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Route param is undefined #15088

@Tyler-Petrov

Description

@Tyler-Petrov

Describe the bug

Svelte.Params.error.mp4

Route param is undefined when +page.svelte is rendered.

Reproduction

experimental.async should be enabled.

Hover the first link then click the first button. This will add the key url query parameter with the value of value. Updating this query parameter will trigger the render of routes/[index]/+page.svelte even though there is no index. This behavior doesn't happen if the /?key=value link is hovered before the buttons are clicked.

routes/+page.svelte

<script>
    import { goto } from "$app/navigation";
</script>

<a href="/1">Navigate to /1</a>
<a href="/?key=value">Navigate to /?key=value</a>

<button onclick={() => {
    goto("/?key=value", {
        replaceState: true,
    })
}}>
    Go to /?key=value
</button>
<button onclick={() => {
    goto("/", {
        replaceState: true,
    })
}}>
    Go to /
</button>

routes/[index]/+page.svelte

<script lang="ts">
    import { untrack } from "svelte";
    import { page } from "$app/state";

    let { params } = $props();

    $inspect({
        name: "index",
        index: params.index,
        url: untrack(() => page.url.href),
    });
</script>

<p>Index {params.index}</p>

Logs

+page.svelte:10 {name: 'index', index: '1', url: 'http://localhost:5000/1'}
+page.svelte:10 {name: 'index', index: undefined, url: 'http://localhost:5000/'}
+page.svelte:10 {name: 'index', index: undefined, url: 'http://localhost:5000/?key=value'}

System Info

System:
    OS: Linux 6.6 Arch Linux
    CPU: (16) x64 Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
    Memory: 7.69 GB / 15.57 GB
    Container: Yes
    Shell: 5.2.37 - /usr/bin/bash
  Binaries:
    Node: 22.17.0 - /home/tpetrov/.nvm/versions/node/v22.17.0/bin/node
    npm: 10.9.2 - /home/tpetrov/.nvm/versions/node/v22.17.0/bin/npm
    pnpm: 10.24.0 - /home/tpetrov/.local/share/pnpm/pnpm
    bun: 1.3.2 - /home/tpetrov/.bun/bin/bun
  npmPackages:
    @sveltejs/adapter-auto: ^7.0.0 => 7.0.0 
    @sveltejs/kit: ^2.49.1 => 2.49.2 
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1 
    svelte: ^5.45.8 => 5.45.8 
    vite: ^7.2.6 => 7.3.0

Severity

serious, but I can work around it

Additional Information

No response

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