diff --git a/gateway/platforms/whatsapp_common.py b/gateway/platforms/whatsapp_common.py
index 228cb530f8..1ac99c8839 100644
--- a/gateway/platforms/whatsapp_common.py
+++ b/gateway/platforms/whatsapp_common.py
@@ -18,7 +18,10 @@ import re
 from pathlib import Path
 from typing import Any, Dict, Optional
 
-from gateway.platforms._shared import extra_or_secret as _extra_or_wsecret, get_scoped_secret as _get_wsecret
+from gateway.platforms._shared import (
+    decode_json_list_literal as _decode_json_list_literal, extra_or_secret as _extra_or_wsecret,
+    get_scoped_secret as _get_wsecret
+)
 from gateway.platforms.access_policy_mixin import OwnAccessPolicyMixin
 
 
@@ -99,9 +102,10 @@ class WhatsAppBehaviorMixin(OwnAccessPolicyMixin):
 
     @staticmethod
     def _coerce_allow_list(raw) -> set[str]:
-        """Parse allow_from / group_allow_from from config (list) or env var (CSV)."""
+        """Parse allow_from / group_allow_from from config (list or JSON-list string) or env var (CSV)."""
         if raw is None:
             return set()
+        raw = _decode_json_list_literal(raw)
         parts = raw if isinstance(raw, list) else str(raw).split(",")
         return {str(part).strip() for part in parts if str(part).strip()}
 
diff --git a/plugins/platforms/dingtalk/adapter.py b/plugins/platforms/dingtalk/adapter.py
index fabfe31f48..a4406fa16e 100644
--- a/plugins/platforms/dingtalk/adapter.py
+++ b/plugins/platforms/dingtalk/adapter.py
@@ -50,8 +50,8 @@ from gateway.platforms.helpers import MessageDeduplicator, compile_mention_patte
 from gateway.platforms.base import BasePlatformAdapter, SendResult
 from gateway.platforms.event import MessageEvent
 from gateway.platforms._shared import (
-    apply_yaml_bridge as _apply_yaml_bridge, extra_or_secret as _extra_or_secret,
-    get_scoped_secret as _get_scoped_secret, send_error
+    apply_yaml_bridge as _apply_yaml_bridge, decode_json_list_literal as _decode_json_list_literal,
+    extra_or_secret as _extra_or_secret, get_scoped_secret as _get_scoped_secret, send_error
 )
 from plugins.platforms.dingtalk.inbound import collect_download_codes, extract_media, extract_text
 
@@ -73,7 +73,8 @@ _NO_LOCAL_UPLOAD = "DingTalk session webhook replies do not support local %s. On
 
 
 def _csv_set(raw: Any) -> Set[str]:
-    """Split a list or comma-separated string into a set of stripped, non-empty items."""
+    """Split a list, JSON-list string or comma-separated string into a set of stripped, non-empty items."""
+    raw = _decode_json_list_literal(raw)
     parts = raw if isinstance(raw, list) else str(raw).split(",")
     return {str(part).strip() for part in parts if str(part).strip()}
 
diff --git a/plugins/platforms/discord/adapter.py b/plugins/platforms/discord/adapter.py
index b36bb1ebcb..0355161850 100644
--- a/plugins/platforms/discord/adapter.py
+++ b/plugins/platforms/discord/adapter.py
@@ -272,8 +272,9 @@ from gateway.platforms.base import (
 from gateway.platforms.event import MessageEvent, MessageType, ProcessingOutcome
 from tools.url_safety import is_safe_url
 from gateway.platforms._shared import (
-    env_is_connected as _env_is_connected, extra_or_secret as _extra_or_secret,
-    platform_gate_env as _scoped_gate_env, send_error, yaml_env_setter as _yaml_env_setter
+    decode_json_list_literal as _decode_json_list_literal, env_is_connected as _env_is_connected,
+    extra_or_secret as _extra_or_secret, platform_gate_env as _scoped_gate_env, send_error,
+    yaml_env_setter as _yaml_env_setter
 )
 
 # Every refusal (slash command, approval button, picker, prompt) says the same thing.
@@ -2160,17 +2161,14 @@ class DiscordAdapter(DiscordMediaMixin, BasePlatformAdapter):
         free-response channels by default; ``channels: "*"`` scans every text channel."""
         configured = self.config.extra.get("missed_message_backfill")
         if isinstance(configured, dict) and "channels" in configured:
-            raw = configured.get("channels")
-            if isinstance(raw, list):
-                return {str(item).strip() for item in raw if str(item).strip()}
-            raw = str(raw or "")
-            if raw.strip():
-                return {item.strip() for item in raw.split(",") if item.strip()}
+            channels = self._gate_csv_set(configured.get("channels"))
+            if channels:
+                return channels
         raw = self._gate_env("DISCORD_MISSED_MESSAGE_BACKFILL_CHANNELS")
         if not raw.strip():
             allowed = self._get_allowed_channels()
             return allowed | self._discord_free_response_channels()
-        return {item.strip() for item in raw.split(",") if item.strip()}
+        return self._gate_csv_set(raw)
 
