lambda function url response wrapped in json

[Jimmy-Z]

I basically copied the example code from
https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/examples/http-basic-lambda/src/main.rs
(but changed it to json, trying to see if I returns json, maybe lambda won't wrap it, I was obviously wrong)

Sorry if this is a stupid question, I'm very new to lambda.

use lambda_http::{
    Error, IntoResponse, Request,
    run, service_fn, tracing,
    http::{Response, StatusCode},
};

async fn function_handler(_event: Request) -> Result<impl IntoResponse, Error> {
    let resp = Response::builder()
        .status(StatusCode::OK)
        .header("Content-Type", "application/json")
        .body("{\"msg\": \"you're welcome (5)\"}".to_string())
        .map_err(Box::new)?;
    Ok(resp)
}

#[tokio::main]
async fn main() -> Result<(), Error> {
    // required to enable CloudWatch error logging by the runtime
    tracing::init_default_subscriber();

    run(service_fn(function_handler)).await
}

the response:

> GET / HTTP/1.1
> Host: absricps3wfevsoe5vixfsjlw40bvyiy.lambda-url.ap-northeast-1.on.aws
> User-Agent: curl/8.9.1
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< Date: Wed, 25 Jun 2025 12:13:05 GMT
< Content-Type: application/octet-stream
< Transfer-Encoding: chunked
< Connection: keep-alive
< x-amzn-RequestId: 218395f8-5843-48c6-af8c-6e6021ec46f7
< X-Amzn-Trace-Id: Root=1-685be7d1-486ec7aa00d2b11979f3e3da;Parent=75601cdbc0a879bc;Sampled=0;Lineage=1:9749c154:0
<
{"statusCode":200,"headers":{"content-type":"application/json"},"multiValueHeaders":{},"body":"{\"msg\": \"you're welcome (5)\"}","isBase64Encoded":false,"cookies":[]}* Connection #0 to host absricps3wfevsoe5vixfsjlw40bvyiy.lambda-url.ap-northeast-1.on.aws left intact

[Jimmy-Z]

Turns out it's because of streaming, if I set it back to buffered, it's not wrapped anymore.