diff --git a/src/agents/tool.py b/src/agents/tool.py
index 4bcb0f670c..0d88f33719 100644
--- a/src/agents/tool.py
+++ b/src/agents/tool.py
@@ -1715,6 +1715,8 @@ def is_responses_tool_search_surface(tool: Tool) -> bool:
     """Return True when a tool can be exposed through hosted Responses tool search."""
     if isinstance(tool, FunctionTool):
         return tool.defer_loading or get_explicit_function_tool_namespace(tool) is not None
+    if isinstance(tool, CustomTool):
+        return tool.defer_loading
     if isinstance(tool, HostedMCPTool):
         return bool(tool.tool_config.get("defer_loading"))
     return False
@@ -1729,6 +1731,8 @@ def is_required_tool_search_surface(tool: Tool) -> bool:
     """Return True when a tool requires ToolSearchTool() to stay reachable."""
     if isinstance(tool, FunctionTool):
         return tool.defer_loading
+    if isinstance(tool, CustomTool):
+        return tool.defer_loading
     if isinstance(tool, HostedMCPTool):
         return bool(tool.tool_config.get("defer_loading"))
     return False
@@ -1763,7 +1767,8 @@ def validate_responses_tool_search_configuration(
         raise UserError(
             "ToolSearchTool() requires at least one searchable Responses surface: a "
             "tool_namespace(...) function tool, a deferred-loading function tool "
-            "(`function_tool(..., defer_loading=True)`), or a deferred-loading hosted MCP "
+            "(`function_tool(..., defer_loading=True)`), a deferred-loading custom tool "
+            "(`CustomTool(..., defer_loading=True)`), or a deferred-loading hosted MCP "
             "server (`HostedMCPTool(tool_config={..., 'defer_loading': True})`)."
         )
 
diff --git a/tests/models/test_openai_responses.py b/tests/models/test_openai_responses.py
index 9808f443ba..af210435ab 100644
--- a/tests/models/test_openai_responses.py
+++ b/tests/models/test_openai_responses.py
@@ -11,7 +11,12 @@
 import httpx2
 import pytest
 from openai import NOT_GIVEN, APIConnectionError, AsyncOpenAI, RateLimitError, omit
-from openai.types.responses import Response, ResponseCompletedEvent, ResponseErrorEvent
+from openai.types.responses import (
+    Response,
+    ResponseCompletedEvent,
+    ResponseCustomToolCall,
+    ResponseErrorEvent,
+)
 from openai.types.responses.response import IncompleteDetails
 from openai.types.responses.response_create_params import ContextManagement, PromptCacheOptions
 from openai.types.responses.response_usage import ResponseUsage
@@ -23,6 +28,7 @@
     AsyncComputer,
     Computer,
     ComputerTool,
+    CustomTool,
     ImageGenerationTool,
     ModelSettings,
     ModelTracing,
@@ -1773,6 +1779,67 @@ def __init__(self):
     assert called_kwargs["tools"] == [{"type": "tool_search"}]
 
 
+@pytest.mark.allow_call_model_methods
+@pytest.mark.asyncio
+async def test_deferred_custom_tool_is_searchable_and_invocable() -> None:
+    invoked_inputs: list[str] = []
+    sent_tools: list[Any] = []
+
+    class DummyResponses:
+        def __init__(self) -> None:
+            self.call_count = 0
+
+        async def create(self, **kwargs):
+            self.call_count += 1
+            if self.call_count == 1:
+                sent_tools.extend(kwargs["tools"])
+                return get_response_obj(
+                    [
+                        ResponseCustomToolCall(
+                            type="custom_tool_call",
+                            id="ctc_1",
+                            call_id="call_1",
+                            name="raw_editor",
+                            input="hello",
+                        )
+                    ]
+                )
+            return get_response_obj([])
+
+    class DummyResponsesClient:
+        def __init__(self):
+            self.responses = DummyResponses()
+
+    def on_invoke_tool(_ctx: Any, raw_input: str) -> str:
+        invoked_inputs.append(raw_input)
+        return "edited"
+
+    deferred_tool = CustomTool(
+        name="raw_editor",
+        description="Edit raw text.",
+        on_invoke_tool=on_invoke_tool,
+        defer_loading=True,
+    )
+    model = OpenAIResponsesModel(
+        model="gpt-5.4",
+        openai_client=DummyResponsesClient(),  # type: ignore[arg-type]
+    )
+    agent = Agent(name="test", model=model, tools=[ToolSearchTool(), deferred_tool])
+
+    await Runner.run(agent, "hi")
+
+    assert sent_tools == [
+        {"type": "tool_search"},
+        {
+            "type": "custom",
+            "name": "raw_editor",
+            "description": "Edit raw text.",
+            "defer_loading": True,
+        },
+    ]
+    assert invoked_inputs == ["hello"]
+
+
 @pytest.mark.allow_call_model_methods
 @pytest.mark.asyncio
 async def test_ga_computer_tool_does_not_require_preview_metadata() -> None:
diff --git a/tests/models/test_openai_responses_converter.py b/tests/models/test_openai_responses_converter.py
index 9c64107892..eb1aa0267f 100644
--- a/tests/models/test_openai_responses_converter.py
+++ b/tests/models/test_openai_responses_converter.py
@@ -36,6 +36,7 @@
     AgentOutputSchema,
     Computer,
     ComputerTool,
+    CustomTool,
     FileSearchTool,
     Handoff,
     HostedMCPTool,
@@ -848,6 +849,39 @@ def test_convert_tools_top_level_deferred_function_with_tool_search() -> None:
     ]
 
 
+def test_convert_tools_deferred_custom_tool_requires_tool_search() -> None:
+    deferred_tool = CustomTool(
+        name="raw_editor",
+        description="Edit raw text.",
+        on_invoke_tool=lambda _ctx, raw_input: raw_input,
+        defer_loading=True,
+    )
+
+    with pytest.raises(UserError, match="ToolSearchTool\\(\\)"):
+        Converter.convert_tools(tools=[deferred_tool], handoffs=[])
+
+
+def test_convert_tools_deferred_custom_tool_with_tool_search() -> None:
+    deferred_tool = CustomTool(
+        name="raw_editor",
+        description="Edit raw text.",
+        on_invoke_tool=lambda _ctx, raw_input: raw_input,
+        defer_loading=True,
+    )
+
+    converted = Converter.convert_tools(tools=[deferred_tool, ToolSearchTool()], handoffs=[])
+
+    assert converted.tools == [
+        {
+            "type": "custom",
+            "name": "raw_editor",
+            "description": deferred_tool.description,
+            "defer_loading": True,
+        },
+        {"type": "tool_search"},
+    ]
+
+
 def test_convert_tools_preserves_tool_search_config_fields() -> None:
     deferred_tool = function_tool(
         lambda city: city,