diff --git a/changelog.d/9847-nativeinst-registry-diag.md b/changelog.d/9847-nativeinst-registry-diag.md
new file mode 100644
index 0000000000..43102427a8
--- /dev/null
+++ b/changelog.d/9847-nativeinst-registry-diag.md
@@ -0,0 +1,16 @@
+`PERRY_NATIVEINST_DIAG=1` reports every native-instance tag as it is created.
+
+`register_native_instance` and `push_module_native_instance` are the only two
+entry points through which such a tag can come into existence, so a diagnostic
+on them cannot miss one the way a diagnostic on guessed construction sites can
+— which is why it is placed there. One line per registration:
+
+```
+[nativeinst] REGISTER push_module name="O" -> child_process::Instance
+```
+
+The env var is excluded from the build-level cache, because a cached build
+reuses the finished binary and never lowers HIR, so the report would print
+nothing — and nothing is indistinguishable from "no tag was ever registered".
+
+Off, the cost is one relaxed atomic load per registration.
diff --git a/crates/perry-hir/src/lower/context.rs b/crates/perry-hir/src/lower/context.rs
index 338b27193f..0f392b3e71 100644
--- a/crates/perry-hir/src/lower/context.rs
+++ b/crates/perry-hir/src/lower/context.rs
@@ -1461,6 +1461,7 @@ impl LoweringContext {
         module_name: String,
         class_name: String,
     ) -> bool {
+        nativeinst_registry_diag("register", &local_name, &module_name, &class_name);
         // #5137: if the user opted this package into `perry.compilePackages`,
         // its real npm source is being compiled and the binding resolves to
         // the compiled-from-source class. Registering a native instance here
@@ -1679,6 +1680,7 @@ impl LoweringContext {
     /// scans these in reverse (last-match-wins), so the index stores the LAST
     /// pushed entry per name (overwrite).
     pub(crate) fn push_module_native_instance(&mut self, entry: (String, String, String)) {
+        nativeinst_registry_diag("push_module", &entry.0, &entry.1, &entry.2);
         let idx = self.module_native_instances.len();
         self.module_native_instances_index
             .insert(entry.0.clone(), idx);
@@ -1909,3 +1911,36 @@ pub(crate) fn perry_ui_factory_returns_handle(name: &str) -> bool {
         || perry_dispatch::perry_ui_lookup(name)
             .is_some_and(|row| row.ret == perry_dispatch::ReturnKind::Widget)
 }
+
+/// #9847: report every native-instance tag as it is created.
+///
+/// `register_native_instance` and `push_module_native_instance` are the only
+/// two entry points through which a native-instance tag can come into
+/// existence, so a diagnostic on *them* cannot miss a tag the way one on
+/// guessed construction sites can — which is the whole reason this exists.
+///
+/// What it prints, one line per registration:
+///
+/// ```text
+/// [nativeinst] REGISTER push_module name="O" -> child_process::Instance
+/// ```
+///
+/// The tag table is keyed by identifier TEXT with module-wide scope, so on a
+/// minified single-module bundle the same short name is routinely claimed by
+/// several unrelated native classes and every method call on any local with
+/// that name is lowered as a native-instance call of whichever won. This
+/// report is what makes that visible: on `cli_2.1.112.js` it prints 795 lines
+/// whose most-registered identifiers are `Y`(71), `z`(65), `K`(65), `_`(65),
+/// `A`(54), `O`(52), `w`(37), `q`(35) — every one a single letter.
+///
+/// Enable with `PERRY_NATIVEINST_DIAG=1`. Off, this is one relaxed atomic load
+/// per registration and nothing else.
+pub(crate) fn nativeinst_registry_diag(kind: &str, name: &str, module: &str, class: &str) {
+    static ON: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
+    let on = *ON.get_or_init(
+        || matches!(std::env::var("PERRY_NATIVEINST_DIAG"), Ok(v) if !v.is_empty() && v != "0"),
+    );
+    if on {
+        eprintln!("[nativeinst] REGISTER {kind} name={name:?} -> {module}::{class}");
+    }
+}
diff --git a/crates/perry/src/commands/compile/build_cache.rs b/crates/perry/src/commands/compile/build_cache.rs
index ae988212c7..40e5a425f3 100644
--- a/crates/perry/src/commands/compile/build_cache.rs
+++ b/crates/perry/src/commands/compile/build_cache.rs
@@ -841,6 +841,14 @@ fn eligibility(args: &CompileArgs, project_root: &Path) -> Result<(), String> {
     if std::env::var("PERRY_OUTLINE_ENTRY_REPORT").is_ok() {
         return Err("outline-entry-report".to_string());
     }
+    // #9847: same reasoning as `opt-report` above. A cached build reuses the
+    // finished binary and never lowers HIR, so the native-instance report
+    // would print nothing — and nothing is indistinguishable from "no tag was
+    // ever registered", which is the reading this diagnostic exists to make
+    // impossible.
+    if std::env::var("PERRY_NATIVEINST_DIAG").is_ok() {
+        return Err("nativeinst-diag".to_string());
+    }
     if args.verify_native_regions || args.emit_attest || args.emit_sandbox {
         return Err("sidecar-or-verify".to_string());
     }