Problem with reserving lambda execution concurrency limits

Question

## Summary of the initial problem

I am getting an error when trying to reserve one concurrent execution lambda for a new account. Since it is a new account that has not been used much, it's concurrency limit is set to 10. I have more than 10 lambdas in that account, so trying to reserve one lambda will set the `UnreservedConcurrentExecution` to below it's minimum value of 10. The picture attached demonstrates the error.

## Current Workaround

We have experienced this problem with other new accounts, and the current workaround is to raise a support request to increase the lambda concurrency limit for the account. Then we can redeploy the code. Due to what I think is an AWS-placed limit I can only increase the concurrency execution limit to 1000, even though I only need about 200.

## The real problem

Whilst that fixes things in the short term I have few issues with this approach.

1. It has been almost 48 hours after I have raised the current request, and it has not yet been resolved. This is now blocking our production pipeline.
2. AWS needs to have a minimum of 10 unreserved executions across all lambdas. The AWS account default is 10 lambdas. If I want to reserve even one lambda for executions I will cross that 10 unreserved limit. This can be mitigated by increasing the concurrent executions to 1000, but to be honest I don't even need 1000, all I want is 200 or so. But AWS is forcing me to increase it by 900. Surely the minimum number of unreserved lambdas must be below that of the account default?
3. For newer accounts we have made a request template so that the support request will be raised at the start of the account creation. However, this leads us back to the first problem where it will still take many hours, maybe days to complete. This feels like an unnecessary blocker once again.

## The solution I am seeking

What I would like is either

1. A different workaround that does not take almost two days to implement
2. A change in the default lambda concurrent executions limit so that it is greater than 10, allowing me to reserve just one lambda.

Thanks,

Sukhraj.

AWS Support - Roman · Answer

Hi there.

I apologize for the inconvenience regarding your quota increase request.

I've located your case and shared your sentiments internally. Correspondence will be shared with you once our Support team completes their review via our Support Center: http://go.aws/support-center.

— Roman Z.

AWS Support - Randi · Answer

Hi Sukhraj,

We don't have a private chat option available on re:Post. You can provide the info for the other account on your open case and our internal teams can take a look at both.

\- Randi S.

Sukhraj · Answer

Hello, any updated on the above?

re:Post Agent · Answer

You've identified a real constraint in AWS Lambda's concurrency model. When you reserve concurrency for a function, AWS requires that at least 100 units of concurrency remain unreserved across your account - not 10 as you mentioned. This is a safety mechanism to ensure other functions can still execute. With a new account's default limit of 10, you cannot reserve any concurrency without first increasing your account limit.

Regarding your specific situation:

**Understanding the Limits:**
You can reserve up to your account's total concurrency limit minus 100. So with a limit of 10, reservation isn't possible. Even if you increase to 1000, you can only reserve up to 900 units, with 100 remaining unreserved for other functions.

**Immediate Options:**
Unfortunately, there isn't a faster workaround than requesting a limit increase through the AWS Support Center. The service quota increase process is the standard path for raising concurrency limits. You can check the status of your existing request or add correspondence through the AWS Support Center at https://console.aws.amazon.com/support/.

**About Your Request:**
When requesting an increase, you can specify the exact limit you need. If you only need 200 total concurrent executions, you can request that specific amount rather than jumping to 1000. The key is that whatever limit you receive, 100 units must remain unreserved.

**For Future Accounts:**
Your proactive approach of requesting limit increases during account creation is sound. While the timing can vary, having this as part of your account setup process is the right strategy. You might also consider reviewing your AWS Support plan options, as different support tiers offer different response time commitments for service limit increases.

The 100-unit unreserved minimum is an architectural safeguard that applies regardless of account age or default limits, so this constraint will persist across all accounts.  
**Sources**  
[Configuring reserved concurrency for a function - AWS Lambda](https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html)  
[Use PutFunctionConcurrency with a CLI - AWS Lambda](https://docs.aws.amazon.com/lambda/latest/dg/example_lambda_PutFunctionConcurrency_section.html)  
[Use PutFunctionConcurrency with a CLI - AWS SDK Code Examples](https://docs.aws.amazon.com/code-library/latest/ug/lambda_example_lambda_PutFunctionConcurrency_section.html)

Problem with reserving lambda execution concurrency limits

Summary of the initial problem

Current Workaround

The real problem

The solution I am seeking

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