diff --git a/litellm/llms/anthropic/experimental_pass_through/adapters/handler.py b/litellm/llms/anthropic/experimental_pass_through/adapters/handler.py
index d16f5afb45ca..ac55aac80621 100644
--- a/litellm/llms/anthropic/experimental_pass_through/adapters/handler.py
+++ b/litellm/llms/anthropic/experimental_pass_through/adapters/handler.py
@@ -27,6 +27,16 @@
 if TYPE_CHECKING:
     pass
 
+
+# Anthropic-only fields that the translator above already maps into the
+# OpenAI-format completion_kwargs (output_config → reasoning_effort /
+# response_format, etc.). They must be filtered out of the raw
+# extra_kwargs re-merge below or non-Anthropic backends reject the call
+# with 400 "Extra inputs are not permitted". Add new entries here when
+# extending AnthropicMessagesRequestOptionalParams with another Anthropic-
+# specific key.
+ANTHROPIC_ONLY_REQUEST_KEYS: frozenset[str] = frozenset({"output_config"})
+
 ########################################################
 # init adapter
 ANTHROPIC_ADAPTER = AnthropicAdapter()
@@ -202,8 +212,12 @@ def _prepare_completion_kwargs(
             request_data["output_format"] = output_format
 
         # Extract output_config from extra_kwargs so the translator can use it
-        # (e.g. output_config.effort for adaptive thinking → reasoning_effort)
-        extra_kwargs = extra_kwargs or {}
+        # (e.g. output_config.effort for adaptive thinking → reasoning_effort,
+        # output_config.format → response_format for structured outputs).
+        # Use explicit None check rather than `or {}` so an explicit empty dict
+        # caller-passed argument is preserved (matters for tests that drive
+        # the fallback inference path).
+        extra_kwargs = extra_kwargs if extra_kwargs is not None else {}
         if "output_config" in extra_kwargs:
             request_data["output_config"] = extra_kwargs["output_config"]
 
@@ -225,8 +239,23 @@ def _prepare_completion_kwargs(
                 "include_usage": True,
             }
 
-        excluded_keys = {"anthropic_messages"}
-        extra_kwargs = extra_kwargs or {}
+        # Keys that must NOT be forwarded as raw extras into the OpenAI-format
+        # ``completion_kwargs`` after translation. The translator above has
+        # already consumed the meaningful parts of these inputs (e.g.
+        # ``output_config.format`` → ``response_format``, ``output_config.effort``
+        # → ``reasoning_effort`` for non-Claude targets). Re-adding the raw
+        # Anthropic-shaped key here causes 400 "Extra inputs are not permitted"
+        # on non-Anthropic backends (Azure OpenAI, Fireworks, Bedrock Nova,
+        # etc.) and is silently lossy on Anthropic-family targets, which would
+        # see the translated key ``response_format`` AND a duplicate, conflicting
+        # ``output_config``.
+        #
+        # Maintainability: when adding a new Anthropic-only request param to
+        # ``AnthropicMessagesRequestOptionalParams``, also extend
+        # ``ANTHROPIC_ONLY_REQUEST_KEYS`` here so it doesn't silently leak.
+        excluded_keys = ANTHROPIC_ONLY_REQUEST_KEYS | {"anthropic_messages"}
+        # NOTE: extra_kwargs was already coerced from None to {} at the top of
+        # this method (line ~220). It is guaranteed to be a dict here.
         for key, value in extra_kwargs.items():
             if (
                 key == "litellm_logging_obj"
diff --git a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
index 20fa4f125de1..f7bc67ccd3ac 100644
--- a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
@@ -664,7 +664,7 @@ def translate_anthropic_messages_to_openai(  # noqa: PLR0915
 
