vercel
Large HTTP body truncated in API route (using vercel-ai-sdk)
Link to the code that reproduces this issue
https://github.com/needle-ai/needle-mcp
To Reproduce
Use Vercel AI SDK's useChat to stream messages. It makes an HTTP request to /api/chat. Request body gets large (around 10kb) since we add many things into the context. However the root cause is not AI SDK since we see the same error for other API routes that dont use AI SDK but have a large payload.
Requests fail with the following error message
SyntaxError: Unexpected token 'a', "are as a S"... is not valid JSON
at JSON.parse (<anonymous>)
at g (/home/bun/app/.next/server/chunks/my-app_260fae17._.js:46:2376)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
at async /home/bun/app/.next/server/chunks/my-app_260fae17._.js:46:902
at async /home/bun/app/.next/server/chunks/my-app_4aac2b5c._.js:2785:9624
at async r$.do (/home/bun/app/node_modules/next/dist/compiled/next-server/app-route-turbo.runtime.prod.js:23:20293)
at async r$.handle (/home/bun/app/node_modules/next/dist/compiled/next-server/app-route-turbo.runtime.prod.js:23:25083)
at async u (/home/bun/app/.next/server/chunks/my-app_260fae17._.js:46:5991)
at async r$.handleResponse (/home/bun/app/node_modules/next/dist/compiled/next-server/app-route-turbo.runtime.prod.js:19:73806)
at async c (/home/bun/app/.next/server/chunks/my-app_260fae17._.js:46:7029)
I traced the line, and figured out that NextRequest.json() is failing due to malformed json body, I examined and saw that it's partial json.
This issue started happening immediately after we updated from NextJS 14 to 16.
Current vs. Expected behavior
Current behaviour: Payload is truncated. Expected behaviour: Request body is not truncated.
Provide environment information
1. Build in `standalone` mode (deployed in GCP VM)
2. `bun` version: 1.2.12
3. `node` version: 23
4. `next` version: 16.0.1
Which area(s) are affected? (Select all that apply)
Dynamic Routes
Which stage(s) are affected? (Select all that apply)
Other (Deployed)
Additional context
No response
Update: I've drilled down into the issue and realized the truncated body issue happens only when I use the middleware/proxy in nodejs mode. When I switch back to the edge mode the problem disappears.
I am not sure what would be the intended difference between these 2 if i am targeting such deployment (standalone, docker).
But the takeaway message is edge works fine while nodejs not.
Hi everyone,
On my side, I've encountered the same JSON.parse problem but with server actions. The only way for me to fix it was to switch back to middleware.ts file instead of proxy.ts in order to switch back to edge runtime. The problem disappeared after that.
I'm running Next.js version 16.0.3.
Wanted to echo the comments above, just for visibility. Same thing on my end, trying to use server actions with proxy, large response body gets truncated and fails to parse. Middleware set to edge runtime works, but I'd like to use latest config if possible.
Same issue.
In my case, I tracked it down to following:
export async function GET(request: Request) {
// load ret from upstream server
// retBody is a string of length 5592
return new Response(retBody, ...);
}
// response shows up client truncated length ~1kb
...and the issue was ultimately that the response we're proxying has content-length header causing Next to truncate the response.
If you are proxying an upstream response: remove content-encoding and content-length to avoid this problem.
