diff --git a/src/tracker/api/messages.py b/src/tracker/api/messages.py
index da9c263..8887f46 100644
--- a/src/tracker/api/messages.py
+++ b/src/tracker/api/messages.py
@@ -73,7 +73,7 @@ def _message_out(m: Message) -> dict:
         "task_id": str(m.task_id) if m.task_id else None,
         "parent_id": str(m.parent_id) if m.parent_id else None,
         "author_type": m.author_type,
-        "author_id": str(m.author_id),
+        "author_id": str(m.author_id) if m.author_id else None,
         "author": {"id": str(m.author.id), "slug": m.author.slug, "name": m.author.name} if m.author else None,
         "content": m.content,
         "mentions": m.mentions or [],
@@ -164,8 +164,8 @@ async def create_message(req: MessageCreate, request: Request, db: AsyncSession
     msg = result2.scalar_one()
 
     # Get author name for broadcast (already loaded via relationship)
-    author_name = msg.author.name if msg.author else "Unknown"
-    author_slug = msg.author.slug if msg.author else "unknown"
+    author_name = msg.author.name if msg.author else "System"
+    author_slug = msg.author.slug if msg.author else "system"
 
     # Broadcast via WebSocket
     from tracker.ws.manager import manager
diff --git a/src/tracker/api/tasks.py b/src/tracker/api/tasks.py
index 42c21eb..599628a 100644
--- a/src/tracker/api/tasks.py
+++ b/src/tracker/api/tasks.py
@@ -144,14 +144,13 @@ async def _system_message(
     now = datetime.datetime.now(datetime.timezone.utc)
 
     chat_id = await _get_project_chat_id(db, task.project_id)
-    actor_id = actor.id if actor else None
     actor_slug = actor.slug if actor else "system"
 
-    # 1. Task comment
+    # 1. Task comment (author=None for system, actor tracked in task_actions)
     task_msg = Message(
         task_id=task.id,
         author_type=AuthorType.SYSTEM,
-        author_id=actor_id,
+        author_id=None,
         content=task_text,
         mentions=[],
     )
@@ -163,7 +162,7 @@ async def _system_message(
         chat_msg = Message(
             chat_id=chat_id,
             author_type=AuthorType.SYSTEM,
-            author_id=actor_id,
+            author_id=None,
             content=chat_text,
             mentions=[],
         )
@@ -177,8 +176,9 @@ async def _system_message(
         "task_id": str(task.id),
         "task_key": key,
         "author_type": AuthorType.SYSTEM,
-        "author_id": str(actor_id) if actor_id else None,
-        "author_slug": actor_slug,
+        "author_id": None,
+        "author_slug": "system",
+        "author": None,
         "content": task_text,
         "created_at": task_msg.created_at.isoformat() if task_msg.created_at else now.isoformat(),
     })
@@ -189,11 +189,12 @@ async def _system_message(
             "id": str(chat_msg.id),
             "chat_id": str(chat_id),
             "author_type": AuthorType.SYSTEM,
-            "author_id": str(actor_id) if actor_id else None,
-            "author_slug": actor_slug,
+            "author_id": None,
+            "author_slug": "system",
+            "author": None,
             "content": chat_text,
             "created_at": chat_msg.created_at.isoformat() if chat_msg.created_at else now.isoformat(),
-        }, author_slug=actor_slug)
+        }, author_slug="system")
 
 
 def _task_out(t: Task, project_slug: str = "") -> dict:
diff --git a/src/tracker/models/chat.py b/src/tracker/models/chat.py
index aa2a64d..94a9df4 100644
--- a/src/tracker/models/chat.py
+++ b/src/tracker/models/chat.py
@@ -42,7 +42,7 @@ class Message(Base):
 
     # Author
     author_type: Mapped[str] = mapped_column(String(20), nullable=False)  # human | agent | system
-    author_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("members.id"), nullable=False)
+    author_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("members.id"), nullable=True)
 
     # Content
     content: Mapped[str] = mapped_column(Text, nullable=False)