diff --git a/docs/guides/mattermost-ai-agent.md b/docs/guides/mattermost-ai-agent.md
index f399d5da7..af049e02b 100644
--- a/docs/guides/mattermost-ai-agent.md
+++ b/docs/guides/mattermost-ai-agent.md
@@ -41,6 +41,7 @@ Merge this snippet into `~/.nanobot/config.json`:
       "token": "YOUR_MATTERMOST_TOKEN",
       "teamId": "YOUR_TEAM_ID",
       "groupPolicy": "mention",
+      "groupPolicyInThread": "open",
       "replyInThread": true,
       "dm": {
         "policy": "allowlist"
@@ -51,7 +52,15 @@ Merge this snippet into `~/.nanobot/config.json`:
 ```
 
 `teamId` scopes the channel to a Mattermost team. Keep `groupPolicy` as
-`mention` for the first test.
+`mention` for the first test. `groupPolicyInThread` can be `"mention"`,
+`"open"`, or `"allowlist"` and controls messages that reply inside a
+thread. If it is omitted, it inherits `groupPolicy`, preserving the behavior
+of existing configurations. Set it to `"open"` explicitly when follow-up
+messages in threads should not require another @mention.
+
+When `groupPolicy` is `"allowlist"`, `groupAllowFrom` remains the outer
+channel boundary for root posts and thread replies. A thread policy cannot open
+a channel that is not on that allowlist.
 
 Mattermost DMs are open by default. Setting `dm.policy` to `"allowlist"` with no
 `dm.allowFrom` entries makes new DM senders receive a pairing code. Approve the
@@ -93,8 +102,8 @@ Then DM the bot again, or mention it in a channel where the bot has access:
 - If DMs are ignored, review the `dm` policy and pairing approval state.
 - If channel messages are ignored, confirm the bot is mentioned and belongs to
   the team/channel.
-- If thread replies are surprising, review `replyInThread` and
-  `includeThreadContext`.
+- If thread replies are surprising, review `groupPolicyInThread`,
+  `replyInThread`, and `includeThreadContext`.
 
 ## Next: memory, automations, MCP tools
 
diff --git a/nanobot/channels/mattermost/manifest.py b/nanobot/channels/mattermost/manifest.py
index 575e09565..4061a3513 100644
--- a/nanobot/channels/mattermost/manifest.py
+++ b/nanobot/channels/mattermost/manifest.py
@@ -10,6 +10,7 @@ SETUP_SPEC = ChannelSetupSpec(
         "token": field("secret"),
         "teamId": field(),
         "groupPolicy": field("enum", choices=GROUP_POLICIES, default="mention"),
+        "groupPolicyInThread": field("enum", choices=GROUP_POLICIES, default="mention"),
         "allowFrom": field("list"),
     },
     required=required_fields("serverUrl", "token"),
diff --git a/nanobot/channels/mattermost/runtime.py b/nanobot/channels/mattermost/runtime.py
index 141c82452..6cd32fce3 100644
--- a/nanobot/channels/mattermost/runtime.py
+++ b/nanobot/channels/mattermost/runtime.py
@@ -9,7 +9,7 @@ from pathlib import Path
 from typing import Any, cast
 
 import httpx
-from pydantic import Field
+from pydantic import Field, model_validator
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
@@ -60,6 +60,22 @@ class MattermostConfig(Base):
     send_tool_hints: bool = True
     dm: MattermostDMConfig = Field(default_factory=MattermostDMConfig)
 
+    @model_validator(mode="before")
+    @classmethod
+    def _inherit_thread_policy(cls, data: Any) -> Any:
+        """Preserve the existing group policy unless a thread override is set."""
