diff --git a/tests/tool_use/test_tool_choice_required.py b/tests/tool_use/test_tool_choice_required.py
index fc868bbcab..26978fbb7c 100644
--- a/tests/tool_use/test_tool_choice_required.py
+++ b/tests/tool_use/test_tool_choice_required.py
@@ -410,6 +410,101 @@ class TestNonFunctionToolsSkipped:
             get_json_schema_from_tools(tools=[WEB_SEARCH_TOOL], tool_choice="required")
 
 
+class TestMalformedToolSchemaDefs:
+    """Malformed `$defs` in caller-supplied tool parameters is a 400, not a 500."""
+
+    @staticmethod
+    def _tool(params: dict) -> ChatCompletionToolsParam:
+        return TypeAdapter(ChatCompletionToolsParam).validate_python(
+            {
+                "type": "function",
+                "function": {
+                    "name": "get_weather",
+                    "description": "Get the weather",
+                    "parameters": params,
+                },
+            }
+        )
+
+    @pytest.mark.parametrize(
+        "defs",
+        [
+            pytest.param(None, id="null"),
+            pytest.param([], id="list"),
+            pytest.param("nope", id="string"),
+            pytest.param(1, id="int"),
+        ],
+    )
+    def test_non_object_defs_is_a_client_error(self, defs):
+        tools = [
+            self._tool(
+                {
+                    "type": "object",
+                    "properties": {"a": {"type": "string"}},
+                    "$defs": defs,
+                }
+            )
+        ]
+
+        with pytest.raises(VLLMValidationError) as exc_info:
+            get_json_schema_from_tools(tools=tools, tool_choice="required")
+
+        assert exc_info.value.parameter == "tools"
+        assert "$defs" in str(exc_info.value)
+        assert "get_weather" in str(exc_info.value)
+
+    def test_duplicate_def_names_is_a_client_error(self):
+        """Also caller-caused, and it used to raise a plain `ValueError`."""
+        tools = [
+            self._tool(
+                {
+                    "type": "object",
+                    "properties": {"a": {"$ref": "#/$defs/D"}},
+                    "$defs": {"D": {"type": "string"}},
+                }
+            ),
+            self._tool(
+                {
+                    "type": "object",
+                    "properties": {"b": {"$ref": "#/$defs/D"}},
+                    "$defs": {"D": {"type": "integer"}},
+                }
+            ),
+        ]
+
+        with pytest.raises(VLLMValidationError) as exc_info:
+            get_json_schema_from_tools(tools=tools, tool_choice="required")
+
+        assert exc_info.value.parameter == "tools"
+        assert "multiple schemas" in str(exc_info.value)
+
+    def test_well_formed_defs_still_hoisted(self):
+        """The type check must not reject the shape it is guarding."""
+        tools = [
+            self._tool(
+                {
+                    "type": "object",
+                    "properties": {"a": {"$ref": "#/$defs/D"}},
+                    "$defs": {"D": {"type": "string"}},
+                }
+            )
+        ]
+
+        schema = get_json_schema_from_tools(tools=tools, tool_choice="required")
+
+        assert isinstance(schema, dict)
+        assert schema["$defs"] == {"D": {"type": "string"}}
+
+    def test_absent_defs_still_works(self):
+        tools = [
+            self._tool({"type": "object", "properties": {"a": {"type": "string"}}})
+        ]
+
+        schema = get_json_schema_from_tools(tools=tools, tool_choice="required")
+
+        assert isinstance(schema, dict)
+
+
 class TestParallelToolCallsConstraint:
     """`parallel_tool_calls=false` must be enforced by the decoding grammar.
 
diff --git a/vllm/tool_parsers/utils.py b/vllm/tool_parsers/utils.py
index 03bcf1b652..86d68a9b51 100644
--- a/vllm/tool_parsers/utils.py
+++ b/vllm/tool_parsers/utils.py
@@ -368,15 +368,22 @@ def _get_tool_schema_defs(
 ) -> dict:
     all_defs: dict[str, dict[str, Any]] = {}
     for tool in tools:
-        _, params = _extract_tool_info(tool)
+        name, params = _extract_tool_info(tool)
         if params is None:
             continue
         defs = params.pop("$defs", {})
+        if not isinstance(defs, dict):
+            raise VLLMValidationError(
+                f"`$defs` in the parameters of tool '{name}' must be an "
+                f"object, got {type(defs).__name__}.",
+                parameter="tools",
+            )
         for def_name, def_schema in defs.items():
             if def_name in all_defs and all_defs[def_name] != def_schema:
-                raise ValueError(
+                raise VLLMValidationError(
                     f"Tool definition '{def_name}' has multiple schemas, "
-                    "which is not supported."
+                    "which is not supported.",
+                    parameter="tools",
                 )
             all_defs[def_name] = def_schema
     return all_defs