diff --git a/src/__tests__/extension-polyfill.spec.ts b/src/__tests__/extension-polyfill.spec.ts
new file mode 100644
index 00000000000..76e5be9dc33
--- /dev/null
+++ b/src/__tests__/extension-polyfill.spec.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect, vi } from "vitest"
+
+describe("getDefaultHighWaterMark polyfill", () => {
+	it("should add getDefaultHighWaterMark if it doesn't exist", () => {
+		// Mock the stream module without getDefaultHighWaterMark
+		const mockStream: any = {}
+
+		// Execute the polyfill code
+		if (!mockStream.getDefaultHighWaterMark) {
+			mockStream.getDefaultHighWaterMark = function (objectMode: boolean): number {
+				return objectMode ? 16 : 16 * 1024
+			}
+		}
+
+		// Verify the function was added
+		expect(mockStream.getDefaultHighWaterMark).toBeDefined()
+		expect(typeof mockStream.getDefaultHighWaterMark).toBe("function")
+	})
+
+	it("should return correct values for objectMode", () => {
+		const mockStream: any = {}
+		mockStream.getDefaultHighWaterMark = function (objectMode: boolean): number {
+			return objectMode ? 16 : 16 * 1024
+		}
+
+		// Test objectMode = true
+		expect(mockStream.getDefaultHighWaterMark(true)).toBe(16)
+
+		// Test objectMode = false
+		expect(mockStream.getDefaultHighWaterMark(false)).toBe(16 * 1024)
+	})
+
+	it("should not override existing getDefaultHighWaterMark", () => {
+		// Mock stream with existing getDefaultHighWaterMark
+		const originalFunction = vi.fn((objectMode: boolean) => (objectMode ? 32 : 32 * 1024))
+		const mockStream: any = {
+			getDefaultHighWaterMark: originalFunction,
+		}
+
+		// Execute polyfill check
+		if (!mockStream.getDefaultHighWaterMark) {
+			mockStream.getDefaultHighWaterMark = function (objectMode: boolean): number {
+				return objectMode ? 16 : 16 * 1024
+			}
+		}
+
+		// Verify original function is preserved
+		expect(mockStream.getDefaultHighWaterMark).toBe(originalFunction)
+		expect(mockStream.getDefaultHighWaterMark(true)).toBe(32)
+		expect(mockStream.getDefaultHighWaterMark(false)).toBe(32 * 1024)
+	})
+})
diff --git a/src/extension.ts b/src/extension.ts
index bd43bcbf8a8..0250b58cfb1 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -1,3 +1,16 @@
+// Polyfill for getDefaultHighWaterMark to support older VS Code versions
+// VS Code 1.85.x uses Node.js 16.14.2 which doesn't have getDefaultHighWaterMark
+// (introduced in Node.js 16.7.0). This polyfill ensures compatibility.
+// This must be added before any imports that might use the stream module.
+// See: https://github.com/RooCodeInc/Roo-Code/issues/6240
+if (!require("stream").getDefaultHighWaterMark) {
+	const stream = require("stream")
+	stream.getDefaultHighWaterMark = function (objectMode: boolean): number {
+		// Default values from Node.js source
+		return objectMode ? 16 : 16 * 1024
+	}
+}
+
 import * as vscode from "vscode"
 import * as dotenvx from "@dotenvx/dotenvx"
 import * as path from "path"