diff --git a/docs/world api.md b/docs/world api.md
index fd8e0988e567..f82ef40a98f8 100644
--- a/docs/world api.md	
+++ b/docs/world api.md	
@@ -738,8 +738,9 @@ def generate_output(self, output_directory: str) -> None:
 
 If the game client needs to know information about the generated seed, a preferred method of transferring the data
 is through the slot data. This is filled with the `fill_slot_data` method of your world by returning
-a `Dict[str, Any]`, but, to not waste resources, should be limited to data that is absolutely necessary. Slot data is
-sent to your client once it has successfully [connected](network%20protocol.md#connected).
+a `dict` with `str` keys that can be serialized with json.
+But, to not waste resources, it should be limited to data that is absolutely necessary. Slot data is sent to your client
+once it has successfully [connected](network%20protocol.md#connected).
 If you need to know information about locations in your world, instead of propagating the slot data, it is preferable
 to use [LocationScouts](network%20protocol.md#locationscouts), since that data already exists on the server. The most
 common usage of slot data is sending option results that the client needs to be aware of.
diff --git a/worlds/AutoWorld.py b/worlds/AutoWorld.py
index b282c7deb8bd..dd0f46f6a6d1 100644
--- a/worlds/AutoWorld.py
+++ b/worlds/AutoWorld.py
@@ -7,8 +7,8 @@
 import sys
 import time
 from dataclasses import make_dataclass
-from typing import Any, Callable, ClassVar, Dict, Set, Tuple, FrozenSet, List, Optional, TYPE_CHECKING, TextIO, Type, \
-    Union
+from typing import (Any, Callable, ClassVar, Dict, FrozenSet, List, Mapping,
+                    Optional, Set, TextIO, Tuple, TYPE_CHECKING, Type, Union)
 
 from Options import PerGameCommonOptions
 from BaseClasses import CollectionState
@@ -365,13 +365,19 @@ def generate_output(self, output_directory: str) -> None:
         If you need any last-second randomization, use self.random instead."""
         pass
 
-    def fill_slot_data(self) -> Dict[str, Any]:  # json of WebHostLib.models.Slot
-        """Fill in the `slot_data` field in the `Connected` network package.
+    def fill_slot_data(self) -> Mapping[str, Any]:  # json of WebHostLib.models.Slot
+        """What is returned from this function will be in the `slot_data` field
+        in the `Connected` network package.
+        It should be a `dict` with `str` keys, and should be serializable with json.
+
         This is a way the generator can give custom data to the client.
         The client will receive this as JSON in the `Connected` response.
 
         The generation does not wait for `generate_output` to complete before calling this.
         `threading.Event` can be used if you need to wait for something from `generate_output`."""
+        # The reason for the `Mapping` type annotation, rather than `dict`
+        # is so that type checkers won't worry about the mutability of `dict`,
+        # so you can have more specific typing in your world implementation.
         return {}
 
     def extend_hint_information(self, hint_data: Dict[int, Dict[int, str]]):