All Skills > Feature Setup > Span Streaming (JavaScript)

Sentry Span Streaming Migration (JavaScript)

Migrate from the default transaction-based trace lifecycle (static) to span streaming (stream), where spans are sent individually as they complete instead of being batched into a transaction at the end.

This skill covers the JavaScript SDK (Browser, Node.js, Bun, Deno, Cloudflare). For Python, see Span Streaming (Python).

Invoke This Skill When

• User asks to "enable span streaming" or "migrate to span streaming" in a JavaScript project
• User wants to switch from transaction-based to streamed span delivery
• User mentions traceLifecycle, spanStreamingIntegration, or withStreamedSpan
• User wants lower latency span delivery or per-span processing

---

Phase 1: Detect

1.1 Detect Environment

# Detect if browser, server, or both
grep -rn "from '@sentry/browser'\|from '@sentry/react'\|from '@sentry/vue'\|from '@sentry/angular'\|from '@sentry/svelte'\|from '@sentry/nextjs'\|from '@sentry/nuxt'\|from '@sentry/sveltekit'\|from '@sentry/remix'\|from '@sentry/solidstart'\|from '@sentry/astro'\|from '@sentry/react-router'" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null | head -20

grep -rn "from '@sentry/node'\|from '@sentry/bun'\|from '@sentry/deno'\|from '@sentry/cloudflare'" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null | head -20

1.2. Find Existing Sentry Config

# Find Sentry.init calls
grep -rn "Sentry\.init\|init({" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null | head -20

# Find beforeSendSpan usage
grep -rn "beforeSendSpan" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null

# Find beforeSendTransaction usage
grep -rn "beforeSendTransaction" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null

# Find ignoreSpans usage
grep -rn "ignoreSpans" --include="*.ts" --include="*.js" --include="*.tsx" --include="*.jsx" --include="*.mjs" -l 2>/dev/null

1.3 Classify Environment

Based on detection results, classify each Sentry.init call as:

---

Phase 2: Migrate

Prerequisites: Sentry JavaScript SDK >=10.53.1 with tracing enabled (tracesSampleRate or tracesSampler configured).

Apply changes to each Sentry.init call. Work through each file identified above.

2.1 Enable Span Streaming

Server-Side SDKs

Add traceLifecycle: 'stream' to Sentry.init():

// Before
Sentry.init({
  dsn: '...',
  tracesSampleRate: 1.0,
});

// After
Sentry.init({
  dsn: '...',
  tracesSampleRate: 1.0,
  traceLifecycle: 'stream',
});

Browser-Side SDKs

Add Sentry.spanStreamingIntegration() to the integrations array. The integration automatically enables traceLifecycle: 'stream' — you do not need to set it manually.

// Before
Sentry.init({
  dsn: '...',
  integrations: [
    Sentry.browserTracingIntegration(),
  ],
  tracesSampleRate: 1.0,
});

// After
Sentry.init({
  dsn: '...',
  integrations: [
    Sentry.spanStreamingIntegration(),
    Sentry.browserTracingIntegration(),
  ],
  tracesSampleRate: 1.0,
});

The order of spanStreamingIntegration() relative to other integrations does not matter.

Framework SDKs (Client + Server)

Apply the browser migration to client config files and the server migration to server config files. Common patterns:

2.2 Migrate beforeSendSpan

If the user has a beforeSendSpan callback, it must be wrapped with Sentry.withStreamedSpan() to work in streaming mode. Without this wrapper, the SDK falls back to static mode.

The callback shape also changes:

• description is now name
• data is now attributes
• The span object is StreamedSpanJSON instead of SpanJSON

// Before (static mode)
Sentry.init({
  beforeSendSpan: (span) => {
    if (span.description?.includes('/health')) {
      span.description = '[filtered]';
    }
    // 'data' contains span attributes
    delete span.data?.['http.request.body'];
    return span;
  },
});

// After (streaming mode)
Sentry.init({
  beforeSendSpan: Sentry.withStreamedSpan((span) => {
    if (span.name?.includes('/health')) {
      span.name = '[filtered]';
    }
    // 'attributes' replaces 'data'
    if (span.attributes) {
      delete span.attributes['http.request.body'];
    }
    return span;
  }),
});

Key differences in the callback:

Returning null from beforeSendSpan does not drop the span — it is ignored and a warning is logged.

2.3 Remove or Replace beforeSendTransaction

beforeSendTransaction has no effect in streaming mode. Spans are sent individually, not batched into transactions.

// Before
Sentry.init({
  beforeSendTransaction: (event) => {
    // This entire callback is ignored in streaming mode
    if (event.transaction === '/health') {
      return null;
    }
    return event;
  },
});

Migration paths depending on what beforeSendTransaction was used for:

Remove the beforeSendTransaction option from Sentry.init() after migrating its logic.

