diff --git a/lib/ash_phoenix/form/auto.ex b/lib/ash_phoenix/form/auto.ex
index b4db758..57a0286 100644
--- a/lib/ash_phoenix/form/auto.ex
+++ b/lib/ash_phoenix/form/auto.ex
@@ -212,6 +212,7 @@ defmodule AshPhoenix.Form.Auto do
               prepare_source: prepare_source,
               transform_params: transform_params,
               embed?: true,
+              union?: true,
               save_updates?: false,
               forms:
                 Keyword.new(
diff --git a/lib/ash_phoenix/form/form.ex b/lib/ash_phoenix/form/form.ex
index 9f5b2fd..ab229aa 100644
--- a/lib/ash_phoenix/form/form.ex
+++ b/lib/ash_phoenix/form/form.ex
@@ -1664,6 +1664,7 @@ defmodule AshPhoenix.Form do
                             id: form.id <> "_#{key}_#{index}"
                           )
                       end
+                      |> clear_embed_source_errors(opts)
 
                     Map.update(forms, key, [new_form], &(&1 ++ [new_form]))
 
@@ -1682,6 +1683,7 @@ defmodule AshPhoenix.Form do
                       )
                       |> Map.put(:name, form.name <> "[#{key}][#{index}]")
                       |> Map.put(:id, form.id <> "_#{key}_#{index}")
+                      |> clear_embed_source_errors(opts)
 
                     Map.update(forms, key, [validated], fn nested_forms ->
                       nested_forms ++
@@ -1722,6 +1724,7 @@ defmodule AshPhoenix.Form do
                       accessing_from: opts[:managed_relationship],
                       prepare_source: opts[:prepare_source]
                     )
+                    |> clear_embed_source_errors(opts)
 
                   Map.put(forms, key, new_form)
                 else
@@ -1758,6 +1761,7 @@ defmodule AshPhoenix.Form do
                       as: form.name <> "[#{key}]",
                       id: form.id <> "_#{key}"
                     )
+                    |> clear_embed_source_errors(opts)
 
                   Map.put(forms, key, new_form)
                 end
@@ -2393,6 +2397,23 @@ defmodule AshPhoenix.Form do
     end
   end
 
+  # The parent action casts and validates embedded params inline, so running
+  # the embed's own action would produce a second, duplicate validation that
+  # bypasses parent-level logic. We still need the standalone source for
+  # rendering; `carry_over_errors/2` replays any real errors from the parent.
+  #
+  # Unions are excluded: they rewrite params into an `%Ash.Union{}` struct
+  # before handing them up, so the parent can't catch input errors and the
+  # standalone validation is the only line of defense. `union?` is set by
+  # `AshPhoenix.Form.Auto.unions/3` to mark these forms explicitly.
+  defp clear_embed_source_errors(form, opts) do
+    if opts[:embed?] && !opts[:union?] && form.source do
+      %{form | source: %{form.source | errors: [], valid?: true}}
+    else
+      form
+    end
+  end
+
   defp carry_over_errors(form, additional_errors \\ nil) do
     errors = List.wrap(form.source.errors)
 
diff --git a/test/form_test.exs b/test/form_test.exs
index 6489131..44f933e 100644
--- a/test/form_test.exs
+++ b/test/form_test.exs
@@ -1464,6 +1464,66 @@ defmodule AshPhoenix.FormTest do
       assert is_map(all_raw_errors)
       assert Map.has_key?(all_raw_errors, [])
     end
+
+    test "embedded forms are not validated in isolation when parent cast_input allows empty values" do
+      form =
+        Author
+        |> Form.for_create(:create, forms: [auto?: true])
+        |> Form.add_form(:address, params: %{})
+        |> Form.validate(%{
+          "email" => "me@example.com",
+          "address" => %{"line1" => "", "city" => "", "postcode" => ""}
+        })
+
+      assert Form.errors(form, for_path: :all) == %{}
+      assert form.valid?
+    end
+
+    test "embedded forms still surface parent errors for partial input" do
+      form =
+        Author
+        |> Form.for_create(:create, forms: [auto?: true])
+        |> Form.add_form(:address, params: %{})
+        |> Form.validate(%{
+          "email" => "me@example.com",
+          "address" => %{"line1" => "1 Main St", "city" => "", "postcode" => ""}
+        })
+
+      errors = Form.errors(form, for_path: [:address])
+      assert {:postcode, _} = List.keyfind(errors, :postcode, 0)
+      refute form.valid?
+    end
+
+    test "manage_relationship forms still validate standalone on validate" do
+      form =
+        Comment
+        |> Form.for_create(:create, forms: [auto?: true])
+        |> Form.add_form(:post, params: %{})
+        |> Form.validate(%{"text" => "comment text", "post" => %{}})
+
+      assert Form.errors(form, for_path: [:post]) == [text: "is required"]
+      refute form.valid?
+    end
+
+    test "update form with existing embedded data is valid without changes" do
+      author = %Author{
+        id: Ash.UUID.generate(),
+        email: "me@example.com",
+        address: %AshPhoenix.Test.Address{
+          line1: "1 Main St",
+          city: "Sydney",
+          postcode: "2000"
+        }
+      }
+
+      form =
+        author
+        |> Form.for_update(:update, forms: [auto?: true])
+        |> Form.validate(%{"email" => "new@example.com"})
+
+      assert Form.errors(form, for_path: :all) == %{}
+      assert form.valid?
+    end
   end
 
   describe "data" do
diff --git a/test/support/resources/address.ex b/test/support/resources/address.ex
new file mode 100644
index 0000000..236de5d
--- /dev/null
+++ b/test/support/resources/address.ex
@@ -0,0 +1,26 @@
+# SPDX-FileCopyrightText: 2020 ash_phoenix contributors <https://github.com/ash-project/ash_phoenix/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshPhoenix.Test.Address do
+  @moduledoc false
+  use Ash.Resource, data_layer: :embedded
+
+  attributes do
+    attribute :line1, :string, allow_nil?: false, public?: true
+    attribute :city, :string, allow_nil?: false, public?: true
+    attribute :postcode, :string, allow_nil?: false, public?: true
+  end
+
+  defoverridable cast_input: 2
+
+  def cast_input(value, constraints) when is_map(value) and not is_struct(value) do
+    if Enum.all?(Map.values(value), &(&1 in [nil, ""])) do
+      {:ok, nil}
+    else
+      super(value, constraints)
+    end
+  end
+
+  def cast_input(value, constraints), do: super(value, constraints)
+end
diff --git a/test/support/resources/author.ex b/test/support/resources/author.ex
index e62876a..6545514 100644
--- a/test/support/resources/author.ex
+++ b/test/support/resources/author.ex
@@ -16,6 +16,7 @@ defmodule AshPhoenix.Test.Author do
   attributes do
     uuid_primary_key(:id)
     attribute(:email, :string, allow_nil?: false, public?: true)
+    attribute(:address, AshPhoenix.Test.Address, allow_nil?: true, public?: true)
   end
 
   actions do