# Configure SigV4a for S3 Multi-Region Access Points in AWS SDK

> Learn how to configure SigV4a for S3 Multi-Region Access Points in the AWS SDK for JavaScript v3. Install the necessary package and import it for effective client creation.

- Repository: [Amazon Web Services/agent-toolkit-for-aws](https://github.com/aws/agent-toolkit-for-aws)
- Tags: how-to-guide
- Published: 2026-07-03

---

**To configure SigV4a for S3 Multi-Region Access Points in the AWS SDK for JavaScript (v3), install either `@aws-sdk/signature-v4-crt` for Node.js or `@aws-sdk/signature-v4a` for browser support, then import the package solely for its side effects before creating your S3 client.**

The AWS SDK for JavaScript (v3) requires **SigV4a**—a multi-region signing algorithm—to authenticate requests to S3 Multi-Region Access Points (MRAP) and certain other features like S3 Object Integrity with specific checksum types. According to the `aws/agent-toolkit-for-aws` repository, this specialized signing implementation produces signatures valid across any AWS region, which is essential when your bucket ARN contains an MRAP alias instead of a single region identifier.

## Why SigV4a Is Required for S3 Multi-Region Access Points

Standard SigV4 signatures are region-specific, but MRAPs route requests to the closest available region automatically. When using an MRAP ARN such as `arn:aws:s3::123456789012:accesspoint/mfzwi23gnjvgw.mrap`, the SDK must generate signatures that AWS can validate in any region. The [`skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md) file in the repository specifies that SigV4a solves this by creating multi-region signatures that remain valid across the global AWS infrastructure.

## Choose Between the Two SigV4a Implementations

The SDK provides two optional SigV4a implementations, but you must install only one:

### Option 1: @aws-sdk/signature-v4-crt (Node.js Only)

This implementation utilizes the AWS Common Runtime (CRT) libraries for superior performance. It is the recommended choice for Node.js applications where bundle size is less critical than execution speed.

- **Platform**: Node.js only
- **Performance**: Faster (native CRT libraries)
- **Bundle size**: Smaller

### Option 2: @aws-sdk/signature-v4a (Universal)

This pure-JavaScript implementation works in both Node.js and browser environments but adds approximately 300 KB to your bundle and operates slower than the CRT version.

- **Platform**: Node.js and browsers
- **Performance**: Slower (pure JavaScript)
- **Bundle size**: Larger (~300 KB additional)

**Important**: If both packages are present, the CRT version takes precedence automatically, as documented in [`skills/core-skills/aws-sdk-js-v3-usage/SKILL.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/SKILL.md).

## Step-by-Step Configuration

Follow these steps to enable SigV4a support in your application:

### Install the Dependency

Choose the package appropriate for your runtime environment:

```bash

# For Node.js (recommended)

npm install @aws-sdk/signature-v4-crt

```

```bash

# For browser or universal applications

npm install @aws-sdk/signature-v4a

```

### Add the Side-Effect Import

Import the package solely for its side effects before creating any AWS client. According to the reference documentation in [`skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md), this import registers the signer with the SDK internally but does not expose any symbols for direct use.

```javascript
// For CRT (Node.js)
import "@aws-sdk/signature-v4-crt";

```

```javascript
// For pure JavaScript (Node.js + browsers)
import "@aws-sdk/signature-v4a";

```

### Reference the MRAP ARN

When constructing S3 commands, use the full MRAP ARN in the `Bucket` field instead of a standard bucket name.

```javascript
const bucketArn = "arn:aws:s3::123456789012:accesspoint/mfzwi23gnjvgw.mrap";

```

## Complete Implementation Examples

### CRT Implementation (Node.js)

This example demonstrates the optimal Node.js configuration using the CRT-based signer:

```javascript
// Register the CRT signer via side-effect import
import "@aws-sdk/signature-v4-crt";

import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";

const client = new S3Client({ region: "us-east-1" });

await client.send(new PutObjectCommand({
  Bucket: "arn:aws:s3::123456789012:accesspoint/mfzwi23gnjvgw.mrap",
  Key: "my-key",
  Body: "hello",
}));

