diff --git a/litellm/litellm_core_utils/fallback_generalizations.py b/litellm/litellm_core_utils/fallback_generalizations.py
index 9b5c369b56e..25ea1bff613 100644
--- a/litellm/litellm_core_utils/fallback_generalizations.py
+++ b/litellm/litellm_core_utils/fallback_generalizations.py
@@ -53,7 +53,9 @@ O(number of rules); callers must only invoke them on a cache miss.
 
 import logging
 import re
+from collections.abc import Mapping
 from dataclasses import dataclass
+from types import MappingProxyType
 from typing import Final
 
 verbose_logger: Final = logging.getLogger("LiteLLM")
@@ -209,9 +211,7 @@ class _FallbackGeneralizations:
             return None
         return {key: value for model_info in matched for key, value in model_info.items()}
 
-    def match_fill_missing(
-        self, model: str, provider: str
-    ) -> dict[str, object] | None:  # mutable-ok: preserve the existing dict return contract
+    def match_fill_missing(self, model: str, provider: str) -> Mapping[str, object] | None:
         if not model or not provider:
             return None
         matched = tuple(
@@ -221,9 +221,9 @@ class _FallbackGeneralizations:
         )
         if not matched:
             return None
-        fill_missing: Final[dict[str, object]] = {  # mutable-ok: preserve the existing dict merge input
-            key: value for model_info in matched for key, value in model_info.items() if key != PROVIDER_KEY
-        }
+        fill_missing: Final[Mapping[str, object]] = MappingProxyType(
+            {key: value for model_info in matched for key, value in model_info.items() if key != PROVIDER_KEY}
+        )
         return fill_missing or None
 
 
@@ -263,9 +263,7 @@ def match_capability_generalizations(model: str) -> dict | None:
     return _registry.match_capabilities(model)
 
 
-def match_fill_missing_generalizations(
-    model: str, provider: str
-) -> dict[str, object] | None:  # mutable-ok: preserve the existing dict return contract
+def match_fill_missing_generalizations(model: str, provider: str) -> Mapping[str, object] | None:
     """Return flagged capability rules matching ``model`` for ``provider``.
 
     Later rules override earlier ones on key conflicts. Only rules listing