diff --git a/REQUIREMENTS.md b/REQUIREMENTS.md
index 7752dd3..c926d63 100644
--- a/REQUIREMENTS.md
+++ b/REQUIREMENTS.md
@@ -884,6 +884,9 @@ All forms support returning `impl IntoEffect` and compose through tuples.
 ### req: convention/008
 008	`data-slhx-disable-while-pending` disables the triggering form controls or button while the request is active and restores them afterward.
 
+### req: convention/009
+009	Unknown `data-slhx-*` authoring attributes are build errors with a suggested fix. slhx-owned attributes are a checked contract, not a silent extension namespace; opaque/integration islands should use explicit allowed attributes or their own non-slhx `data-*` names.
+
 ---
 
 ## multipart
diff --git a/slhx-build/src/lib.rs b/slhx-build/src/lib.rs
index 94dc7ed..6747371 100644
--- a/slhx-build/src/lib.rs
+++ b/slhx-build/src/lib.rs
@@ -150,6 +150,7 @@ impl Resources {
             };
 
             reject_selector_target_attrs(path, &node.attrs)?;
+            reject_unknown_slhx_attrs(path, &node.attrs)?;
 
             if let Some(class_attr) = static_attr(&node.attrs, "class") {
                 for token in class_tokens(&class_attr) {
@@ -968,6 +969,49 @@ fn reject_selector_target_attrs(path: &Path, attrs: &[SurfaceAttribute]) -> io::
     Ok(())
 }
 
+fn reject_unknown_slhx_attrs(path: &Path, attrs: &[SurfaceAttribute]) -> io::Result<()> {
+    for attr in attrs {
+        let name = attr.name.as_str();
+        if name.starts_with("data-slhx-") && !known_slhx_attr(name) {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                format!(
+                    "{}: unknown slhx attribute `{name}`; check the spelling or use a non-slhx data-* attribute for app-specific metadata",
+                    path.display()
+                ),
+            ));
+        }
+    }
+    Ok(())
+}
+
+fn known_slhx_attr(name: &str) -> bool {
+    matches!(
+        name,
+        "data-slhx-root"
+            | "data-slhx-sse"
+            | "data-slhx-st"
+            | "data-slhx-handle"
+            | "data-slhx-slot"
+            | "data-slhx-form"
+            | "data-slhx-atom"
+            | "data-slhx-key"
+            | "data-slhx-on"
+            | "data-slhx-pending-class"
+            | "data-slhx-indicator"
+            | "data-slhx-confirm"
+            | "data-slhx-debounce"
+            | "data-slhx-throttle"
+            | "data-slhx-every"
+            | "data-slhx-disable-while-pending"
+            | "data-slhx-policy"
+            | "data-slhx-nav"
+            | "data-slhx-boost"
+            | "data-slhx-error-for"
+            | "data-slhx-island"
+    )
+}
+
 fn reject_unkeyed_loop(
     surface: &SurfaceDocument,
     mut scope: ScopeId,
@@ -1394,6 +1438,29 @@ fn main() {{
         }
     }
 
+    #[test]
+    fn rejects_unknown_slhx_authoring_attrs() {
+        // req: convention/009 req: diagnostics/002
+        let base = test_dir("slhx-build-unknown-slhx-attr-test");
+        let templates = base.join("templates");
+        let out = base.join("out");
+        let _ = std::fs::remove_dir_all(&base);
+        std::fs::create_dir_all(&templates).unwrap();
+        std::fs::write(
+            templates.join("typo.heml"),
+            r#"<button data-slhx-handle="save" data-slhx-pendig-class="busy">Save</button>"#,
+        )
+        .unwrap();
+
+        let err = app().template_dir(&templates).out_dir(&out).run().unwrap_err();
+        assert!(err.to_string().contains("data-slhx-pendig-class"), "missing attr in diagnostic: {err}");
+        assert!(err.to_string().contains("unknown slhx attribute"), "missing unknown-attr diagnostic: {err}");
+        assert!(err.to_string().contains("check the spelling"), "missing spelling guidance: {err}");
+        assert!(err.to_string().contains("non-slhx data-*"), "missing app metadata guidance: {err}");
+
+        let _ = std::fs::remove_dir_all(&base);
+    }
+
     #[test]
     fn generated_lowering_injects_progressive_form_handle_and_key() {
         // req: form/002, req: wire/001