diff --git a/crates/gc-extract/tests/disc.rs b/crates/gc-extract/tests/disc.rs
index f3bdab5..f8d1a56 100644
--- a/crates/gc-extract/tests/disc.rs
+++ b/crates/gc-extract/tests/disc.rs
@@ -748,6 +748,7 @@ fn tfrag_textures_decode_to_plausible_images() {
     }
 
     let mut decoded = 0usize;
+    let mut alphas: std::collections::BTreeMap<u8, usize> = std::collections::BTreeMap::new();
     let mut opaque = 0usize;
     let mut multi_colour = 0usize;
     let mut flat = 0usize;
@@ -801,6 +802,9 @@ fn tfrag_textures_decode_to_plausible_images() {
             if op == texels as usize {
                 opaque += 1;
             }
+            for px in t.rgba.chunks_exact(4) {
+                *alphas.entry(px[3]).or_insert(0usize) += 1;
+            }
         }
     }
 
@@ -822,6 +826,30 @@ fn tfrag_textures_decode_to_plausible_images() {
         opaque_pct * 100.0,
         detail_pct * 100.0
     );
+
+    // The 0x80 convention, measured on the disc rather than asserted in prose.
+    // Every decoded alpha must be a value gs::alpha_to_8 can produce -- and 254
+    // cannot be produced by it, since (v*255+64)/128 never lands on 254 for an
+    // integer v. 254 is precisely what the old `a * 2` produced for a raw 0x7F,
+    // so this line fails if that bug is ever reintroduced or the two expressions
+    // of the rule drift apart again.
+    let legal: std::collections::HashSet<u8> =
+        (0..=255u8).map(gc_formats::gs::alpha_to_8).collect();
+    let illegal: Vec<(u8, usize)> = alphas
+        .iter()
+        .filter(|(a, _)| !legal.contains(a))
+        .map(|(a, n)| (*a, *n))
+        .collect();
+    eprintln!("decoded alpha histogram: {alphas:?}");
+    assert!(
+        illegal.is_empty(),
+        "{illegal:?} are not producible by gs::alpha_to_8 -- the CLUT alpha \
+         conversion and the GS rule have diverged again"
+    );
+    assert!(
+        alphas.contains_key(&0xff),
+        "no fully opaque texel decoded; the 0x80 -> opaque rule is not exercised"
+    );
 }
 
 /// LOD linkage: every LOD-01 and LOD-0 vertex-info entry must resolve to a