fix(nextjs): Update bundler detection (#17976)

Test were failing due to missing value injection, because the bundler
was incorrectly detected.

We can rely on `process.env.TURBOPACK` being set, confirmed this with
Vercel.

So this PR

- simplifies the bundler detection by just checking the env var
- brings back webpack dev tests

closes
https://linear.app/getsentry/issue/FE-618/webpack-breaks-instrumentation-for-dev-mode-in-next-16

Author: chargome

dev-packages/e2e-tests/test-applications/nextjs-16/package.json

@@ -18,7 +18,7 @@
     "test:build-canary": "pnpm install && pnpm add next@canary && pnpm build",
     "test:build-canary-webpack": "pnpm install && pnpm add next@canary && pnpm build-webpack",
     "test:assert": "pnpm test:prod && pnpm test:dev",
-    "test:assert-webpack": "pnpm test:prod"
+    "test:assert-webpack": "pnpm test:prod && pnpm test:dev-webpack"
   },
   "dependencies": {
     "@sentry/nextjs": "latest || *",

packages/nextjs/src/config/util.ts

@@ -109,66 +109,20 @@ export function supportsNativeDebugIds(version: string): boolean {
 }
 
 /**
- * Checks if the current Next.js version uses Turbopack as the default bundler.
- * Starting from Next.js 15.6.0-canary.38, turbopack became the default for `next build`.
+ * Determines which bundler is actually being used based on environment variables,
+ * and CLI flags.
  *
- * @param version - Next.js version string to check.
- * @returns true if the version uses Turbopack by default
+ * @returns 'turbopack' or 'webpack'
  */
-export function isTurbopackDefaultForVersion(version: string): boolean {
-  if (!version) {
-    return false;
-  }
-
-  const { major, minor, prerelease } = parseSemver(version);
-
-  if (major === undefined || minor === undefined) {
-    return false;
-  }
+export function detectActiveBundler(): 'turbopack' | 'webpack' {
+  const turbopackEnv = process.env.TURBOPACK;
 
-  // Next.js 16+ uses turbopack by default
-  if (major >= 16) {
-    return true;
-  }
+  // Check if TURBOPACK env var is set to a truthy value (excluding falsy strings like 'false', '0', '')
+  const isTurbopackEnabled = turbopackEnv && turbopackEnv !== 'false' && turbopackEnv !== '0';
 
-  // For Next.js 15, only canary versions 15.6.0-canary.40+ use turbopack by default
-  // Stable 15.x releases still use webpack by default
-  if (major === 15 && minor >= 6 && prerelease && prerelease.startsWith('canary.')) {
-    if (minor >= 7) {
-      return true;
-    }
-    const canaryNumber = parseInt(prerelease.split('.')[1] || '0', 10);
-    if (canaryNumber >= 40) {
-      return true;
-    }
-  }
-
-  return false;
-}
-
-/**
- * Determines which bundler is actually being used based on environment variables,
- * CLI flags, and Next.js version.
- *
- * @param nextJsVersion - The Next.js version string
- * @returns 'turbopack', 'webpack', or undefined if it cannot be determined
- */
-export function detectActiveBundler(nextJsVersion: string | undefined): 'turbopack' | 'webpack' | undefined {
-  if (process.env.TURBOPACK || process.argv.includes('--turbo')) {
+  if (isTurbopackEnabled || process.argv.includes('--turbo')) {
     return 'turbopack';
-  }
-
-  // Explicit opt-in to webpack via --webpack flag
-  if (process.argv.includes('--webpack')) {
+  } else {
     return 'webpack';
   }
-
-  // Fallback to version-based default behavior
-  if (nextJsVersion) {
-    const turbopackIsDefault = isTurbopackDefaultForVersion(nextJsVersion);
-    return turbopackIsDefault ? 'turbopack' : 'webpack';
-  }
-
-  // Unlikely but at this point, we just assume webpack for older behavior
-  return 'webpack';
 }

packages/nextjs/src/config/withSentryConfig.ts

@@ -261,7 +261,7 @@ function getFinalConfigObject(
     nextMajor = major;
   }
 
-  const activeBundler = detectActiveBundler(nextJsVersion);
+  const activeBundler = detectActiveBundler();
   const isTurbopack = activeBundler === 'turbopack';
   const isWebpack = activeBundler === 'webpack';
   const isTurbopackSupported = supportsProductionCompileHook(nextJsVersion ?? '');

packages/nextjs/test/config/util.test.ts

@@ -213,117 +213,6 @@ describe('util', () => {
     });
   });
 