     @staticmethod
     def translate_anthropic_thinking_to_reasoning_effort(
-        thinking: Dict[str, Any]
+        thinking: Dict[str, Any],
     ) -> Optional[str]:
         """
         Translate Anthropic's thinking parameter to OpenAI's reasoning_effort.
@@ -1081,10 +1081,23 @@ def _translate_output_format_to_openai(
         anthropic_message_request: AnthropicMessagesRequest,
         new_kwargs: ChatCompletionRequest,
     ) -> None:
-        """Translate output_format to response_format when applicable."""
-        if "output_format" not in anthropic_message_request:
-            return
-        output_format = anthropic_message_request["output_format"]
+        """Translate Anthropic structured-output config to OpenAI ``response_format``.
+
+        Accepts either the legacy top-level ``output_format`` field OR the
+        newer ``output_config.format`` (sub-key on ``output_config``) so that
+        both shapes flow through to non-Anthropic backends as
+        ``response_format``. Without the ``output_config.format`` branch,
+        callers using the new Anthropic Structured Outputs API would have
+        their schema silently dropped on the adapter path — only the legacy
+        top-level ``output_format`` was being mapped.
+
+        ``output_format`` takes precedence when both are provided.
+        """
+        output_format: Any = anthropic_message_request.get("output_format")
+        if not output_format:
+            output_config = anthropic_message_request.get("output_config")
+            if isinstance(output_config, dict):
+                output_format = output_config.get("format")
         if not output_format:
             return
         response_format = self.translate_anthropic_output_format_to_openai(
diff --git a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
index 5c3bbf61ee2c..d450f7a46351 100644
--- a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
+++ b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/experimental_pass_through/transformation.py
@@ -13,6 +13,7 @@
 from litellm.types.router import GenericLiteLLMParams
 
 from ....vertex_llm_base import VertexBase
+from ..output_params_utils import sanitize_vertex_anthropic_output_params
 
 
 class VertexAIPartnerModelsAnthropicMessagesConfig(AnthropicMessagesConfig, VertexBase):
@@ -158,12 +159,10 @@ def transform_anthropic_messages_request(
             "model", None
         )  # do not pass model in request body to vertex ai
 
-        anthropic_messages_request.pop(
-            "output_format", None
-        )  # do not pass output_format in request body to vertex ai - vertex ai does not support output_format as yet
-
-        anthropic_messages_request.pop(
-            "output_config", None
-        )  # do not pass output_config in request body to vertex ai - vertex ai does not support output_config
+        # Vertex AI Claude accepts ``output_config.format`` (structured outputs)
+        # and ``output_format``, but rejects ``output_config.effort`` with 400
+        # "Extra inputs are not permitted". Sanitize in place so the supported
+        # bits flow through.
+        sanitize_vertex_anthropic_output_params(anthropic_messages_request)
 
         return anthropic_messages_request
diff --git a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/output_params_utils.py b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/output_params_utils.py
new file mode 100644
index 000000000000..982d8edbf205
--- /dev/null
+++ b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/output_params_utils.py
@@ -0,0 +1,50 @@
+"""
+Shared sanitization for ``output_config`` / ``output_format`` on Vertex AI
+Claude. Lives in its own module so both the chat-completion transformation
+(``transformation.py``) and the Messages pass-through transformation
+(``experimental_pass_through/transformation.py``) can import it without
+forming a cycle through the parent module's heavier imports.
+
+CodeQL flagged the ``..transformation`` import path as a potential cyclic
+import; extracting the helper into a leaf module resolves the warning and
+keeps the parent module's import surface narrow.
+"""
+
+# Keys inside ``output_config`` that Vertex AI Claude does not accept.
+# Today only ``effort`` triggers "Extra inputs are not permitted"; add new
+# entries here as Vertex parity drifts. Keep this list narrow — anything
+# Vertex DOES accept (e.g. ``format`` for structured outputs) must be
+# preserved so callers can rely on Anthropic-native features.
+VERTEX_UNSUPPORTED_OUTPUT_CONFIG_KEYS: frozenset = frozenset({"effort"})
+
+
+def sanitize_vertex_anthropic_output_params(data: dict) -> None:
+    """
+    Strip Vertex-unsupported keys from ``output_config`` /
+    ``output_format`` in-place; forward whatever remains.
+
+    Behavior:
+      * ``output_config`` containing only unsupported keys (e.g. ``effort``
+        alone) is removed entirely so the request body has no empty dict.
+      * ``output_config`` containing a mix of supported + unsupported keys
+        has the unsupported subset filtered out and the rest forwarded.
+      * ``output_config`` that is supported in full passes through unchanged.
+      * ``output_format`` is forwarded as-is (Vertex AI Claude accepts it).
+      * Non-dict values for ``output_config`` are dropped to avoid sending
+        malformed payloads downstream.
+    """
+    output_config = data.get("output_config")
+    if output_config is None:
+        return
+    if not isinstance(output_config, dict):
+        data.pop("output_config", None)
+        return
+    sanitized = {
+        k: v
+        for k, v in output_config.items()
+        if k not in VERTEX_UNSUPPORTED_OUTPUT_CONFIG_KEYS
+    }
+    if sanitized:
+        data["output_config"] = sanitized
+    else:
+        data.pop("output_config", None)
diff --git a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/transformation.py b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/transformation.py
index 504914c47964..914c7e92e5ed 100644
--- a/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/transformation.py
+++ b/litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/transformation.py
@@ -10,6 +10,7 @@
 from litellm.types.utils import ModelResponse
 
