diff --git a/apps/studio/lib/router-url-shape.test.tsx b/apps/studio/lib/router-url-shape.test.tsx
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+import {
+  createMemoryHistory,
+  createRootRoute,
+  createRoute,
+  createRouter,
+} from '@tanstack/react-router'
+import { renderQueryString } from 'nuqs/adapters/custom'
+import { describe, expect, it } from 'vitest'
+
+import { resolveSearchOrHashOnlyTarget } from '../compat/next/router'
+import { buildSearchUpdateTarget } from './nuqs-tanstack-adapter'
+import { parseSearch, stringifySearch } from './router-search-params'
+
+// Integration guard for the family of URL-shape regressions from the
+// TanStack migration: drives a real router (with the app's custom search
+// codec) through the navigation shapes the compat shims and the nuqs
+// adapter produce, and asserts the built hrefs are Next-shaped — no JSON
+// coercion/quoting, no dropped repeated keys, no injected trailing slash,
+// no doubled `#`.
+describe('URL shape end-to-end against a real router', () => {
+  it('produces Next-shaped URLs for every fixed producer', async () => {
+    const rootRoute = createRootRoute({})
+    const routes = ['/project/$ref/advisors/security', '/project/$ref/auth/providers', '/org'].map(
+      (p) => createRoute({ getParentRoute: () => rootRoute, path: p })
+    )
+    rootRoute.addChildren(routes)
+    const router = createRouter({
+      routeTree: rootRoute,
+      history: createMemoryHistory({
+        initialEntries: [
+          '/project/abc/advisors/security?preset=security&filter=a:eq:1&filter=b:eq:2',
+        ],
+      }),
+      parseSearch,
+      stringifySearch,
+    })
+    await router.load()
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const build = (opts: any) => router.buildLocation(opts).href
+
+    // Custom codec: strings and repeated-key arrays, no JSON coercion.
+    expect(router.state.location.search).toEqual({
+      preset: 'security',
+      filter: ['a:eq:1', 'b:eq:2'],
+    })
+
+    // Compat router push({ query }) resolves to a ?-only target; must stay
+    // on the exact current pathname (no `/advisors/security/?...`).
+    const target = resolveSearchOrHashOnlyTarget('?preset=WARN', router.state.location.pathname)
+    expect(build({ to: target })).toBe('/project/abc/advisors/security?preset=WARN')
+
+    // nuqs write over the whole page query — same guarantee.
+    const params = new URLSearchParams(router.state.location.searchStr)
+    params.set('preset', 'perf')
+    expect(
+      build({
+        to: buildSearchUpdateTarget(router.state.location.pathname, renderQueryString(params)),
+        hash: (h: string | undefined) => h ?? '',
+      })
+    ).toBe('/project/abc/advisors/security?preset=perf&filter=a:eq:1&filter=b:eq:2')
+
+    // nuqs clear-all keeps the exact pathname.
+    expect(
+      build({
+        to: buildSearchUpdateTarget(
+          router.state.location.pathname,
+          renderQueryString(new URLSearchParams())
+        ),
+      })
+    ).toBe('/project/abc/advisors/security')
+
+    // A bare fragment (the Link shim's hash shape) produces a single '#'.
+    expect(build({ to: '/org', hash: 'invoices' })).toBe('/org#invoices')
+  })
+})