diff --git a/app/controllers/sessions/magic_links_controller.rb b/app/controllers/sessions/magic_links_controller.rb
index 32257d9415..d2b25594c5 100644
--- a/app/controllers/sessions/magic_links_controller.rb
+++ b/app/controllers/sessions/magic_links_controller.rb
@@ -46,7 +46,7 @@ def sign_in(magic_link)
 
       respond_to do |format|
         format.html { redirect_to after_sign_in_url(magic_link) }
-        format.json { render json: { session_token: session_token } }
+        format.json { render json: { session_token: session_token, requires_signup_completion: requires_signup_completion?(magic_link) } }
       end
     end
 
@@ -68,7 +68,7 @@ def invalid_code
     end
 
     def after_sign_in_url(magic_link)
-      if magic_link.for_sign_up?
+      if requires_signup_completion?(magic_link)
         new_signup_completion_path
       else
         after_authentication_url
@@ -82,4 +82,8 @@ def rate_limit_exceeded
         format.json { render json: { message: rate_limit_exceeded_message }, status: :too_many_requests }
       end
     end
+
+    def requires_signup_completion?(magic_link)
+      magic_link.for_sign_up?
+    end
 end
diff --git a/app/controllers/signups/completions_controller.rb b/app/controllers/signups/completions_controller.rb
index 4fcaaabd36..2d8c3693c1 100644
--- a/app/controllers/signups/completions_controller.rb
+++ b/app/controllers/signups/completions_controller.rb
@@ -11,10 +11,9 @@ def create
     @signup = Signup.new(signup_params)
 
     if @signup.complete
-      flash[:welcome_letter] = true
-      redirect_to landing_url(script_name: @signup.account.slug)
+      welcome_to_account
     else
-      render :new, status: :unprocessable_entity
+      invalid_signup
     end
   end
 
@@ -22,4 +21,22 @@ def create
     def signup_params
       params.expect(signup: %i[ full_name ]).with_defaults(identity: Current.identity)
     end
+
+    def welcome_to_account
+      respond_to do |format|
+        format.html do
+          flash[:welcome_letter] = true
+          redirect_to landing_url(script_name: @signup.account.slug)
+        end
+
+        format.json { render json: { account_id: @signup.account.external_account_id }, status: :created }
+      end
+    end
+
+    def invalid_signup
+      respond_to do |format|
+        format.html { render :new, status: :unprocessable_entity }
+        format.json { render json: { errors: @signup.errors.full_messages }, status: :unprocessable_entity }
+      end
+    end
 end
diff --git a/test/controllers/sessions/magic_links_controller_test.rb b/test/controllers/sessions/magic_links_controller_test.rb
index 83b228416e..edcb22d3df 100644
--- a/test/controllers/sessions/magic_links_controller_test.rb
+++ b/test/controllers/sessions/magic_links_controller_test.rb
@@ -80,7 +80,7 @@ class Sessions::MagicLinksControllerTest < ActionDispatch::IntegrationTest
     assert MagicLink.exists?(expired_link.id), "Expired magic link should not be consumed"
   end
 
-  test "create via JSON" do
+  test "create via JSON for sign in" do
     identity = identities(:david)
     magic_link = identity.send_magic_link
 
@@ -89,6 +89,20 @@ class Sessions::MagicLinksControllerTest < ActionDispatch::IntegrationTest
       post session_magic_link_path(format: :json), params: { code: magic_link.code }
       assert_response :success
       assert @response.parsed_body["session_token"].present?
+      assert_equal false, @response.parsed_body["requires_signup_completion"]
+    end
+  end
+
+  test "create via JSON for sign up" do
+    identity = identities(:david)
+    magic_link = identity.send_magic_link(for: :sign_up)
+
+    untenanted do
+      post session_path(format: :json), params: { email_address: identity.email_address }
+      post session_magic_link_path(format: :json), params: { code: magic_link.code }
+      assert_response :success
+      assert @response.parsed_body["session_token"].present?
+      assert_equal true, @response.parsed_body["requires_signup_completion"]
     end
   end
 
diff --git a/test/controllers/signup/completions_controller_test.rb b/test/controllers/signup/completions_controller_test.rb
index 126ae6acfb..ccba9e8d4f 100644
--- a/test/controllers/signup/completions_controller_test.rb
+++ b/test/controllers/signup/completions_controller_test.rb
@@ -55,4 +55,30 @@ class Signup::CompletionsControllerTest < ActionDispatch::IntegrationTest
       assert_select "li", text: "Full name can't be blank"
     end
   end
+
+  test "create via JSON" do
+    untenanted do
+      post signup_completion_path(format: :json), params: {
+        signup: {
+          full_name: @signup.full_name
+        }
+      }
+    end
+
+    assert_response :created
+    assert @response.parsed_body["account_id"].present?
+  end
+
+  test "create via JSON with blank name" do
+    untenanted do
+      post signup_completion_path(format: :json), params: {
+        signup: {
+          full_name: ""
+        }
+      }
+    end
+
+    assert_response :unprocessable_entity
+    assert_includes @response.parsed_body["errors"], "Full name can't be blank"
+  end
 end