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Emulated AWS cloud services (S3, SQS, Cognito, IAM, STS) for local development and testing. Use when the user needs to interact with AWS API endpoints locally, test S3 bucket and object operations, emulate SQS queues and messages, test Cognito sign-up, sign-in, JWTs, or Identity Pools, manage IAM users/roles/access keys, test STS assume role, or work without hitting real AWS APIs. Triggers include "AWS emulator", "emulate AWS", "mock S3", "local SQS", "test IAM", "emulate S3", "AWS locally", "STS assume role", or any task requiring local AWS service emulation.

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vercel-labs/emulate
Works withAWS
See how to install

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SKILL.md (original)

Review the instructions the agent will receive before you install.

AWS Emulator

S3, SQS, Cognito, IAM, and STS emulation with AWS SDK-compatible S3 paths. S3 uploads and downloads preserve arbitrary binary payloads, including raw byte lengths and ETags. SQS supports AWS JSON 1.0 and XML Query protocols; IAM and STS use XML Query. All state is in-memory.

Start

Local listeners bind to 127.0.0.1 by default. For access from containers or other machines, pass --host 0.0.0.0 to the CLI or hostname: '0.0.0.0' to createEmulator. Configure --base-url or baseUrl for advertised URLs reachable by those clients.

# AWS only
npx emulate --service aws

# Default port (when run alone)
# http://localhost:4000

Or programmatically:

import { createEmulator } from 'emulate'

const aws = await createEmulator({ service: 'aws', port: 4006 })
// aws.url === 'http://localhost:4006'

Auth

Pass tokens as Authorization: Bearer <token>. Scoped permissions use s3:*, sqs:*, iam:*, sts:* patterns.

curl http://localhost:4006/ \
  -H "Authorization: Bearer test_token_admin"

Pointing Your App at the Emulator

Environment Variable

AWS_EMULATOR_URL=http://localhost:4006

AWS SDK v3

import { S3Client } from '@aws-sdk/client-s3'

const s3 = new S3Client({
  endpoint: process.env.AWS_EMULATOR_URL,
  region: 'us-east-1',
  credentials: {
    accessKeyId: 'AKIAIOSFODNN7EXAMPLE',
    secretAccessKey: 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY',
  },
  forcePathStyle: true,
})

import { SQSClient } from '@aws-sdk/client-sqs'

const sqs = new SQSClient({
  endpoint: `${process.env.AWS_EMULATOR_URL}/sqs`,
  region: 'us-east-1',
  credentials: {
    accessKeyId: 'AKIAIOSFODNN7EXAMPLE',
    secretAccessKey: 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY',
  },
})

import { IAMClient } from '@aws-sdk/client-iam'

const iam = new IAMClient({
  endpoint: `${process.env.AWS_EMULATOR_URL}/iam`,
  region: 'us-east-1',
  credentials: {
    accessKeyId: 'AKIAIOSFODNN7EXAMPLE',
    secretAccessKey: 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY',
  },
})

Seed Config

aws:
  region: us-east-1
  s3:
    buckets:
      - name: my-app-bucket
      - name: my-app-uploads
        region: eu-west-1
  sqs:
    queues:
      - name: my-app-events
      - name: my-app-dlq
        visibility_timeout: 60
      - name: my-app-orders.fifo
        fifo: true
  iam:
    users:
      - user_name: developer
        create_access_key: true
      - user_name: readonly-user
    roles:
      - role_name: lambda-execution-role
        description: Role for Lambda function execution
        assume_role_policy: '{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"Service":"lambda.amazonaws.com"},"Action":"sts:AssumeRole"}]}'

Default seed (always created): S3 bucket emulate-default, SQS queue emulate-default-queue, IAM user admin with access key pair (AKIAIOSFODNN7EXAMPLE / wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY).

API Endpoints

S3

S3 routes use root paths matching the real AWS S3 wire format. Legacy /s3/ prefixed paths are also supported.

# List all buckets
curl http://localhost:4006/ \
  -H "Authorization: Bearer $TOKEN"

# Create bucket
curl -X PUT http://localhost:4006/my-bucket \
  -H "Authorization: Bearer $TOKEN"

# Delete bucket (must be empty)
curl -X DELETE http://localhost:4006/my-bucket \
  -H "Authorization: Bearer $TOKEN"

# Head bucket (check existence, get region)
curl -I http://localhost:4006/my-bucket \
  -H "Authorization: Bearer $TOKEN"

# List objects (with prefix, delimiter, pagination)
curl "http://localhost:4006/my-bucket?prefix=uploads/&delimiter=/&max-keys=100" \
  -H "Authorization: Bearer $TOKEN"

# Put object
curl -X PUT http://localhost:4006/my-bucket/path/to/file.txt \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: text/plain" \
  -H "x-amz-meta-author: test" \
  --data-binary "file contents"

# Get object
curl http://localhost:4006/my-bucket/path/to/file.txt \
  -H "Authorization: Bearer $TOKEN"

# Head object (metadata only)
curl -I http://localhost:4006/my-bucket/path/to/file.txt \
  -H "Authorization: Bearer $TOKEN"

