diff --git a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
index 98c181f152..65f23af44f 100644
--- a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
+++ b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
@@ -76,7 +76,6 @@
 
 ANTHROPIC_DEFAULT_MAX_TOKENS: Final[int] = 1024
 BETA_FLAGS: Final[list[str]] = ["mcp-client-2025-04-04", "code-execution-2025-08-25"]
-STRUCTURED_OUTPUTS_BETA_FLAG: Final[str] = "structured-outputs-2025-11-13"
 
 ResponseModelT = TypeVar("ResponseModelT", bound=BaseModel | None, default=None)
 AnthropicAsyncClient = AsyncAnthropic | AsyncAnthropicBedrock | AsyncAnthropicFoundry | AsyncAnthropicVertex
@@ -632,12 +631,17 @@ def _prepare_options(
         if tools_config := self._prepare_tools_for_anthropic(options):
             run_options.update(tools_config)
 
-        # response_format - use native output_format for structured outputs
+        # response_format - emit Anthropic's GA ``output_config.format`` shape.
+        # The deprecated ``output_format`` parameter (gated by the
+        # ``structured-outputs-2025-11-13`` beta flag) produced concatenated /
+        # malformed JSON when combined with tools — the GA path does not.
+        # Merge into any caller-supplied ``output_config`` so e.g. the
+        # adaptive-thinking ``effort`` setting survives the transformation.
         response_format = options.get("response_format")
         if response_format is not None:
-            run_options["output_format"] = self._prepare_response_format(response_format)
-            # Add the structured outputs beta flag
-            run_options["betas"].add(STRUCTURED_OUTPUTS_BETA_FLAG)
+            output_config = dict(run_options.get("output_config") or {})
+            output_config["format"] = self._prepare_response_format(response_format)
+            run_options["output_config"] = output_config
 
         return run_options
 
@@ -657,7 +661,7 @@ def _prepare_betas(self, options: Mapping[str, Any]) -> set[str]:
         }
 
     def _prepare_response_format(self, response_format: type[BaseModel] | dict[str, Any]) -> dict[str, Any]:
-        """Prepare the output_format parameter for structured output.
+        """Build the ``output_config.format`` payload for Anthropic structured outputs.
 
         Args:
             response_format: Either a Pydantic model class or a dict with the schema specification.
@@ -665,7 +669,8 @@ def _prepare_response_format(self, response_format: type[BaseModel] | dict[str,
                 or direct format with "schema" key, or the raw schema dict itself.
 
         Returns:
-            A dictionary representing the output_format for Anthropic's structured outputs.
+            A ``{"type": "json_schema", "schema": ...}`` dict — the value placed
+            under ``output_config["format"]`` on the GA structured-outputs path.
         """
         if isinstance(response_format, dict):
             if "json_schema" in response_format:
diff --git a/python/packages/anthropic/tests/test_anthropic_client.py b/python/packages/anthropic/tests/test_anthropic_client.py
index 0cfec3423c..fcdedd4700 100644
--- a/python/packages/anthropic/tests/test_anthropic_client.py
+++ b/python/packages/anthropic/tests/test_anthropic_client.py
@@ -1800,6 +1800,74 @@ class TestModel(BaseModel):
     assert "properties" in result["schema"]
 
 
+async def test_prepare_options_uses_output_config_for_response_format(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """``response_format`` is forwarded as GA ``output_config.format`` (not the deprecated ``output_format``).
+
+    The deprecated ``output_format`` parameter, gated by the
+    ``structured-outputs-2025-11-13`` beta flag, produced concatenated /
+    malformed JSON when combined with tools. The GA ``output_config`` shape
+    works correctly with tools, so we emit that and no longer set the beta
+    flag.
+    """
+
+    class StructuredOut(BaseModel):
+        answer: str
+
+    client = create_test_anthropic_client(mock_anthropic_client)
+    messages = [Message(role="user", contents=["Hello"])]
+    chat_options = ChatOptions(max_tokens=100, response_format=StructuredOut)
+
+    run_options = client._prepare_options(messages, chat_options)
+
+    assert "output_format" not in run_options
+    assert "output_config" in run_options
+    fmt = run_options["output_config"]["format"]
+    assert fmt["type"] == "json_schema"
+    assert fmt["schema"]["additionalProperties"] is False
+    assert "answer" in fmt["schema"]["properties"]
+    # The deprecated structured-outputs beta flag is no longer needed on the
+    # GA path and must not leak into ``betas``.
+    assert "structured-outputs-2025-11-13" not in run_options["betas"]
+
+
+async def test_prepare_options_preserves_caller_supplied_output_config_effort(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """A caller-supplied ``output_config.effort`` (e.g. adaptive thinking) survives the format merge."""
+
+    class StructuredOut(BaseModel):
+        answer: str
+
+    client = create_test_anthropic_client(mock_anthropic_client)
+    messages = [Message(role="user", contents=["Hello"])]
+    # ``output_config`` is provider-specific; pass it through additional kwargs
+    # the way a caller would when configuring adaptive thinking.
+    run_options = client._prepare_options(
+        messages,
+        ChatOptions(max_tokens=100, response_format=StructuredOut),
+        output_config={"effort": "high"},
+    )
+
+    output_config = run_options["output_config"]
+    assert output_config["effort"] == "high"
+    assert output_config["format"]["type"] == "json_schema"
+    assert "answer" in output_config["format"]["schema"]["properties"]
+
+
+async def test_prepare_options_no_response_format_omits_output_config(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """Without ``response_format``, no ``output_config`` is added implicitly."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+    messages = [Message(role="user", contents=["Hello"])]
+    run_options = client._prepare_options(messages, ChatOptions(max_tokens=100))
+
+    assert "output_config" not in run_options
+    assert "output_format" not in run_options
+
+
 # Message Preparation Tests