diff --git a/sdk/typescript/src/__tests__/base64.test.ts b/sdk/typescript/src/__tests__/base64.test.ts
new file mode 100644
index 00000000..dddbfed5
--- /dev/null
+++ b/sdk/typescript/src/__tests__/base64.test.ts
@@ -0,0 +1,80 @@
+import { afterEach, describe, expect, it } from "bun:test";
+import { decodeBase64ToUtf8, encodeUtf8ToBase64 } from "../base64";
+
+type Base64Global = "atob" | "btoa";
+
+const originalAtobDescriptor = Object.getOwnPropertyDescriptor(globalThis, "atob");
+const originalBtoaDescriptor = Object.getOwnPropertyDescriptor(globalThis, "btoa");
+
+function setGlobalCapability(name: Base64Global, value: typeof globalThis.atob | undefined) {
+  Object.defineProperty(globalThis, name, {
+    configurable: true,
+    value,
+    writable: true,
+  });
+}
+
+function restoreGlobalCapability(name: Base64Global, descriptor: PropertyDescriptor | undefined) {
+  if (descriptor) {
+    Object.defineProperty(globalThis, name, descriptor);
+    return;
+  }
+
+  Reflect.deleteProperty(globalThis, name);
+}
+
+afterEach(() => {
+  restoreGlobalCapability("atob", originalAtobDescriptor);
+  restoreGlobalCapability("btoa", originalBtoaDescriptor);
+});
+
+describe("base64 UTF-8 helpers", () => {
+  it("encodes known ASCII fixtures without Node Buffer", () => {
+    expect(encodeUtf8ToBase64("hello")).toBe("aGVsbG8=");
+    expect(encodeUtf8ToBase64("MicroAI Paygate")).toBe("TWljcm9BSSBQYXlnYXRl");
+  });
+
+  it("round-trips ASCII, accented, non-Latin, and emoji text", () => {
+    const cases = [
+      "plain ASCII receipt",
+      "Cr\u00e8me br\u00fbl\u00e9e",
+      "\u0928\u092e\u0938\u094d\u0924\u0947 \u092d\u0941\u0917\u0924\u093e\u0928",
+      "paid receipt \u{1f680}\u2705",
+    ];
+
+    for (const value of cases) {
+      expect(decodeBase64ToUtf8(encodeUtf8ToBase64(value))).toBe(value);
+    }
+  });
+
+  it("decodes valid UTF-8 base64 fixtures", () => {
+    expect(decodeBase64ToUtf8("SGVsbG8sIHdvcmxkIQ==")).toBe("Hello, world!");
+    expect(decodeBase64ToUtf8("4pyF")).toBe("\u2705");
+  });
+
+  it("encodes empty text to an empty base64 payload", () => {
+    expect(encodeUtf8ToBase64("")).toBe("");
+  });
+
+  it("rejects empty, whitespace-padded, invalid-character, and invalid-padding input", () => {
+    for (const value of ["", " aGVsbG8=", "aGVsbG8= ", "abc@", "ab=", "abc==", "a==="]) {
+      expect(() => decodeBase64ToUtf8(value)).toThrow("invalid base64");
+    }
+  });
+
+  it("reports unavailable decoding capability and restores globals", () => {
+    setGlobalCapability("atob", undefined);
+
+    expect(() => decodeBase64ToUtf8("aGVsbG8=")).toThrow(
+      "base64 decoding is unavailable in this runtime",
+    );
+  });
+
+  it("reports unavailable encoding capability and restores globals", () => {
+    setGlobalCapability("btoa", undefined);
+
+    expect(() => encodeUtf8ToBase64("hello")).toThrow(
+      "base64 encoding is unavailable in this runtime",
+    );
+  });
+});