`RouteId` is not a valid param for `resolve()`

[marekdedic]

Describe the bug

Hi, starting somewhere on 2.50+ (cannot remember exact version, sorry), I started getting strange errors, where a manually type RouteId cannot be passed as a first argument to resolve(), even though it should explicitly work with that as per the docs.

Reproduction

The linked Stackblitz doesn't boot, so a more full repro:

<script lang="ts">
  import type { RouteId } from "$app/types";

  import { goto } from "$app/navigation";
  import { resolve } from "$app/paths";

  void goto(
    resolve("whatever" as RouteId),
  );
</script>

Logs

I get this when running svelte-check:

Error: Argument of type '["/" | "/blog" | "/blog/hyperparameter-optimization-for-gnns-with-transfer-learning" | "/publications" | "/teaching" | "/teaching/LAL1" | "/teaching/LAL1/2021-winter" | ... 19 more ... | "/vyuka"]' is not assignable to parameter of type '[route: "/"] | [route: "/blog"] | [route: "/blog/hyperparameter-optimization-for-gnns-with-transfer-learning"] | [route: "/publications"] | [route: "/teaching"] | [route: "/teaching/LAL1"] | [route: "/teaching/LAL1/2021-winter"] | ... 19 more ... | [route: ...]'.
  Type '["/" | "/blog" | "/blog/hyperparameter-optimization-for-gnns-with-transfer-learning" | "/publications" | "/teaching" | "/teaching/LAL1" | "/teaching/LAL1/2021-winter" | ... 19 more ... | "/vyuka"]' is not assignable to type '[route: "/vyuka"]'.
    Type '"/" | "/blog" | "/blog/hyperparameter-optimization-for-gnns-with-transfer-learning" | "/publications" | "/teaching" | "/teaching/LAL1" | "/teaching/LAL1/2021-winter" | ... 19 more ... | "/vyuka"' is not assignable to type '"/vyuka"'.
      Type '"/"' is not assignable to type '"/vyuka"'. (ts)
      void goto(
        resolve(`/teaching/${course?.slug ?? ""}/${newVersion}` as RouteId),
      );

System Info

❯ npx envinfo --system --binaries --browsers --npmPackages "{svelte,@sveltejs/*,vite}"

  System:
    OS: Linux 6.17 Ubuntu 25.10 25.10 (Questing Quokka)
    CPU: (24) x64 AMD Ryzen AI 9 HX 370 w/ Radeon 890M
    Memory: 14.28 GB / 30.13 GB
    Container: Yes
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 24.11.0 - /usr/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 11.8.0 - /usr/bin/npm
    pnpm: 10.30.0 - /usr/bin/pnpm
  Browsers:
    Firefox: 148.0
    Firefox Developer Edition: 148.0
  npmPackages:
    @sveltejs/adapter-static: ^3.0.10 => 3.0.10
    @sveltejs/enhanced-img: ^0.10.3 => 0.10.3
    @sveltejs/kit: ^2.54.0 => 2.54.0
    @sveltejs/vite-plugin-svelte: ^6.2.4 => 6.2.4
    svelte: ^5.53.6 => 5.53.6
    vite: ^7.3.1 => 7.3.1

Severity

serious, but I can work around it

Additional Information

I was waiting for 2.54 and tried RouteIdWithSearchOrParams, but that doesn't fix it either.

[shimscho]

We hit the same issue in our project (SvelteKit 2.54.0, ~35 files affected). When passing a variable typed as Pathname or RouteId (the full generated union) to resolve(), TypeScript fails to distribute ResolveArgs<T> correctly over the large union — resulting in Expected 2 arguments, but got 1 (TS2554).

String literals work fine (resolve('/about') ✓), but variables don't (resolve(somePathname) ✗).

Workaround: Module augmentation in src/app.d.ts — adds a plain string fallback overload:

declare module '$app/paths' {
  export function resolve(pathname: string): string;
}

• String literals still match the generic overload first → full type safety preserved
• Variable strings fall back to the simple overload → compiles correctly
• Zero runtime code, no wrapper functions needed