fix(anthropic): keep the type-discipline budget flat

Build the candidate indices as a tuple, take the already-marked messages as a
Container with a frozenset default, and say why the one remaining set is mutable.

litellm/integrations/anthropic_cache_control_hook.py

@@ -12,7 +12,7 @@ Supported for both `v1/chat/completions` (via the prompt-management hook) and
 import copy
 import os
 import re
-from collections.abc import Iterable, Mapping, Sequence
+from collections.abc import Container, Iterable, Mapping, Sequence
 from typing import TYPE_CHECKING, Any, Final, cast
 from urllib.parse import urlparse
 
@@ -439,7 +439,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         """
         used_blocks = AnthropicCacheControlHook.count_request_cache_breakpoints(messages)
 
-        taken: set[int] = set()
+        taken: set[int] = set()  # mutable-ok: the messages this request has already marked
         limit_reached = False
         for point in points:
             if used_blocks >= max_blocks:
@@ -481,7 +481,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
 
     @staticmethod
     def _resolve_target_indices(
-        point: CacheControlMessageInjectionPoint, messages: list[AllMessageValues], taken: set[int] | None = None
+        point: CacheControlMessageInjectionPoint, messages: list[AllMessageValues], taken: Container[int] = frozenset()
     ) -> list[int]:
         """Resolve which message indices an injection point targets.
 
@@ -490,7 +490,6 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         point turns two configured breakpoints into one, and the four exist so that a
         prefix which stops matching at one can still match at an earlier one.
         """
-        already_marked: Final = taken or set()
         _targetted_index: Final[int | str | None] = point.get("index", None)
         targetted_index: int | None = None
         if isinstance(_targetted_index, str):
@@ -505,11 +504,11 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         # that role's turns, so {role: assistant, index: -1} is the last assistant
         # turn rather than the last message.
         targetted_role: Final = point.get("role", None)
-        candidates: Final = [
+        candidates: Final = tuple(
             index
             for index, message in enumerate(messages)
             if targetted_role is None or message.get("role") == targetted_role
-        ]
+        )
 
         # Case 1: Target by role alone
         if targetted_index is None:
@@ -538,7 +537,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         position = targetted_index
         while position >= 0:
             index = candidates[position]
-            if index not in already_marked and _message_accepts_cache_control(messages[index]):
+            if index not in taken and _message_accepts_cache_control(messages[index]):
                 return [index]
             position -= 1
         return []