fix(schema): stringify non-string enum values in Google tool format

lib/req_llm/schema.ex

@@ -662,7 +662,11 @@ defmodule ReqLLM.Schema do
   @spec to_google_format(ReqLLM.Tool.t()) :: map()
   def to_google_format(%ReqLLM.Tool{} = tool) do
     json_schema = to_json(tool.parameter_schema)
-    parameters = deep_delete_keys(json_schema, @google_forbidden_schema_keys)
+
+    parameters =
+      json_schema
+      |> deep_delete_keys(@google_forbidden_schema_keys)
+      |> stringify_enums()
 
     %{
       "name" => tool.name,
@@ -1073,6 +1077,21 @@ defmodule ReqLLM.Schema do
 
   defp deep_delete_keys(value, _keys), do: value
 
+  defp stringify_enums(map) when is_map(map) do
+    Map.new(map, fn
+      {"enum", values} when is_list(values) ->
+        {"enum", Enum.map(values, &to_string/1)}
+
+      {k, v} ->
+        {k, stringify_enums(v)}
+    end)
+  end
+
+  defp stringify_enums(list) when is_list(list),
+    do: Enum.map(list, &stringify_enums/1)
+
+  defp stringify_enums(value), do: value
+
   defp key_variants(keys) when is_list(keys), do: Enum.flat_map(keys, &key_variants/1)
   defp key_variants(key) when is_binary(key), do: [key, String.to_atom(key)]
   defp key_variants(key) when is_atom(key), do: [key, Atom.to_string(key)]

test/req_llm/schema_test.exs

@@ -502,6 +502,37 @@ defmodule ReqLLM.SchemaTest do
       assert result["parameters"]["required"] == ["tags"]
     end
 
+    test "to_google_format/1 stringifies non-string enum values" do
+      tool = %ReqLLM.Tool{
+        name: "set_level",
+        description: "Set a level",
+        parameter_schema: %{
+          "type" => "object",
+          "properties" => %{
+            "level" => %{
+              "type" => "integer",
+              "enum" => [1, 2, 3],
+              "description" => "The level"
+            },
+            "mode" => %{
+              "type" => "string",
+              "enum" => ["fast", "slow"],
+              "description" => "The mode"
+            }
+          },
+          "required" => ["level"]
+        },
+        callback: fn _ -> {:ok, "ok"} end
+      }
+
+      result = Schema.to_google_format(tool)
+      level = result["parameters"]["properties"]["level"]
+      mode = result["parameters"]["properties"]["mode"]
+
+      assert level["enum"] == ["1", "2", "3"]
+      assert mode["enum"] == ["fast", "slow"]
+    end
+
     test "to_google_format/1 strips atom-keyed forbidden fields recursively" do
       atom_keyed_schema = %{
         :type => :object,