diff --git a/slhx-build/src/lib.rs b/slhx-build/src/lib.rs
index d1f8961..43ceda1 100644
--- a/slhx-build/src/lib.rs
+++ b/slhx-build/src/lib.rs
@@ -332,7 +332,8 @@ fn __slhx_lower_html(html: &str, table: &[(&str, &str, u32)]) -> ::std::string::
         };
         out = __slhx_replace_attr(out, attr, name, runtime_attr, *id);
     }
-    out
+    out = __slhx_lower_static_attr(out, "data-slhx-key", "data-key");
+    __slhx_inject_handle_inputs(out)
 }
 
 fn __slhx_replace_attr(mut html: ::std::string::String, attr: &str, name: &str, runtime_attr: &str, id: u32) -> ::std::string::String {
@@ -342,6 +343,55 @@ fn __slhx_replace_attr(mut html: ::std::string::String, attr: &str, name: &str,
     let single = ::std::format!("{attr}='{name}'");
     html.replace(&single, &replacement)
 }
+
+fn __slhx_lower_static_attr(mut html: ::std::string::String, attr: &str, runtime_attr: &str) -> ::std::string::String {
+    html = html.replace(&::std::format!("{attr}=\""), &::std::format!("{runtime_attr}=\""));
+    html.replace(&::std::format!("{attr}='"), &::std::format!("{runtime_attr}='"))
+}
+
+fn __slhx_inject_handle_inputs(html: ::std::string::String) -> ::std::string::String {
+    let mut out = ::std::string::String::with_capacity(html.len());
+    let mut rest = html.as_str();
+    while let Some(start) = rest.find("<form") {
+        out.push_str(&rest[..start]);
+        rest = &rest[start..];
+        let Some(open_end) = rest.find('>') else {
+            out.push_str(rest);
+            return out;
+        };
+        let opening = &rest[..=open_end];
+        out.push_str(opening);
+        rest = &rest[open_end + 1..];
+
+        let Some(handle_id) = __slhx_attr(opening, "data-hid") else {
+            continue;
+        };
+        let form_body_end = rest.find("</form>").unwrap_or(rest.len());
+        let form_body = &rest[..form_body_end];
+        if !__slhx_has_handle_input(form_body) {
+            out.push_str(&::std::format!("<input type=\"hidden\" name=\"__h\" value=\"{}\">", handle_id));
+        }
+    }
+    out.push_str(rest);
+    out
+}
+
+fn __slhx_has_handle_input(html: &str) -> bool {
+    html.contains("name=\"__h\"") || html.contains("name='__h'")
+}
+
+fn __slhx_attr(tag: &str, attr: &str) -> Option<::std::string::String> {
+    let attr_at = tag.find(attr)?;
+    let after_attr = &tag[attr_at + attr.len()..];
+    let after_equals = after_attr.trim_start().strip_prefix('=')?.trim_start();
+    let quote = after_equals.chars().next()?;
+    if quote != '\"' && quote != '\'' {
+        return None;
+    }
+    let value = &after_equals[quote.len_utf8()..];
+    let end = value.find(quote)?;
+    Some(value[..end].to_owned())
+}
 "#);
     }
 
@@ -660,6 +710,7 @@ mod tests {
         assert!(generated.contains("data-slhx-form"));
         assert!(generated.contains("data-sid"));
         assert!(generated.contains("data-fid"));
+        assert!(generated.contains("__slhx_inject_handle_inputs"));
 
         let syms = std::fs::read_to_string(out.join("slhx.syms")).unwrap();
         assert!(syms.contains("atom\t"));
@@ -688,6 +739,69 @@ mod tests {
         let _ = std::fs::remove_dir_all(&base);
     }
 
+    #[test]
+    fn generated_lowering_injects_progressive_form_handle_and_key() {
+        // req: form/002, req: wire/001
+        let base = test_dir("slhx-build-lowering-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("todo.heml"),
+            r#"<form data-slhx-handle="create"><input name="title"></form><li data-slhx-slot="row" data-slhx-key="7"></li>"#,
+        )
+        .unwrap();
+
+        app().template_dir(&templates).out_dir(&out).run().unwrap();
+        let generated = std::fs::read_to_string(out.join("slhx.generated.rs")).unwrap();
+        assert!(generated.contains("fn __slhx_lower_html"));
+        let source = format!(
+            r###"
+#![allow(dead_code)]
+mod slhx {{
+    #[derive(Clone, Copy)] pub struct BuildFingerprint;
+    impl BuildFingerprint {{ pub const fn from_parts(_: &[u32]) -> Self {{ Self }} }}
+    #[derive(Clone, Copy)] pub struct Slot<T>(::std::marker::PhantomData<T>);
+    impl<T> Slot<T> {{ pub const fn new(_: u32) -> Self {{ Self(::std::marker::PhantomData) }} }}
+    #[derive(Clone, Copy)] pub struct KeyedSlot<K, T>(::std::marker::PhantomData<(K, T)>);
+    impl<K, T> KeyedSlot<K, T> {{ pub const fn new(_: u32) -> Self {{ Self(::std::marker::PhantomData) }} }}
+    #[derive(Clone, Copy)] pub struct Handle<T>(::std::marker::PhantomData<T>);
+    impl<T> Handle<T> {{ pub const fn new(_: u32) -> Self {{ Self(::std::marker::PhantomData) }} }}
+    #[derive(Clone, Copy)] pub struct Atom<T>(::std::marker::PhantomData<T>);
+    impl<T> Atom<T> {{ pub const fn new(_: u32) -> Self {{ Self(::std::marker::PhantomData) }} }}
+    #[derive(Clone, Copy)] pub struct Form<T>(::std::marker::PhantomData<T>);
+    impl<T> Form<T> {{ pub const fn new(_: u32) -> Self {{ Self(::std::marker::PhantomData) }} }}
+    pub struct FormContract {{ pub fields: &'static [FormField] }}
+    pub struct FormField {{ pub name: &'static str, pub kind: FormControlKind, pub required: bool }}
+    pub enum FormControlKind {{ Text, Number {{ min: Option<&'static str>, max: Option<&'static str>, step: Option<&'static str> }}, Checkbox, Radio, Select {{ multiple: bool }}, TextArea, File, Hidden, Submit, Other {{ tag: &'static str, input_type: Option<&'static str> }} }}
+}}
+{generated}
+fn main() {{
+    let html = lower_html(r#"<form data-slhx-handle="create"><input name="title"></form><li data-slhx-slot="row" data-slhx-key="7"></li>"#);
+    assert!(html.contains(r#"data-hid="#));
+    assert!(html.contains(r#"name="__h""#));
+    assert!(html.contains(r#"data-key="7""#));
+    assert!(!html.contains("data-slhx-key"));
+}}
+"###,
+        );
+        let source_path = base.join("lowering.rs");
+        let bin_path = base.join("lowering-bin");
+        std::fs::write(&source_path, source).unwrap();
+        let status = std::process::Command::new("rustc")
+            .arg(&source_path)
+            .arg("-o")
+            .arg(&bin_path)
+            .status()
+            .unwrap();
+        assert!(status.success());
+        let status = std::process::Command::new(&bin_path).status().unwrap();
+        assert!(status.success());
+
+        let _ = std::fs::remove_dir_all(&base);
+    }
+
     fn test_dir(prefix: &str) -> PathBuf {
         std::env::temp_dir().join(format!("{prefix}-{}", std::process::id()))
     }