sentry-junior · Nov 5, 2026
diff --git a/.size-limit.js b/.size-limit.js
@@ -64,6 +64,18 @@ module.exports = [
     gzip: true,
     disablePlugins: ['@size-limit/esbuild'],
   },
+  // esbuild drops dead code differently from webpack + terser, so measure one bundle with it too.
+  {
+    name: '@sentry/browser (incl. Tracing) - esbuild with treeshaking flags',
+    path: 'packages/browser/build/npm/esm/prod/index.js',
+    import: createImport('init', 'browserTracingIntegration'),
+    gzip: true,
+    disablePlugins: ['@size-limit/webpack'],
+    modifyEsbuildConfig: function (config) {
+      config.define = { __SENTRY_DEBUG__: 'false' };
+      return config;
+    },
+  },
   {
     name: '@sentry/browser (incl. Tracing + Span Streaming)',
     path: 'packages/browser/build/npm/esm/prod/index.js',

diff --git a/packages/core/src/tracing/idleSpan.ts b/packages/core/src/tracing/idleSpan.ts
@@ -339,11 +339,16 @@ export function startIdleSpan(startSpanOptions: StartSpanOptions, options: Parti
       const spanEndedBeforeFinalTimeout = childEndTimestamp - childStartTimestamp <= timeoutWithMarginOfError;
 
       if (DEBUG_BUILD) {
-        const stringifiedSpan = JSON.stringify(childSpan, undefined, 2);
         if (!spanStartedBeforeIdleSpanEnd) {
-          debug.log('[Tracing] Discarding span since it happened after idle span was finished', stringifiedSpan);
+          debug.log(
+            '[Tracing] Discarding span since it happened after idle span was finished',
+            JSON.stringify(childSpan, undefined, 2),
+          );
         } else if (!spanEndedBeforeFinalTimeout) {
-          debug.log('[Tracing] Discarding span since it finished after idle span final timeout', stringifiedSpan);
+          debug.log(
+            '[Tracing] Discarding span since it finished after idle span final timeout',
+            JSON.stringify(childSpan, undefined, 2),
+          );
         }
       }
 

diff --git a/packages/core/src/tracing/logSpans.ts b/packages/core/src/tracing/logSpans.ts
@@ -3,57 +3,58 @@ import type { Span } from '../types/span';
 import { debug } from '../utils/debug-logger';
 import { getRootSpan, spanIsSampled, spanToStaticSpanJSON } from '../utils/spanUtils';
 
+// A ternary instead of an early return, so that esbuild can drop the debug-only body in non-debug builds.
 /**
  * Print a log message for a started span.
  */
-export function logSpanStart(span: Span): void {
-  if (!DEBUG_BUILD) return;
-
-  const {
-    description = '< unknown name >',
-    op = '< unknown op >',
-    parent_span_id: parentSpanId,
-  } = spanToStaticSpanJSON(span);
-  const { spanId } = span.spanContext();
-
-  const sampled = spanIsSampled(span);
-  const rootSpan = getRootSpan(span);
-  const isRootSpan = rootSpan === span;
-
-  const header = `[Tracing] Starting ${sampled ? 'sampled' : 'unsampled'} ${isRootSpan ? 'root ' : ''}span`;
-
-  const infoParts: string[] = [`op: ${op}`, `name: ${description}`, `ID: ${spanId}`];
-
-  if (parentSpanId) {
-    infoParts.push(`parent ID: ${parentSpanId}`);
-  }
-
-  if (!isRootSpan) {
-    const { op, description } = spanToStaticSpanJSON(rootSpan);
-    infoParts.push(`root ID: ${rootSpan.spanContext().spanId}`);
-    if (op) {
-      infoParts.push(`root op: ${op}`);
-    }
-    if (description) {
-      infoParts.push(`root description: ${description}`);
-    }
-  }
-
-  debug.log(`${header}
+export const logSpanStart: (span: Span) => void = DEBUG_BUILD
+  ? span => {
+      const {
+        description = '< unknown name >',
+        op = '< unknown op >',
+        parent_span_id: parentSpanId,
+      } = spanToStaticSpanJSON(span);
+      const { spanId } = span.spanContext();
+
+      const sampled = spanIsSampled(span);
+      const rootSpan = getRootSpan(span);
+      const isRootSpan = rootSpan === span;
+
+      const header = `[Tracing] Starting ${sampled ? 'sampled' : 'unsampled'} ${isRootSpan ? 'root ' : ''}span`;
+
+      const infoParts: string[] = [`op: ${op}`, `name: ${description}`, `ID: ${spanId}`];
+
+      if (parentSpanId) {
+        infoParts.push(`parent ID: ${parentSpanId}`);
+      }
+
+      if (!isRootSpan) {
+        const { op, description } = spanToStaticSpanJSON(rootSpan);
+        infoParts.push(`root ID: ${rootSpan.spanContext().spanId}`);
+        if (op) {
+          infoParts.push(`root op: ${op}`);
+        }
+        if (description) {
+          infoParts.push(`root description: ${description}`);
+        }
+      }
+
+      debug.log(`${header}
   ${infoParts.join('\n  ')}`);
-}
+    }
+  : () => {};
 