 from ....anthropic.chat.transformation import AnthropicConfig
+from .output_params_utils import sanitize_vertex_anthropic_output_params
 
 
 class VertexAIError(Exception):
@@ -105,11 +106,12 @@ def transform_request(
 
         data.pop("model", None)  # vertex anthropic doesn't accept 'model' parameter
 
-        # VertexAI doesn't support output_format parameter, remove it if present
-        data.pop("output_format", None)
-
-        # VertexAI doesn't support output_config parameter, remove it if present
-        data.pop("output_config", None)
+        # Vertex AI Claude accepts ``output_config.format`` (structured outputs /
+        # JSON Schema) but NOT ``output_config.effort`` — sending ``effort`` to
+        # Vertex returns 400 "Extra inputs are not permitted". Sanitize in place:
+        # forward the structured-output bits, drop the unsupported keys.
+        # Same treatment for the legacy top-level ``output_format`` field.
+        sanitize_vertex_anthropic_output_params(data)
 
         tools = optional_params.get("tools")
         tool_search_used = self.is_tool_search_used(tools)
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_handler_output_config_passthrough.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_handler_output_config_passthrough.py
new file mode 100644
index 000000000000..615dc5cfebce
--- /dev/null
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_handler_output_config_passthrough.py
@@ -0,0 +1,232 @@
+"""
+Regression tests for output_config passthrough through the Anthropic
+``/v1/messages`` → ``/chat/completions`` adapter.
+
+Background — what was broken:
+* When a client sent ``output_config`` to ``/v1/messages`` and the request
+  was routed to a non-Anthropic backend (Azure OpenAI, Fireworks, Bedrock
+  Nova, etc.), the adapter forwarded the raw Anthropic-shaped ``output_config``
+  field as-is into the OpenAI-format ``completion_kwargs``. The non-Anthropic
+  backend then rejected the request with 400 "Extra inputs are not permitted".
+* The translator above the re-merge already extracts the meaningful parts of
+  ``output_config`` (``format`` → ``response_format``, ``effort`` →
+  ``reasoning_effort`` for non-Claude targets), so re-adding the raw key was
+  always either redundant (Anthropic-family) or harmful (non-Anthropic).
+
+Tests cover (consolidating PRs #23706 and #22727):
+1. ``output_config`` is excluded from the post-translation re-merge.
+2. ``ANTHROPIC_ONLY_REQUEST_KEYS`` constant is exported and contains
+   ``output_config`` so future maintainers know where to extend it.
+3. The translator-extracted fields (``response_format`` / ``reasoning_effort``)
+   are still present after the strip — the strip removes only the raw
+   Anthropic-shaped duplicate.
+4. Helper-level coverage for empty ``extra_kwargs`` (PR #22727 Greptile P2 —
+   the original ``or {}`` pattern silently substituted a default and prevented
+   the fallback inference path from being exercised).
+"""
+
+import os
+import sys
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+# Anchor sys.path to this file's location — not the working-directory-relative
+# pattern Greptile flagged on PR #23706. Resolves correctly regardless of
+# where pytest is invoked from.
+sys.path.insert(
+    0, os.path.abspath(os.path.join(os.path.dirname(__file__), "../../../../../.."))
+)
+
+from litellm.llms.anthropic.experimental_pass_through.adapters.handler import (
+    ANTHROPIC_ONLY_REQUEST_KEYS,
+    LiteLLMMessagesToCompletionTransformationHandler,
+)
+
+MESSAGES = [{"role": "user", "content": "hello"}]
+
+
+def _call_prepare(extra_kwargs, model="gpt-4o", output_format=None, **overrides):
+    """
+    Drive ``_prepare_completion_kwargs`` with the minimum scaffolding needed.
+
+    ``output_format`` is a top-level parameter on the function, so callers
+    pass it explicitly here rather than tucking it into ``extra_kwargs``.
+
+    Uses an explicit-None check on ``extra_kwargs`` so callers can test the
+    falsy-empty-dict path. The fallback ``or {}`` pattern PR #22727 used here
+    masked the no-extra-kwargs case from ever exercising the test's intent.
