vercel dev does not route requests to Rust Serverless Functions when SvelteKit is the framework

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Current behavior

Running vercel dev locally, SvelteKit intercepts all requests, including /api/* routes. The Rust serverless functions are never compiled or invoked. SvelteKit’s dev server takes ownership of the entire request
pipeline, so the conventional api/ directory routing that works in production never kicks in.

Expected behavior

/api/* requests should be routed to compiled Rust serverless functions, and all other requests should be served by SvelteKit’s dev server — the same way it works in production on Vercel.

Steps to reproduce

  1. Create a SvelteKit project with @sveltejs/adapter-vercel
  2. Add Rust serverless functions in api/ directory (using vercel_runtime)
  3. Run vercel dev
  4. Try to access http://localhost:3000/api/hello — SvelteKit handles it instead of the Rust function

What I’ve tried

  1. Plain vercel dev — SvelteKit is detected as the framework and serves everything. Rust functions are ignored.
  2. Adding explicit builds / functions in vercel.json — No effect locally, the framework override still takes priority.
  3. Vite proxy — Proxying /api to another port works for mocking, but doesn’t actually run the Rust binaries, defeating the purpose of e2e local dev.

Project information

  • Framework: SvelteKit (@sveltejs/adapter-vercel v6)
  • Serverless Functions: Rust (compiled binaries in api/ via vercel_runtime)
  • vercel.json: {} (empty, relying on convention-based routing)
  • Project structure:
    demo/
    ├── api/ ← Rust serverless functions (hello.rs, time.rs, etc.)
    ├── Cargo.toml ← each .rs file is a [[bin]] target
    ├── src/ ← SvelteKit app
    ├── svelte.config.js
    ├── vite.config.ts
    └── package.json
    Each Rust function uses vercel_runtime (via a proc macro) and compiles into a standalone binary. In production on Vercel, /api/hello, /api/time, etc. are served by these Rust binaries, and everything else is handled by
    SvelteKit. This works perfectly in production.

Question

Is there a supported way to run vercel dev so that both SvelteKit and Rust serverless functions work simultaneously? Or is this a known limitation of vercel dev with framework-detected projects + non-Node serverless
functions?
Currently my workaround is running e2e tests against a deployed preview URL, but local iteration would be much faster.

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