0.02-second cadence and cancellation
+check. Clocks and sleeps are injectable; tests must not patch the shared stdlib
+time module.
+The channel caller wraps its injected sleep to warn once after 50 contention
+retries; the helper remains responsible for error classification and cadence.
 
 `parse_retry_after_date_ms(raw, now=...)` owns HTTP-date parsing for Claude, Codex
 and LLM error middleware. Interpret timezone-less asctime as UTC and apply the
diff --git a/backend/tests/test_channel_runtime_config_store_lock.py b/backend/tests/test_channel_runtime_config_store_lock.py
new file mode 100644
index 0000000000..6b57ff24e1
--- /dev/null
+++ b/backend/tests/test_channel_runtime_config_store_lock.py
@@ -0,0 +1,249 @@
+"""Behavior tests for the cross-process file lock in the runtime config store.
+
+Windows' blocking lock mode (``msvcrt.LK_LOCK``) gives up after roughly ten
+seconds and surfaces contention as ``Permission denied``, while the whole
+point of the sidecar lock is to serialize concurrent Gateway workers'
+read-modify-write cycles of one store file. The Windows fallback must retry
+the non-blocking acquisition until the region frees, mirroring the extensions
+manager's lock.
+"""
+
+from __future__ import annotations
+
+import errno
+import logging
+import os
+import sys
+import threading
+import time
+from types import SimpleNamespace
+
+import pytest
+
+from app.channels import runtime_config_store
+from app.channels.runtime_config_store import ChannelRuntimeConfigStore, _runtime_config_file_lock
+
+TELEGRAM = {"enabled": True, "bot_token": "tg-token"}
+
+
+class _ContendedFakeMsvcrt:
+    """Fake ``msvcrt`` whose lock region stays busy for the first N attempts.
+
+    Records every ``locking`` call so tests can assert the retry cadence and
+    the final unlock, independently of the real Windows semantics.
+    """
+
+    LK_UNLCK = 0
+    LK_LOCK = 1
+    LK_NBLCK = 2
+
+    def __init__(self, fail_times: int, error_number: int = errno.EDEADLK) -> None:
+        self.fail_times = fail_times
+        self.error_number = error_number
+        self.attempts: list[int] = []
+        self.unlocked = 0
+
+    def locking(self, fd: int, mode: int, nbytes: int) -> None:
+        self.attempts.append(mode)
+        if mode == self.LK_UNLCK:
+            self.unlocked += 1
+            return None
+        blocking_attempts = len(self.attempts) - self.unlocked
+        if blocking_attempts <= self.fail_times:
+            raise OSError(self.error_number, "Lock contended")
+        return None
+
+
+class _BlockingModeFakeMsvcrt:
+    """Fake ``msvcrt`` that fails only the blocking ``LK_LOCK`` mode.
+
+    Models a region contended past the blocking mode's patience: the
+    unpatched module calls ``LK_LOCK`` once and fails, while a retry loop
+    succeeds on its first non-blocking attempt.
+    """
+
+    LK_UNLCK = 0
+    LK_LOCK = 1
+    LK_NBLCK = 2
+
+    def __init__(self) -> None:
+        self.attempts: list[int] = []
+        self.unlocked = 0
+
+    def locking(self, fd: int, mode: int, nbytes: int) -> None:
+        self.attempts.append(mode)
+        if mode == self.LK_UNLCK:
+            self.unlocked += 1
+            return None
+        if mode == self.LK_LOCK:
+            raise OSError(errno.EDEADLK, "Resource deadlock avoided")
+        return None
+
+
+@pytest.mark.parametrize("error_number", [errno.EACCES, errno.EAGAIN, errno.EDEADLK])
+def test_file_lock_retries_until_contention_clears(tmp_path, monkeypatch, caplog, error_number) -> None:
+    """Contention is retried on the Windows fallback instead of failing."""
