diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 1d1c9b029..cb5107ab1 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -1,6 +1,9 @@
 # Changelog
 
 ## [Unreleased]
+### Added
+
+- Added Buffer.toBase64() polyfill for Bun compatibility to enable base64 encoding of buffers
 
 ## [12.18.0] - 2026-02-21
 
diff --git a/packages/coding-agent/src/utils/image-resize.ts b/packages/coding-agent/src/utils/image-resize.ts
index 748026824..123d0692a 100644
--- a/packages/coding-agent/src/utils/image-resize.ts
+++ b/packages/coding-agent/src/utils/image-resize.ts
@@ -37,6 +37,11 @@ function pickSmaller(
 	return a.buffer.length <= b.buffer.length ? a : b;
 }
 
+/** Polyfill for Buffer.toBase64, technically since it derives from Uint8Array it should exist but Bun reasons... */
+Buffer.prototype.toBase64 = function (this: Buffer) {
+	return new Uint8Array(this.buffer, this.byteOffset, this.byteLength).toBase64();
+};
+
 /**
  * Resize an image to fit within the specified max dimensions and file size.
  * Returns the original image if it already fits within the limits.
diff --git a/packages/coding-agent/test/image-b64poly.test.ts b/packages/coding-agent/test/image-b64poly.test.ts
new file mode 100644
index 000000000..00d975fec
--- /dev/null
+++ b/packages/coding-agent/test/image-b64poly.test.ts
@@ -0,0 +1,14 @@
+import { describe, expect, it } from "bun:test";
+
+describe("Buffer.toBase64", async () => {
+	await import("../src/utils/image-resize");
+
+	it("should be defined", () => {
+		expect(Buffer.prototype.toBase64).toBeDefined();
+	});
+
+	it("should return a base64 string", () => {
+		const buffer = Buffer.from("Hello, world!");
+		expect(buffer.toBase64()).toBe("SGVsbG8sIHdvcmxkIQ==");
+	});
+});