+    """
+    return LiteLLMMessagesToCompletionTransformationHandler._prepare_completion_kwargs(
+        max_tokens=overrides.get("max_tokens", 1024),
+        messages=overrides.get("messages", MESSAGES),
+        model=model,
+        metadata=None,
+        stop_sequences=None,
+        stream=False,
+        system=None,
+        temperature=None,
+        thinking=None,
+        tool_choice=None,
+        tools=None,
+        top_k=None,
+        top_p=None,
+        output_format=output_format,
+        extra_kwargs=extra_kwargs,
+    )
+
+
+class TestAnthropicOnlyRequestKeysExport:
+    """The exclusion list must be a public, named constant for maintainability —
+    Greptile P2 on PR #23706: ``excluded_keys`` was silently growing as a
+    point-fix pattern. A named module-level constant gives reviewers a single
+    grep target when extending Anthropic-only fields."""
+
+    def test_constant_exposed(self):
+        assert isinstance(ANTHROPIC_ONLY_REQUEST_KEYS, frozenset)
+
+    def test_contains_output_config(self):
+        assert "output_config" in ANTHROPIC_ONLY_REQUEST_KEYS
+
+
+class TestOutputConfigStrippedFromCompletionKwargs:
+    """``output_config`` must not survive the post-translation re-merge into
+    ``completion_kwargs`` regardless of the target provider — the translator
+    has already consumed its meaningful parts."""
+
+    def test_output_config_with_effort_is_stripped(self):
+        extra_kwargs = {
+            "custom_llm_provider": "azure",
+            "output_config": {"effort": "high"},
+        }
+
+        result = _call_prepare(extra_kwargs=extra_kwargs)
+
+        # Returns (completion_kwargs, original_messages, ...) — first element
+        # is the dict we care about.
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+        assert "output_config" not in completion_kwargs, (
+            "Raw output_config must not be forwarded — non-Anthropic backends "
+            "reject it with 400 'Extra inputs are not permitted'"
+        )
+
+    def test_output_config_format_translated_to_response_format(self):
+        """When ``output_config`` carries structured-output ``format``, the
+        translator now maps it to OpenAI's ``response_format`` so non-Anthropic
+        backends see the schema in their native shape. The raw
+        ``output_config`` key is still stripped from ``completion_kwargs`` —
+        only the translated ``response_format`` survives.
+
+        Before this PR, only the legacy top-level ``output_format`` was
+        translated; ``output_config.format`` was silently dropped on the
+        adapter path even when the schema was correctly supplied (issue
+        flagged by Greptile review of the initial fix).
+        """
+        schema = {
+            "type": "object",
+            "additionalProperties": False,
+            "properties": {"name": {"type": "string"}},
+        }
+        extra_kwargs = {
+            "custom_llm_provider": "azure",
+            "output_config": {"format": {"type": "json_schema", "schema": schema}},
+        }
+
+        result = _call_prepare(extra_kwargs=extra_kwargs)
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+
+        # Raw Anthropic-shaped key is gone (would 400 on non-Anthropic backends).
+        assert "output_config" not in completion_kwargs
+        # Translated OpenAI-shaped key is present so the schema actually
+        # reaches the downstream backend.
+        assert "response_format" in completion_kwargs, (
+            "output_config.format must be translated to response_format — "
+            "without this, structured-output schemas are silently dropped on "
+            "the adapter path"
+        )
+
+    def test_output_format_top_level_still_translates(self):
+        """Regression guard: the legacy top-level ``output_format`` field must
+        continue to translate to ``response_format``. The new
+        ``output_config.format`` path must not break this existing behavior."""
+        schema = {"type": "object", "properties": {"name": {"type": "string"}}}
+        result = _call_prepare(
+            extra_kwargs={"custom_llm_provider": "azure"},
