diff --git a/crates/ml-alpha/src/data/loader.rs b/crates/ml-alpha/src/data/loader.rs
index 972d0f628..c9f601801 100644
--- a/crates/ml-alpha/src/data/loader.rs
+++ b/crates/ml-alpha/src/data/loader.rs
@@ -471,14 +471,24 @@ impl MultiHorizonLoader {
         let n_files = self.files_loaded.len();
         let file_idx = self.rng.gen_range(0..n_files);
         let lf = &self.files_loaded[file_idx];
-        let needed = self.cfg.multi_resolution.total_positions() + max_horizon;
+        // Multi-resolution anchor sampling has TWO bounds:
+        //   * Upper:  anchor + total + max_horizon < snapshots.len()
+        //             (room for the forward-label window after the input edge)
+        //   * Lower:  anchor + total >= lookback
+        //             (room to look back across coarsest aggregation scale)
+        // For the default 3-scale config (total=32, lookback=1510) the lower
+        // bound is binding: anchor must be ≥ lookback - total = 1478.
+        let total = self.cfg.multi_resolution.total_positions();
+        let lookback = self.cfg.multi_resolution.required_lookback_ticks();
+        let min_anchor = lookback.saturating_sub(total);
+        let max_anchor = lf.snapshots.len().saturating_sub(total + max_horizon);
         anyhow::ensure!(
-            lf.snapshots.len() > needed,
-            "file too short for total_positions={} max_horizon={}: {} <= {}",
-            self.cfg.multi_resolution.total_positions(), max_horizon, lf.snapshots.len(), needed
+            max_anchor > min_anchor,
+            "file too short for multi-resolution(total={total}, lookback={lookback}) max_horizon={max_horizon}: \
+             snapshots.len()={} cannot span [{min_anchor}, {max_anchor})",
+            lf.snapshots.len()
         );
-        let max_anchor = lf.snapshots.len() - needed;
-        let anchor: usize = self.rng.gen_range(0..max_anchor);
+        let anchor: usize = self.rng.gen_range(min_anchor..max_anchor);
 
         let mut labels: [Vec<f32>; N_HORIZONS] = Default::default();
         let mut outcome_prof_long: [Vec<f32>; N_HORIZONS] = Default::default();