fix(anthropic): keep two injection points as two cache breakpoints

The second point resolved to the message the first one had just marked, found it
marked and was dropped. Two configured breakpoints became one, and the four exist
so that a prefix which stops matching at one can still match at an earlier one.

Carry the messages already marked in this request into the walk, so a point that
arrives on one keeps going back instead of being spent there.

litellm/integrations/anthropic_cache_control_hook.py

@@ -439,6 +439,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         """
         used_blocks = AnthropicCacheControlHook.count_request_cache_breakpoints(messages)
 
+        taken: set[int] = set()
         limit_reached = False
         for point in points:
             if used_blocks >= max_blocks:
@@ -449,7 +450,9 @@ class AnthropicCacheControlHook(CustomPromptManagement):
                 type="ephemeral"
             )
 
-            for target_index in AnthropicCacheControlHook._resolve_target_indices(point=point, messages=messages):
+            for target_index in AnthropicCacheControlHook._resolve_target_indices(
+                point=point, messages=messages, taken=taken
+            ):
                 if used_blocks >= max_blocks:
                     limit_reached = True
                     break
@@ -463,6 +466,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
                 )
                 if AnthropicCacheControlHook._message_has_cache_control(messages[target_index]):
                     used_blocks += 1
+                    taken.add(target_index)
 
             if limit_reached:
                 break
@@ -477,9 +481,16 @@ class AnthropicCacheControlHook(CustomPromptManagement):
 
     @staticmethod
     def _resolve_target_indices(
-        point: CacheControlMessageInjectionPoint, messages: list[AllMessageValues]
+        point: CacheControlMessageInjectionPoint, messages: list[AllMessageValues], taken: set[int] | None = None
     ) -> list[int]:
-        """Resolve which message indices an injection point targets."""
+        """Resolve which message indices an injection point targets.
+
+        ``taken`` is the messages an earlier point in the same request already marked.
+        The walk goes past them: arriving on one, finding it marked and dropping the
+        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):
@@ -527,7 +538,7 @@ class AnthropicCacheControlHook(CustomPromptManagement):
         position = targetted_index
         while position >= 0:
             index = candidates[position]
-            if _message_accepts_cache_control(messages[index]):
+            if index not in already_marked and _message_accepts_cache_control(messages[index]):
                 return [index]
             position -= 1
         return []

tests/unit/integrations/test_anthropic_cache_control_hook.py

@@ -4061,3 +4061,30 @@ async def test_anthropic_cache_control_hook_walks_back_off_a_thinking_only_turn(
         "content": [{"type": "text", "text": "a1", "cache_control": {"type": "ephemeral"}}],
     }
     assert marked[3] == {"role": "assistant", "content": [{"type": "thinking", "thinking": "t", "signature": "s"}]}
+
+
+@pytest.mark.asyncio
+async def test_anthropic_cache_control_hook_two_points_stay_two_breakpoints(monkeypatch: pytest.MonkeyPatch):
+    """
+    The second point walks past the message the first one marked. Arriving on it,
+    finding it marked and dropping the point turns two configured breakpoints into one,
+    and the four exist so a prefix that stops matching at one can match at an earlier one.
+    """
+    marked = await _marked_messages(
+        [
+            {"role": "user", "content": "open"},
+            {"role": "assistant", "content": "ack"},
+            {"role": "user", "content": ""},
+        ],
+        [{"location": "message", "index": -1}, {"location": "message", "index": -2}],
+        monkeypatch,
+    )
+
+    assert marked == [
+        {"role": "user", "content": [{"type": "text", "text": "open", "cache_control": {"type": "ephemeral"}}]},
+        {"role": "assistant", "content": [{"type": "text", "text": "ack", "cache_control": {"type": "ephemeral"}}]},
+        {
+            "role": "user",
+            "content": [{"type": "text", "text": "[System: Empty message content sanitised to satisfy protocol]"}],
+        },
+    ]