fix(nextjs): Skip re instrumentating on generate phase of experimental build mode

packages/nextjs/src/config/withSentryConfig.ts

@@ -15,6 +15,7 @@ import { getNextjsVersion } from './util';
 import { constructWebpackConfigFunction } from './webpack';
 
 let showedExportModeTunnelWarning = false;
+let showedExperimentalBuildModeWarning = false;
 
 /**
  * Modifies the passed in Next.js configuration with automatic build-time instrumentation and source map upload.
@@ -67,6 +68,27 @@ function getFinalConfigObject(
     }
   }
 
+  if (process.argv.includes('--experimental-build-mode')) {
+    if (!showedExperimentalBuildModeWarning) {
+      showedExperimentalBuildModeWarning = true;
+      // eslint-disable-next-line no-console
+      console.warn(
+        '[@sentry/nextjs] The Sentry Next.js SDK does not currently fully support next build --experimental-build-mode',
+      );
+    }
+    if (process.argv.includes('generate')) {
+      // Next.js v15.3.0-canary.1 splits the experimental build into two phases:
+      // 1. compile: Code compilation
+      // 2. generate: Environment variable inlining and prerendering (We don't instrument this phase, we inline in the compile phase)
+      //
+      // We assume a single “full” build and reruns Webpack instrumentation in both phases.
+      // During the generate step it collides with Next.js’s inliner
+      // producing malformed JS and build failures.
+      // We skip Sentry processing during generate to avoid this issue.
+      return incomingUserNextConfigObject;
+    }
+  }
+
   setUpBuildTimeVariables(incomingUserNextConfigObject, userSentryOptions, releaseName);
 
   const nextJsVersion = getNextjsVersion();

packages/nextjs/test/config/withSentryConfig.test.ts

@@ -50,4 +50,29 @@ describe('withSentryConfig', () => {
 
     expect(exportedNextConfigFunction).toHaveBeenCalledWith(defaultRuntimePhase, defaultsObject);
   });
+
+  it('handles experimental build mode correctly', () => {
+    const originalArgv = process.argv;
+    const consoleWarnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+    try {
+      process.argv = [...originalArgv, '--experimental-build-mode'];
+      materializeFinalNextConfig(exportedNextConfig);
+
+      expect(consoleWarnSpy).toHaveBeenCalledWith(
+        '[@sentry/nextjs] The Sentry Next.js SDK does not currently fully support next build --experimental-build-mode',
+      );
+
+      // Generate phase
+      process.argv = [...process.argv, 'generate'];
+      const generateConfig = materializeFinalNextConfig(exportedNextConfig);
+
+      expect(generateConfig).toEqual(exportedNextConfig);
+
+      expect(consoleWarnSpy).toHaveBeenCalledTimes(1);
+    } finally {
+      process.argv = originalArgv;
+      consoleWarnSpy.mockRestore();
+    }
+  });
 });