# Delete object
curl -X DELETE http://localhost:4006/my-bucket/path/to/file.txt \
  -H "Authorization: Bearer $TOKEN"

# Copy object
curl -X PUT http://localhost:4006/dest-bucket/copy.txt \
  -H "Authorization: Bearer $TOKEN" \
  -H "x-amz-copy-source: /source-bucket/original.txt"

SQS

Current AWS SDK clients use the /sqs endpoint shown above. SQS accepts AWS JSON 1.0 requests at POST /sqs/ with Content-Type: application/x-amz-json-1.0 and X-Amz-Target: AmazonSQS.<Action>, returning JSON results and errors. XML Query requests with an Action form parameter remain supported. Queue operations also accept the returned QueueUrl as the endpoint.

# List queues using AWS JSON 1.0
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/x-amz-json-1.0" \
  -H "X-Amz-Target: AmazonSQS.ListQueues" \
  -d '{}'

# Create queue
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "Action=CreateQueue&QueueName=my-queue"

# Create queue with attributes
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "Action=CreateQueue&QueueName=my-queue&Attribute.1.Name=VisibilityTimeout&Attribute.1.Value=30"

# List queues
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ListQueues"

# List queues with prefix filter
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ListQueues&QueueNamePrefix=my-"

# Get queue URL
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetQueueUrl&QueueName=my-queue"

# Get queue attributes
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetQueueAttributes&QueueUrl=<queue_url>"

# Send message
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=SendMessage&QueueUrl=<queue_url>&MessageBody=Hello+World"

# Send message with attributes
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=SendMessage&QueueUrl=<queue_url>&MessageBody=Hello&MessageAttribute.1.Name=type&MessageAttribute.1.Value.DataType=String&MessageAttribute.1.Value.StringValue=greeting"

# Receive messages
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ReceiveMessage&QueueUrl=<queue_url>&MaxNumberOfMessages=5"

# Delete message
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=DeleteMessage&QueueUrl=<queue_url>&ReceiptHandle=<receipt_handle>"

# Purge queue
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=PurgeQueue&QueueUrl=<queue_url>"

# Delete queue
curl -X POST http://localhost:4006/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=DeleteQueue&QueueUrl=<queue_url>"

IAM

All IAM operations use POST /iam/ with Action as a form-urlencoded parameter.

# Create user
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=CreateUser&UserName=new-user"

# Get user
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetUser&UserName=new-user"

# List users
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ListUsers"

# Delete user
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=DeleteUser&UserName=new-user"

# Create access key
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=CreateAccessKey&UserName=developer"

# List access keys
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ListAccessKeys&UserName=developer"

# Delete access key
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=DeleteAccessKey&UserName=developer&AccessKeyId=AKIA..."

# Create role
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=CreateRole&RoleName=my-role&AssumeRolePolicyDocument={}"

# Get role
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetRole&RoleName=my-role"

# List roles
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ListRoles"

# Delete role
curl -X POST http://localhost:4006/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=DeleteRole&RoleName=my-role"

Cognito

Use CognitoIdentityProviderClient with endpoint /cognito-idp and CognitoIdentityClient with /cognito-identity. Both accept AWS JSON 1.1 and work with the official AWS SDK v3:

import { CognitoIdentityProviderClient, InitiateAuthCommand } from "@aws-sdk/client-cognito-identity-provider";

const cognito = new CognitoIdentityProviderClient({
  endpoint: "http://localhost:4007/cognito-idp",
  region: "us-east-1",
  credentials: { accessKeyId: "local-access-key", secretAccessKey: "local-secret" },
});
const result = await cognito.send(new InitiateAuthCommand({
  ClientId: "emulateclient",
  AuthFlow: "USER_PASSWORD_AUTH",
  AuthParameters: { USERNAME: "developer", PASSWORD: "LocalPass123!" },
}));

A default User Pool us-east-1_emulate, public app client emulateclient, and Identity Pool us-east-1:00000000-0000-4000-8000-000000000001 are created without users. Seed a confirmed account for the example above:

aws:
  cognito:
    user_pools:
      - id: us-east-1_emulate
        confirmation_code: "123456"
        users:
          - username: developer
            password: LocalPass123!
            attributes:
              email: dev@example.com
              email_verified: "true"
              name: Developer

User pools also accept name and clients with client_id, optional client_secret, exact redirect_uris, and allowed scopes. Users accept status (CONFIRMED, UNCONFIRMED, or FORCE_CHANGE_PASSWORD) and enabled. Passwords require at least eight characters and are stored as salted hashes. Confidential clients require Cognito's HMAC SecretHash or SECRET_HASH for SDK flows.