+            output_format={"type": "json_schema", "schema": schema},
+        )
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+
+        assert "response_format" in completion_kwargs
+
+    def test_output_format_takes_precedence_over_output_config_format(self):
+        """When both top-level ``output_format`` and ``output_config.format``
+        are present, the legacy top-level ``output_format`` wins. Documents
+        which one the translator picks rather than leaving it implementation-
+        defined."""
+        winning_schema = {
+            "type": "object",
+            "properties": {"top_level": {"type": "string"}},
+        }
+        losing_schema = {
+            "type": "object",
+            "properties": {"nested": {"type": "string"}},
+        }
+        result = _call_prepare(
+            extra_kwargs={
+                "custom_llm_provider": "azure",
+                "output_config": {
+                    "format": {"type": "json_schema", "schema": losing_schema}
+                },
+            },
+            output_format={"type": "json_schema", "schema": winning_schema},
+        )
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+
+        assert "response_format" in completion_kwargs
+        # Verify the winning_schema (top-level output_format) was used,
+        # not the losing one nested under output_config.
+        rendered = str(completion_kwargs["response_format"])
+        assert "top_level" in rendered
+        assert "nested" not in rendered
+
+    def test_other_extra_kwargs_still_passed_through(self):
+        """Regression guard: the strip must be narrow. Unrelated fields like
+        ``api_key`` / ``timeout`` continue to flow through."""
+        extra_kwargs = {
+            "custom_llm_provider": "azure",
+            "output_config": {"effort": "high"},
+            "timeout": 30,
+            "user": "end-user-123",
+        }
+
+        result = _call_prepare(extra_kwargs=extra_kwargs)
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+
+        assert "output_config" not in completion_kwargs
+        assert completion_kwargs.get("timeout") == 30
+        assert completion_kwargs.get("user") == "end-user-123"
+
+
+class TestEmptyExtraKwargsPath:
+    """Greptile P2 on PR #22727: ``extra_kwargs or {default}`` substitutes a
+    default for an explicitly-passed empty dict, hiding the no-extra-kwargs
+    path. The new explicit-None pattern lets ``extra_kwargs={}`` reach the
+    code under test as written."""
+
+    def test_explicit_empty_dict_does_not_substitute_default(self):
+        # Explicit empty dict must be honored — not silently replaced with a
+        # default that adds back a custom_llm_provider this test wants absent.
+        result = _call_prepare(extra_kwargs={})
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+
+        # No output_config because nothing supplied it.
+        assert "output_config" not in completion_kwargs
+
+    def test_none_extra_kwargs_handled_safely(self):
+        """The signature documents ``extra_kwargs: Optional[Dict] = None``;
+        passing None must not crash with KeyError or AttributeError."""
+        result = _call_prepare(extra_kwargs=None)
+        # Just exercising the path; assert no exception and we get back a
+        # dict-like result.
+        completion_kwargs = result[0] if isinstance(result, tuple) else result
+        assert isinstance(completion_kwargs, dict)
diff --git a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py
index 79fc66a74b8d..d0be476d72ec 100644
--- a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py
+++ b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py
@@ -203,12 +203,15 @@ def test_vertex_ai_anthropic_structured_output_header_not_added():
 def test_vertex_ai_claude_sonnet_4_5_structured_output_fix():
     """
     Test fix for issue #18625: Claude Sonnet 4.5 on VertexAI should use tool-based
-    structured outputs instead of output_format parameter.
+    structured outputs when ``response_format`` is supplied via the OpenAI-compat
+    interface (``map_openai_params``).
 
