diff --git a/crates/aura-cli/src/main.rs b/crates/aura-cli/src/main.rs
index 45358f1..2d5e5de 100644
--- a/crates/aura-cli/src/main.rs
+++ b/crates/aura-cli/src/main.rs
@@ -3078,11 +3078,33 @@ fn blueprint_mc_family(doc: &str, n_seeds: u64, data: &DataSource) -> Result<McF
     // re-runs the shared reduce-mode member path over its own seeded synthetic walk.
     let seeds: Vec<u64> = (1..=n_seeds).collect();
     let base_point: Vec<Scalar> = Vec::new();
-    Ok(monte_carlo(&base_point, &seeds, |seed, _base| {
+    let family = monte_carlo(&base_point, &seeds, |seed, _base| {
         let sources = synthetic_walk_sources(seed);
         let window = window_of(&sources).expect("non-empty synthetic walk");
         run_blueprint_member(reload(doc), &[], &space, sources, window, seed, pip, &topo)
-    }))
+    });
+    // Silent-vacuous MC guard (refuse-don't-guess, C10): with >= 2 seeds, if every draw's
+    // metrics are bit-identical to the first, no seed reached a distinguishable realization —
+    // the strategy never warmed over the fixed synthetic walk (e.g. a lookback as deep as the
+    // walk is long), so the "distribution" is a single point masquerading as a family: a wrong
+    // result with no error. Compare `metrics`, not the whole `RunReport` — the manifest's
+    // `seed` differs per draw by construction, so a whole-report compare could never detect the
+    // collapse; the metrics are the realization the seed is meant to move. A single-draw MC
+    // (n == 1) is trivially "all identical" and is NOT this cross-seed condition, so it passes.
+    if family.draws.len() >= 2
+        && family
+            .draws
+            .iter()
+            .all(|d| d.report.metrics == family.draws[0].report.metrics)
+    {
+        return Err(
+            "mc is vacuous: every seed produced an identical result — the strategy never warmed \
+             over the synthetic walk, so no seed reached a distinguishable realization; use a \
+             shallower-lookback blueprint or a longer walk"
+                .to_string(),
+        );
+    }
+    Ok(family)
 }
 
 /// `aura sweep <blueprint.json> --axis <name>=<csv> …`: sweep a loaded signal over its
@@ -5486,6 +5508,24 @@ mod tests {
         assert!(m[0] != m[1] || m[1] != m[2], "seeds must yield differing realizations");
     }
 
+    #[test]
+    fn blueprint_mc_family_rejects_vacuous_deep_lookback() {
+        // Property: the mc family builder REFUSES a silent-vacuous Monte-Carlo — one where
+        // every per-seed draw collapses to a bit-identical realization — by RETURNING a named
+        // error rather than an `Ok` family that looks like a real (but indistinguishable)
+        // distribution. A CLOSED deep-lookback signal whose slow SMA length (60) equals the
+        // fixed 60-bar synthetic walk never warms, so every seed yields zero trades and thus
+        // identical metrics; that is a wrong result with no error (C10 refuse-don't-guess).
+        let deep = stage1_signal(Some(2), Some(60)); // slow len == walk len -> never warms
+        let doc = blueprint_to_json(&deep).expect("serializes");
+        let err = blueprint_mc_family(&doc, 3, &DataSource::Synthetic)
+            .expect_err("a vacuous (all-identical) Monte-Carlo is rejected, not returned");
+        assert!(
+            err.contains("vacuous") || err.contains("identical"),
+            "names the vacuous/degenerate condition: {err}"
+        );
+    }
+
     #[test]
     fn blueprint_mc_family_rejects_an_open_blueprint() {
         // Property: the mc family builder REFUSES an open blueprint (free knobs) by RETURNING