diff --git a/backend/internal/repository/account_repo.go b/backend/internal/repository/account_repo.go
index 6ab4446ba4..212523a4fb 100644
--- a/backend/internal/repository/account_repo.go
+++ b/backend/internal/repository/account_repo.go
@@ -3358,6 +3358,12 @@ func (r *accountRepository) FindByExtraField(ctx context.Context, key string, va
 // ListDueUpstreamBillingProbeAccounts bounds result hydration and network work
 // to limit. PostgreSQL must still filter and order all enabled candidates;
 // MATERIALIZED avoids repeating the defensive timestamp parse expression.
+// Go writes next_probe_at via RFC3339Nano (up to 9 fractional digits) while
+// jsonpath datetime() parses at most microseconds, so fractions beyond 6
+// digits are trimmed first — mirroring ListDueOllamaCloudUsageAccounts.
+// Without this, every nanosecond timestamp is treated as malformed and the
+// fail-open ordering pins the cycle to the lowest account IDs, starving the
+// rest of the pool.
 func (r *accountRepository) ListDueUpstreamBillingProbeAccounts(ctx context.Context, now time.Time, limit int) ([]service.Account, error) {
 	if limit <= 0 {
 		return []service.Account{}, nil
@@ -3387,7 +3393,11 @@ func (r *accountRepository) ListDueUpstreamBillingProbeAccounts(ctx context.Cont
 				jsonb_path_query_first_tz(
 					jsonb_build_object(
 						'value',
-						replace(regexp_replace(next_probe_at, 'Z$', '+00:00'), 'T', ' ')
+						replace(regexp_replace(regexp_replace(
+							next_probe_at,
+							'(\.[0-9]{6})[0-9]+(Z|[+-][0-9]{2}:[0-9]{2})$',
+							'\1\2'
+						), 'Z$', '+00:00'), 'T', ' ')
 					),
 					'$.value.datetime()',
 					'{}'::jsonb,
diff --git a/backend/internal/repository/account_repo_upstream_billing_probe_due_integration_test.go b/backend/internal/repository/account_repo_upstream_billing_probe_due_integration_test.go
index 64d29553fe..104771af03 100644
--- a/backend/internal/repository/account_repo_upstream_billing_probe_due_integration_test.go
+++ b/backend/internal/repository/account_repo_upstream_billing_probe_due_integration_test.go
@@ -49,3 +49,107 @@ func TestListDueUpstreamBillingProbeAccountsHandlesInvalidCalendarDate(t *testin
 	require.Equal(t, invalidID, accounts[0].ID)
 	require.Equal(t, dueID, accounts[1].ID)
 }
+
+func insertUpstreamBillingProbeAccount(ctx context.Context, t *testing.T, tx sqlQueryer, name, nextProbeAt string) int64 {
+	t.Helper()
+	var id int64
+	extra := fmt.Sprintf(`{
+		"upstream_billing_probe_enabled": true,
+		"upstream_billing_probe": {"status": "ok", "next_probe_at": %q}
+	}`, nextProbeAt)
+	err := scanSingleRow(ctx, tx, `
+		INSERT INTO accounts (name, platform, type, status, extra)
+		VALUES ($1, 'openai', $2, 'active', $3::jsonb)
+		RETURNING id
+	`, []any{name, service.AccountTypeAPIKey, extra}, &id)
+	require.NoError(t, err)
+	return id
+}
+
+// rfc3339WithFraction renders t in UTC with an explicit fractional-second
+// suffix, e.g. "2026-07-25T17:29:00.123456789Z". The probes' Go writer uses
+// RFC3339Nano, so stored fractions carry up to 9 digits.
+func rfc3339WithFraction(t time.Time, fraction string) string {
+	return fmt.Sprintf("%s.%sZ", t.UTC().Format("2006-01-02T15:04:05"), fraction)
+}
+
+// Regression for the scheduled-probe starvation bug: Go persists next_probe_at
+// via RFC3339Nano (7-9 fractional digits), which jsonpath datetime() cannot
+// parse. Before the trim fix every such row was treated as malformed and
+// fail-open "due", so the cycle always returned the lowest account IDs and
+// higher IDs were never probed.
+func TestListDueUpstreamBillingProbeAccountsParsesNanosecondTimestamps(t *testing.T) {
+	ctx := context.Background()
+	tx := testEntTx(t)
+	repo := newAccountRepositoryWithSQL(tx.Client(), tx, nil)
+	now := time.Date(2026, time.July, 25, 17, 30, 0, 0, time.UTC)
+	_, err := tx.ExecContext(ctx, `
+		UPDATE accounts
+		SET extra = extra - 'upstream_billing_probe_enabled' - 'upstream_billing_probe'
+	`)
+	require.NoError(t, err)
+
+	// 22 low-ID accounts that are NOT due yet, stored with 9 fractional digits
+	// exactly as the legacy writer produced them (no migration).