-  describe('isTurbopackDefaultForVersion', () => {
-    describe('returns true for versions where turbopack is default', () => {
-      it.each([
-        // Next.js 16+ stable versions
-        ['16.0.0', 'Next.js 16.0.0 stable'],
-        ['16.0.1', 'Next.js 16.0.1 stable'],
-        ['16.1.0', 'Next.js 16.1.0 stable'],
-        ['16.2.5', 'Next.js 16.2.5 stable'],
-
-        // Next.js 16+ pre-release versions
-        ['16.0.0-rc.1', 'Next.js 16.0.0-rc.1'],
-        ['16.0.0-canary.1', 'Next.js 16.0.0-canary.1'],
-        ['16.1.0-beta.2', 'Next.js 16.1.0-beta.2'],
-
-        // Next.js 17+
-        ['17.0.0', 'Next.js 17.0.0'],
-        ['18.0.0', 'Next.js 18.0.0'],
-        ['20.0.0', 'Next.js 20.0.0'],
-
-        // Next.js 15.6.0-canary.40+ (boundary case)
-        ['15.6.0-canary.40', 'Next.js 15.6.0-canary.40 (exact threshold)'],
-        ['15.6.0-canary.41', 'Next.js 15.6.0-canary.41'],
-        ['15.6.0-canary.42', 'Next.js 15.6.0-canary.42'],
-        ['15.6.0-canary.100', 'Next.js 15.6.0-canary.100'],
-
-        // Next.js 15.7+ canary versions
-        ['15.7.0-canary.1', 'Next.js 15.7.0-canary.1'],
-        ['15.7.0-canary.50', 'Next.js 15.7.0-canary.50'],
-        ['15.8.0-canary.1', 'Next.js 15.8.0-canary.1'],
-        ['15.10.0-canary.1', 'Next.js 15.10.0-canary.1'],
-      ])('returns true for %s (%s)', version => {
-        expect(util.isTurbopackDefaultForVersion(version)).toBe(true);
-      });
-    });
-
-    describe('returns false for versions where webpack is still default', () => {
-      it.each([
-        // Next.js 15.6.0-canary.39 and below
-        ['15.6.0-canary.39', 'Next.js 15.6.0-canary.39 (just below threshold)'],
-        ['15.6.0-canary.36', 'Next.js 15.6.0-canary.36'],
-        ['15.6.0-canary.38', 'Next.js 15.6.0-canary.38'],
-        ['15.6.0-canary.0', 'Next.js 15.6.0-canary.0'],
-
-        // Next.js 15.6.x stable releases (NOT canary)
-        ['15.6.0', 'Next.js 15.6.0 stable'],
-        ['15.6.1', 'Next.js 15.6.1 stable'],
-        ['15.6.2', 'Next.js 15.6.2 stable'],
-        ['15.6.10', 'Next.js 15.6.10 stable'],
-
-        // Next.js 15.6.x rc releases (NOT canary)
-        ['15.6.0-rc.1', 'Next.js 15.6.0-rc.1'],
-        ['15.6.0-rc.2', 'Next.js 15.6.0-rc.2'],
-
-        // Next.js 15.7+ stable releases (NOT canary)
-        ['15.7.0', 'Next.js 15.7.0 stable'],
-        ['15.8.0', 'Next.js 15.8.0 stable'],
-        ['15.10.0', 'Next.js 15.10.0 stable'],
-
-        // Next.js 15.5 and below (all versions)
-        ['15.5.0', 'Next.js 15.5.0'],
-        ['15.5.0-canary.100', 'Next.js 15.5.0-canary.100'],
-        ['15.4.1', 'Next.js 15.4.1'],
-        ['15.0.0', 'Next.js 15.0.0'],
-        ['15.0.0-canary.1', 'Next.js 15.0.0-canary.1'],
-
-        // Next.js 14.x and below
-        ['14.2.0', 'Next.js 14.2.0'],
-        ['14.0.0', 'Next.js 14.0.0'],
-        ['14.0.0-canary.50', 'Next.js 14.0.0-canary.50'],
-        ['13.5.0', 'Next.js 13.5.0'],
-        ['13.0.0', 'Next.js 13.0.0'],
-        ['12.0.0', 'Next.js 12.0.0'],
-      ])('returns false for %s (%s)', version => {
-        expect(util.isTurbopackDefaultForVersion(version)).toBe(false);
-      });
-    });
-
-    describe('edge cases', () => {
-      it.each([
-        ['', 'empty string'],
-        ['invalid', 'invalid version string'],
-        ['15', 'missing minor and patch'],
-        ['15.6', 'missing patch'],
-        ['not.a.version', 'completely invalid'],
-        ['15.6.0-alpha.1', 'alpha prerelease (not canary)'],
-        ['15.6.0-beta.1', 'beta prerelease (not canary)'],
-      ])('returns false for %s (%s)', version => {
-        expect(util.isTurbopackDefaultForVersion(version)).toBe(false);
-      });
-    });
-
-    describe('canary number parsing edge cases', () => {
-      it.each([
-        ['15.6.0-canary.', 'canary with no number'],
-        ['15.6.0-canary.abc', 'canary with non-numeric value'],
-        ['15.6.0-canary.38.extra', 'canary with extra segments'],
-      ])('handles malformed canary versions: %s (%s)', version => {
-        // Should not throw, just return appropriate boolean
-        expect(() => util.isTurbopackDefaultForVersion(version)).not.toThrow();
-      });
-
-      it('handles canary.40 exactly (boundary)', () => {
-        expect(util.isTurbopackDefaultForVersion('15.6.0-canary.40')).toBe(true);
-      });
-
-      it('handles canary.39 exactly (boundary)', () => {
-        expect(util.isTurbopackDefaultForVersion('15.6.0-canary.39')).toBe(false);
-      });
-    });
-  });
-
   describe('detectActiveBundler', () => {
     const originalArgv = process.argv;
     const originalEnv = process.env;
@@ -341,52 +230,26 @@ describe('util', () => {
 
