diff --git a/.changeset/webpack-runtime-watch-only.md b/.changeset/webpack-runtime-watch-only.md
new file mode 100644
index 0000000..59ab772
--- /dev/null
+++ b/.changeset/webpack-runtime-watch-only.md
@@ -0,0 +1,5 @@
+---
+"agoda-devfeedback-webpack": patch
+---
+
+Only inject the DevFeedback client runtime module in watch mode (dev server). The browser snippet was previously added to every compilation's initial chunks, including one-off production/CI builds and SSR bundles — it broke html-webpack-plugin template evaluation (`window is not defined` in node:vm) and would crash SSR bundles running in Node. The generated client code also now guards on `typeof window`/`document` as a defensive measure.
diff --git a/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.spec.ts b/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.spec.ts
index ec5d206..b610eb5 100644
--- a/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.spec.ts
+++ b/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.spec.ts
@@ -18,6 +18,7 @@ const mockedSendBuildData = sendBuildData as unknown as ReturnType<typeof vi.fn>
  */
 const createMockCompiler = () => {
   return {
+    watchMode: false,
     hooks: {
       watchRun: {
         tap: vi.fn(),
@@ -67,13 +68,27 @@ describe('WebpackBuildStatsPlugin', () => {
       'WebpackBuildStatsPlugin',
       expect.any(Function),
     );
-    // The plugin also uses `compilation.tap(...)` internally
+  });
+
+  it('should inject the client runtime module in watch mode', () => {
+    mockedCompiler.watchMode = true;
+    plugin.apply(mockedCompiler as unknown as Compiler);
+
     expect(mockedCompiler.hooks.compilation.tap).toHaveBeenCalledWith(
       'WebpackBuildStatsPlugin',
       expect.any(Function),
     );
   });
 
+  it('should not inject the client runtime module for one-off builds', () => {
+    mockedCompiler.watchMode = false;
+    plugin.apply(mockedCompiler as unknown as Compiler);
+
+    // One-off builds (production/CI/SSR) must not get the browser runtime —
+    // it breaks html-webpack-plugin template evaluation and SSR bundles.
+    expect(mockedCompiler.hooks.compilation.tap).not.toHaveBeenCalled();
+  });
+
   it('should send the correct build stats data on done', async () => {
     // Arrange
     const plugin = new WebpackBuildStatsPlugin('my custom identifier');
diff --git a/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.ts b/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.ts
index 534ab27..a4f9b71 100644
--- a/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.ts
+++ b/packages/webpack-plugin/src/lib/webpack-build-stats-plugin.ts
@@ -96,23 +96,31 @@ export class WebpackBuildStatsPlugin {
     });
 
     /**
-     * 4) Inject client code as a runtime module
+     * 4) Inject client code as a runtime module — watch mode only.
+     *
+     * The client snippet exists to report HMR / DOM-update events during local
+     * development, so it only makes sense when webpack is watching (dev server).
+     * Injecting it into one-off builds (production, CI, SSR) breaks consumers:
+     * html-webpack-plugin child compilations evaluate the template in node:vm
+     * where `window` is undefined, and SSR bundles execute the runtime in Node.
      */
-    compiler.hooks.compilation.tap(
-      'WebpackBuildStatsPlugin',
-      (compilation: Compilation) => {
-        compilation.hooks.afterChunks.tap('WebpackBuildStatsPlugin', (chunks) => {
-          for (const chunk of chunks) {
-            if (chunk.canBeInitial()) {
-              compilation.addRuntimeModule(
-                chunk,
-                new DevFeedbackRuntimeModule(() => this.generateClientCode()),
-              );
+    if (compiler.watchMode) {
+      compiler.hooks.compilation.tap(
+        'WebpackBuildStatsPlugin',
+        (compilation: Compilation) => {
+          compilation.hooks.afterChunks.tap('WebpackBuildStatsPlugin', (chunks) => {
+            for (const chunk of chunks) {
+              if (chunk.canBeInitial()) {
+                compilation.addRuntimeModule(
+                  chunk,
+                  new DevFeedbackRuntimeModule(() => this.generateClientCode()),
+                );
+              }
             }
-          }
-        });
-      },
-    );
+          });
+        },
+      );
+    }
   }
 
   /**
@@ -165,6 +173,11 @@ export class WebpackBuildStatsPlugin {
 
     return `
       (function(){
+        // Defensive: never execute outside a browser (e.g. SSR bundles or
+        // html-webpack-plugin template evaluation in node:vm).
+        if (typeof window === 'undefined' || typeof document === 'undefined') {
+          return;
+        }
         window.__BUILD_START__ = ${buildStartTime};
 
         var ws = new WebSocket('${wsUrl}');