DynamoDB - Sporadic ValidationException when calling the BatchGetItem operation: The provided key element does not match the schema #4709
Description
Describe the bug
Summary
We are observing sporadic ValidationException errors when calling DynamoDB BatchGetItem via boto3 / botocore (through PynamoDB) even though the request keys are valid and match the table schema.
The same request (same keys, same table, same IAM role) will usually succeed, but sometimes fail. When the failing batch is split into smaller batches or retried, the requests succeed immediately.
Error
ValidationException: The provided key element does not match the schema
This error occurs on BatchGetItem. The request is not retried automatically by botocore (as expected for validation errors).
Environment
• AWS Lambda (Python 3.13)
• Region: eu-central-1
• DynamoDB table with single hash key (no sort key)
• Hash key attribute type: S
• boto3/botocore: (provided by Lambda)
What we have verified
• Same AWS account, role, and region for successful and failing requests
• Table key schema is correct and consistent
• Hash key name and type match exactly (S)
• No duplicate keys in the batch
• Very small batch sizes can trigger the issue (e.g. 3 keys)
• Individual GetItem calls succeed
• The same keys succeed when retried or when split into smaller batches
Example
A batch request with multiple valid keys intermittently fails:
{
"RequestItems": {
"<table-name>": {
"Keys": [
{"id": {"S": "<uuid-1>"}},
{"id": {"S": "<uuid-2>"}},
{"id": {"S": "<uuid-3>"}}
]
}
}
}
The same keys succeed immediately when split:
• Batch 1: 1 key → success
• Batch 2: remaining keys → success
No schema, IAM, or request changes between attempts.
Regression Issue
- Select this option if this issue appears to be a regression.
Expected Behavior
Request succeeds or returns meaningful and correct error message
Current Behavior
Schema mismatch error despite correct schema.
Reproduction Steps
We are not sure how to reproduce.
Possible Solution
No response
Additional Information/Context
No response
SDK version used
Lambda Python 3.13 runtime
Environment details (OS name and version, etc.)
Lambda Python 3.13 runtime