diff --git a/lib/membership/member.ex b/lib/membership/member.ex
index ee0622d..d29a759 100644
--- a/lib/membership/member.ex
+++ b/lib/membership/member.ex
@@ -561,14 +561,16 @@ defmodule Mv.Membership.Member do
     )
   end
 
-  # Builds search filter for custom field values using LIKE on JSONB
-  # Note: LIKE on JSONB is not index-optimized, may be slow with many custom fields
+  # Builds search filter for custom field values using ILIKE on JSONB
+  # Note: ILIKE on JSONB is not index-optimized, may be slow with many custom fields
   # This is a fallback for substring matching in custom fields (e.g., phone numbers)
   # Uses ->> operator which always returns TEXT directly (no need for -> + ::text fallback)
+  # Important: `id` must be passed as parameter to correctly reference the outer members table
   defp build_custom_field_filter(pattern) do
     expr(
       fragment(
-        "EXISTS (SELECT 1 FROM custom_field_values WHERE member_id = id AND (value->>'_union_value' LIKE ? OR value->>'value' LIKE ?))",
+        "EXISTS (SELECT 1 FROM custom_field_values WHERE member_id = ? AND (value->>'_union_value' ILIKE ? OR value->>'value' ILIKE ?))",
+        id,
         ^pattern,
         ^pattern
       )
diff --git a/test/membership/member_search_with_custom_fields_test.exs b/test/membership/member_search_with_custom_fields_test.exs
index 3b1b3b9..6711df8 100644
--- a/test/membership/member_search_with_custom_fields_test.exs
+++ b/test/membership/member_search_with_custom_fields_test.exs
@@ -544,4 +544,159 @@ defmodule Mv.Membership.MemberSearchWithCustomFieldsTest do
       # Substring matching for custom fields may need to be implemented separately
     end
   end
+
+  describe "custom field substring search (ILIKE)" do
+    test "finds member by prefix of custom field value", %{
+      member1: member1,
+      string_field: string_field
+    } do
+      # Create custom field value with a distinct word
+      {:ok, _cfv} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member1.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "Premium"}
+        })
+        |> Ash.create()
+
+      # Test prefix searches - should all find the member
+      for prefix <- ["Premium", "Premiu", "Premi", "Prem", "Pre"] do
+        results =
+          Member
+          |> Member.fuzzy_search(%{query: prefix})
+          |> Ash.read!()
+
+        assert Enum.any?(results, fn m -> m.id == member1.id end),
+               "Prefix '#{prefix}' should find member with custom field 'Premium'"
+      end
+    end
+
+    test "custom field search is case-insensitive", %{
+      member1: member1,
+      string_field: string_field
+    } do
+      # Create custom field value
+      {:ok, _cfv} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member1.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "GoldMember"}
+        })
+        |> Ash.create()
+
+      # Test case variations - should all find the member
+      for variant <- [
+            "GoldMember",
+            "goldmember",
+            "GOLDMEMBER",
+            "GoLdMeMbEr",
+            "gold",
+            "GOLD",
+            "Gold"
+          ] do
+        results =
+          Member
+          |> Member.fuzzy_search(%{query: variant})
+          |> Ash.read!()
+
+        assert Enum.any?(results, fn m -> m.id == member1.id end),
+               "Case variant '#{variant}' should find member with custom field 'GoldMember'"
+      end
+    end
+
+    test "finds member by suffix/middle of custom field value", %{
+      member1: member1,
+      string_field: string_field
+    } do
+      # Create custom field value
+      {:ok, _cfv} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member1.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "ActiveMember"}
+        })
+        |> Ash.create()
+
+      # Test suffix and middle substring searches
+      for substring <- ["Member", "ember", "tiveMem", "ctive"] do
+        results =
+          Member
+          |> Member.fuzzy_search(%{query: substring})
+          |> Ash.read!()
+
+        assert Enum.any?(results, fn m -> m.id == member1.id end),
+               "Substring '#{substring}' should find member with custom field 'ActiveMember'"
+      end
+    end
+
+    test "finds correct member among multiple with different custom field values", %{
+      member1: member1,
+      member2: member2,
+      member3: member3,
+      string_field: string_field
+    } do
+      # Create different custom field values for each member
+      {:ok, _cfv1} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member1.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "Beginner"}
+        })
+        |> Ash.create()
+
+      {:ok, _cfv2} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member2.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "Advanced"}
+        })
+        |> Ash.create()
+
+      {:ok, _cfv3} =
+        CustomFieldValue
+        |> Ash.Changeset.for_create(:create, %{
+          member_id: member3.id,
+          custom_field_id: string_field.id,
+          value: %{"_union_type" => "string", "_union_value" => "Expert"}
+        })
+        |> Ash.create()
+
+      # Search for "Begin" - should only find member1
+      results_begin =
+        Member
+        |> Member.fuzzy_search(%{query: "Begin"})
+        |> Ash.read!()
+
+      assert length(results_begin) == 1
+      assert List.first(results_begin).id == member1.id
+
+      # Search for "Advan" - should only find member2
+      results_advan =
+        Member
+        |> Member.fuzzy_search(%{query: "Advan"})
+        |> Ash.read!()
+
+      assert length(results_advan) == 1
+      assert List.first(results_advan).id == member2.id
+
+      # Search for "Exper" - should only find member3
+      results_exper =
+        Member
+        |> Member.fuzzy_search(%{query: "Exper"})
+        |> Ash.read!()
+
+      assert length(results_exper) == 1
+      assert List.first(results_exper).id == member3.id
+    end
+
+    # Note: Legacy format (type/value) is supported via the SQL ILIKE query on value->>'value'
+    # This is tested implicitly by the migration trigger which handles both formats.
+    # The Ash union type only accepts the new format (_union_type/_union_value) for creation,
+    # but the search works on existing legacy data.
+  end
 end