diff --git a/src/strands/session/file_session_manager.py b/src/strands/session/file_session_manager.py
index 9df86e17a..14e71d07c 100644
--- a/src/strands/session/file_session_manager.py
+++ b/src/strands/session/file_session_manager.py
@@ -86,7 +86,13 @@ def _get_message_path(self, session_id: str, agent_id: str, message_id: int) ->
             message_id: Index of the message
         Returns:
             The filename for the message
+
+        Raises:
+            ValueError: If message_id is not an integer.
         """
+        if not isinstance(message_id, int):
+            raise ValueError(f"message_id=<{message_id}> | message id must be an integer")
+        
         agent_path = self._get_agent_path(session_id, agent_id)
         return os.path.join(agent_path, "messages", f"{MESSAGE_PREFIX}{message_id}.json")
 
diff --git a/src/strands/session/s3_session_manager.py b/src/strands/session/s3_session_manager.py
index d15e6e3bd..da1735e35 100644
--- a/src/strands/session/s3_session_manager.py
+++ b/src/strands/session/s3_session_manager.py
@@ -113,11 +113,16 @@ def _get_message_path(self, session_id: str, agent_id: str, message_id: int) ->
             session_id: ID of the session
             agent_id: ID of the agent
             message_id: Index of the message
-            **kwargs: Additional keyword arguments for future extensibility.
 
         Returns:
             The key for the message
+            
+        Raises:
+            ValueError: If message_id is not an integer.
         """
+        if not isinstance(message_id, int):
+            raise ValueError(f"message_id=<{message_id}> | message id must be an integer")
+            
         agent_path = self._get_agent_path(session_id, agent_id)
         return f"{agent_path}messages/{MESSAGE_PREFIX}{message_id}.json"
 
diff --git a/tests/strands/session/test_file_session_manager.py b/tests/strands/session/test_file_session_manager.py
index a89222b7e..036591924 100644
--- a/tests/strands/session/test_file_session_manager.py
+++ b/tests/strands/session/test_file_session_manager.py
@@ -224,14 +224,14 @@ def test_read_messages_with_new_agent(file_manager, sample_session, sample_agent
     file_manager.create_session(sample_session)
     file_manager.create_agent(sample_session.session_id, sample_agent)
 
-    result = file_manager.read_message(sample_session.session_id, sample_agent.agent_id, "nonexistent_message")
+    result = file_manager.read_message(sample_session.session_id, sample_agent.agent_id, 999)
 
     assert result is None
 
 
 def test_read_nonexistent_message(file_manager, sample_session, sample_agent):
     """Test reading a message that doesnt exist."""
-    result = file_manager.read_message(sample_session.session_id, sample_agent.agent_id, "nonexistent_message")
+    result = file_manager.read_message(sample_session.session_id, sample_agent.agent_id, 999)
     assert result is None
 
 
@@ -390,3 +390,21 @@ def test__get_session_path_invalid_session_id(session_id, file_manager):
 def test__get_agent_path_invalid_agent_id(agent_id, file_manager):
     with pytest.raises(ValueError, match=f"agent_id={agent_id} | id cannot contain path separators"):
         file_manager._get_agent_path("session1", agent_id)
+
+
+@pytest.mark.parametrize(
+    "message_id",
+    [
+        "../../../secret",
+        "../../attack", 
+        "../escape",
+        "path/traversal",
+        "not_an_int",
+        None,
+        [],
+    ],
+)
+def test__get_message_path_invalid_message_id(message_id, file_manager):
+    """Test that message_id that is not an integer raises ValueError."""
+    with pytest.raises(ValueError, match=r"message_id=<.*> \| message id must be an integer"):
+        file_manager._get_message_path("session1", "agent1", message_id)
diff --git a/tests/strands/session/test_s3_session_manager.py b/tests/strands/session/test_s3_session_manager.py
index 71bff3050..50fb303f7 100644
--- a/tests/strands/session/test_s3_session_manager.py
+++ b/tests/strands/session/test_s3_session_manager.py
@@ -251,7 +251,7 @@ def test_read_nonexistent_message(s3_manager, sample_session, sample_agent, samp
     s3_manager.create_agent(sample_session.session_id, sample_agent)
 
     # Read message
-    result = s3_manager.read_message(sample_session.session_id, sample_agent.agent_id, "nonexistent_message")
+    result = s3_manager.read_message(sample_session.session_id, sample_agent.agent_id, 999)
 
     assert result is None
 
@@ -356,3 +356,21 @@ def test__get_session_path_invalid_session_id(session_id, s3_manager):
 def test__get_agent_path_invalid_agent_id(agent_id, s3_manager):
     with pytest.raises(ValueError, match=f"agent_id={agent_id} | id cannot contain path separators"):
         s3_manager._get_agent_path("session1", agent_id)
+
+
+@pytest.mark.parametrize(
+    "message_id",
+    [
+        "../../../secret",
+        "../../attack", 
+        "../escape",
+        "path/traversal",
+        "not_an_int",
+        None,
+        [],
+    ],
+)
+def test__get_message_path_invalid_message_id(message_id, s3_manager):
+    """Test that message_id that is not an integer raises ValueError."""
+    with pytest.raises(ValueError, match=r"message_id=<.*> \| message id must be an integer"):
+        s3_manager._get_message_path("session1", "agent1", message_id)