diff --git a/sql/141-switch-speed-drift-guard.sql b/sql/141-switch-speed-drift-guard.sql
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+-- sql/141-switch-speed-drift-guard.sql
+-- Preventive close of the THIRD direction of the same drift-gap class fixed in
+-- sql/126 (compat_speed_guard, guards compatibility writes) and sql/138
+-- (compat_speed_guard_on_transceiver_change, guards transceivers.speed_gbps
+-- changes). The "no physically-impossible compatibility rows" invariant is
+-- derived from THREE tables' data: compatibility.status, transceivers.speed_gbps,
+-- and switches.max_speed_gbps/ports_config (via v_switch_effective_max). Only
+-- two of those three write paths were guarded.
+--
+-- Audit 2026-07-17 found switches.max_speed_gbps/ports_config are currently
+-- write-once (set at scrape/seed time, sql/125's one-time backfill) — no live
+-- job updates them today, so this direction is NOT actively exploited yet. But
+-- the documented open item "110 switches with understated ports_config need
+-- datasheet enrichment" ([[tip-switch-database-audit-2026-07-06]]) describes
+-- exactly the future job that WOULD trigger this gap: correcting a switch's
+-- max_speed_gbps or ports_config downward after compat rows already exist would
+-- silently leave now-impossible rows marked 'compatible', same failure mode as
+-- sql/138, just from the other table. Closing it now, before that automation
+-- ships, rather than waiting for the next backlog surprise.
+--
+-- Same "no false positives" design as 126/138: only algorithmic (non-curated)
+-- rows are ever forced to incompatible; curated rows are trusted as-is.
+
+create or replace function compat_speed_guard_on_switch_change() returns trigger
+language plpgsql as $$
+declare
+  new_ceiling numeric;
+begin
+  if NEW.max_speed_gbps is distinct from OLD.max_speed_gbps
+     or NEW.ports_config is distinct from OLD.ports_config then
+    select greatest(
+             coalesce(NEW.max_speed_gbps, 0),
+             coalesce(fn_ports_max_gbps(NEW.ports_config), 0),
+             coalesce((select max(t.speed_gbps)
+                         from compatibility c join transceivers t on t.id = c.transceiver_id
+                        where c.switch_id = NEW.id and c.status = 'compatible'
+                          and c.verification_method in ('vendor_matrix','vendor_compat')
+                          and t.speed_gbps > 0), 0)
+           )
+      into new_ceiling;
+
+    if new_ceiling > 0 then
+      update compatibility c
+         set status = 'incompatible',
+             notes  = case when c.notes is null or c.notes = ''
+                           then 'auto(141): transceiver speed exceeds switch max port speed (post-hoc drift)'
+                           else c.notes || ' | auto(141): transceiver speed exceeds switch max port speed (post-hoc drift)' end
+        from transceivers t
+       where t.id = c.transceiver_id
+         and c.switch_id = NEW.id
+         and c.status = 'compatible'
+         and c.verification_method = 'spec_match'
+         and t.speed_gbps is not null
+         and t.speed_gbps > new_ceiling;
+    end if;
+  end if;
+  return NEW;
+end $$;
+
+comment on function compat_speed_guard_on_switch_change() is
+  'AFTER UPDATE OF max_speed_gbps/ports_config on switches: re-validates existing compatible spec_match compatibility rows for this switch against the (re-derived) evidence-based ceiling. Third leg of the compat_speed_guard family (126=compatibility writes, 138=transceiver speed changes, this=switch capacity changes). Never touches curated rows. Currently a no-op in practice -- nothing updates these columns live yet -- this is preventive, closing the gap before the planned switch datasheet-enrichment automation ships.';
+
+drop trigger if exists trg_compat_speed_guard_on_switch_change on switches;
+create trigger trg_compat_speed_guard_on_switch_change
+  after update of max_speed_gbps, ports_config on switches
+  for each row execute function compat_speed_guard_on_switch_change();
+
+insert into _migrations (filename, applied_at)
+select '141-switch-speed-drift-guard.sql', now()
+where not exists (select 1 from _migrations where filename = '141-switch-speed-drift-guard.sql');