diff --git a/agent/context_manager/manager.py b/agent/context_manager/manager.py
index 85e96af0f..1c7a54263 100644
--- a/agent/context_manager/manager.py
+++ b/agent/context_manager/manager.py
@@ -257,9 +257,9 @@ def _load_system_prompt(
             f"Tools={len(tool_specs)}]"
         )
 
-    def add_message(self, message: Message, token_count: int = None) -> None:
+    def add_message(self, message: Message, token_count: int | None = None) -> None:
         """Add a message to the history"""
-        if token_count:
+        if token_count is not None:
             self.running_context_usage = token_count
         self.items.append(message)
         if self.on_message_added:
diff --git a/tests/unit/test_context_manager.py b/tests/unit/test_context_manager.py
new file mode 100644
index 000000000..237fcb68d
--- /dev/null
+++ b/tests/unit/test_context_manager.py
@@ -0,0 +1,16 @@
+"""Unit tests for the context manager."""
+
+from agent.context_manager.manager import ContextManager
+
+
+def test_add_message_accepts_zero_token_count():
+    """A token_count of 0 should update running_context_usage, not be treated as missing."""
+    manager = ContextManager(tool_specs=[])
+    manager.running_context_usage = 100
+
+    class _Message:
+        role = "user"
+        content = "hi"
+
+    manager.add_message(_Message(), token_count=0)
+    assert manager.running_context_usage == 0