     This test verifies that:
-    1. Claude Sonnet 4.5 uses tool-based structured outputs on VertexAI
-    2. output_format parameter is removed from the final request
-    3. The fix prevents "Extra inputs are not permitted" error
+    1. Claude Sonnet 4.5 uses tool-based structured outputs when ``response_format``
+       is given to the OpenAI-compat path (the path that triggered #18625).
+    2. ``output_format`` is forwarded to Vertex AI when present — Vertex now
+       accepts the field; the prior blanket-strip behavior was the silent drop
+       of Anthropic Structured Outputs that this PR fixes.
     """
     config = VertexAIAnthropicConfig()
 
@@ -294,11 +297,15 @@ def mock_transform_request(
             headers={},
         )
 
-        # Verify that output_format was removed (fixes the "Extra inputs are not permitted" error)
+        # output_format is now forwarded to Vertex (Vertex parity has shifted —
+        # it accepts the field and uses it to enforce the JSON schema). The
+        # prior behavior silently stripped it, hiding Structured Outputs from
+        # callers who explicitly requested them.
+        assert "output_format" in final_data
+        assert final_data["output_format"]["type"] == "json_schema"
         assert (
-            "output_format" not in final_data
-        ), "output_format should be removed for VertexAI"
-        assert "model" not in final_data, "model should be removed for VertexAI"
+            "model" not in final_data
+        ), "model is still stripped (Vertex routes by URL)"
         assert "tools" in final_data, "tools should still be present"
         assert "tool_choice" in final_data, "tool_choice should still be present"
 
@@ -491,28 +498,22 @@ def test_vertex_ai_partner_models_anthropic_remove_prompt_caching_scope_beta_hea
     ), "Header should be removed if no supported values remain"
 
 
-def test_vertex_ai_anthropic_output_config_dropped():
+def test_vertex_ai_anthropic_output_config_effort_only_dropped():
     """
-    Test that output_config parameter is dropped from Vertex AI Anthropic requests.
-
-    Vertex AI does not support the output_config parameter (used for effort settings
-    in Anthropic API). This test ensures it's properly removed to prevent
-    "Extra inputs are not permitted" errors.
+    ``output_config`` containing only ``effort`` (an Anthropic-only key Vertex
+    rejects with "Extra inputs are not permitted") is dropped entirely so the
+    request body has no empty dict.
     """
     config = VertexAIAnthropicConfig()
 
     messages = [{"role": "user", "content": "What is 2+2?"}]
-    headers = {}
+    headers: dict = {}
 
-    # Simulate optional_params with output_config that would be passed in
     optional_params = {
         "max_tokens": 1024,
-        "output_config": {
-            "effort": "high"  # This is Anthropic-specific and not supported by Vertex AI
-        },
+        "output_config": {"effort": "high"},
     }
 
-    # Call transform_request which should drop output_config
     result = config.transform_request(
         model="claude-3-5-sonnet-20241022",
         messages=messages,
@@ -521,54 +522,144 @@ def test_vertex_ai_anthropic_output_config_dropped():
         headers=headers,
     )
 
