diff --git a/lib/ash_phoenix/form/auto.ex b/lib/ash_phoenix/form/auto.ex
index c743097..62c3705 100644
--- a/lib/ash_phoenix/form/auto.ex
+++ b/lib/ash_phoenix/form/auto.ex
@@ -177,8 +177,12 @@ defmodule AshPhoenix.Form.Auto do
 
         transform_params =
           if fake_embedded? do
-            fn form, _params, _type ->
-              AshPhoenix.Form.value(form, :value)
+            fn form, params, type ->
+              if type == :nested do
+                AshPhoenix.Form.value(form, :value)
+              else
+                params
+              end
             end
           end
 
@@ -244,11 +248,11 @@ defmodule AshPhoenix.Form.Auto do
 
         Params:
 
-        #{inspect(params)}
+        #{inspect(params, pretty: true)}
 
         Available types:
 
-        #{inspect(constraints[:types])}
+        #{inspect(constraints[:types], pretty: true)}
         """
 
       {_key, config} ->
@@ -266,13 +270,16 @@ defmodule AshPhoenix.Form.Auto do
         raise """
         Got no "_union_type" parameter, and no union type had a tag & tag_value pair matching the params.
 
+        If you are adding a form, select a type using `params: %{"_union_type" => "type_name"}`, or if one
+        or more of your types is using a tag you can set that tag with `params: %{"tag" => "tag_value"}`.
+
         Params:
 
-        #{inspect(params)}
+        #{inspect(params, pretty: true)}
 
         Available types:
 
-        #{inspect(constraints[:types])}
+        #{inspect(constraints[:types], pretty: true)}
         """
 
       {_key, config} ->
@@ -823,6 +830,8 @@ defmodule AshPhoenix.Form.Auto do
     |> Keyword.new()
   end
 
+  defp union?({:array, type}), do: union?(type)
+
   defp union?(type) do
     if Ash.Type.NewType.new_type?(type) do
       union?(Ash.Type.NewType.subtype_of(type))
diff --git a/lib/ash_phoenix/form/form.ex b/lib/ash_phoenix/form/form.ex
index 96f0ed0..1ce260d 100644
--- a/lib/ash_phoenix/form/form.ex
+++ b/lib/ash_phoenix/form/form.ex
@@ -1182,7 +1182,7 @@ defmodule AshPhoenix.Form do
                     Map.update(forms, key, [new_form], &(&1 ++ [new_form]))
 
                   matching_form ->
-                    opts = update_opts(opts, matching_form.data, form_params)
+                    opts = update_opts(opts, matching_form.data, params)
 
                     validated =
                       validate(matching_form, params,
@@ -2889,6 +2889,13 @@ defmodule AshPhoenix.Form do
         fields -> Keyword.put(hidden, :_touched, fields)
       end
 
+    hidden =
+      if form.params["_union_type"] do
+        Keyword.put(hidden, :_union_type, form.params["_union_type"])
+      else
+        hidden
+      end
+
     if form.params["_index"] && form.params["_index"] != "" do
       Keyword.put(hidden, :_index, form.params["_index"])
     else
diff --git a/test/auto_form_test.exs b/test/auto_form_test.exs
index 384f015..80fb8fc 100644
--- a/test/auto_form_test.exs
+++ b/test/auto_form_test.exs
@@ -2,7 +2,8 @@ defmodule AshPhoenix.AutoFormTest do
   use ExUnit.Case
 
   alias AshPhoenix.Form.Auto
-  alias AshPhoenix.Test.Post
+  alias AshPhoenix.Test.{Api, Post}
+  import Phoenix.HTML.Form, only: [form_for: 2]
   import AshPhoenix.Form, only: [update_opts: 2]
 
   test "it works for simple relationships" do
@@ -53,6 +54,80 @@ defmodule AshPhoenix.AutoFormTest do
     assert validated.source.arguments[:comment_ids] == [1]
   end
 
+  describe "single unions" do
+    test "a form can be added for a union" do
+        Post
+        |> AshPhoenix.Form.for_create(:create,
+          api: Api,
+          forms: [
+            auto?: true
+          ]
+        )
+        |> AshPhoenix.Form.add_form(:union, params: %{"type" => "foo"})
+        |> form_for("action")
+    end
+
+    test "a form can be removed from a union" do
+      form =
+        Post
+        |> AshPhoenix.Form.for_create(:create,
+          api: Api,
+          forms: [
+            auto?: true
+          ]
+        )
+        |> AshPhoenix.Form.add_form(:union, params: %{"type" => "foo"})
+        |> form_for("action")
+
+      AshPhoenix.Form.remove_form(form, [:union])
+    end
+
+    test "a form can be added for a non-embedded type" do
+      Post
+      |> AshPhoenix.Form.for_create(:create,
+        api: Api,
+        forms: [
+          auto?: true
+        ],
+        params: %{
+          "text" => "foobar"
+        }
+      )
+      |> AshPhoenix.Form.add_form(:union, params: %{"_union_type" => "bar", "value" => 10})
+      |> AshPhoenix.Form.submit!()
+    end
+  end
+
+  describe "list unions" do
+    test "a form can be added for a union" do
+      Post
+      |> AshPhoenix.Form.for_create(:create,
+        api: Api,
+        forms: [
+          auto?: true
+        ]
+      )
+      |> AshPhoenix.Form.add_form(:union_array, params: %{"type" => "foo"})
+      |> form_for("action")
+    end
+
+    test "a form can be removed from a union" do
+      form =
+      Post
+      |> AshPhoenix.Form.for_create(:create,
+        api: Api,
+        forms: [
+          auto?: true
+        ]
+      )
+      |> AshPhoenix.Form.add_form(:union_array, params: %{"type" => "foo"})
+      |> form_for("action")
+
+      AshPhoenix.Form.remove_form(form, [:union_array, 0])
+    end
+  end
+
+
   defp auto_forms(resource, action) do
     [forms: Auto.auto(resource, action)]
   end
diff --git a/test/support/resources/post.ex b/test/support/resources/post.ex
index 120a1e9..675c264 100644
--- a/test/support/resources/post.ex
+++ b/test/support/resources/post.ex
@@ -10,6 +10,7 @@ defmodule AshPhoenix.Test.Post do
     uuid_primary_key(:id)
     attribute(:text, :string, allow_nil?: false)
     attribute(:union, AshPhoenix.Test.UnionValue)
+    attribute(:union_array, {:array, AshPhoenix.Test.UnionValue})
     attribute(:title, :string)
   end