I confirmed the behavior of ENIs when running Amazon Bedrock AgentCore Runtime in VPC mode

from fastapi import FastAPI

app = FastAPI()

@app.get("/ping")
def ping() -> dict[str, str]:
    return {"status": "ok"}

@app.post("/invocations")
def invocations(payload: dict) -> dict:
    return {"echo": payload}

FROM public.ecr.aws/docker/library/python:3.12-slim

WORKDIR /app

COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt

COPY app.py ./

EXPOSE 8080

CMD ["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "8080"]

import * as path from 'path';
import * as cdk from 'aws-cdk-lib';
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as ecrAssets from 'aws-cdk-lib/aws-ecr-assets';
import * as agentcore from '@aws-cdk/aws-bedrock-agentcore-alpha';
import { Construct } from 'constructs';

export interface AgentcoreEniTestStackProps extends cdk.StackProps {
  readonly runtimeCount: number;
}

export class AgentcoreEniTestStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props: AgentcoreEniTestStackProps) {
    super(scope, id, props);

    const vpc = new ec2.Vpc(this, 'AgentCoreVpc', {
      maxAzs: 2,
      natGateways: 1,
      subnetConfiguration: [
        { name: 'public', subnetType: ec2.SubnetType.PUBLIC, cidrMask: 24 },
        { name: 'private', subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS, cidrMask: 24 },
      ],
    });

    const agentSg = new ec2.SecurityGroup(this, 'AgentCoreSg', {
      vpc,
      description: 'Shared SG for AgentCore Runtime ENI test',
      allowAllOutbound: true,
    });

    const artifact = agentcore.AgentRuntimeArtifact.fromAsset(
      path.join(__dirname, '..', 'agent'),
      { platform: ecrAssets.Platform.LINUX_ARM64 },
    );

    for (let i = 0; i < props.runtimeCount; i++) {
      new agentcore.Runtime(this, `AgentRuntime${i}`, {
        runtimeName: `eni_test_runtime_${i}`,
        agentRuntimeArtifact: artifact,
        networkConfiguration: agentcore.RuntimeNetworkConfiguration.usingVpc(this, {
          vpc,
          vpcSubnets: { subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS },
          securityGroups: [agentSg],
        }),
      });
    }
  }
}

npx cdk deploy --profile private --context runtimeCount=10 --require-approval never

Outputs:
AgentcoreEniTestV2.VpcId = vpc-xxxxxxxxxxxxxxxxx
AgentcoreEniTestV2.SecurityGroupId = sg-xxxxxxxxxxxxxxxxx
AgentcoreEniTestV2.ImageUri = <ACCOUNT_ID>.dkr.ecr.ap-northeast-1.amazonaws.com/cdk-hnb659fds-container-assets-<ACCOUNT_ID>-ap-northeast-1:xxxxxxxx...

aws bedrock-agentcore-control list-agent-runtimes --profile private --region ap-northeast-1 \
  --query 'agentRuntimes[?starts_with(agentRuntimeName, `eni_test_runtime_`)].{Name:agentRuntimeName,Status:status}' \
  --output table

---------------------------------------
|         ListAgentRuntimes           |
+----------------------+--------------+
|         Name         |   Status     |
+----------------------+--------------+
|  eni_test_runtime_9  |   READY      |
|  eni_test_runtime_8  |   READY      |
|  eni_test_runtime_7  |   READY      |
|  eni_test_runtime_6  |   READY      |
|  eni_test_runtime_5  |   READY      |
|  eni_test_runtime_4  |   READY      |
|  eni_test_runtime_3  |   READY      |
|  eni_test_runtime_2  |   READY      |
|  eni_test_runtime_1  |   READY      |
|  eni_test_runtime_0  |   READY      |
+----------------------+--------------+

aws ec2 describe-network-interfaces --profile private --region ap-northeast-1 \
  --filters "Name=group-id,Values=sg-xxxxxxxxxxxxxxxxx" \
  --query 'NetworkInterfaces[].{Id:NetworkInterfaceId,Subnet:SubnetId,Ip:PrivateIpAddress,AZ:AvailabilityZone,InterfaceType:InterfaceType,Status:Status}' \
  --output table

-----------------------------------------------------------------------------------------------------------------------
|                                                DescribeNetworkInterfaces                                            |
+------------------+------------------------+----------------+-------------+----------+----------------------------+
|       AZ         |          Id            | InterfaceType  |     Ip      | Status   |          Subnet            |
+------------------+------------------------+----------------+-------------+----------+----------------------------+
|  ap-northeast-1c |  eni-aaaaaaaaaaaaaaaaa |  agentic_ai    |  10.0.3.160 |  in-use  |  subnet-xxxxxxxxxxxxxxxxx  |
|  ap-northeast-1a |  eni-bbbbbbbbbbbbbbbbb |  agentic_ai    |  10.0.2.185 |  in-use  |  subnet-yyyyyyyyyyyyyyyyy  |
+------------------+------------------------+----------------+-------------+----------+----------------------------+

