diff --git a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
index bf425637b56..75a8acdfcc3 100644
--- a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
@@ -859,7 +859,17 @@ class LiteLLMAnthropicMessagesAdapter:
         """
         new_tools: List[ChatCompletionToolParam] = []
         tool_name_mapping: Dict[str, str] = {}
-        mapped_tool_params = ["name", "input_schema", "description", "cache_control"]
+        # "type" is the Anthropic tool type (e.g. "custom"); it must not be
+        # merged into the OpenAI function `parameters` schema below, or it
+        # overwrites the real parameters.type ("object") and the provider
+        # rejects the request. See #30557.
+        mapped_tool_params = [
+            "name",
+            "input_schema",
+            "description",
+            "cache_control",
+            "type",
+        ]
 
         for idx, tool in enumerate(tools):
             # Check if this is an Anthropic-native tool that should be kept as-is
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
index 76aa3a9c6aa..0300b6f3f51 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
@@ -2747,3 +2747,23 @@ def test_translate_openai_response_to_anthropic_with_polyfill_both_compaction_an
     cm = result.get("context_management")
     assert cm is not None
     assert cm["applied_edits"][0]["type"] == "compact_20260112"
+
+
+def test_translate_anthropic_tools_to_openai_preserves_parameters_type():
+    """Regression for #30557: the Anthropic tool `type` ("custom") must not be
+    merged into the OpenAI function `parameters`, overwriting parameters.type."""
+    adapter = LiteLLMAnthropicMessagesAdapter()
+    tools = [
+        {
+            "type": "custom",
+            "name": "get_weather",
+            "description": "Get weather",
+            "input_schema": {"type": "object", "properties": {}},
+        }
+    ]
+
+    new_tools, _ = adapter.translate_anthropic_tools_to_openai(tools=tools)
+
+    params = new_tools[0]["function"]["parameters"]
+    assert params["type"] == "object"
+    assert new_tools[0]["type"] == "function"