```

### Pure-JavaScript Implementation (Browser)

Use this approach for browser-based applications or when you need universal runtime support:

```javascript
// Register the JS signer via side-effect import
import "@aws-sdk/signature-v4a";

import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";

const client = new S3Client({ region: "us-east-1" });

await client.send(new PutObjectCommand({
  Bucket: "arn:aws:s3::123456789012:accesspoint/mfzwi23gnjvgw.mrap",
  Key: "my-key",
  Body: "hello",
}));

```

## Critical Implementation Rules

The [`plugins/aws-core/skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/plugins/aws-core/skills/aws-sdk-js-v3-usage/references/sigv4a.md) file emphasizes these mandatory constraints:

- **Side-effect imports only**: Never import specific symbols from the SigV4a packages. The import statement must exist solely to trigger the registration of the signer with the SDK's internal middleware stack.
- **Single implementation**: Do not install both `@aws-sdk/signature-v4-crt` and `@aws-sdk/signature-v4a` simultaneously. If both are present, the CRT version silently takes precedence, potentially causing confusion in browser environments where CRT is unsupported.
- **No client configuration changes**: After importing the signer, create the `S3Client` instance normally without additional configuration parameters. The SDK automatically detects and uses the registered SigV4a implementation when encountering MRAP ARNs.

## Summary

- **SigV4a** is mandatory for S3 Multi-Region Access Points because it generates signatures valid across all AWS regions.
- Install **only one** implementation: `@aws-sdk/signature-v4-crt` for Node.js performance or `@aws-sdk/signature-v4a` for browser compatibility.
- Import the package using **side-effect syntax** (`import "@aws-sdk/signature-v4-crt"`) to register the signer before creating any AWS clients.
- Use the full **MRAP ARN** in the `Bucket` parameter when making S3 API calls.
- Refer to [`skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md) and [`skills/core-skills/aws-sdk-js-v3-usage/SKILL.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/SKILL.md) in the `aws/agent-toolkit-for-aws` repository for canonical implementation details.

## Frequently Asked Questions

### What happens if I install both SigV4a packages?

If both `@aws-sdk/signature-v4-crt` and `@aws-sdk/signature-v4a` are present in your project dependencies, the CRT version automatically takes precedence during SDK initialization. This occurs because the SDK's internal resolution logic prioritizes the native implementation, which can lead to runtime errors in browser environments where the CRT libraries are unavailable.

### Can I use SigV4a in browser environments?

Yes, but only with the `@aws-sdk/signature-v4a` package. The `@aws-sdk/signature-v4-crt` implementation relies on native Node.js bindings and cannot execute in browsers. Be aware that the pure-JavaScript implementation increases your bundle size by approximately 300 KB compared to the CRT version.

### Do I need to configure the S3 client differently when using SigV4a?

No additional client configuration is required. After importing the SigV4a package for its side effects, instantiate the `S3Client` with your standard configuration options. The SDK automatically detects MRAP ARNs in request parameters and applies the multi-region signing algorithm transparently.

### Where can I find the official documentation for SigV4a in the AWS SDK?

The canonical documentation resides in the `aws/agent-toolkit-for-aws` repository. Consult [`skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/references/sigv4a.md) for detailed implementation guidance and [`skills/core-skills/aws-sdk-js-v3-usage/SKILL.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/skills/core-skills/aws-sdk-js-v3-usage/SKILL.md) for the broader SDK usage context. An identical reference exists at [`plugins/aws-core/skills/aws-sdk-js-v3-usage/references/sigv4a.md`](https://github.com/aws/agent-toolkit-for-aws/blob/main/plugins/aws-core/skills/aws-sdk-js-v3-usage/references/sigv4a.md) for the core plugin variant.