     def _missed_message_backfill_number(self, key: str, env_key: str, default, cast, lo, hi=None):
         """Numeric ``missed_message_backfill.<key>`` (dict extra wins over env), clamped to [lo, hi]."""
@@ -4832,6 +4830,7 @@ class DiscordAdapter(DiscordMediaMixin, BasePlatformAdapter):
     def _gate_csv_set(raw) -> set:
         if raw is None:
             return set()
+        raw = _decode_json_list_literal(raw)
         if isinstance(raw, list):
             return {str(part).strip() for part in raw if str(part).strip()}
         return {part.strip() for part in str(raw).split(",") if part.strip()}
@@ -4934,13 +4933,7 @@ class DiscordAdapter(DiscordMediaMixin, BasePlatformAdapter):
         raw = self.config.extra.get("free_response_channels")
         if raw is None:
             raw = self._gate_env("DISCORD_FREE_RESPONSE_CHANNELS")
-        if isinstance(raw, list):
-            return {str(part).strip() for part in raw if str(part).strip()}
-        # YAML parses a bare numeric value as int; str() any scalar before splitting.
-        s = str(raw).strip() if raw is not None else ""
-        if s:
-            return {part.strip() for part in s.split(",") if part.strip()}
-        return set()
+        return self._gate_csv_set(raw)
 
     def _raw_mentioned_user_ids(self, message: Any) -> set:
         """Extract user-mention IDs (``<@ID>`` and legacy ``<@!ID>``) from raw content,
diff --git a/tests/plugins/platforms/test_bracket_list_allowlists.py b/tests/plugins/platforms/test_bracket_list_allowlists.py
new file mode 100644
index 0000000000..d0194f522a
--- /dev/null
+++ b/tests/plugins/platforms/test_bracket_list_allowlists.py
@@ -0,0 +1,54 @@
+"""Allowlists stored as a JSON-list *string* are honoured by every platform gate (issue #76457).
+
+Configs written by ``hermes config set KEY '["-100","-200"]'`` before the writer learned
+to emit YAML lists hold the literal ``'["-100","-200"]'`` as a string. The Telegram gate
+already decodes that shape (``gateway/platforms/_shared.py::decode_json_list_literal``);
+the Discord / WhatsApp / DingTalk gates comma-split it into one bogus entry that matches
+nothing, silently locking out every allowlisted chat or user.
+"""
+
+from gateway.config import Platform, PlatformConfig
+
+BRACKET_LIST = '["-100", "-200"]'
+
+
+def _discord(extra):
+    from plugins.platforms.discord.adapter import DiscordAdapter
+
+    adapter = object.__new__(DiscordAdapter)
+    adapter.platform = Platform.DISCORD
+    adapter.config = PlatformConfig(enabled=True, token="x", extra=dict(extra))
+    adapter._gate_env_snapshot = None
+    return adapter
+
+
+def _whatsapp(extra):
+    from plugins.platforms.whatsapp.adapter import WhatsAppAdapter
+
+    adapter = object.__new__(WhatsAppAdapter)
+    adapter.platform = Platform.WHATSAPP
+    adapter.config = PlatformConfig(enabled=True, extra=dict(extra))
+    return adapter
+
+
+def _dingtalk(extra):
+    from plugins.platforms.dingtalk.adapter import DingTalkAdapter
+
+    return DingTalkAdapter(PlatformConfig(enabled=True, extra=dict(extra)))
+
+
+def test_bracket_list_string_is_decoded_by_every_platform_gate():
+    assert _discord({"allowed_channels": BRACKET_LIST})._get_allowed_channels() == {"-100", "-200"}
+    assert _discord({"free_response_channels": BRACKET_LIST})._discord_free_response_channels() == {"-100", "-200"}
+    assert _whatsapp({"free_response_chats": BRACKET_LIST})._whatsapp_free_response_chats() == {"-100", "-200"}
+    dingtalk = _dingtalk({"allowed_chats": BRACKET_LIST, "allowed_users": '["Alice"]'})
+    assert dingtalk._dingtalk_allowed_chats() == {"-100", "-200"}
+    assert dingtalk._allowed_users == {"alice"}
+
+
+def test_plain_csv_and_yaml_lists_keep_their_meaning():
+    assert _discord({"allowed_channels": "-100, -200"})._get_allowed_channels() == {"-100", "-200"}
+    assert _discord({"allowed_channels": ["-100", "-200"]})._get_allowed_channels() == {"-100", "-200"}
+    assert _whatsapp({"free_response_chats": "a,b"})._whatsapp_free_response_chats() == {"a", "b"}
+    # Malformed JSON is not a list: it stays on the legacy comma-split path.
+    assert _dingtalk({"allowed_chats": "[not-json"})._dingtalk_allowed_chats() == {"[not-json"}