diff --git a/crates/rex_build/src/client_bundle.rs b/crates/rex_build/src/client_bundle.rs
index 76977cb..46266c0 100644
--- a/crates/rex_build/src/client_bundle.rs
+++ b/crates/rex_build/src/client_bundle.rs
@@ -6,7 +6,10 @@ use crate::page_exports::{detect_data_strategy, detect_has_static_paths};
 use anyhow::Result;
 use rex_core::{ProjectConfig, RexConfig};
 use rex_router::ScanResult;
-use rolldown_common::Output;
+use rolldown_common::{
+    ManualCodeSplittingOptions, MatchGroup, MatchGroupName, MatchGroupTest, Output,
+};
+use rolldown_utils::js_regex::HybridRegex;
 use std::collections::HashMap;
 use std::fs;
 use std::path::{Path, PathBuf};
@@ -224,13 +227,29 @@ if (!window.__REX_NAVIGATING__) {{
     // Append user-defined aliases from rex.config build.alias
     client_aliases.extend(project_config.build.resolved_aliases(&config.project_root));
 
+    // Force react/react-dom into a stable vendor chunk so HMR doesn't
+    // re-initialize React and break the running component tree.
+    let vendor_chunk = MatchGroup {
+        name: MatchGroupName::Static("vendor".to_string()),
+        test: Some(MatchGroupTest::Regex(
+            HybridRegex::new("node_modules[\\\\/](react|react-dom)[\\\\/]")
+                .expect("valid vendor regex"),
+        )),
+        priority: Some(100),
+        ..Default::default()
+    };
+
     let options = rolldown::BundlerOptions {
         input: Some(inputs),
         cwd: Some(config.project_root.clone()),
         format: Some(rolldown::OutputFormat::Esm),
         dir: Some(output_dir.to_string_lossy().to_string()),
         entry_filenames: Some(format!("[name]-{hash}.js").into()),
-        chunk_filenames: Some(format!("chunk-[name]-{hash}.js").into()),
+        // Use rolldown's content-based [hash] for shared chunks so that stable
+        // dependencies (e.g. the vendor/React chunk) keep the same filename
+        // across HMR rebuilds — the browser reuses the cached ESM module and
+        // React stays a singleton instead of being re-initialized.
+        chunk_filenames: Some("chunk-[name]-[hash].js".to_string().into()),
         asset_filenames: Some(format!("[name]-{hash}.[ext]").into()),
         platform: Some(rolldown::Platform::Browser),
         module_types: Some(module_types),
@@ -238,6 +257,10 @@ if (!window.__REX_NAVIGATING__) {{
         define: Some(define.iter().cloned().collect()),
         tsconfig: Some(rolldown_common::TsConfig::Auto(true)),
         treeshake: crate::rsc_build_config::react_treeshake_options(),
+        manual_code_splitting: Some(ManualCodeSplittingOptions {
+            groups: Some(vec![vendor_chunk]),
+            ..Default::default()
+        }),
         resolve: Some(rolldown::ResolveOptions {
             alias: Some(client_aliases),
             extensions: Some(vec![
diff --git a/crates/rex_build/tests/vendor_chunk_tests.rs b/crates/rex_build/tests/vendor_chunk_tests.rs
new file mode 100644
index 0000000..a214781
--- /dev/null
+++ b/crates/rex_build/tests/vendor_chunk_tests.rs
@@ -0,0 +1,88 @@
+#![allow(clippy::unwrap_used)]
+
+mod common;
+
+use common::setup_test_project;
+use rex_build::build_bundles;
+use rex_core::ProjectConfig;
+use std::fs;
+
+/// Test that React is isolated into vendor chunk(s) with content-based hashes.
+///
+/// Regression test for HMR crash: when React was co-bundled into shared app chunks,
+/// HMR would re-initialize React, breaking the running component tree with
+/// "Cannot read properties of null (reading 'useState')".
+#[tokio::test]
+async fn test_vendor_chunk_isolates_react() {
+    let (_tmp, config, scan) = setup_test_project(
+        &[
+            (
+                "index.tsx",
+                "export default function Home() { return <div>Home</div>; }",
+            ),
+            (
+                "about.tsx",
+                "export default function About() { return <div>About</div>; }",
+            ),
+        ],
+        None,
+    );
+    let result = build_bundles(&config, &scan, &ProjectConfig::default())
+        .await
+        .unwrap();
+
+    let client_dir = config.client_build_dir();
+    let build_hash = &result.build_id[..8];
+
+    // Find vendor chunks in the output directory (rolldown may split react
+    // and react-dom into separate vendor chunks)
+    let vendor_chunks: Vec<_> = fs::read_dir(&client_dir)
+        .unwrap()
+        .flatten()
+        .filter(|e| e.file_name().to_string_lossy().starts_with("chunk-vendor-"))
+        .collect();
+    assert!(
+        !vendor_chunks.is_empty(),
+        "should have at least 1 vendor chunk"
+    );
+
+    // At least one vendor chunk should contain React's createElement
+    let vendor_has_react = vendor_chunks.iter().any(|c| {
+        fs::read_to_string(c.path())
+            .unwrap()
+            .contains("createElement")
+    });
+    assert!(vendor_has_react, "vendor chunk(s) should contain React");
+
+    // Vendor chunks should use content-based hash, NOT the build_id hash
+    for chunk in &vendor_chunks {
+        let name = chunk.file_name().to_string_lossy().to_string();
+        assert!(
+            !name.contains(build_hash),
+            "vendor chunk should use content hash, not build_id hash: {name}"
+        );
+    }
+
+    // Page entry chunks should NOT contain React inline
+    let index_js = fs::read_to_string(client_dir.join(format!("index-{build_hash}.js"))).unwrap();
+    assert!(
+        !index_js.contains("function createElement"),
+        "page chunk should not bundle React inline — it should import from vendor chunk"
+    );
+
+    // Page entry chunks should import from the vendor chunk
+    assert!(
+        index_js.contains("chunk-vendor-"),
+        "page chunk should import from vendor chunk"
+    );
+
+    // Manifest should list vendor chunk(s) as shared chunks
+    assert!(
+        result
+            .manifest
+            .shared_chunks
+            .iter()
+            .any(|c| c.starts_with("chunk-vendor-")),
+        "manifest should track vendor as a shared chunk"
+    );
+}