CodeBuild Spec Sandboxes for AI Coding Agents: Ephemeral Builds Without ECS Sprawl

# buildspec.yml — checked into the repo the agent is patching
version: 0.2
env:
 variables:
 NODE_OPTIONS: "--max-old-space-size=2048"
 # ❌ Do not put SaaS tokens in plaintext env here — use Secrets Manager
 # secrets-manager:
 # GITHUB_TOKEN: agent/$TENANT/github_pr_read:token
phases:
 install:
 runtime-versions:
 nodejs: 20
 commands:
 - npm ci --ignore-scripts
 pre_build:
 commands:
 - npm run lint
 build:
 commands:
 - npm test -- --ci
 - npm run typecheck
artifacts:
 files:
 - "coverage/**/*"
 - "junit.xml"
 name: agent-build-$(date +%Y%m%d%H%M%S)

import {
 CodeBuildClient,
 StartBuildCommand,
 BatchGetBuildsCommand,
} from "@aws-sdk/client-codebuild";

const cb = new CodeBuildClient({});

export async function runPrSandboxBuild(input: {
 projectName: string;
 sourceVersion: string; // commit SHA
 tenantId: string;
 idempotencyKey: string;
}) {
 // ✅ Idempotency: if you already started this key, return prior build id
 const start = await cb.send(
 new StartBuildCommand({
 projectName: input.projectName,
 sourceVersion: input.sourceVersion,
 environmentVariablesOverride: [
 { name: "TENANT_ID", value: input.tenantId, type: "PLAINTEXT" },
 {
 name: "IDEMPOTENCY_KEY",
 value: input.idempotencyKey,
 type: "PLAINTEXT",
 },
 ],
 // Optional: buildspec override only for allowlisted extra steps
 })
 );

 const buildId = start.build?.id;
 if (!buildId) throw new Error("no_build_id");

 // Poll with timeout — do not hold a Bedrock turn open forever;
 // prefer async tool + EventBridge like agent async tools pattern
 for (let i = 0; i < 60; i++) {
 await new Promise((r) => setTimeout(r, 5000));
 const got = await cb.send(
 new BatchGetBuildsCommand({ ids: [buildId] })
 );
 const build = got.builds?.[0];
 const status = build?.buildStatus;
 if (status && status !== "IN_PROGRESS") {
 return {
 buildId,
 status,
 logLink: build?.logs?.deepLink,
 artifacts: build?.artifacts?.location,
 };
 }
 }
 return { buildId, status: "TIMEOUT_WAITING", artifacts: null };
}

{
 "Version": "2012-10-17",
 "Statement": [
 {
 "Sid": "ReadOnlySourceBucket",
 "Effect": "Allow",
 "Action": ["s3:GetObject", "s3:GetObjectVersion"],
 "Resource": "arn:aws:s3:::acme-agent-src/tenant-${tenant}/*"
 },
 {
 "Sid": "WriteArtifactsPrefix",
 "Effect": "Allow",
 "Action": ["s3:PutObject"],
 "Resource": "arn:aws:s3:::acme-agent-artifacts/tenant-${tenant}/*"
 },
 {
 "Sid": "Logs",
 "Effect": "Allow",
 "Action": [
 "logs:CreateLogGroup",
 "logs:CreateLogStream",
 "logs:PutLogEvents"
 ],
 "Resource": "arn:aws:logs:*:*:log-group:/aws/codebuild/agent-*"
 },
 {
 "Sid": "DenyPrivilegeEscalation",
 "Effect": "Deny",
 "Action": [
 "iam:PassRole",
 "sts:AssumeRole",
 "ec2:Create*",
 "lambda:Create*",
 "sagemaker:*"
 ],
 "Resource": "*"
 }
 ]
}

// CDK-ish environment block
environment: {
 buildImage: codebuild.LinuxBuildImage.STANDARD_7_0,
 privileged: false, // ✅ default for agent sandboxes
 computeType: codebuild.ComputeType.SMALL,
 // vpc: optional — only if tests need private RDS / internal npm
}

// ✅ Tool result stays small
return {
 status: "FAILED",
 failedPhase: "BUILD",
 summary: "3 tests failed in auth.test.ts",
 logLink,
 artifactUri,
};