From 8a71c80d23cf75ffee4169bc614f9f3727f4bb85 Mon Sep 17 00:00:00 2001
From: Alex Hunt <huntie@meta.com>
Date: Thu, 4 Dec 2025 11:23:55 -0800
Subject: [PATCH] Exclude legacy Hermes targets from single host assertion
 (#54784)

Summary:

### Context

This is a follow-up to D86201689 following user feedback on `0.83.0-rc.2`: https://github.com/react-native-community/discussions-and-proposals/discussions/954#discussioncomment-15138992.

The problem was that we were incorrectly attributing any and all `IInspector::addPage()` calls to represent a new React Native Host being registered. However, this API also remains common to the legacy Hermes `ConnectionDemux` runtime target setup (which does not represent a host target).

### This diff

We now fork this code path internally based on `InspectorTargetCapabilities.prefersFuseboxFrontend`, in order to track `registeredHostsCount` accurately (i.e. **only** for HostTarget page registrations).

Changelog:
[General][Fixed] - React Native DevTools: Fix a bug where we would incorrectly flag apps using additonal Hermes runtimes (e.g. Reanimated) as being multi-host

Reviewed By: hoxyq

Differential Revision: D88386623
---
 .../ReactCommon/jsinspector-modern/HostAgent.cpp  |  4 ++--
 .../jsinspector-modern/InspectorInterfaces.cpp    | 15 ++++++++++-----
 .../jsinspector-modern/InspectorInterfaces.h      |  4 ++--
 .../jsinspector-modern/TracingAgent.cpp           |  2 +-
 4 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/packages/react-native/ReactCommon/jsinspector-modern/HostAgent.cpp b/packages/react-native/ReactCommon/jsinspector-modern/HostAgent.cpp
index 813770d51ad5..95b86ad8cfbd 100644
--- a/packages/react-native/ReactCommon/jsinspector-modern/HostAgent.cpp
+++ b/packages/react-native/ReactCommon/jsinspector-modern/HostAgent.cpp
@@ -147,7 +147,7 @@ class HostAgent::Impl final {
     if (InspectorFlags::getInstance().getNetworkInspectionEnabled()) {
       if (req.method == "Network.enable") {
         auto& inspector = getInspectorInstance();
-        if (inspector.getSystemState().registeredPagesCount > 1) {
+        if (inspector.getSystemState().registeredHostsCount > 1) {
           frontendChannel_(
               cdp::jsonError(
                   req.id,
@@ -231,7 +231,7 @@ class HostAgent::Impl final {
               "ReactNativeApplication.metadataUpdated",
               createHostMetadataPayload(hostMetadata_)));
       auto& inspector = getInspectorInstance();
-      bool isSingleHost = inspector.getSystemState().registeredPagesCount <= 1;
+      bool isSingleHost = inspector.getSystemState().registeredHostsCount <= 1;
       if (!isSingleHost) {
         emitSystemStateChanged(isSingleHost);
       }
diff --git a/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.cpp b/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.cpp
index b0d2f7a5ffd4..065dd0e53c3b 100644
--- a/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.cpp
+++ b/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.cpp
@@ -67,8 +67,8 @@ class InspectorImpl : public IInspector {
    public:
     explicit SystemStateListener(InspectorSystemState& state) : state_(state) {}
 
-    void onPageAdded(int /*pageId*/) override {
-      state_.registeredPagesCount++;
+    void unstable_onHostTargetAdded() override {
+      state_.registeredHostsCount++;
     }
 
    private:
@@ -94,6 +94,7 @@ class InspectorImpl : public IInspector {
     ConnectFunc connectFunc_;
     InspectorTargetCapabilities capabilities_;
   };
+
   mutable std::mutex mutex_;
   int nextPageId_{1};
   std::map<int, Page> pages_;
@@ -142,9 +143,13 @@ int InspectorImpl::addPage(
       pageId,
       Page{pageId, description, vm, std::move(connectFunc), capabilities});
 
-  for (const auto& listenerWeak : listeners_) {
-    if (auto listener = listenerWeak.lock()) {
-      listener->onPageAdded(pageId);
+  // Strong assumption: If prefersFuseboxFrontend is set, the page added is a
+  // HostTarget and not a legacy Hermes runtime target.
+  if (capabilities.prefersFuseboxFrontend) {
+    for (const auto& listenerWeak : listeners_) {
+      if (auto listener = listenerWeak.lock()) {
+        listener->unstable_onHostTargetAdded();
+      }
     }
   }
 
diff --git a/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.h b/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.h
index 3117b216ac2d..aa256a4bf7d0 100644
--- a/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.h
+++ b/packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.h
@@ -53,7 +53,7 @@ using InspectorPage = InspectorPageDescription;
 
 struct InspectorSystemState {
   /** The total count of pages registered during the app lifetime. */
-  int registeredPagesCount;
+  int registeredHostsCount;
 };
 
 /// IRemoteConnection allows the VM to send debugger messages to the client.
@@ -83,7 +83,7 @@ class JSINSPECTOR_EXPORT ILocalConnection : public IDestructible {
 class JSINSPECTOR_EXPORT IPageStatusListener : public IDestructible {
  public:
   virtual ~IPageStatusListener() = 0;
-  virtual void onPageAdded(int /*pageId*/) {}
+  virtual void unstable_onHostTargetAdded() {}
   virtual void onPageRemoved(int /*pageId*/) {}
 };
 
diff --git a/packages/react-native/ReactCommon/jsinspector-modern/TracingAgent.cpp b/packages/react-native/ReactCommon/jsinspector-modern/TracingAgent.cpp
index 823c56ecfaa9..b8cc1cc1ab1b 100644
--- a/packages/react-native/ReactCommon/jsinspector-modern/TracingAgent.cpp
+++ b/packages/react-native/ReactCommon/jsinspector-modern/TracingAgent.cpp
@@ -54,7 +54,7 @@ TracingAgent::~TracingAgent() {
 bool TracingAgent::handleRequest(const cdp::PreparsedRequest& req) {
   if (req.method == "Tracing.start") {
     auto& inspector = getInspectorInstance();
-    if (inspector.getSystemState().registeredPagesCount > 1) {
+    if (inspector.getSystemState().registeredHostsCount > 1) {
       frontendChannel_(
           cdp::jsonError(
               req.id,