 /**
  * Print a log message for an ended span.
  */
-export function logSpanEnd(span: Span): void {
-  if (!DEBUG_BUILD) return;
-
-  const { description = '< unknown name >', op = '< unknown op >' } = spanToStaticSpanJSON(span);
-  const { spanId } = span.spanContext();
-  const rootSpan = getRootSpan(span);
-  const isRootSpan = rootSpan === span;
-
-  const msg = `[Tracing] Finishing "${op}" ${isRootSpan ? 'root ' : ''}span "${description}" with ID ${spanId}`;
-  debug.log(msg);
-}
+export const logSpanEnd: (span: Span) => void = DEBUG_BUILD
+  ? span => {
+      const { description = '< unknown name >', op = '< unknown op >' } = spanToStaticSpanJSON(span);
+      const { spanId } = span.spanContext();
+      const rootSpan = getRootSpan(span);
+      const isRootSpan = rootSpan === span;
+
+      const msg = `[Tracing] Finishing "${op}" ${isRootSpan ? 'root ' : ''}span "${description}" with ID ${spanId}`;
+      debug.log(msg);
+    }
+  : () => {};
diff --git a/packages/core/src/utils/dsn.ts b/packages/core/src/utils/dsn.ts
@@ -80,43 +80,42 @@ function dsnFromComponents(components: DsnComponents): DsnComponents {
   };
 }
 
-function validateDsn(dsn: DsnComponents): boolean {
-  if (!DEBUG_BUILD) {
-    return true;
-  }
-
-  const { port, projectId, protocol } = dsn;
+// A ternary instead of an early return, so that esbuild can drop the debug-only body in non-debug builds.
+const validateDsn: (dsn: DsnComponents) => boolean = DEBUG_BUILD
+  ? dsn => {
+      const { port, projectId, protocol } = dsn;
+
+      const requiredComponents: ReadonlyArray<keyof DsnComponents> = ['protocol', 'publicKey', 'host', 'projectId'];
+      const hasMissingRequiredComponent = requiredComponents.find(component => {
+        if (!dsn[component]) {
+          debug.error(`Invalid Sentry Dsn: ${component} missing`);
+          return true;
+        }
+        return false;
+      });
+
+      if (hasMissingRequiredComponent) {
+        return false;
+      }
+
+      if (!projectId.match(/^\d+$/)) {
+        debug.error(`Invalid Sentry Dsn: Invalid projectId ${projectId}`);
+        return false;
+      }
+
+      if (!isValidProtocol(protocol)) {
+        debug.error(`Invalid Sentry Dsn: Invalid protocol ${protocol}`);
+        return false;
+      }
+
+      if (port && isNaN(parseInt(port, 10))) {
+        debug.error(`Invalid Sentry Dsn: Invalid port ${port}`);
+        return false;
+      }
 
