The type annotation change from ChatMessageStoreState | None to MutableMapping[str, Any] | ChatMessageStoreState | None is correct and necessary to support custom stores. However, this is a breaking change to the public API since code that accesses chat_message_store_state.messages directly may now fail if it's a dict instead of a ChatMessageStoreState object.

Consider documenting this breaking change more prominently in the PR description and potentially adding a BREAKING prefix to the PR title as mentioned in the contribution checklist.

Creating a temporary ChatMessageStoreState instance with no arguments may not be reliable for determining the standard fields. The 'type' field is dynamically added by SerializationMixin.to_dict() and will vary depending on whether the instance has custom type attributes. This could lead to incorrect detection of custom stores.

Instead, you should explicitly check for the known standard fields: {'type', 'messages'}. This is more robust and doesn't depend on runtime object instantiation.

The detection logic for custom stores may incorrectly classify stores that only differ in the 'type' field value as standard stores. For example, if a custom store serializes as {"type": "custom_store_state", "messages": []}, it will be treated as a standard store because it has no extra fields beyond 'type' and 'messages', and will be converted to ChatMessageStoreState with type "chat_message_store_state", losing the original custom type information.

To fix this, also check if the 'type' field value (when present) differs from 'chat_message_store_state'. If it does, treat it as a custom store and preserve as dict. For example:

if extra_fields or (
    "type" in chat_message_store_state
    and chat_message_store_state["type"] != "chat_message_store_state"
):
    # Custom store with additional fields or different type
    self.chat_message_store_state = chat_message_store_state

The error message says "Cannot deserialize custom message store without providing a message_store instance" but this check is triggered even for standard stores that have a 'messages' field. The condition should be checking if this is actually a custom store (no 'messages' field) before raising this error.

Consider restructuring the logic to: (1) check if 'messages' exists, if yes create standard ChatMessageStore, (2) if no 'messages' but has 'type' field matching known custom types, raise this error, (3) otherwise raise a generic "Invalid state" error.

This re-raises AgentThreadException but then has a catch-all for other exceptions. The problem is that if the code at line 500 (ChatMessageStoreState.from_dict) raises an AgentThreadException, it will be caught by line 512 and re-raised. However, if line 501 (ChatMessageStore construction) fails with an AgentThreadException, it will also be caught and re-raised. This double-catch pattern is confusing and may not preserve the original exception context properly.

Consider removing the re-raise at line 512-513 and letting AgentThreadExceptions propagate naturally, or handle them separately if you need to wrap them with additional context.

The same issue exists here as in the deserialize method. The error message "Cannot create custom message store from state" is misleading because this branch is also reached for standard stores that happen to be stored as dicts. The check at line 556 verifies that 'messages' exists, so if it doesn't exist, you're treating it as a custom store error. However, it could also just be an invalid standard store state.

The error messaging should be more precise about what went wrong.

Same maintainability issue as in the deserialize method. The double exception handling with re-raise of AgentThreadException followed by a generic catch-all creates confusing control flow. Consider simplifying this error handling pattern.