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 4be4c2d5e7..1e92754857 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
@@ -159,6 +159,6 @@ class VertexAIPartnerModelsAnthropicMessagesConfig(AnthropicMessagesConfig, Vert
             "model", None
         )  # do not pass model in request body to vertex ai
 
-        sanitize_vertex_anthropic_output_params(anthropic_messages_request)
+        sanitize_vertex_anthropic_output_params(anthropic_messages_request, model)
 
         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
index a33ad67778..280cc1c888 100644
--- 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
@@ -10,23 +10,38 @@ 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.
-# Add an entry only when a 400 "Extra inputs are not permitted" is
-# reproducible against the live Vertex endpoint.
+# Keys inside ``output_config`` that Vertex AI Claude rejects regardless of
+# the target model. Add an entry only when a 400 "Extra inputs are not
+# permitted" is reproducible against the live Vertex endpoint for every model.
 VERTEX_UNSUPPORTED_OUTPUT_CONFIG_KEYS: frozenset = frozenset()
 
 
-def sanitize_vertex_anthropic_output_params(data: dict) -> None:
+def _model_accepts_output_config_effort(model: str) -> bool:
+    """Whether ``model`` accepts ``output_config.effort`` on Vertex.
+
+    Opus/Sonnet 4.6+ advertise ``supports_output_config`` (or a reasoning
+    effort level) and accept it; Haiku 4.5 advertises neither and 400s on
+    ``output_config.effort: Extra inputs are not permitted``. Imported lazily
+    so this stays a leaf module (see module docstring).
+    """
+    from litellm.llms.anthropic.chat.transformation import AnthropicConfig
+
+    return AnthropicConfig._model_supports_effort_param(model)
+
+
+def sanitize_vertex_anthropic_output_params(data: dict, model: str) -> 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_config.effort`` is dropped for models that don't accept it
+        (e.g. Haiku 4.5) and forwarded for those that do (Opus/Sonnet 4.6+).
+        Clients like Claude Code inject it into every Messages payload, so the
+        gate has to live here rather than rely on the caller.
+      * Keys in ``VERTEX_UNSUPPORTED_OUTPUT_CONFIG_KEYS`` are always filtered.
+      * ``output_config`` left empty after filtering is removed so the request
+        body has no empty dict.
       * ``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.
@@ -37,11 +52,19 @@ def sanitize_vertex_anthropic_output_params(data: dict) -> None:
     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
-    }
+
+    drop_keys = set(VERTEX_UNSUPPORTED_OUTPUT_CONFIG_KEYS)
+    if "effort" in output_config and not _model_accepts_output_config_effort(model):
+        from litellm._logging import verbose_logger
+
+        verbose_logger.debug(
+            "Dropping unsupported output_config.effort for vertex_ai model=%s "
+            "(no supports_output_config in the model map)",
+            model,
+        )
+        drop_keys.add("effort")
+
+    sanitized = {k: v for k, v in output_config.items() if k not in drop_keys}
     if sanitized:
         data["output_config"] = sanitized
     else:
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 4627d9f6df..c852909d47 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
@@ -106,7 +106,7 @@ class VertexAIAnthropicConfig(AnthropicConfig):
 
         data.pop("model", None)  # vertex anthropic doesn't accept 'model' parameter
 
-        sanitize_vertex_anthropic_output_params(data)
+        sanitize_vertex_anthropic_output_params(data, model)
 
         tools = optional_params.get("tools")
         tool_search_used = self.is_tool_search_used(tools)
diff --git a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
index b8cd65d3c9..6f4bb4e59c 100644
--- a/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
+++ b/tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
@@ -313,6 +313,40 @@ def test_transform_anthropic_messages_request_removes_scope_from_cache_control()
     assert result["messages"][0]["content"][0]["cache_control"]["type"] == "ephemeral"
 
