readResponseWithSizeLimit can replace streamed size-limit errors with cancellation errors

[teamleaderleo]

Description

readResponseWithSizeLimit() can correctly detect that a streamed response exceeded maxBytes, then replace that DownloadError if reader cleanup fails.

The streamed path currently ends with:

finally {
  try {
    await reader.cancel();
  } finally {
    reader.releaseLock();
  }
}

The adjacent cancelResponseBody() helper already ignores cancellation failures to preserve the original rejection, including on the Content-Length size-limit path.

Reproduction

Against current main at ff0f708e917e3f556f889ed9158b396a17f3828c, a native ReadableStream can reproduce the issue:

const cancelError = new Error('cancel failed');

const body = new ReadableStream<Uint8Array>({
  start(controller) {
    controller.enqueue(new Uint8Array([1, 2]));
  },
  cancel() {
    return Promise.reject(cancelError);
  },
});

const response = {
  headers: new Headers(),
  body,
} as Response;

await readResponseWithSizeLimit({
  response,
  url: 'https://example.com/file',
  maxBytes: 1,
});

The two-byte chunk exceeds the one-byte limit, so the operation should fail with a DownloadError.

Current result:

Error: cancel failed

Expected result:

DownloadError: Download of https://example.com/file exceeded maximum size of 1 bytes.

Expected behavior

Cancellation should still be attempted and the reader lock released, but a cancellation failure shouldn't replace the DownloadError that triggered cleanup.

AI SDK Version

ai: 7.0.56

Code of Conduct

• I agree to follow this project's Code of Conduct

[ai-sdk-factory]

Identify issue type

Identified as Bug
Confidence: High

Rationale

This is a concrete error-handling defect: cleanup failure masks the intended size-limit error, producing incorrect observable behavior.

Evidence

• A streamed response exceeding maxBytes correctly triggers a DownloadError, but a rejection from reader.cancel() replaces it with Error: cancel failed.
• The reproduction demonstrates the incorrect error on AI SDK 7.0.56 and current main.
• The adjacent cleanup helper reportedly already preserves the original rejection by ignoring cancellation failures.