diff --git a/supabase/migrations/20260902160000_parties_substrate.sql b/supabase/migrations/20260902160000_parties_substrate.sql
index dd1640fd..b6656ac5 100644
--- a/supabase/migrations/20260902160000_parties_substrate.sql
+++ b/supabase/migrations/20260902160000_parties_substrate.sql
@@ -306,6 +306,9 @@ GRANT EXECUTE ON FUNCTION public.ensure_party(uuid, uuid, text, text, text, text
 -- party; nothing is merged on name. Rows with an empty name (three exist on
 -- prod: one supplier, two customers) keep party_id NULL: ensure_party
 -- refuses a nameless party, and the suggestion pipeline names them later.
+-- Rows in a company archived by a migration reset (company_migration_resets
+-- .source_company_id) are frozen by block_migration_reset_source_mutation
+-- and are skipped too: that archive is immutable by design.
 DO $$
 DECLARE
   r record;
@@ -315,6 +318,7 @@ BEGIN
       FROM public.suppliers
      WHERE party_id IS NULL AND company_id IS NOT NULL
        AND nullif(btrim(name), '') IS NOT NULL
+       AND company_id NOT IN (SELECT source_company_id FROM public.company_migration_resets)
      ORDER BY created_at, id
   LOOP
     UPDATE public.suppliers
@@ -327,6 +331,7 @@ BEGIN
       FROM public.customers
      WHERE party_id IS NULL AND company_id IS NOT NULL
        AND nullif(btrim(name), '') IS NOT NULL
+       AND company_id NOT IN (SELECT source_company_id FROM public.company_migration_resets)
      ORDER BY created_at, id
   LOOP
     UPDATE public.customers
diff --git a/tests/pg/parties-substrate.pg.test.ts b/tests/pg/parties-substrate.pg.test.ts
index 1527b427..564abe14 100644
--- a/tests/pg/parties-substrate.pg.test.ts
+++ b/tests/pg/parties-substrate.pg.test.ts
@@ -139,6 +139,29 @@ describe('parties substrate (pg)', () => {
     expect(visibleToStranger).toBe(0)
   })
 
+  it('a supplier in a company archived by a migration reset cannot take a party_id (why the backfill skips it)', async () => {
+    const frozen = await seedCompany()
+    const replacement = await seedCompany()
+    const { rows } = await getPool().query<{ id: string }>(
+      `INSERT INTO public.suppliers (company_id, user_id, name) VALUES ($1, $2, 'Frozen Supplier') RETURNING id`,
+      [frozen.companyId, frozen.userId],
+    )
+    await getPool().query(
+      `INSERT INTO public.company_migration_resets (source_company_id, replacement_company_id, actor_id, reason, confirmation_snapshot, source_counts)
+       VALUES ($1, $2, $3, 'pg test: the party backfill must skip archived companies', '{}'::jsonb, '{}'::jsonb)`,
+      [frozen.companyId, replacement.companyId, frozen.userId],
+    )
+    const partyId = await getPool().query<{ id: string }>(
+      `SELECT public.ensure_party($1, $2, 'Frozen Supplier', NULL, 'company', 'backfill') AS id`,
+      [replacement.companyId, replacement.userId],
+    )
+    await expect(
+      getPool().query(`UPDATE public.suppliers SET party_id = $1 WHERE id = $2`, [partyId.rows[0]!.id, rows[0]!.id]),
+    ).rejects.toMatchObject({ code: 'P0001' })
+    const still = await getPool().query<{ party_id: string | null }>(`SELECT party_id FROM public.suppliers WHERE id = $1`, [rows[0]!.id])
+    expect(still.rows[0]!.party_id).toBeNull()
+  })
+
   it('ensure_party refuses a p_user_id that is not the authenticated caller', async () => {
     const mine = await seedCompany()
     const other = await seedCompany()
diff --git a/tests/pg/upgrade/seed.sql b/tests/pg/upgrade/seed.sql
index b3b481a9..19db9b21 100644
--- a/tests/pg/upgrade/seed.sql
+++ b/tests/pg/upgrade/seed.sql
@@ -105,4 +105,74 @@ VALUES
   ('44444444-4444-4444-4444-444444444403', '6570', 123.45, 0),
   ('44444444-4444-4444-4444-444444444403', '1930', 0, 123.45);
 
+-- Edge rows that broke real backfills on production (2026-09-02, the party
+-- substrate migration failed twice on prod after passing here): a supplier
+-- and a customer with an empty name, and a company archived by a migration
+-- reset whose rows block_migration_reset_source_mutation makes immutable.
+-- Any migration that UPDATEs every supplier or customer must survive both.
+INSERT INTO public.suppliers (id, company_id, user_id, name)
+VALUES (
+  '77777777-7777-7777-7777-777777777701',
+  '22222222-2222-2222-2222-222222222222',
+  '11111111-1111-1111-1111-111111111111',
+  ''
+);
+
+INSERT INTO public.customers (id, company_id, user_id, name)
+VALUES (
+  '77777777-7777-7777-7777-777777777702',
+  '22222222-2222-2222-2222-222222222222',
+  '11111111-1111-1111-1111-111111111111',
+  ''
+);
+
+-- Guarded: company_migration_resets arrived 2026-08-18 and a merge-base older
+-- than that lacks the table. Then the archived company simply is not seeded.
+DO $$
+BEGIN
+  IF to_regclass('public.company_migration_resets') IS NULL THEN
+    RETURN;
+  END IF;
+
+  INSERT INTO public.companies (id, name, entity_type, created_by)
+  VALUES (
+    '88888888-8888-8888-8888-888888888801',
+    'Arkiverade Bolaget AB',
+    'aktiebolag',
+    '11111111-1111-1111-1111-111111111111'
+  );
+  INSERT INTO public.companies (id, name, entity_type, created_by)
+  VALUES (
+    '88888888-8888-8888-8888-888888888802',
+    'Ersattningsbolaget AB',
+    'aktiebolag',
+    '11111111-1111-1111-1111-111111111111'
+  );
+  INSERT INTO public.suppliers (id, company_id, user_id, name)
+  VALUES (
+    '88888888-8888-8888-8888-888888888811',
+    '88888888-8888-8888-8888-888888888801',
+    '11111111-1111-1111-1111-111111111111',
+    'Frusen Leverantor AB'
+  );
+  INSERT INTO public.customers (id, company_id, user_id, name)
+  VALUES (
+    '88888888-8888-8888-8888-888888888812',
+    '88888888-8888-8888-8888-888888888801',
+    '11111111-1111-1111-1111-111111111111',
+    'Frusen Kund AB'
+  );
+  -- Last, so the rows above are written before the archive freezes them.
+  INSERT INTO public.company_migration_resets
+    (source_company_id, replacement_company_id, actor_id, reason, confirmation_snapshot, source_counts)
+  VALUES (
+    '88888888-8888-8888-8888-888888888801',
+    '88888888-8888-8888-8888-888888888802',
+    '11111111-1111-1111-1111-111111111111',
+    'pg-upgrade fixture: archived source company for migration edge cases',
+    '{}'::jsonb,
+    '{}'::jsonb
+  );
+END $$;
+
 COMMIT;