+    fake = _ContendedFakeMsvcrt(fail_times=3, error_number=error_number)
+    monkeypatch.setattr(runtime_config_store, "fcntl", None, raising=False)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", fake, raising=False)
+    sleeps: list[float] = []
+    monkeypatch.setattr(runtime_config_store, "time", SimpleNamespace(sleep=sleeps.append))
+
+    entered = False
+    with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+        entered = True
+
+    assert entered
+    assert fake.attempts == [fake.LK_NBLCK] * 4 + [fake.LK_UNLCK]
+    assert fake.unlocked == 1
+    assert sleeps == [0.2] * 3
+    assert not caplog.records
+
+
+@pytest.mark.parametrize("error_number", [errno.EBADF, errno.EINVAL, errno.EIO, errno.EPERM, errno.ENOSYS, None])
+def test_file_lock_propagates_permanent_errors_without_waiting_or_unlocking(tmp_path, monkeypatch, error_number) -> None:
+    primary = OSError(error_number, "Lock unavailable")
+    calls = []
+    sleeps = []
+
+    def locking(fd, mode, size):
+        calls.append(mode)
+        if len(calls) > 1:
+            pytest.fail("A failed acquisition must not be retried or unlocked")
+        raise primary
+
+    monkeypatch.setattr(runtime_config_store, "fcntl", None)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", SimpleNamespace(LK_NBLCK=2, LK_UNLCK=0, locking=locking), raising=False)
+    monkeypatch.setattr(runtime_config_store, "time", SimpleNamespace(sleep=sleeps.append))
+
+    with pytest.raises(OSError) as caught:
+        with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+            pytest.fail("A failed acquisition must not enter the protected section")
+
+    assert caught.value is primary
+    assert calls == [2]
+    assert sleeps == []
+
+
+@pytest.mark.parametrize("fail_times", [49, 50, 100])
+def test_file_lock_warns_once_for_prolonged_contention(tmp_path, monkeypatch, caplog, fail_times) -> None:
+    fake = _ContendedFakeMsvcrt(fail_times=fail_times)
+    sleeps = []
+    monkeypatch.setattr(runtime_config_store, "fcntl", None)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", fake, raising=False)
+    monkeypatch.setattr(runtime_config_store, "time", SimpleNamespace(sleep=sleeps.append))
+
+    with caplog.at_level(logging.WARNING, logger=runtime_config_store.__name__):
+        with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+            pass
+
+    warnings = [record for record in caplog.records if record.name == runtime_config_store.__name__]
+    assert len(warnings) == (1 if fail_times >= 50 else 0)
+    if warnings:
+        assert str(tmp_path / ".runtime-config.json.lock") in warnings[0].getMessage()
+        assert "50" in warnings[0].getMessage()
+    assert sleeps == [0.2] * fail_times
+    assert fake.unlocked == 1
+
+
+def test_file_lock_diagnostic_failure_does_not_abort_the_wait(tmp_path, monkeypatch) -> None:
+    fake = _ContendedFakeMsvcrt(fail_times=51)
+    warnings = []
+    sleeps = []
+
+    def failing_warning(*args, **kwargs):
+        warnings.append(args)
+        raise RuntimeError("Log handler failed")
+
+    monkeypatch.setattr(runtime_config_store, "fcntl", None)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", fake, raising=False)
+    monkeypatch.setattr(runtime_config_store, "time", SimpleNamespace(sleep=sleeps.append))
+    monkeypatch.setattr(runtime_config_store.logger, "warning", failing_warning)
+
+    with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+        pass
+
+    assert len(warnings) == 1
+    assert sleeps == [0.2] * 51
+    assert fake.unlocked == 1
+
+
+def test_file_lock_creates_the_sidecar_byte_on_the_windows_fallback(tmp_path, monkeypatch) -> None:
+    """The empty sidecar file still gets its lockable byte before locking."""
+    fake = _ContendedFakeMsvcrt(fail_times=0)
+    monkeypatch.setattr(runtime_config_store, "fcntl", None, raising=False)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", fake, raising=False)
+
+    with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+        pass
+
+    assert (tmp_path / ".runtime-config.json.lock").read_bytes() == b"\0"
+
+
+def test_file_lock_posix_branch_still_flocks(tmp_path, monkeypatch) -> None:
+    """The POSIX branch keeps using ``fcntl.flock`` unchanged."""
+
+    class _FakeFcntl:
+        LOCK_EX = 2
+        LOCK_UN = 8
+
+        def __init__(self) -> None:
+            self.ops: list[tuple[int, int]] = []
+
+        def flock(self, fd: int, operation: int) -> None:
+            self.ops.append((fd, operation))
+
+    fake_fcntl = _FakeFcntl()
+    monkeypatch.setattr(runtime_config_store, "fcntl", fake_fcntl, raising=False)
+
+    entered = False
+    with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+        entered = True
+
+    assert entered
+    assert [operation for _, operation in fake_fcntl.ops] == [_FakeFcntl.LOCK_EX, _FakeFcntl.LOCK_UN]
+
+
+@pytest.mark.skipif(os.name != "nt", reason="exercises the Windows import path directly")
+def test_windows_fallback_survives_contention_the_blocking_mode_cannot(tmp_path, monkeypatch) -> None:
+    """Direct regression: the fallback retries contention instead of failing.
+
+    ``sys.modules`` catches the function-local ``import msvcrt`` the unpatched
+    module performs; the module attribute covers the fixed module's global
+    binding, so this test demonstrates the base failure mode (one ``LK_LOCK``
+    call raising ``OSError``) and the fixed retry behavior on either structure.
+    """
+    fake = _BlockingModeFakeMsvcrt()
+    monkeypatch.setitem(sys.modules, "msvcrt", fake)
+    monkeypatch.setattr(runtime_config_store, "msvcrt", fake, raising=False)
+
+    with _runtime_config_file_lock(tmp_path / "runtime-config.json"):
+        pass
+
+    assert fake.attempts == [fake.LK_NBLCK, fake.LK_UNLCK]
+    assert fake.unlocked == 1
+
+
+def test_save_serializes_with_a_peer_holding_the_real_lock(tmp_path) -> None:
+    """A real peer holding the sidecar lock delays the save instead of failing it.
+
+    The advisory file lock is per open file, so two lock contexts on one path
+    exclude each other inside this process, as the peer tests for #6844 note.
+    """
+    store_path = tmp_path / "channels" / "runtime-config.json"
+    store_path.parent.mkdir(parents=True, exist_ok=True)
+    holder_ready = threading.Event()
+
+    def hold():
+        with _runtime_config_file_lock(store_path):
+            holder_ready.set()
+            time.sleep(0.6)
+
+    holder = threading.Thread(target=hold)
+    holder.start()
+    assert holder_ready.wait(timeout=5)
+
+    ChannelRuntimeConfigStore(store_path).set_provider_config("telegram", TELEGRAM)
+
+    holder.join(timeout=5)
+    assert ChannelRuntimeConfigStore(store_path).get_provider_config("telegram") == TELEGRAM