diff --git a/crates/clawhdf5-wasm/tests/h5py_interop.rs b/crates/clawhdf5-wasm/tests/h5py_interop.rs
index a70d8bd..7ef60ab 100644
--- a/crates/clawhdf5-wasm/tests/h5py_interop.rs
+++ b/crates/clawhdf5-wasm/tests/h5py_interop.rs
@@ -166,9 +166,16 @@ fn check_file(dir: &Path, file: &str, exp: &Value) {
 
     for (path, want) in exp["datasets"].as_object().unwrap() {
         let kind = want["kind"].as_str().unwrap();
-        let a = r
-            .read(path, None)
-            .unwrap_or_else(|e| panic!("{file}:{path}: {e}"));
+        let a = match (r.read(path, None), want["unavailable"].as_str()) {
+            (Ok(a), _) => a,
+            // A filter this build may lack (zstd: the wasm build has it
+            // off, a workspace build may unify it on) must fail clearly.
+            (Err(e), Some(why)) => {
+                assert!(e.contains(why), "{file}:{path}: {e}");
+                continue;
+            }
+            (Err(e), None) => panic!("{file}:{path}: {e}"),
+        };
         assert_eq!(a.shape, shape(&want["shape"]), "{file}:{path} shape");
         let (got_kind, got) = data_strings(&a.data);
         assert_eq!(got_kind, kind, "{file}:{path} kind");
diff --git a/examples/wasm-viewer/test/make_fixture.py b/examples/wasm-viewer/test/make_fixture.py
index c8d023a..8c7722c 100644
--- a/examples/wasm-viewer/test/make_fixture.py
+++ b/examples/wasm-viewer/test/make_fixture.py
@@ -175,10 +175,12 @@ def describe(path):
                     walk(key.rstrip("/") + "/" + n, o)
             elif obj.dtype.names:
                 expected["errors"][key] = "compound"
-            elif key == "/zstd":
-                expected["errors"][key] = "unsupported filter: 32015"
             else:
                 expected["datasets"][key] = entry(obj, slab_for(obj))
+                if key == "/zstd":
+                    # Refused by the wasm build (no zstd); a native build
+                    # that unifies in clawhdf5-format/zstd reads it.
+                    expected["datasets"][key]["unavailable"] = "unsupported filter: 32015"
 
         walk("/", f)
     return expected
diff --git a/examples/wasm-viewer/test/test.mjs b/examples/wasm-viewer/test/test.mjs
index 1386f4c..80ca30a 100644
--- a/examples/wasm-viewer/test/test.mjs
+++ b/examples/wasm-viewer/test/test.mjs
@@ -69,6 +69,12 @@ for (const [name, exp] of Object.entries(expected)) {
   for (const [path, want] of Object.entries(exp.datasets)) {
     const ctx = `${name}:${path}`;
     eq(file.kind(path), "dataset", `${ctx} kind`);
+    if (want.unavailable) {
+      // The wasm build has no zstd (it links C): a clear error, no data.
+      assert.throws(() => file.read(path), (e) => e.message.includes(want.unavailable), ctx);
+      checks++;
+      continue;
+    }
     const info = file.info(path);
     eq([...info.shape, ...info.elementShape], want.shape, `${ctx} info shape`);
     const r = file.read(path);