diff --git a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
index 3ec953b5bb1..367362c7032 100644
--- a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
+++ b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
@@ -579,7 +579,9 @@ def _prepare_options(
         """
         # Start with a copy of options, excluding keys we handle separately
         run_options: dict[str, Any] = {
-            k: v for k, v in options.items() if v is not None and k not in {"instructions", "response_format"}
+            k: v
+            for k, v in options.items()
+            if v is not None and k not in {"instructions", "response_format", "additional_beta_flags"}
         }
         # Framework-level options handled elsewhere; do not forward as raw Anthropic request kwargs.
         run_options.pop("allow_multiple_tool_calls", None)
@@ -592,7 +594,9 @@ def _prepare_options(
         # This includes underscore-prefixed internal objects (like _function_middleware_pipeline)
         # and framework kwargs like 'thread' and 'middleware'.
         filtered_kwargs = {
-            k: v for k, v in kwargs.items() if not k.startswith("_") and k not in {"thread", "middleware"}
+            k: v
+            for k, v in kwargs.items()
+            if not k.startswith("_") and k not in {"thread", "middleware", "additional_beta_flags"}
         }
         _apply_option_translations(filtered_kwargs)
         run_options.update(filtered_kwargs)
diff --git a/python/packages/anthropic/tests/test_anthropic_client.py b/python/packages/anthropic/tests/test_anthropic_client.py
index 0992c0ed91f..8876823cc61 100644
--- a/python/packages/anthropic/tests/test_anthropic_client.py
+++ b/python/packages/anthropic/tests/test_anthropic_client.py
@@ -1270,6 +1270,48 @@ async def test_prepare_options_excludes_stream_option(
     assert "stream" not in run_options
 
 
+async def test_prepare_options_consumes_additional_beta_flags(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """Per-run additional_beta_flags must be folded into betas, not forwarded raw.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/5764:
+    the key survived into run_options and was passed straight through to
+    ``AsyncMessages.create()``, which rejects it with
+    ``TypeError: got an unexpected keyword argument 'additional_beta_flags'``.
+    """
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    messages = [Message(role="user", contents=["Hello"])]
+    chat_options: dict[str, Any] = {"additional_beta_flags": ["extended-cache-ttl-2025-04-11"]}
+
+    run_options = client._prepare_options(messages, chat_options)
+
+    assert "additional_beta_flags" not in run_options
+    assert "extended-cache-ttl-2025-04-11" in run_options["betas"]
+
+
+async def test_prepare_options_drops_additional_beta_flags_passed_as_kwarg(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """additional_beta_flags must also be excluded when passed as a raw kwarg,
+    not just via the options dict.
+
+    Flagged in code review on the fix for #5764: the initial fix only excluded
+    the key from the options-dict copy, but filtered_kwargs (built from
+    **kwargs at the call site) had no equivalent exclusion, so
+    ``_prepare_options(messages, {}, additional_beta_flags=[...])`` would still
+    forward the raw key and reproduce the same TypeError.
+    """
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    messages = [Message(role="user", contents=["Hello"])]
+
+    run_options = client._prepare_options(messages, {}, additional_beta_flags=["extended-cache-ttl-2025-04-11"])
+
+    assert "additional_beta_flags" not in run_options
+
+
 async def test_prepare_options_filters_internal_kwargs(
     mock_anthropic_client: MagicMock,
 ) -> None: