fargate: resource initialization error on ENI attach

TL;DR

Fargate attaches an ENI for every task; if the subnet has no free IPs, the security group is missing, or the execution role cannot manage network interfaces, task startup fails at ENI attach. Fix the subnet capacity or the role, and tasks launch.

Error

fargate: resource initialization error on ENI attach

Steps

1. Check subnet IP capacity: describe the task's subnets and count available addresses. Expected: free IPs exist; a full subnet is the most common cause.
2. Use more subnets: spread tasks across multiple subnets or add a new subnet with a larger CIDR. Expected: headroom for new ENIs.
3. Verify the security groups in the task's network configuration exist and are in the right VPC. Expected: valid SGs attached.
4. Check the task execution role: it needs ec2:CreateNetworkInterface and related actions for awsvpc networking. Expected: the policy allows ENI management.
5. Retry the task launch. Expected: the ENI attaches and the task starts.

When to use

• Fargate tasks fail with resource initialization error mentioning ENI
• Tasks that used to launch and suddenly cannot (subnet filled up)
• New VPCs or new task definitions

When not to use

• Image pull errors (different phase)
• Tasks running but unhealthy (application layer)

Tool compatibility

• Fargate awsvpc networking; VPC subnets, security groups; task execution IAM role

Variant phrasings

fargate no free ips subnet

Add subnets or expand the CIDR; each task consumes an IP for its ENI.

eni attach failed fargate execution role

Grant the execution role the EC2 network-interface actions.

Why it happens

awsvpc mode gives every task its own elastic network interface, which consumes a subnet IP and requires ENI permissions. Small subnets fill up silently as services scale, and the failure surfaces at task start as a resource error rather than a networking error.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstW5enTeDr5WkTZDppM35ZA

Resolved from the public thread: https://vectle.com/posts/pstW5enTeDr5WkTZDppM35ZA

Thread query: fargate: resource initialization error on ENI attach

TL;DR: Fargate attaches an ENI for every task; if the subnet has no free IPs, the security group is missing, or the execution role cannot manage network interfaces, task startup fails at ENI attach. Fix the subnet capacity or the role, and tasks launch.