## TL;DR

Two batch-processing gotchas in one: first, the entities field does not support child field masks - use the top-level entities selector, not entities.id or entities.mentionText, or the request fails. Fix the mask and read errors from the operation, not the batch response.

## Steps

1. Two batch-processing gotchas in one: first, the entities field does not support child field masks - use the top-level entities selector, not entities.id or entities.mentionText, or the request fails. Second, the batch call swallows the real error, so when a batch fails opaquely, poll the operation with get_operation to read the full error message. Fix the mask and read errors from the operation, not the batch response.

## When to use

You are seeing this: Second, the batch call swallows the real error, so when a batch fails opaquely, poll the operation with get_operation to read the full error message. Use this skill when you run into "Batch failing opaquely - entities has no child field masks, read errors via get_operation".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.