  • SignUp, ConfirmSignUp, ResendConfirmationCode, ForgotPassword, and ConfirmForgotPassword model account confirmation and recovery. Codes expire after ten minutes and use the pool's confirmation_code fixture, default 123456. No email or SMS is sent. Inspect active codes at /_inspector?tab=cognito; seeded unconfirmed users can request a code with ResendConfirmationCode.
  • InitiateAuth supports USER_PASSWORD_AUTH, REFRESH_TOKEN_AUTH, and REFRESH_TOKEN. AdminInitiateAuth also supports ADMIN_USER_PASSWORD_AUTH. Admin-created temporary passwords return NEW_PASSWORD_REQUIRED, completed with RespondToAuthChallenge or AdminRespondToAuthChallenge. Challenge sessions expire after three minutes and are bound to the user and client.
  • GetUser, ChangePassword, GlobalSignOut, and RevokeToken use issued tokens. AdminCreateUser (with TemporaryPassword), AdminGetUser, AdminConfirmSignUp, AdminSetUserPassword, AdminEnableUser, AdminDisableUser, AdminDeleteUser, and paginated ListUsers support test setup and administration.

Tokens are RS256 JWTs with Cognito token_use claims. ID tokens identify the app client in aud; access tokens use client_id. ID and access tokens expire after one hour; refresh tokens after thirty days. Sign-out, revocation, disabling, deletion, and password resets invalidate affected sessions. Users, sessions, identities, and signing keys participate in snapshots, restore, and reset.

The local issuer is http://localhost:4007/cognito-idp/USER_POOL_ID. Discovery and public keys are served at ISSUER/.well-known/openid-configuration and ISSUER/.well-known/jwks.json. Configure token verifiers with this local issuer and JWKS. ISSUER/oauth2/authorize presents a local user picker, ISSUER/oauth2/token exchanges authorization codes or refresh tokens, and ISSUER/oauth2/userInfo returns scoped profile claims. Authorization codes expire after five minutes, are single-use, and bind the client, exact redirect URI, and optional S256 PKCE challenge. Client authentication accepts client_secret_basic or client_secret_post. The default client permits http://localhost:3000/callback.

Identity Pools support GetId and GetCredentialsForIdentity. Supply the ID token in Logins under cognito-idp.REGION.amazonaws.com/USER_POOL_ID. The default Identity Pool accepts the default User Pool and app client. Custom aws.cognito.identity_pools entries take id, optional name, providers: [{ user_pool_id, client_id }], and allow_unauthenticated_identities (default false). Identities remain stable for the same authenticated subject. Credentials expire after one hour; login signatures, expiry, audience, and identity ownership are checked.

SRP, MFA, external identity providers, Lambda triggers, pool/client management APIs, ListUsers filters, and Identity Pool role selection are not simulated. Administrative requests follow the AWS emulator's permissive credential behavior: SigV4 signatures and IAM policies are not evaluated. Identity Pool credentials are generated test credentials, without role or policy evaluation.

STS

All STS operations use POST /sts/ with Action as a form-urlencoded parameter.

# Get caller identity
curl -X POST http://localhost:4006/sts/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetCallerIdentity"

# Assume role
curl -X POST http://localhost:4006/sts/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=AssumeRole&RoleArn=arn:aws:iam::123456789012:role/my-role&RoleSessionName=my-session"

Inspector

# HTML dashboard (shows S3, SQS, IAM state)
curl http://localhost:4006/_inspector?tab=s3
curl http://localhost:4006/_inspector?tab=sqs
curl http://localhost:4006/_inspector?tab=iam

Common Patterns

Upload and Retrieve an Object

TOKEN="test_token_admin"
BASE="http://localhost:4006"

# Create bucket
curl -X PUT $BASE/my-data \
  -H "Authorization: Bearer $TOKEN"

# Upload file
curl -X PUT $BASE/my-data/config.json \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  --data-binary '{"key": "value"}'

# Download file
curl $BASE/my-data/config.json \
  -H "Authorization: Bearer $TOKEN"

Send and Receive SQS Messages

TOKEN="test_token_admin"
BASE="http://localhost:4006"

# Get queue URL
QUEUE_URL=$(curl -s -X POST $BASE/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=GetQueueUrl&QueueName=emulate-default-queue" | grep -oP '<QueueUrl>\K[^<]+')

# Send message
curl -X POST $BASE/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=SendMessage&QueueUrl=$QUEUE_URL&MessageBody=Hello+from+emulate"

# Receive messages
curl -X POST $BASE/sqs/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=ReceiveMessage&QueueUrl=$QUEUE_URL&MaxNumberOfMessages=1"

Create IAM User with Access Key

TOKEN="test_token_admin"
BASE="http://localhost:4006"

# Create user
curl -X POST $BASE/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=CreateUser&UserName=ci-user"

# Generate access key
curl -X POST $BASE/iam/ \
  -H "Authorization: Bearer $TOKEN" \
  -d "Action=CreateAccessKey&UserName=ci-user"

Custom emulators alongside built-ins

Use npx emulate init --custom inventory to scaffold a third-party API emulator and test. Register it in emulate.config.ts with defineConfig from emulate, alongside built-in entries such as { emulator: "aws" }. Run npx emulate start --watch to reload imports and inspect custom state at the printed /_emulate URL. Successful reloads reset the run to seed. Existing flat seed configs still work; --config selects an explicit file. For authoring and testing third-party API emulators, see https://emulate.dev/docs/custom-emulators.

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