diff --git a/litellm/llms/anthropic/common_utils.py b/litellm/llms/anthropic/common_utils.py
index db540e5441d..964186fd76a 100644
--- a/litellm/llms/anthropic/common_utils.py
+++ b/litellm/llms/anthropic/common_utils.py
@@ -289,6 +289,13 @@ class AnthropicModelInfo(BaseLLMModelInfo):
                 status_code=400,
             )
 
+    @staticmethod
+    def _strip_version_suffix(model: str) -> str:
+        at = model.rfind("@")
+        if at > 0:
+            return model[:at]
+        return model
+
     @staticmethod
     def _model_map_lookup_candidates(model: str) -> List[str]:
         """Model-map keys to try for ``model``: the id itself, the same id with a
@@ -324,6 +331,7 @@ class AnthropicModelInfo(BaseLLMModelInfo):
                 _DATED_RELEASE_SUFFIX_RE.sub("", cand),
                 _DOTTED_VERSION_RE.sub(r"\1-\2", cand),
                 _strip_bedrock_id_suffixes(cand),
+                AnthropicModelInfo._strip_version_suffix(cand),
             )
         )
         return list(dict.fromkeys((*primary, *normalized)))
diff --git a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
index a2fbb68bbb9..f7a234c8ae8 100644
--- a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
+++ b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
@@ -772,7 +772,6 @@ class TestProxyOAuthHeaderForwarding:
         self,
     ):
         """OAuth Authorization header IS forwarded when x-litellm-api-key was used for proxy auth."""
-        from unittest.mock import patch
 
         from starlette.datastructures import Headers
 
@@ -1724,3 +1723,48 @@ class TestClaudeOpus48AdaptiveThinking:
         from litellm.llms.anthropic.common_utils import AnthropicModelInfo
 
         assert AnthropicModelInfo._is_adaptive_thinking_model(model) is False
+
+
+class TestDefaultSuffixAdaptiveThinking:
+    """@default-suffixed Vertex AI model names (e.g. vertex_ai/claude-opus-4-8@default)
+    must resolve as adaptive thinking. Before the fix, _model_map_lookup_candidates
+    never stripped the @default suffix, so the lookup fell through to the bare
+    model name without @default, which may or may not have the flag, and for
+    provider-prefixed forms the lookup always missed (issue #31760)."""
+
+    @pytest.mark.parametrize(
+        "model",
+        [
+            "vertex_ai/claude-opus-4-8@default",
+            "vertex_ai/claude-sonnet-4-6@default",
+            "vertex_ai/claude-opus-4-7@default",
+            "vertex_ai/claude-opus-4-6@default",
+            "vertex_ai/claude-fable-5@default",
+        ],
+    )
+    def test_default_suffix_models_are_adaptive_thinking(
+        self, local_model_cost_map, model: str
+    ) -> None:
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        assert AnthropicModelInfo._is_adaptive_thinking_model(model) is True, (
+            f"{model} not classified as adaptive thinking. "
+            "Check _model_map_lookup_candidates strips @default suffix."
+        )
+
+    @pytest.mark.parametrize(
+        "model,expected_bare",
+        [
+            ("vertex_ai/claude-opus-4-8@default", "claude-opus-4-8"),
+            ("vertex_ai/claude-sonnet-4-6@default", "claude-sonnet-4-6"),
+        ],
+    )
+    def test_lookup_candidates_include_bare_name(
+        self, model: str, expected_bare: str
+    ) -> None:
+        from litellm.llms.anthropic.common_utils import AnthropicModelInfo
+
+        candidates = AnthropicModelInfo._model_map_lookup_candidates(model)
+        assert expected_bare in candidates, (
+            f"Expected '{expected_bare}' in candidates for '{model}', got: {candidates}"
+        )