     it('returns turbopack when TURBOPACK env var is set', () => {
       process.env.TURBOPACK = '1';
-      expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
-    });
-
-    it('returns webpack when --webpack flag is present', () => {
-      process.argv.push('--webpack');
-      expect(util.detectActiveBundler('16.0.0')).toBe('webpack');
-    });
-
-    it('returns turbopack for Next.js 16+ by default', () => {
-      expect(util.detectActiveBundler('16.0.0')).toBe('turbopack');
-      expect(util.detectActiveBundler('17.0.0')).toBe('turbopack');
-    });
-
-    it('returns turbopack for Next.js 15.6.0-canary.40+', () => {
-      expect(util.detectActiveBundler('15.6.0-canary.40')).toBe('turbopack');
-      expect(util.detectActiveBundler('15.6.0-canary.50')).toBe('turbopack');
+      expect(util.detectActiveBundler()).toBe('turbopack');
     });
 
-    it('returns webpack for Next.js 15.6.0 stable', () => {
-      expect(util.detectActiveBundler('15.6.0')).toBe('webpack');
+    it('returns turbopack when TURBOPACK env var is set to auto', () => {
+      process.env.TURBOPACK = 'auto';
+      expect(util.detectActiveBundler()).toBe('turbopack');
     });
 
-    it('returns webpack for Next.js 15.5.x and below', () => {
-      expect(util.detectActiveBundler('15.5.0')).toBe('webpack');
-      expect(util.detectActiveBundler('15.0.0')).toBe('webpack');
-      expect(util.detectActiveBundler('14.2.0')).toBe('webpack');
+    it('returns webpack when TURBOPACK env var is undefined', () => {
+      process.env.TURBOPACK = undefined;
+      expect(util.detectActiveBundler()).toBe('webpack');
     });
 
-    it('returns webpack when version is undefined', () => {
-      expect(util.detectActiveBundler(undefined)).toBe('webpack');
+    it('returns webpack when TURBOPACK env var is false', () => {
+      process.env.TURBOPACK = 'false';
+      expect(util.detectActiveBundler()).toBe('webpack');
     });
 
-    it('prioritizes TURBOPACK env var over version detection', () => {
-      process.env.TURBOPACK = '1';
-      expect(util.detectActiveBundler('14.0.0')).toBe('turbopack');
-    });
-
-    it('prioritizes --webpack flag over version detection', () => {
-      process.argv.push('--webpack');
-      expect(util.detectActiveBundler('16.0.0')).toBe('webpack');
-    });
-
-    it('prioritizes TURBOPACK env var over --webpack flag', () => {
-      process.env.TURBOPACK = '1';
-      process.argv.push('--webpack');
-      expect(util.detectActiveBundler('15.5.0')).toBe('turbopack');
+    it('returns webpack when TURBOPACK env var is not set', () => {
+      expect(util.detectActiveBundler()).toBe('webpack');
     });
   });
 });