Error: dynamodb item size limit error

DynamoDB rejects a write when the resulting item exceeds 400 KB, including attribute names and values. This is a fixed per-item limit, not a table capacity setting. An UpdateItem that makes an existing item too large can fail just like a PutItem.

1. Measure the resulting stored item, including nested values and UTF-8 attribute names. JSON wire size and stored size differ; base64 transport expansion is not the stored size of binary data.
2. Remove redundant data or compress a large payload into a Binary attribute. Compressed content cannot be queried with ordinary document-path filters.
3. Split a large document into related items under a shared partition key and distinct sort keys. Design how readers reconstruct it and how updates remain consistent.
4. Store large objects in S3 and retain their keys and metadata in DynamoDB. Handle cleanup and partial failures because S3 and DynamoDB do not share an atomic transaction.

Flattening a map is not a guaranteed size reduction and can duplicate long attribute names. Also check the additional item-size constraints for projected local secondary index entries. Use the item size calculator as an estimate and leave room for future updates.

AWS describes these alternatives in large-item storage guidance.