What a provider does

After Upload SDK validates the reported file information and generates a storage key, the selected provider:

Creates a signed upload policy or token
Chooses the provider upload endpoint
Adds the required multipart form fields
Applies constraints supported by that provider
Returns a short-lived prepared upload

For example:

const storageProfiles = defineStorageProfiles({
  uploads: awsS3({
    bucket: process.env.AWS_BUCKET!,
    region: process.env.AWS_REGION!,
    credentials: {
      accessKeyId: process.env.AWS_ACCESS_KEY_ID!,
      secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY!,
    },
  }),
});

Here, awsS3() is the provider integration.

The surrounding uploads name is the storage profile. Storage profiles are explained separately in Storage Profiles.

What changes between providers

Each provider uses its own signing method and upload fields.

AWS S3

AWS S3 creates a signed multipart POST policy.

The returned fields can include:

The object key
A signed policy
AWS signature fields
The declared content type
Metadata
Upload-size constraints

ImageKit

ImageKit creates a signed upload request.

The returned fields can include:

A filename and folder
A signed token
Expiration
MIME checks
Metadata
Overwrite settings

The exact configuration and returned fields are covered in each provider guide.

What stays the same

Your application uses the same SDK flow with both providers:

const preparedUpload = await uploader.prepareUpload(
  "avatar",
  input,
);

The client also follows the same multipart upload pattern:

const formData = new FormData();

for (const [name, value] of Object.entries(
  preparedUpload.fields,
)) {
  formData.append(name, value);
}

formData.append("file", file);

await fetch(preparedUpload.url, {
  method: preparedUpload.method,
  headers: preparedUpload.headers,
  body: formData,
});

The provider changes the returned URL and fields. The overall upload flow stays the same.

Keep provider credentials on the server

Provider configuration contains private credentials used to sign uploads.

Keep it in server-only code.

The client receives short-lived signed upload instructions, not your AWS or ImageKit private credentials.

Next steps

AWS S3: Configure S3 and understand its signed POST fields.
ImageKit: Configure ImageKit and understand its signed upload fields.
Storage Profiles: Organize provider configurations by name.

Providers Overview