-    # Verify output_config was removed
     assert (
         "output_config" not in result
-    ), "output_config should be dropped from Vertex AI Anthropic requests"
-
-    # Verify other parameters are preserved
-    assert result["max_tokens"] == 1024, "max_tokens should be preserved"
-    assert "messages" in result, "messages should be present"
+    ), "output_config containing only effort must be dropped"
+    assert result["max_tokens"] == 1024
+    assert "messages" in result
 
 
-def test_vertex_ai_anthropic_output_format_and_output_config_both_dropped():
+def test_vertex_ai_anthropic_output_config_format_passes_through():
+    """
+    ``output_config`` containing structured-output ``format`` is FORWARDED to
+    Vertex AI Claude — Vertex now accepts it and uses it for JSON Schema
+    enforcement. Previously the entire field was being silently stripped, so
+    Anthropic Structured Outputs never engaged on Vertex even when callers
+    requested it.
     """
-    Test that both output_format and output_config are dropped from Vertex AI requests.
+    config = VertexAIAnthropicConfig()
+    messages = [{"role": "user", "content": "Return a person object."}]
+
+    output_config = {
+        "format": {
+            "type": "json_schema",
+            "schema": {
+                "type": "object",
+                "additionalProperties": False,
+                "properties": {
+                    "name": {"type": "string"},
+                    "age": {"type": "integer"},
+                },
+            },
+        }
+    }
+    optional_params = {"max_tokens": 1024, "output_config": output_config}
+
+    result = config.transform_request(
+        model="claude-3-5-sonnet-20241022",
+        messages=messages,
+        optional_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert result["output_config"] == output_config
 
-    This ensures that even if both parameters somehow make it to the transform_request,
-    they are properly cleaned up before sending to Vertex AI.
+
+def test_vertex_ai_anthropic_output_config_format_plus_effort_strips_only_effort():
+    """
+    Greptile P1 on PR #23396: when ``output_config`` contains BOTH ``format``
+    and ``effort``, the prior conditional-passthrough logic forwarded the
+    full dict including the unsupported ``effort`` key, reproducing the
+    400 error the fix was meant to resolve. Only ``effort`` (and any future
+    Vertex-unsupported keys) should be filtered; ``format`` must survive.
     """
     config = VertexAIAnthropicConfig()
+    messages = [{"role": "user", "content": "Return a person object."}]
+
+    output_config = {
+        "format": {
+            "type": "json_schema",
+            "schema": {
+                "type": "object",
+                "additionalProperties": False,
+                "properties": {"name": {"type": "string"}},
+            },
+        },
+        "effort": "high",
+    }
+    optional_params = {"max_tokens": 1024, "output_config": output_config}
 
+    result = config.transform_request(
+        model="claude-3-5-sonnet-20241022",
+        messages=messages,
+        optional_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert "output_config" in result
+    assert (
+        "effort" not in result["output_config"]
+    ), "effort must be stripped — Vertex returns 400 on unknown keys"
+    assert result["output_config"]["format"] == output_config["format"]
+
+
+def test_vertex_ai_anthropic_output_config_non_dict_dropped():
+    """Defensive: if ``output_config`` is somehow not a dict, drop it rather
+    than forwarding malformed data downstream."""
+    config = VertexAIAnthropicConfig()
+    messages = [{"role": "user", "content": "hi"}]
+    optional_params = {"max_tokens": 64, "output_config": "not-a-dict"}
+
+    result = config.transform_request(
+        model="claude-3-5-sonnet-20241022",
+        messages=messages,
+        optional_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert "output_config" not in result
+
+
+def test_vertex_ai_anthropic_output_format_preserved_output_config_effort_dropped():
+    """
+    When the request carries both ``output_format`` (top-level structured
+    outputs) AND an ``output_config`` whose only useful key for Vertex is
+    ``effort``: ``output_format`` must be forwarded (Vertex accepts it),
+    while ``output_config`` is dropped because Vertex returns 400 on
+    ``effort``. This replaces the old "drop both" behavior, which was the
+    silent strip the bug report flagged.
+    """
+    config = VertexAIAnthropicConfig()
     messages = [{"role": "user", "content": "Extract structured data"}]
-    headers = {}
 
