fix: prevent stack overflow in toBase64 for large inputs

src/internal/utils/base64.ts

@@ -15,7 +15,14 @@ export const toBase64 = (data: string | Uint8Array | null | undefined): string =
   }
 
   if (typeof btoa !== 'undefined') {
-    return btoa(String.fromCharCode.apply(null, data as any));
+    // Process in chunks to avoid stack overflow with large inputs.
+    // String.fromCharCode.apply has a max argument count (~65536).
+    let binary = '';
+    const chunkSize = 8192;
+    for (let i = 0; i < data.length; i += chunkSize) {
+      binary += String.fromCharCode(...data.subarray(i, Math.min(i + chunkSize, data.length)));
+    }
+    return btoa(binary);
   }
 
   throw new AnthropicError('Cannot generate base64 string; Expected `Buffer` or `btoa` to be defined');

tests/base64.test.ts

@@ -50,6 +50,24 @@ describe.each(['Buffer', 'atob'])('with %s', (mode) => {
     });
   });
 
+  test('toBase64 handles large input without stack overflow', () => {
+    // 200KB of data - would cause "Maximum call stack size exceeded"
+    // with the old String.fromCharCode.apply/spread approach
+    const size = 200 * 1024;
+    const largeData = new Uint8Array(size);
+    for (let i = 0; i < size; i++) {
+      largeData[i] = i % 256;
+    }
+
+    const encoded = toBase64(largeData);
+    expect(typeof encoded).toBe('string');
+    expect(encoded.length).toBeGreaterThan(0);
+
+    // Round-trip: decode and verify it matches
+    const decoded = fromBase64(encoded);
+    expect(decoded).toEqual(largeData);
+  });
+
   test('fromBase64', () => {
     const testCases = [
       {