diff --git a/core/dr-pipeline/src/lib.rs b/core/dr-pipeline/src/lib.rs
index a763ef9..a651c3a 100644
--- a/core/dr-pipeline/src/lib.rs
+++ b/core/dr-pipeline/src/lib.rs
@@ -107,12 +107,38 @@ mod tests {
         let g = fully_active();
         assert!(!g.is_neutral());
         let shader = g.compose();
-        // Counted against the chain rather than a literal, so adding an
-        // operation does not require editing this test.
+
+        // The chain has two kinds of operation in it and they arrive in
+        // different places: a point operation is a block in the fused shader,
+        // while a neighbourhood operation is a pass of the detail chain and
+        // contributes no fused block at all — it reads pixels it is not
+        // writing, and a fused fragment is handed a colour with no coordinate.
+        //
+        // So the assertion is that each operation reaches exactly one of the
+        // two, checked against the chain rather than a literal, and phrased so
+        // that adding either kind extends it without an edit here.
+        let scale = g.render_scale((4000, 3000), (4000, 3000));
+        let detail = g.compose_detail(scale);
+        let mut fused_blocks = 0;
+        for desc in g.descriptors() {
+            let id = desc.id.0;
+            let point = shader.source.contains(&format!("---- {id} ----"));
+            let neighbourhood = detail
+                .passes
+                .iter()
+                .any(|p| p.label.starts_with(&format!("{id}/")));
+            assert!(
+                point ^ neighbourhood,
+                "{id} reaches {} of the two stages; an active operation \
+                 belongs to exactly one",
+                if point { "both" } else { "neither" }
+            );
+            fused_blocks += usize::from(point);
+        }
         assert_eq!(
             shader.source.matches("---- ").count(),
-            g.descriptors().len(),
-            "every operation in the chain should appear"
+            fused_blocks,
+            "the fused shader carries a block nothing in the chain asked for"
         );
     }