2.4 Configure ignoreSpans (Optional)

ignoreSpans works in both static and streaming modes, but the filter is evaluated at different points in the span lifecycle:

• Streaming mode: evaluated when the span starts. Only data available at span start — the span name and the attributes set at creation — is taken into account.
• Static mode: evaluated when the root span ends. Only data available at that point — the span name and attributes — is taken into account.

In both modes, a match prevents the span from being recorded or sent. Because matching can run as early as span start (streaming), only the span name and attributes set when the span begins are guaranteed to be available — do not rely on attributes added later in the span's lifetime.

Sentry.init({
  traceLifecycle: 'stream',
  ignoreSpans: [
    // String match against span name
    '/health',
    '/ready',

    // RegExp match against span name
    /^OPTIONS /,

    // Object filter — all conditions must match
    {
      op: 'middleware.handle',
      name: /^corsMiddleware/,
    },

    // Filter by attributes (string = substring match, RegExp for patterns)
    {
      op: 'http.server',
      attributes: {
        'http.route': /^\/internal\//,
      },
    },
  ],
});

Filter object properties:

When multiple properties are specified in a filter object, all must match for the span to be ignored.

2.5 Set Up Browser Profiling (Optional)

When using span streaming in the browser, use the v2 profiling options — not the legacy profilesSampleRate. The legacy option is deprecated and does not integrate with the span streaming lifecycle.

Add browserProfilingIntegration() and configure the two v2 options:

// Before (legacy profiling — do NOT use with span streaming)
Sentry.init({
  integrations: [
    Sentry.browserTracingIntegration(),
  ],
  tracesSampleRate: 1.0,
  profilesSampleRate: 0.5, // deprecated
});

// After (v2 profiling with span streaming)
Sentry.init({
  integrations: [
    Sentry.spanStreamingIntegration(),
    Sentry.browserTracingIntegration(),
    Sentry.browserProfilingIntegration(),
  ],
  tracesSampleRate: 1.0,
  profileSessionSampleRate: 1.0,
  profileLifecycle: 'trace',
});

v2 profiling options:

profileLifecycle: 'trace' requires tracing to be enabled (tracesSampleRate or tracesSampler). The profiler starts when a root span begins and stops when no sampled root spans remain. Profile chunks are sent independently every 60 seconds or when the last root span ends.

Do not mix legacy and v2 options. If profilesSampleRate is set, profileSessionSampleRate has no effect and the SDK logs a warning.

---

Phase 3: Verify

After applying changes, verify the migration works correctly.

3.1 Build Check

# TypeScript check
npx tsc --noEmit 2>&1 | head -30

# Build
npm run build 2>&1 | tail -20

3.2 Runtime Verification

Instruct the user to verify in their browser devtools or server logs:

1. Check network tab: Span envelopes should appear as individual requests with content type application/vnd.sentry.items.span.v2+json rather than transaction envelopes
2. Check Sentry dashboard: Spans should appear in the Traces view shortly after they complete, without waiting for the full transaction to finish
3. Check for fallback warnings: If the SDK logs warnings about falling back to static mode, the beforeSendSpan callback is likely missing the withStreamedSpan wrapper

3.3 Common Issues

---

Quick Reference

Minimal Server Setup

import * as Sentry from '@sentry/node';

Sentry.init({
  dsn: '__DSN__',
  tracesSampleRate: 1.0,
  traceLifecycle: 'stream',
});

Minimal Browser Setup

import * as Sentry from '@sentry/browser';

Sentry.init({
  dsn: '__DSN__',
  integrations: [
    Sentry.spanStreamingIntegration(),
    Sentry.browserTracingIntegration(),
  ],
  tracesSampleRate: 1.0,
});

Browser Setup with Profiling

import * as Sentry from '@sentry/browser';

Sentry.init({
  dsn: '__DSN__',
  integrations: [
    Sentry.spanStreamingIntegration(),
    Sentry.browserTracingIntegration(),
    Sentry.browserProfilingIntegration(),
  ],
  tracesSampleRate: 1.0,
  profileSessionSampleRate: 1.0,
  profileLifecycle: 'trace',
});

Full Migration Checklist

• [ ] SDK version is >=10.53.1
• [ ] Server configs: added traceLifecycle: 'stream'
• [ ] Browser configs: added spanStreamingIntegration()
• [ ] beforeSendSpan callbacks wrapped with Sentry.withStreamedSpan()
• [ ] beforeSendSpan callbacks updated: description -> name, data -> attributes
• [ ] beforeSendTransaction logic migrated to beforeSendSpan or ignoreSpans
• [ ] beforeSendTransaction removed from config
• [ ] (If profiling) Replaced profilesSampleRate with profileSessionSampleRate + profileLifecycle
• [ ] (If profiling) Added browserProfilingIntegration() to integrations
• [ ] Build passes with no type errors
• [ ] Spans visible in Sentry dashboard