Resolving the BODY_SIZE_LIMIT for File Uploads on AWS Amplify Hosting with SvelteKit SSR
2025-07-16 21:03 (3 months ago)
ytyng
Problem overview
In an application deployed to AWS Amplify Hosting using SvelteKit + amplify-adapter, the following error occurred when uploading an image file:
Upload error: SvelteKitError: Content-length of 605227 exceeds limit of 524288 bytes.A 566 KB file failed because of a 512 KB (524,288 bytes) limit.
Environment
- SvelteKit
- amplify-adapter 0.2.0
- AWS Amplify Hosting (SSR)
- Node.js Lambda environment
Investigation process
1. Initial investigation
In a normal SvelteKit setup, the body size limit should be configurable via the BODY_SIZE_LIMIT environment variable.
# .env
BODY_SIZE_LIMIT=52428800 # 50MB2. Checking environment variables
- AWS Amplify environment variable settings ✅
- AWS Systems Manager Parameter Store ✅
.envfile ✅
Despite being set correctly everywhere, process.env.BODY_SIZE_LIMIT was undefined in the Lambda environment.
3. Understanding SvelteKit’s environment variable system
SvelteKit handles environment variables as follows:
$env/static/private- read from.envat build timeprocess.env- runtime environment variables
I found that $env/static/private could read the value, but process.env returned undefined.
4. Internal implementation of amplify-adapter
Inspecting the amplify-adapter source, handler.js contained the following:
const body_size_limit = parseInt(env('BODY_SIZE_LIMIT', '524288'));The env function is defined in env.js and reads environment variables from process.env:
function env(name, fallback) {
const prefixed = "" + name;
return prefixed in process.env ? process.env[prefixed] : fallback;
}Solution
Attempts and experiments
- svelte.config.js option — amplify-adapter does not support a
bodySizeLimitoption - Custom implementation in hooks.server.ts — the upload was blocked before SvelteKit hooks ran
- Using dotenv — reading a
.envfile reliably in the Lambda environment proved tricky
Final solution
I decided to directly modify amplify-adapter’s env.js.
1. Create a patch file
I created patches/env.js to force BODY_SIZE_LIMIT to 50 MB:
/* global "" */
const expected = new Set([
'SOCKET_PATH',
'HOST',
'PORT',
'ORIGIN',
'XFF_DEPTH',
'ADDRESS_HEADER',
'PROTOCOL_HEADER',
'HOST_HEADER',
'BODY_SIZE_LIMIT',
]);
if ("") {
for (const name in process.env) {
if (name.startsWith("")) {
const unprefixed = name.slice("".length);
if (!expected.has(unprefixed)) {
throw new Error(
`You should change envPrefix (${""}) to avoid conflicts with existing environment variables — unexpectedly saw ${name}`
);
}
}
}
}
/**
* @param {string} name
* @param {any} fallback
*/
function env(name, fallback) {
// BODY_SIZE_LIMIT is forced to 50MB (52428800 bytes)
if (name === 'BODY_SIZE_LIMIT') {
return '52428800';
}
const prefixed = "" + name;
return prefixed in process.env ? process.env[prefixed] : fallback;
}
export { env };2. Update the build script
In sh/build-for-amplify.sh, copy the patch file after the build:
# Copy compute assets and prepare the compute environment.
cp .env build/compute/default/.env
cd build/compute/default/
npm i --production
cd -
# Copy patched env.js that fixes BODY_SIZE_LIMIT to 50MB
cp patches/env.js build/compute/default/env.js
echo "✅ env.js patched with BODY_SIZE_LIMIT=52428800 (50MB)"Result
After the fix, uploading the 605 KB file succeeded! 🎉
What I learned
- amplify-adapter constraints: it behaves differently from standard SvelteKit environment variable handling
- Environment variables in Lambda: a
.envfile is not automatically loaded intoprocess.envin the Lambda runtime - Adapter internals: understanding the adapter implementation was necessary to solve the problem
- Simple fixes can be more maintainable: a direct change proved simpler than complex workarounds
Next steps
- If other size limits are needed, update
patches/env.jsaccordingly - Re-apply the patch if amplify-adapter is updated
- Consider contributing the fix upstream to amplify-adapter
References
Summary: The file upload limit in a SvelteKit + amplify-adapter environment was resolved by understanding the adapter’s internals and directly patching its environment loader. This reinforced the importance of knowing the differences in environment variable handling.
We look forward to discussing your development needs.