diff --git a/crates/manager-core/migrations/0037_drop_idx_cron_triggers_due.sql b/crates/manager-core/migrations/0037_drop_idx_cron_triggers_due.sql
new file mode 100644
index 0000000..eeba180
--- /dev/null
+++ b/crates/manager-core/migrations/0037_drop_idx_cron_triggers_due.sql
@@ -0,0 +1,12 @@
+-- F-M-001 (audit 2026-06-07): drop idx_cron_triggers_due.
+--
+-- Created in 0017_cron_triggers.sql with a comment claiming it serves
+-- the scheduler. The actual scheduler query is
+-- `... WHERE t.enabled = TRUE FOR UPDATE OF d SKIP LOCKED` — no
+-- `last_fired_at` predicate. The index is unused; the join is
+-- effectively a full table scan of cron details against enabled
+-- triggers (small N today, so the overhead is invisible — but the
+-- index is also pure write amplification with zero read payoff).
+--
+-- Drop is reversible by re-running the CREATE INDEX from 0017.
+DROP INDEX IF EXISTS idx_cron_triggers_due;