diff --git a/libs/sdk-js/src/tests/sse.test.ts b/libs/sdk-js/src/tests/sse.test.ts
index 51ebbbd0c..b4f7ca7d8 100644
--- a/libs/sdk-js/src/tests/sse.test.ts
+++ b/libs/sdk-js/src/tests/sse.test.ts
@@ -2,26 +2,21 @@ import { Readable } from "node:stream";
 import { IterableReadableStream } from "../utils/stream.js";
 import { BytesLineDecoder, SSEDecoder } from "../utils/sse.js";
 
+const gather = async <T>(stream: ReadableStream<T>): Promise<T[]> => {
+  const results: T[] = [];
+  const iterator = IterableReadableStream.fromReadableStream(stream);
+  for await (const chunk of iterator) results.push(chunk);
+  return results;
+};
+
+const textEncoder = new TextEncoder();
+const textDecoder = new TextDecoder();
+
 describe("BytesLineDecoder", () => {
   const createStream = (chunks: Uint8Array[]) => {
     return Readable.toWeb(Readable.from(chunks)) as ReadableStream<Uint8Array>;
   };
 
-  const gather = async (
-    stream: ReadableStream<Uint8Array>,
-  ): Promise<Uint8Array[]> => {
-    const results: Uint8Array[] = [];
-    for await (const chunk of IterableReadableStream.fromReadableStream(
-      stream,
-    )) {
-      results.push(chunk);
-    }
-    return results;
-  };
-
-  const textEncoder = new TextEncoder();
-  const textDecoder = new TextDecoder();
-
   test("handles single line with newline", async () => {
     const input = createStream([textEncoder.encode("hello\n")]);
     const decoded = input.pipeThrough(new BytesLineDecoder());
@@ -78,25 +73,22 @@ describe("BytesLineDecoder", () => {
     expect(textDecoder.decode(results[0])).toBe("line1");
     expect(textDecoder.decode(results[1])).toBe("line2");
   });
+
+  test("handles stale line", async () => {
+    const input = createStream([textEncoder.encode("hello")]);
+    const decoded = input.pipeThrough(new BytesLineDecoder());
+    const results = await gather(decoded);
+
+    expect(results.length).toBe(1);
+    expect(textDecoder.decode(results[0])).toBe("hello");
+  });
 });
 
 describe("SSEDecoder", () => {
   const createStream = (lines: string[]) => {
-    const encoder = new TextEncoder();
-    const chunks = lines.map((line) => encoder.encode(line));
-    return Readable.toWeb(Readable.from(chunks)) as ReadableStream<Uint8Array>;
-  };
-
-  const collectResults = async (
-    stream: ReadableStream<any>,
-  ): Promise<any[]> => {
-    const results: any[] = [];
-    for await (const chunk of IterableReadableStream.fromReadableStream(
-      stream,
-    )) {
-      results.push(chunk);
-    }
-    return results;
+    return Readable.toWeb(
+      Readable.from(lines.map((line) => textEncoder.encode(line))),
+    ) as ReadableStream<Uint8Array>;
   };
 
   test("decodes simple event", async () => {
@@ -109,7 +101,7 @@ describe("SSEDecoder", () => {
       .pipeThrough(new BytesLineDecoder())
       .pipeThrough(new SSEDecoder());
 
-    const results = await collectResults(decoded);
+    const results = await gather(decoded);
     expect(results.length).toBe(1);
     expect(results[0]).toEqual({
       event: "test",
@@ -127,7 +119,7 @@ describe("SSEDecoder", () => {
       .pipeThrough(new BytesLineDecoder())
       .pipeThrough(new SSEDecoder());
 
-    const results = await collectResults(decoded);
+    const results = await gather(decoded);
     expect(results.length).toBe(1);
     expect(results[0]).toEqual({
       event: "test",
@@ -148,7 +140,7 @@ describe("SSEDecoder", () => {
       .pipeThrough(new BytesLineDecoder())
       .pipeThrough(new SSEDecoder());
 
-    const results = await collectResults(decoded);
+    const results = await gather(decoded);
     expect(results.length).toBe(2);
     expect(results[0]).toEqual({
       event: "test1",
@@ -166,7 +158,7 @@ describe("SSEDecoder", () => {
       .pipeThrough(new BytesLineDecoder())
       .pipeThrough(new SSEDecoder());
 
-    const results = await collectResults(decoded);
+    const results = await gather(decoded);
     expect(results.length).toBe(1);
     expect(results[0]).toEqual({
       event: "test",
@@ -180,7 +172,7 @@ describe("SSEDecoder", () => {
       .pipeThrough(new BytesLineDecoder())
       .pipeThrough(new SSEDecoder());
 
-    const results = await collectResults(decoded);
+    const results = await gather(decoded);
     expect(results.length).toBe(1);
     expect(results[0]).toEqual({
       event: "end",
diff --git a/libs/sdk-js/src/utils/sse.ts b/libs/sdk-js/src/utils/sse.ts
index fb1ff0f59..c3c525113 100644
--- a/libs/sdk-js/src/utils/sse.ts
+++ b/libs/sdk-js/src/utils/sse.ts
@@ -1,10 +1,3 @@
-const mergeArrays = (a: ArrayLike<number>, b: ArrayLike<number>) => {
-  const mergedArray = new Uint8Array(a.length + b.length);
-  mergedArray.set(a);
-  mergedArray.set(b, a.length);
-  return mergedArray;
-};
-
 const CR = "\r".charCodeAt(0);
 const LF = "\n".charCodeAt(0);
 const NULL = "\0".charCodeAt(0);
@@ -15,12 +8,12 @@ const TRAILING_NEWLINE = [CR, LF];
 
 export class BytesLineDecoder extends TransformStream<Uint8Array, Uint8Array> {
   constructor() {
-    let buffer = new Uint8Array();
+    let buffer: Uint8Array[] = [];
     let trailingCr = false;
 
     super({
       start() {
-        buffer = new Uint8Array();
+        buffer = [];
         trailingCr = false;
       },
 
