Don't include top_p for gpt-5.2+ in ChatOpenAIResponses

lib/chat_models/chat_open_ai_responses.ex

@@ -352,7 +352,29 @@ defmodule LangChain.ChatModels.ChatOpenAIResponses do
     |> Utils.conditionally_add_to_map(:tools, get_tools_for_api(openai, tools))
     |> Utils.conditionally_add_to_map(:user, openai.user)
     |> Utils.conditionally_add_to_map(:temperature, openai.temperature)
-    |> Utils.conditionally_add_to_map(:top_p, openai.top_p)
+    |> maybe_add_top_p(openai)
+  end
+
+  # gpt-5.2 and newer do not support the top_p parameter.
+  # Earlier models (gpt-4.x, gpt-5.0, gpt-5.1) accept top_p.
+  defp maybe_add_top_p(map, %ChatOpenAIResponses{model: model, top_p: top_p}) do
+    if supports_top_p?(model) do
+      Utils.conditionally_add_to_map(map, :top_p, top_p)
+    else
+      map
+    end
+  end
+
+  @doc false
+  @spec supports_top_p?(String.t()) :: boolean()
+  def supports_top_p?(model) when is_binary(model) do
+    # Match models known to support top_p. This set is fixed and won't grow.
+    cond do
+      String.starts_with?(model, "gpt-4") -> true
+      String.starts_with?(model, "gpt-5.0") -> true
+      String.starts_with?(model, "gpt-5.1") -> true
+      true -> false
+    end
   end
 
   defp get_tools_for_api(%ChatOpenAIResponses{} = _model, nil), do: []

test/chat_models/chat_open_ai_responses_test.exs

@@ -109,6 +109,66 @@ defmodule LangChain.ChatModels.ChatOpenAIResponsesTest do
       assert {"must be greater than or equal to %{number}", _} = changeset.errors[:temperature]
     end
 
+    test "top_p defaults to 1.0 and is included for gpt-4 models" do
+      {:ok, openai} = ChatOpenAIResponses.new(%{"model" => @test_model})
+      assert openai.top_p == 1.0
+
+      data = ChatOpenAIResponses.for_api(openai, [], [])
+      assert data.top_p == 1.0
+    end
+
+    test "top_p is excluded for gpt-5.2 and newer models" do
+      {:ok, openai} = ChatOpenAIResponses.new(%{"model" => "gpt-5.2", "top_p" => 0.9})
+      assert openai.top_p == 0.9
+
+      data = ChatOpenAIResponses.for_api(openai, [], [])
+      refute Map.has_key?(data, :top_p)
+    end
+
+    test "top_p is included for gpt-5.1 and earlier models" do
+      {:ok, openai} = ChatOpenAIResponses.new(%{"model" => "gpt-5.1", "top_p" => 0.8})
+      data = ChatOpenAIResponses.for_api(openai, [], [])
+      assert data.top_p == 0.8
+
+      {:ok, openai} = ChatOpenAIResponses.new(%{"model" => "gpt-5.0", "top_p" => 0.7})
+      data = ChatOpenAIResponses.for_api(openai, [], [])
+      assert data.top_p == 0.7
+    end
+
+    test "supports_top_p?/1 returns correct values for various models" do
+      assert ChatOpenAIResponses.supports_top_p?("gpt-4")
+      assert ChatOpenAIResponses.supports_top_p?("gpt-4o")
+      assert ChatOpenAIResponses.supports_top_p?("gpt-4o-mini")
+      assert ChatOpenAIResponses.supports_top_p?("gpt-4o-mini-2024-07-18")
+      assert ChatOpenAIResponses.supports_top_p?("gpt-5.0")
+      assert ChatOpenAIResponses.supports_top_p?("gpt-5.1")
+      refute ChatOpenAIResponses.supports_top_p?("gpt-5.2")
+      refute ChatOpenAIResponses.supports_top_p?("gpt-5.3")
+      refute ChatOpenAIResponses.supports_top_p?("gpt-6")
+    end
+
+    test "supports overriding top_p" do
+      {:ok, openai} = ChatOpenAIResponses.new(%{"model" => @test_model, "top_p" => 0.9})
+      assert openai.top_p == 0.9
+
+      data = ChatOpenAIResponses.for_api(openai, [], [])
+      assert data.top_p == 0.9
+    end
+
+    test "returns error for out-of-bounds top_p" do
+      assert {:error, changeset} =
+               ChatOpenAIResponses.new(%{"model" => @test_model, "top_p" => 1.5})
+
+      refute changeset.valid?
+      assert {"must be less than or equal to %{number}", _} = changeset.errors[:top_p]
+
+      assert {:error, changeset} =
+               ChatOpenAIResponses.new(%{"model" => @test_model, "top_p" => -0.1})
+
+      refute changeset.valid?
+      assert {"must be greater than or equal to %{number}", _} = changeset.errors[:top_p]
+    end
+
     test "supports setting reasoning options" do
       {:ok, openai} =
         ChatOpenAIResponses.new(%{