import concurrent.futures
import json
import time
import uuid
from threading import Event, Thread

import boto3

REGION = "ap-northeast-1"
PROFILE = "private"
SG_ID = "sg-xxxxxxxxxxxxxxxxx"
RUNTIME_NAME_PREFIX = "eni_test_runtime_"
CONCURRENCY = 50
DURATION_SEC = 120

session = boto3.Session(profile_name=PROFILE, region_name=REGION)
agentcore_ctl = session.client("bedrock-agentcore-control")
agentcore = session.client("bedrock-agentcore")
ec2 = session.client("ec2")

arns = [
    r["agentRuntimeArn"]
    for r in agentcore_ctl.list_agent_runtimes()["agentRuntimes"]
    if r["agentRuntimeName"].startswith(RUNTIME_NAME_PREFIX)
]

def describe_eni():
    return ec2.describe_network_interfaces(
        Filters=[{"Name": "group-id", "Values": [SG_ID]}]
    )["NetworkInterfaces"]

def invoke_once(idx):
    arn = arns[idx % len(arns)]
    session_id = f"load-{uuid.uuid4().hex}-{idx:08d}"
    resp = agentcore.invoke_agent_runtime(
        agentRuntimeArn=arn,
        runtimeSessionId=session_id,
        payload=json.dumps({"prompt": f"load {idx}"}).encode("utf-8"),
        contentType="application/json",
    )
    resp["response"].read()

uv run --with boto3 python scripts/load_test.py

Target runtimes: 10

=== Before load ===
ENI count: 2
  eni-aaaaaaaaaaaaaaaaa / 10.0.3.160 / ap-northeast-1c / agentic_ai / in-use
  eni-bbbbbbbbbbbbbbbbb / 10.0.2.185 / ap-northeast-1a / agentic_ai / in-use

=== Load result ===
Duration: 130.8s
Total: 2148, succ=768, fail=1380
Error samples:
  - ServiceQuotaExceededException: maxVms limit exceeded for account ...

=== ENI count over time ===
  +   0.1s: ENI=2 IPs=['10.0.3.160', '10.0.2.185']
  +   5.2s: ENI=2 IPs=['10.0.3.160', '10.0.2.185']
  +  10.3s: ENI=2 IPs=['10.0.3.160', '10.0.2.185']
  ...
  + 122.9s: ENI=2 IPs=['10.0.3.160', '10.0.2.185']
  + 128.0s: ENI=2 IPs=['10.0.3.160', '10.0.2.185']

Max ENI observed during load: 2
Post-load ENI count: 2

ServiceQuotaExceededException: maxVms limit exceeded for account XXXXXXXXXXXX.
Please contact AWS Support for more information.

const agentSg = new ec2.SecurityGroup(this, 'AgentCoreSg', { vpc, allowAllOutbound: true });
const agentSgSecondary = new ec2.SecurityGroup(this, 'AgentCoreSgSecondary', { vpc, allowAllOutbound: true });

for (let i = 0; i < props.runtimeCount; i++) {
  const sg = i < (props.secondarySgRuntimeCount ?? 0) ? agentSgSecondary : agentSg;
  new agentcore.Runtime(this, `AgentRuntime${i}`, {
    runtimeName: `${prefix}${i}`,
    agentRuntimeArtifact: artifact,
    networkConfiguration: agentcore.RuntimeNetworkConfiguration.usingVpc(this, {
      vpc,
      vpcSubnets: { subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS },
      securityGroups: [sg],
    }),
  });
}

aws ec2 describe-network-interfaces --profile private --region ap-northeast-1 \
  --filters "Name=group-id,Values=<Primary SG ID>" \
  --query 'NetworkInterfaces[].{Id:NetworkInterfaceId,Ip:PrivateIpAddress,AZ:AvailabilityZone}' \
  --output table

aws ec2 describe-network-interfaces --profile private --region ap-northeast-1 \
  --filters "Name=group-id,Values=<Secondary SG ID>" \
  --query 'NetworkInterfaces[].{Id:NetworkInterfaceId,Ip:PrivateIpAddress,AZ:AvailabilityZone}' \
  --output table

=== Primary SG (Runtime 1-9 shared by 9 instances) ===
+------------------+------------------------+-------------+
|       AZ         |          Id            |     Ip      |
+------------------+------------------------+-------------+
|  ap-northeast-1a |  eni-ccccccccccccccccc |  10.0.2.254 |
|  ap-northeast-1c |  eni-ddddddddddddddddd |  10.0.3.36  |
+------------------+------------------------+-------------+

=== Secondary SG (Runtime 0 only) ===
+------------------+------------------------+-------------+
|       AZ         |          Id            |     Ip      |
+------------------+------------------------+-------------+
|  ap-northeast-1a |  eni-eeeeeeeeeeeeeeeee |  10.0.2.233 |
|  ap-northeast-1c |  eni-fffffffffffffffff |  10.0.3.185 |
+------------------+------------------------+-------------+