diff --git a/nanobot/channels/base.py b/nanobot/channels/base.py
index 2201686..3010373 100644
--- a/nanobot/channels/base.py
+++ b/nanobot/channels/base.py
@@ -89,7 +89,8 @@ class BaseChannel(ABC):
         chat_id: str,
         content: str,
         media: list[str] | None = None,
-        metadata: dict[str, Any] | None = None
+        metadata: dict[str, Any] | None = None,
+        session_key: str | None = None,
     ) -> None:
         """
         Handle an incoming message from the chat platform.
@@ -102,6 +103,7 @@ class BaseChannel(ABC):
             content: Message text content.
             media: Optional list of media URLs.
             metadata: Optional channel-specific metadata.
+            session_key: Optional session key override (e.g. thread-scoped sessions).
         """
         if not self.is_allowed(sender_id):
             logger.warning(
@@ -111,15 +113,14 @@ class BaseChannel(ABC):
             )
             return
         
-        meta = metadata or {}
         msg = InboundMessage(
             channel=self.name,
             sender_id=str(sender_id),
             chat_id=str(chat_id),
             content=content,
             media=media or [],
-            metadata=meta,
-            session_key_override=meta.get("session_key"),
+            metadata=metadata or {},
+            session_key_override=session_key,
         )
         
         await self.bus.publish_inbound(msg)
diff --git a/nanobot/channels/slack.py b/nanobot/channels/slack.py
index 2e91f7b..906593b 100644
--- a/nanobot/channels/slack.py
+++ b/nanobot/channels/slack.py
@@ -193,8 +193,8 @@ class SlackChannel(BaseChannel):
                         "thread_ts": thread_ts,
                         "channel_type": channel_type,
                     },
-                    "session_key": session_key,
                 },
+                session_key=session_key,
             )
         except Exception:
             logger.exception("Error handling Slack message from {}", sender_id)