-    optional_params = {
-        "max_tokens": 2048,
-        "output_format": {
-            "type": "json_schema",
-            "json_schema": {
-                "name": "data",
-                "schema": {
-                    "type": "object",
-                    "properties": {"result": {"type": "string"}},
-                },
+    output_format = {
+        "type": "json_schema",
+        "json_schema": {
+            "name": "data",
+            "schema": {
+                "type": "object",
+                "properties": {"result": {"type": "string"}},
             },
         },
+    }
+
+    optional_params = {
+        "max_tokens": 2048,
+        "output_format": output_format,
         "output_config": {"effort": "high"},
     }
 
-    # Simulate parent class creating test_data with both parameters
-    # (as if the parent transform_request added them)
     test_data = {
         "model": "claude-3-5-sonnet-20241022",
         "messages": messages,
         "max_tokens": 2048,
-        "output_format": optional_params["output_format"],
-        "output_config": optional_params["output_config"],
+        "output_format": output_format,
+        "output_config": {"effort": "high"},
     }
 
-    # Mock the parent transform_request to return data with both parameters
     original_transform = config.__class__.__bases__[0].transform_request
 
     def mock_transform_request(
@@ -584,22 +675,50 @@ def mock_transform_request(
             messages=messages,
             optional_params=optional_params,
             litellm_params={},
-            headers=headers,
+            headers={},
         )
 
-        # Verify both were removed
-        assert (
-            "output_format" not in result
-        ), "output_format should be dropped from Vertex AI requests"
-        assert (
-            "output_config" not in result
-        ), "output_config should be dropped from Vertex AI requests"
-
-        # Verify essential params are preserved
-        assert result["max_tokens"] == 2048, "max_tokens should be preserved"
-        assert "messages" in result, "messages should be present"
-        assert "model" not in result, "model should also be dropped for Vertex AI"
-
+        # output_format flows through unchanged — Vertex AI Claude accepts it.
+        assert result["output_format"] == output_format
+        # output_config containing only ``effort`` is dropped to avoid the
+        # 400 "Extra inputs are not permitted" the silent strip used to mask.
+        assert "output_config" not in result
+        assert result["max_tokens"] == 2048
+        assert "model" not in result, "model is still stripped (Vertex routes by URL)"
     finally:
-        # Restore original method
         config.__class__.__bases__[0].transform_request = original_transform
+
+
+def test_sanitize_vertex_anthropic_output_params_unit():
+    """Direct unit coverage for the helper itself (used by both Vertex
+    Anthropic transformation paths). Mirrors the integration assertions
+    above without going through the full ``transform_request`` stack."""
+    from litellm.llms.vertex_ai.vertex_ai_partner_models.anthropic.output_params_utils import (
+        sanitize_vertex_anthropic_output_params,
+    )
+
+    # No-op when output_config absent.
+    data: dict = {"max_tokens": 8}
+    sanitize_vertex_anthropic_output_params(data)
+    assert data == {"max_tokens": 8}
+
+    # Effort-only → dropped entirely.
+    data = {"output_config": {"effort": "high"}}
+    sanitize_vertex_anthropic_output_params(data)
+    assert "output_config" not in data
+
+    # Format-only → preserved unchanged.
+    fmt = {"format": {"type": "json_schema", "schema": {"type": "object"}}}
+    data = {"output_config": dict(fmt)}
+    sanitize_vertex_anthropic_output_params(data)
+    assert data["output_config"] == fmt
+
+    # Mixed → effort filtered, format kept.
+    data = {"output_config": {"format": fmt["format"], "effort": "high"}}
+    sanitize_vertex_anthropic_output_params(data)
+    assert data["output_config"] == fmt
+
+    # Non-dict → dropped defensively.
+    data = {"output_config": "garbage"}
+    sanitize_vertex_anthropic_output_params(data)
+    assert "output_config" not in data