AWS Fargate Spot: Cheap Ephemeral Sandboxes for Coding Agents

// ✅ Spot capacity provider strategy for agent sandboxes (CDK)
import * as ecs from "aws-cdk-lib/aws-ecs";

const cluster = new ecs.Cluster(this, "AgentSandboxes", { vpc });
cluster.enableFargateCapacityProviders();

const taskDef = new ecs.FargateTaskDefinition(this, "SandboxTask", {
  cpu: 1024,
  memoryLimitMiB: 2048,
  runtimePlatform: {
    cpuArchitecture: ecs.CpuArchitecture.ARM64,
    operatingSystemFamily: ecs.OperatingSystemFamily.LINUX,
  },
});

taskDef.addContainer("sandbox", {
  image: ecs.ContainerImage.fromEcrRepository(repo, "agent-sandbox-1.4.2"),
  logging: ecs.LogDrivers.awsLogs({ streamPrefix: "agent-sandbox" }),
  environment: {
    WORK_BUCKET: workBucket.bucketName,
    // ❌ Never bake GitHub PATs here — use Secrets Manager (next post)
  },
  secrets: {
    GITHUB_TOKEN: ecs.Secret.fromSecretsManager(ghSecret, "token"),
  },
  stopTimeout: cdk.Duration.seconds(120), // align with Spot SIGTERM window
});

// RunTask with Spot
await ecs.send(
  new RunTaskCommand({
    cluster: cluster.clusterName,
    taskDefinition: taskDef.taskDefinitionArn,
    capacityProviderStrategy: [
      { capacityProvider: "FARGATE_SPOT", weight: 1, base: 0 },
      // ✅ optional on-demand fallback for critical tenants
      { capacityProvider: "FARGATE", weight: 0, base: 0 },
    ],
    networkConfiguration: {
      awsvpcConfiguration: {
        subnets: privateSubnets,
        securityGroups: [sandboxSg],
        assignPublicIp: "DISABLED",
      },
    },
    overrides: {
      containerOverrides: [
        {
          name: "sandbox",
          environment: [
            { name: "TENANT_ID", value: tenantId },
            { name: "TURN_ID", value: turnId },
            { name: "PATCH_S3_URI", value: patchUri },
          ],
        },
      ],
    },
  })
);

# ❌ Spot sandbox that treats /workspace as durable source of truth
def apply_and_sleep(repo_dir: str, patch: str):
    apply_patch(repo_dir, patch)
    # agent waits for next chat turn inside the same task
    time.sleep(600)  # burning Spot + inviting interruption mid-idle

#!/usr/bin/env bash
# ✅ entrypoint with Spot-aware drain
set -euo pipefail
CHECKPOINT=""
on_term() {
  echo "spot_sigterm turn=$TURN_ID"
  if [[ -n "${CHECKPOINT}" ]]; then
    aws s3 cp "$CHECKPOINT" "s3://$WORK_BUCKET/tenants/$TENANT_ID/turns/$TURN_ID/checkpoint.tgz"
    aws dynamodb update-item \
      --table-name AgentTurns \
      --key "{\"pk\":{\"S\":\"$TENANT_ID#$TURN_ID\"}}" \
      --update-expression "SET #s = :i, updatedAt = :t" \
      --expression-attribute-names '{"#s":"status"}' \
      --expression-attribute-values "{\":i\":{\"S\":\"interrupted\"},\":t\":{\"S\":\"$(date -u +%FT%TZ)\"}}"
  fi
  exit 0
}
trap on_term TERM

git clone --depth 1 "$REPO_URL" /work/repo
cd /work/repo
aws s3 cp "$PATCH_S3_URI" /tmp/patch.diff
git apply /tmp/patch.diff
pytest -q | tee /tmp/test.out
tar -czf /tmp/out.tgz /tmp/test.out
CHECKPOINT=/tmp/out.tgz
aws s3 cp /tmp/out.tgz "s3://$WORK_BUCKET/tenants/$TENANT_ID/turns/$TURN_ID/result.tgz"

// ✅ task-role fragment — tenant-prefixed objects only
{
  "Effect": "Allow",
  "Action": ["s3:GetObject", "s3:PutObject"],
  "Resource": "arn:aws:s3:::agent-work-prod/tenants/${aws:PrincipalTag/tenant_id}/*"
}