+        if not isinstance(data, dict):
+            return data
+        raw = cast(dict[str, Any], data)
+        if "groupPolicyInThread" in raw or "group_policy_in_thread" in raw:
+            return raw
+        values = dict(raw)
+        values["group_policy_in_thread"] = values.get(
+            "groupPolicy",
+            values.get("group_policy", "mention"),
+        )
+        return values
+
 
 def _server_url_to_ws_url(server_url: str) -> str:
     if server_url.startswith("https://"):
diff --git a/nanobot/channels/mattermost/tests/test_mattermost_channel.py b/nanobot/channels/mattermost/tests/test_mattermost_channel.py
index a1fdefea0..1960e10b3 100644
--- a/nanobot/channels/mattermost/tests/test_mattermost_channel.py
+++ b/nanobot/channels/mattermost/tests/test_mattermost_channel.py
@@ -12,6 +12,7 @@ import pytest
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
+from nanobot.channels.mattermost.manifest import SETUP_SPEC
 from nanobot.channels.mattermost.runtime import (
     MATTERMOST_MAX_MESSAGE_LEN,
     MattermostChannel,
@@ -123,6 +124,25 @@ def test_config_defaults():
     assert config.dm.enabled is True
     assert config.dm.policy == "open"
     assert config.reply_in_thread is True
+    assert config.group_policy_in_thread == "mention"
+
+
+def test_thread_policy_inherits_group_policy_when_omitted():
+    config = MattermostConfig.model_validate({"groupPolicy": "open"})
+    assert config.group_policy_in_thread == "open"
+
+    explicit = MattermostConfig.model_validate({
+        "groupPolicy": "open",
+        "groupPolicyInThread": "mention",
+    })
+    assert explicit.group_policy_in_thread == "mention"
+
+
+def test_setup_contract_exposes_thread_policy():
+    field = SETUP_SPEC.fields["groupPolicyInThread"]
+    assert field.kind == "enum"
+    assert field.choices == {"open", "mention", "allowlist"}
+    assert field.default == "mention"
 
 
 def test_config_camelcase_aliases():
@@ -376,15 +396,16 @@ async def test_group_policy_allowlist():
 
 
 @pytest.mark.asyncio
-async def test_group_policy_in_thread_default_open():
-    """Thread uses open policy by default, so no mention needed in threads."""
+async def test_group_policy_in_thread_defaults_to_group_policy():
+    """Existing configs keep their main-channel behavior in threads."""
     channel, fake = _make_channel({"groupPolicy": "mention"})
     channel._self_username = "nanobot"
     # In a main channel (not thread), mention is required
     assert channel._should_respond_in_channel("hello", "c1", in_thread=False) is False
     assert channel._should_respond_in_channel("@nanobot hello", "c1", in_thread=False) is True
-    # In a thread, no mention needed (default open policy)
-    assert channel._should_respond_in_channel("hello", "c1", in_thread=True) is True
+    # In a thread, the omitted override inherits mention policy.
+    assert channel._should_respond_in_channel("hello", "c1", in_thread=True) is False
+    assert channel._should_respond_in_channel("@nanobot hello", "c1", in_thread=True) is True
 
 
 @pytest.mark.asyncio
@@ -410,6 +431,38 @@ async def test_group_policy_in_thread_open():
     assert channel._should_respond_in_channel("hello", "c1", in_thread=True) is True
 
 
+@pytest.mark.asyncio
+async def test_posted_thread_event_uses_thread_policy():
+    """A real posted event derives thread policy from its root_id."""
+    channel, fake = _make_channel({
+        "groupPolicy": "mention",
+        "groupPolicyInThread": "open",
+        "includeThreadContext": False,
+    })
+    channel._self_id = "bot_id"
+    channel._self_username = "nanobot"
+    with patch.object(channel, "_handle_message", AsyncMock()) as mock_handle:
+        ws_msg = {
+            "event": "posted",
+            "data": {
+                "channel_type": "O",
+                "post": json.dumps({
+                    "id": "reply_1",
+                    "user_id": "user_1",
+                    "channel_id": "channel_1",
+                    "message": "follow up without a mention",
+                    "root_id": "root_1",
+                }),
+            },
+            "broadcast": {},
+        }
+
+        await channel._handle_ws_message(ws_msg)
+
+    mock_handle.assert_awaited_once()
+    assert mock_handle.call_args.kwargs["session_key"] == "mattermost:channel_1:root_1"
+
+
 @pytest.mark.asyncio
 async def test_group_policy_in_thread_allowlist():
     """Thread uses allowlist policy when configured."""