From a876f56cc925e1b4b8e6566a235465ceec191b47 Mon Sep 17 00:00:00 2001
From: Rup Sarmah <sarmah.rup@gmail.com>
Date: Tue, 21 Jul 2026 14:35:18 +0530
Subject: [PATCH] fix(node): count event bytes correctly so the size limits are
 enforced

---
 .changeset/fix-node-event-size-guard.md       |  5 ++++
 .../segmentio/__tests__/context-batch.test.ts | 29 +++++++++++++++++++
 .../src/plugins/segmentio/context-batch.ts    |  2 +-
 3 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fix-node-event-size-guard.md
 create mode 100644 packages/node/src/plugins/segmentio/__tests__/context-batch.test.ts

diff --git a/.changeset/fix-node-event-size-guard.md b/.changeset/fix-node-event-size-guard.md
new file mode 100644
index 000000000..b6716ec55
--- /dev/null
+++ b/.changeset/fix-node-event-size-guard.md
@@ -0,0 +1,5 @@
+---
+'@segment/analytics-node': patch
+---
+
+Fix the per-event and batch size limits, which were not enforced. `ContextBatch.calculateSize` used a broken byte-count regex (`split(/%..|i/)`) that counted only `%XX` escapes plus the letter "i" instead of UTF-8 bytes, so a ~39 KB event measured as ~21 bytes and passed the 32 KB per-event guard. Restored the correct byte count so oversized events are rejected before they are sent.
diff --git a/packages/node/src/plugins/segmentio/__tests__/context-batch.test.ts b/packages/node/src/plugins/segmentio/__tests__/context-batch.test.ts
new file mode 100644
index 000000000..c79596b19
--- /dev/null
+++ b/packages/node/src/plugins/segmentio/__tests__/context-batch.test.ts
@@ -0,0 +1,29 @@
+import { ContextBatch } from '../context-batch'
+import { NodeEventFactory } from '../../../app/event-factory'
+import { Context } from '../../../app/context'
+
+const eventFactory = new NodeEventFactory()
+const pending = (context: Context) => ({ resolver: () => undefined, context })
+
+describe('ContextBatch', () => {
+  it('rejects an event that exceeds the 32 KB per-event size limit', () => {
+    // ~40 KB of JSON, well over the 32 KB per-event cap
+    const event = eventFactory.track(
+      'big',
+      { data: 'a'.repeat(40000) },
+      { userId: 'u' }
+    )
+    const batch = new ContextBatch(100)
+    const result = batch.tryAdd(pending(new Context(event)))
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.message).toContain('exceeds maximum event size')
+    }
+  })
+
+  it('accepts a normal small event', () => {
+    const event = eventFactory.track('small', { ok: true }, { userId: 'u' })
+    const batch = new ContextBatch(100)
+    expect(batch.tryAdd(pending(new Context(event))).success).toBe(true)
+  })
+})
diff --git a/packages/node/src/plugins/segmentio/context-batch.ts b/packages/node/src/plugins/segmentio/context-batch.ts
index 6b1c7b702..10d1faa5b 100644
--- a/packages/node/src/plugins/segmentio/context-batch.ts
+++ b/packages/node/src/plugins/segmentio/context-batch.ts
@@ -53,7 +53,7 @@ export class ContextBatch {
   }
 
   private calculateSize(ctx: Context): number {
-    return encodeURI(JSON.stringify(ctx.event)).split(/%..|i/).length
+    return encodeURI(JSON.stringify(ctx.event)).split(/%..|./).length - 1
   }
 
   getEvents(): SegmentEvent[] {