-  const requiredComponents: ReadonlyArray<keyof DsnComponents> = ['protocol', 'publicKey', 'host', 'projectId'];
-  const hasMissingRequiredComponent = requiredComponents.find(component => {
-    if (!dsn[component]) {
-      debug.error(`Invalid Sentry Dsn: ${component} missing`);
       return true;
     }
-    return false;
-  });
-
-  if (hasMissingRequiredComponent) {
-    return false;
-  }
-
-  if (!projectId.match(/^\d+$/)) {
-    debug.error(`Invalid Sentry Dsn: Invalid projectId ${projectId}`);
-    return false;
-  }
-
-  if (!isValidProtocol(protocol)) {
-    debug.error(`Invalid Sentry Dsn: Invalid protocol ${protocol}`);
-    return false;
-  }
-
-  if (port && isNaN(parseInt(port, 10))) {
-    debug.error(`Invalid Sentry Dsn: Invalid port ${port}`);
-    return false;
-  }
-
-  return true;
-}
+  : () => true;
 
 /**
  * Extract the org ID from a DSN host.

diff --git a/packages/core/test/lib/tracing/logSpans.test.ts b/packages/core/test/lib/tracing/logSpans.test.ts
@@ -0,0 +1,48 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { SentrySpan } from '../../../src/tracing/sentrySpan';
+
+// must be var, because vi.mock hoists
+var debugBuild: boolean = true;
+vi.mock(import('../../../src/debug-build'), () => ({
+  get DEBUG_BUILD() {
+    return debugBuild ?? true;
+  },
+}));
+
+describe('logSpans', () => {
+  afterEach(() => {
+    debugBuild = true;
+    vi.restoreAllMocks();
+    vi.resetModules();
+  });
+
+  it('logs span start and end in debug builds', async () => {
+    const { logSpanEnd, logSpanStart } = await import('../../../src/tracing/logSpans');
+    const { debug } = await import('../../../src/utils/debug-logger');
+    const logSpy = vi.spyOn(debug, 'log').mockImplementation(() => {});
+    const span = new SentrySpan({ name: 'test span', op: 'test.op', sampled: true });
+
+    logSpanStart(span);
+    logSpanEnd(span);
+
+    expect(logSpy).toHaveBeenCalledWith(expect.stringContaining('[Tracing] Starting sampled root span'));
+    expect(logSpy).toHaveBeenCalledWith(
+      expect.stringContaining(
+        `[Tracing] Finishing "test.op" root span "test span" with ID ${span.spanContext().spanId}`,
+      ),
+    );
+  });
+
+  it('is a no-op in non-debug builds', async () => {
+    debugBuild = false;
+    const { logSpanEnd, logSpanStart } = await import('../../../src/tracing/logSpans');
+    const { debug } = await import('../../../src/utils/debug-logger');
+    const logSpy = vi.spyOn(debug, 'log').mockImplementation(() => {});
+    const span = new SentrySpan({ name: 'test span', op: 'test.op', sampled: true });
+
+    logSpanStart(span);
+    logSpanEnd(span);
+
+    expect(logSpy).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/core/test/lib/utils/dsn.test.ts b/packages/core/test/lib/utils/dsn.test.ts
@@ -3,11 +3,12 @@ import { debug } from '../../../src/utils/debug-logger';
 import { dsnToString, extractOrgIdFromClient, extractOrgIdFromDsnHost, makeDsn } from '../../../src/utils/dsn';
 import { getDefaultTestClientOptions, TestClient } from '../../mocks/client';
 
-let mockDebugBuild = true;
+// must be var, because vi.mock hoists
+var mockDebugBuild: boolean = true;
 
 vi.mock('../../../src/debug-build', () => ({
   get DEBUG_BUILD() {
-    return mockDebugBuild;
+    return mockDebugBuild ?? true;
   },
 }));
 
@@ -204,8 +205,10 @@ describe('Dsn', () => {
       expect(dsn?.projectId).toBe('123');
     });
 
-    test('skips validation for non-debug builds', () => {
+    test('skips validation for non-debug builds', async () => {
+      vi.resetModules();
       mockDebugBuild = false;
+      const { makeDsn } = await import('../../../src/utils/dsn');
       const dsn = makeDsn('httx://abc@192.168.1.1/123');
       expect(dsn?.protocol).toBe('httx');
       expect(dsn?.publicKey).toBe('abc');