Is it possible to control vercel runtime via env variable?

[derek-cove]

Question

I'm using app router and want to do something like this to control vercel runtime:

export const runtime = env.USE_EDGE ? 'edge' : 'nodejs';

However, it doesn't work as expected. If I simply put edge and don't have the environment check, it works as expected. But with the environment check, it seems to always go to nodejs, regardless of the value of USE_EDGE. Does vercel do some static analysis or something similar to check the segment config options so you can't use environment variables?

[versecafe]

@derek-cove It needs to be statically analyzable so you can't hide it behind a ternary with an env var, you could do some metaprogramming to swap in the build step to set the runtime if you really need.

[derek-cove]

Thanks @versecafe ! Do you have any docs or an example of how I might do this? I guess the idea is to have my build step modify or create the route.ts file before vercel reads it.

[versecafe]

@derek-cove Yep the way would be to modify the route.ts files, and then next build, you could do something like a bun script that parses app/**/route.ts and then some string parsing maybe have a comment like

// #[EnvSetRuntime]
//

which would get modified into

// #[EnvSetRuntime]
export const runtime = "edge";

Or

// #[EnvSetRuntime]
export const runtime = "nodejs";

And then run next build, after the build step completes parse back over and convert all instances ike

// #[EnvSetRuntime]
export const runtime = "nodejs";

back into

// #[EnvSetRuntime]
//

[derek-cove]

Thank you! Got something working w/ a shell script.

sed -i "s/\/\/ #\[EnvSetRuntime\]/export const runtime = '$runtime';/" "$file"

Notably, this command only works on vercel. When running locally, the command is slightly different sed -i '' ....