lambda.py

import boto3

REQUIRED_TAGS = {"team", "environment", "project"}

def lambda_handler(event, context):
    ec2 = boto3.client("ec2")
    detail = event["detail"]

    if detail["eventName"] != "RunInstances":
        return

    instance_ids = [
        item["instanceId"]
        for item in detail["responseElements"]["instancesSet"]["items"]
    ]

    for instance_id in instance_ids:
        tags_response = ec2.describe_tags(
            Filters=[{"Name": "resource-id", "Values": [instance_id]}]
        )
        existing_tags = {tag["Key"] for tag in tags_response["Tags"]}
        missing = REQUIRED_TAGS - existing_tags

        if missing:
            ec2.create_tags(
                Resources=[instance_id],
                Tags=[{"Key": "cost-compliance", "Value": "missing-tags"}],
            )
            print(f"Instance {instance_id} missing tags: {missing}")