 
+def test_messages_request_strips_effort_for_haiku_45():
+    """Regression: Claude Code (``claude --model claude-haiku-4.5``) sends
+    ``output_config.effort`` in its default Messages payload. Haiku 4.5 on
+    Vertex rejects it with 400 ``output_config.effort: Extra inputs are not
+    permitted``, so the pass-through must strip it for Haiku while keeping it
+    for Opus/Sonnet 4.6+."""
+    config = VertexAIPartnerModelsAnthropicMessagesConfig()
+    messages = [{"role": "user", "content": "Hello"}]
+
+    haiku_result = config.transform_anthropic_messages_request(
+        model="claude-haiku-4-5@20251001",
+        messages=messages,
+        anthropic_messages_optional_request_params={
+            "max_tokens": 1024,
+            "output_config": {"effort": "high"},
+        },
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+    assert "output_config" not in haiku_result
+
+    opus_result = config.transform_anthropic_messages_request(
+        model="claude-opus-4-6",
+        messages=messages,
+        anthropic_messages_optional_request_params={
+            "max_tokens": 1024,
+            "output_config": {"effort": "high"},
+        },
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+    assert opus_result["output_config"] == {"effort": "high"}
+
+
 def test_provider_config_manager_reuses_vertex_anthropic_messages_config_instance():
     """
     Regression test: repeated provider config lookups for the same Vertex Claude model
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 d89d09a4e6..ac2368130d 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
@@ -675,28 +675,60 @@ def test_sanitize_vertex_anthropic_output_params_unit():
         sanitize_vertex_anthropic_output_params,
     )
 
+    supported = "claude-opus-4-6"
+
     # No-op when output_config absent.
     data: dict = {"max_tokens": 8}
-    sanitize_vertex_anthropic_output_params(data)
+    sanitize_vertex_anthropic_output_params(data, supported)
     assert data == {"max_tokens": 8}
 
-    # Effort-only → preserved (Vertex 4.6/4.7 accept it on rawPredict).
+    # Effort-only on a supporting model → preserved (Vertex 4.6/4.7 accept it).
     data = {"output_config": {"effort": "high"}}
-    sanitize_vertex_anthropic_output_params(data)
+    sanitize_vertex_anthropic_output_params(data, supported)
     assert data["output_config"] == {"effort": "high"}
 
     # Format-only → preserved unchanged.
     fmt = {"format": {"type": "json_schema", "schema": {"type": "object"}}}
     data = {"output_config": dict(fmt)}
-    sanitize_vertex_anthropic_output_params(data)
+    sanitize_vertex_anthropic_output_params(data, supported)
     assert data["output_config"] == fmt
 
-    # Mixed → both effort and format kept (no current Vertex-unsupported keys).
+    # Mixed on a supporting model → both effort and format kept.
     data = {"output_config": {"format": fmt["format"], "effort": "high"}}
-    sanitize_vertex_anthropic_output_params(data)
+    sanitize_vertex_anthropic_output_params(data, supported)
     assert data["output_config"] == {"format": fmt["format"], "effort": "high"}
 
     # Non-dict → dropped defensively.
     data = {"output_config": "garbage"}
-    sanitize_vertex_anthropic_output_params(data)
+    sanitize_vertex_anthropic_output_params(data, supported)
     assert "output_config" not in data
+
+
+def test_sanitize_strips_effort_for_haiku_45():
+    """Regression: Haiku 4.5 on Vertex does not support ``output_config.effort``
+    and 400s with ``Extra inputs are not permitted``. Claude Code injects
+    ``effort`` into every Messages payload, so the helper must strip it for
+    models that don't advertise output_config support while leaving it intact
+    for Opus/Sonnet 4.6+."""
+    from litellm.llms.vertex_ai.vertex_ai_partner_models.anthropic.output_params_utils import (
+        sanitize_vertex_anthropic_output_params,
+    )
+
+    haiku = "claude-haiku-4-5@20251001"
+
+    # Effort-only → output_config removed entirely (no empty dict on the wire).
+    data: dict = {"output_config": {"effort": "high"}, "max_tokens": 8}
+    sanitize_vertex_anthropic_output_params(data, haiku)
+    assert "output_config" not in data
+    assert data["max_tokens"] == 8
+
+    # Mixed → effort stripped, format preserved.
+    fmt = {"type": "json_schema", "schema": {"type": "object"}}
+    data = {"output_config": {"effort": "high", "format": fmt}}
+    sanitize_vertex_anthropic_output_params(data, haiku)
+    assert data["output_config"] == {"format": fmt}
+
+    # Same payload on a supporting model keeps effort untouched.
+    data = {"output_config": {"effort": "high"}}
+    sanitize_vertex_anthropic_output_params(data, "vertex_ai/claude-opus-4-6")
+    assert data["output_config"] == {"effort": "high"}