diff --git a/astrbot/core/agent/message.py b/astrbot/core/agent/message.py
index 374ec7dca..6e3a4012d 100644
--- a/astrbot/core/agent/message.py
+++ b/astrbot/core/agent/message.py
@@ -3,7 +3,7 @@
 
 from typing import Any, ClassVar, Literal, cast
 
-from pydantic import BaseModel, GetCoreSchemaHandler
+from pydantic import BaseModel, GetCoreSchemaHandler, model_validator
 from pydantic_core import core_schema
 
 
@@ -145,23 +145,39 @@ class Message(BaseModel):
         "tool",
     ]
 
-    content: str | list[ContentPart]
+    content: str | list[ContentPart] | None = None
     """The content of the message."""
 
+    tool_calls: list[ToolCall] | list[dict] | None = None
+    """The tool calls of the message."""
+
+    tool_call_id: str | None = None
+    """The ID of the tool call."""
+
+    @model_validator(mode="after")
+    def check_content_required(self):
+        # assistant + tool_calls is not None: allow content to be None
+        if self.role == "assistant" and self.tool_calls is not None:
+            return self
+
+        # other all cases: content is required
+        if self.content is None:
+            raise ValueError(
+                "content is required unless role='assistant' and tool_calls is not None"
+            )
+        return self
+
 
 class AssistantMessageSegment(Message):
     """A message segment from the assistant."""
 
     role: Literal["assistant"] = "assistant"
-    content: str | list[ContentPart] | None = None
-    tool_calls: list[ToolCall] | list[dict] | None = None
 
 
 class ToolCallMessageSegment(Message):
     """A message segment representing a tool call."""
 
     role: Literal["tool"] = "tool"
-    tool_call_id: str
 
 
 class UserMessageSegment(Message):