+	notDue := now.Add(time.Hour)
+	for i := 0; i < 22; i++ {
+		insertUpstreamBillingProbeAccount(ctx, t, tx,
+			fmt.Sprintf("probe-nano-not-due-%02d", i),
+			rfc3339WithFraction(notDue.Add(time.Duration(i)*time.Second), "123456789"))
+	}
+	// 3 high-ID accounts that ARE due, with 9/8/7 fractional digits. Their
+	// parsed order must follow due time, not insertion/ID order.
+	dueThird := insertUpstreamBillingProbeAccount(ctx, t, tx,
+		"probe-nano-due-9digits", rfc3339WithFraction(now.Add(-time.Minute), "123456789"))
+	dueFirst := insertUpstreamBillingProbeAccount(ctx, t, tx,
+		"probe-nano-due-8digits", rfc3339WithFraction(now.Add(-3*time.Minute), "12345678"))
+	dueSecond := insertUpstreamBillingProbeAccount(ctx, t, tx,
+		"probe-nano-due-7digits", rfc3339WithFraction(now.Add(-2*time.Minute), "1234567"))
+
+	accounts, err := repo.ListDueUpstreamBillingProbeAccounts(ctx, now, 20)
+	require.NoError(t, err)
+	require.Len(t, accounts, 3)
+	require.Equal(t, dueFirst, accounts[0].ID)
+	require.Equal(t, dueSecond, accounts[1].ID)
+	require.Equal(t, dueThird, accounts[2].ID)
+}
+
+// With more due accounts than the cycle limit, selection must follow the
+// parsed due time so accounts beyond the first <limit> IDs still get their
+// turn; before the fix the same lowest IDs monopolized every cycle.
+func TestListDueUpstreamBillingProbeAccountsSelectsEarliestDueAcrossIDs(t *testing.T) {
+	ctx := context.Background()
+	tx := testEntTx(t)
+	repo := newAccountRepositoryWithSQL(tx.Client(), tx, nil)
+	now := time.Date(2026, time.July, 25, 17, 30, 0, 0, time.UTC)
+	_, err := tx.ExecContext(ctx, `
+		UPDATE accounts
+		SET extra = extra - 'upstream_billing_probe_enabled' - 'upstream_billing_probe'
+	`)
+	require.NoError(t, err)
+
+	// 25 due accounts; the LOWEST IDs carry the LATEST due times, so a
+	// correct query must pick the 20 highest-ID rows here.
+	ids := make([]int64, 0, 25)
+	for i := 0; i < 25; i++ {
+		due := now.Add(-time.Duration(i+1) * time.Minute)
+		ids = append(ids, insertUpstreamBillingProbeAccount(ctx, t, tx,
+			fmt.Sprintf("probe-nano-order-%02d", i),
+			rfc3339WithFraction(due, "123456789")))
+	}
+
+	accounts, err := repo.ListDueUpstreamBillingProbeAccounts(ctx, now, 20)
+	require.NoError(t, err)
+	require.Len(t, accounts, 20)
+	got := make([]int64, 0, len(accounts))
+	for _, account := range accounts {
+		got = append(got, account.ID)
+	}
+	// Earliest due first == highest index first; the five most recently due
+	// (lowest index / lowest IDs) fall outside the limit this cycle.
+	want := make([]int64, 0, 20)
+	for i := 24; i >= 5; i-- {
+		want = append(want, ids[i])
+	}
+	require.Equal(t, want, got)
+}
diff --git a/backend/internal/repository/account_repo_upstream_billing_probe_due_test.go b/backend/internal/repository/account_repo_upstream_billing_probe_due_test.go
index 350b95d4a2..aa1fcb4303 100644
--- a/backend/internal/repository/account_repo_upstream_billing_probe_due_test.go
+++ b/backend/internal/repository/account_repo_upstream_billing_probe_due_test.go
@@ -32,6 +32,7 @@ func TestAccountRepositoryListDueUpstreamBillingProbeAccountsBoundsQuery(t *test
 	require.Contains(t, normalized, "type = 'apikey'")
 	require.Contains(t, normalized, `extra @> '{"upstream_billing_probe_enabled": true}'::jsonb`)
 	require.Contains(t, normalized, "jsonb_path_query_first_tz")
+	require.Contains(t, normalized, `'(\.[0-9]{6})[0-9]+(Z|[+-][0-9]{2}:[0-9]{2})$'`)
 	require.Contains(t, normalized, "parsed AS MATERIALIZED")
 	require.Contains(t, normalized, "parsed_next_probe_at::timestamptz <= $1")
 	require.Contains(t, normalized, "LIMIT $2")