diff --git a/priv/repo/migrations/20260616165309_make_member_user_fks_deferrable.exs b/priv/repo/migrations/20260616165309_make_member_user_fks_deferrable.exs
new file mode 100644
index 0000000..2fa45fb
--- /dev/null
+++ b/priv/repo/migrations/20260616165309_make_member_user_fks_deferrable.exs
@@ -0,0 +1,32 @@
+defmodule Mv.Repo.Migrations.MakeMemberUserFksDeferrable do
+  @moduledoc """
+  Makes the members/users foreign keys DEFERRABLE INITIALLY DEFERRED.
+
+  Concurrent `create_member` transactions take FK `FOR KEY SHARE` (MultiXact)
+  locks on these foreign keys at statement time and can form a cross-transaction
+  lock cycle, producing a PostgreSQL `deadlock_detected` (40P01). Deferring the
+  FK checks to commit time breaks the cycle.
+
+  Constraint deferrability is not tracked by AshPostgres resource snapshots, so
+  this is a hand-written migration with raw `execute/2`. Do not regenerate it
+  via `mix ash_postgres.generate_migrations`.
+  """
+  use Ecto.Migration
+
+  def change do
+    execute(
+      "ALTER TABLE users ALTER CONSTRAINT users_member_id_fkey DEFERRABLE INITIALLY DEFERRED",
+      "ALTER TABLE users ALTER CONSTRAINT users_member_id_fkey NOT DEFERRABLE"
+    )
+
+    execute(
+      "ALTER TABLE users ALTER CONSTRAINT users_role_id_fkey DEFERRABLE INITIALLY DEFERRED",
+      "ALTER TABLE users ALTER CONSTRAINT users_role_id_fkey NOT DEFERRABLE"
+    )
+
+    execute(
+      "ALTER TABLE members ALTER CONSTRAINT members_membership_fee_type_id_fkey DEFERRABLE INITIALLY DEFERRED",
+      "ALTER TABLE members ALTER CONSTRAINT members_membership_fee_type_id_fkey NOT DEFERRABLE"
+    )
+  end
+end