@@ -30,7 +23,7 @@ export class BytesLineDecoder extends TransformStream<Uint8Array, Uint8Array> {
 
         // Handle trailing CR from previous chunk
         if (trailingCr) {
-          text = mergeArrays([CR], text);
+          text = joinArrays([[CR], text]);
           trailingCr = false;
         }
 
@@ -43,44 +36,45 @@ export class BytesLineDecoder extends TransformStream<Uint8Array, Uint8Array> {
         if (!text.length) return;
         const trailingNewline = TRAILING_NEWLINE.includes(text.at(-1)!);
 
-        // Pre-allocate lines array with estimated capacity
-        let lines: Uint8Array[] = [];
+        const lastIdx = text.length - 1;
+        const { lines } = text.reduce<{ lines: Uint8Array[]; from: number }>(
+          (acc, cur, idx) => {
+            if (acc.from > idx) return acc;
 
-        for (let offset = 0; offset < text.byteLength; ) {
-          let idx = text.indexOf(CR, offset);
-          if (idx === -1) idx = text.indexOf(LF, offset);
-          if (idx === -1) {
-            lines.push(text.subarray(offset));
-            break;
-          }
+            if (cur === CR || cur === LF) {
+              acc.lines.push(text.subarray(acc.from, idx));
+              if (cur === CR && text[idx + 1] === LF) {
+                acc.from = idx + 2;
+              } else {
+                acc.from = idx + 1;
+              }
+            }
 
-          lines.push(text.subarray(offset, idx));
-          if (text[idx] === CR && text[idx + 1] === LF) {
-            offset = idx + 2;
-          } else {
-            offset = idx + 1;
-          }
-        }
+            if (idx === lastIdx && acc.from <= lastIdx) {
+              acc.lines.push(text.subarray(acc.from));
+            }
+
+            return acc;
+          },
+          { lines: [], from: 0 },
+        );
 
         if (lines.length === 1 && !trailingNewline) {
-          buffer = mergeArrays(buffer, lines[0]);
+          buffer.push(lines[0]);
           return;
         }
 
         if (buffer.length) {
           // Include existing buffer in first line
-          buffer = mergeArrays(buffer, lines[0]);
-
-          lines = lines.slice(1);
-          lines.unshift(buffer);
-
-          buffer = new Uint8Array();
+          buffer.push(lines[0]);
+          lines[0] = joinArrays(buffer);
+          buffer = [];
         }
 
         if (!trailingNewline) {
           // If the last segment is not newline terminated,
           // buffer it for the next chunk
-          if (lines.length) buffer = lines.pop()!;
+          if (lines.length) buffer = [lines.pop()!];
         }
 
         // Enqueue complete lines
@@ -91,7 +85,7 @@ export class BytesLineDecoder extends TransformStream<Uint8Array, Uint8Array> {
 
       flush(controller) {
         if (buffer.length) {
-          controller.enqueue(buffer);
+          controller.enqueue(joinArrays(buffer));
         }
       },
     });
@@ -106,7 +100,7 @@ interface StreamPart {
 export class SSEDecoder extends TransformStream<Uint8Array, StreamPart> {
   constructor() {
     let event = "";
-    let data: Uint8Array = new Uint8Array();
+    let data: Uint8Array[] = [];
     let lastEventId = "";
     let retry: number | null = null;
 
@@ -120,12 +114,12 @@ export class SSEDecoder extends TransformStream<Uint8Array, StreamPart> {
 
           const sse = {
             event,
-            data: data.length ? JSON.parse(decoder.decode(data)) : null,
+            data: data.length ? decodeArraysToJson(decoder, data) : null,
           };
 
           // NOTE: as per the SSE spec, do not reset lastEventId
           event = "";
-          data = new Uint8Array();
+          data = [];
           retry = null;
 
           controller.enqueue(sse);
@@ -145,7 +139,7 @@ export class SSEDecoder extends TransformStream<Uint8Array, StreamPart> {
         if (fieldName === "event") {
           event = decoder.decode(value);
         } else if (fieldName === "data") {
-          data = mergeArrays(data, value);
+          data.push(value);
         } else if (fieldName === "id") {
           if (value.indexOf(NULL) === -1) lastEventId = decoder.decode(value);
         } else if (fieldName === "retry") {
@@ -158,10 +152,25 @@ export class SSEDecoder extends TransformStream<Uint8Array, StreamPart> {
         if (event) {
           controller.enqueue({
             event,
-            data: data.length ? JSON.parse(decoder.decode(data)) : null,
+            data: data.length ? decodeArraysToJson(decoder, data) : null,
           });
         }
       },
     });
   }
 }
+
+function joinArrays(data: ArrayLike<number>[]) {
+  const totalLength = data.reduce((acc, curr) => acc + curr.length, 0);
+  let merged = new Uint8Array(totalLength);
+  let offset = 0;
+  for (const c of data) {
+    merged.set(c, offset);
+    offset += c.length;
+  }
+  return merged;
+}
+
+function decodeArraysToJson(decoder: TextDecoder, data: ArrayLike<number>[]) {
+  return JSON.parse(decoder.decode(joinArrays(data)));
+}