fix(router-core): stringify parent params for missing route templates

packages/router-core/src/router.ts

@@ -1924,9 +1924,14 @@ export class RouterCore<
       if (destRoute) {
         destRoutes = this.getRouteBranch(destRoute)
       } else if (nextTo.includes('$')) {
-        // Route templates must match routesByPath exactly. A miss here is a
-        // typed destination mismatch, not a concrete URL to route-match.
-        destRoutes = []
+        // Server-only leaf routes can be absent from the client route tree while
+        // their parent route templates are still present. Use the known parent
+        // branch so parent params.stringify hooks still canonicalize the href.
+        destRoutes = getKnownParentRouteBranch(
+          this.routesByPath,
+          nextTo,
+          (route) => this.getRouteBranch(route),
+        )
       } else {
         const destMatchResult = this.getMatchedRoutes(nextTo)
         destRoutes = destMatchResult.matchedRoutes
@@ -3092,6 +3097,29 @@ function validateSearch(validateSearch: AnyValidator, input: unknown): unknown {
   return {}
 }
 
+function getKnownParentRouteBranch(
+  routesByPath: Record<string, AnyRoute | undefined>,
+  path: string,
+  getRouteBranch: (route: AnyRoute) => ReadonlyArray<AnyRoute>,
+): ReadonlyArray<AnyRoute> {
+  let parentPath = trimPathRight(path)
+
+  while (parentPath) {
+    parentPath = parentPath.slice(0, parentPath.lastIndexOf('/')) || '/'
+    const parentRoute = routesByPath[parentPath]
+
+    if (parentRoute) {
+      return getRouteBranch(parentRoute)
+    }
+
+    if (parentPath === '/') {
+      break
+    }
+  }
+
+  return []
+}
+
 /**
  * Build the matched route chain and extract params for a pathname.
  * Falls back to the root route if no specific route is found.

packages/router-core/tests/build-location.test.ts

@@ -1615,6 +1615,37 @@ describe('buildLocation - params edge cases', () => {
     expect(currentRouteLocation.pathname).toBe('/pl')
   })
 
+  test('params.stringify should run for the matched parent of a missing route template', async () => {
+    const rootRoute = new BaseRootRoute({})
+    const postTypeRoute = new BaseRoute({
+      getParentRoute: () => rootRoute,
+      path: '/$postType',
+      params: {
+        parse: ({ postType }: { postType: string }) =>
+          postType === 'articles' ? { postType: 'article' as const } : false,
+        stringify: ({ postType }: { postType: 'article' }) => ({
+          postType: postType === 'article' ? 'articles' : postType,
+        }),
+      },
+    })
+
+    const routeTree = rootRoute.addChildren([postTypeRoute])
+
+    const router = createTestRouter({
+      routeTree,
+      history: createMemoryHistory({ initialEntries: ['/articles'] }),
+    })
+
+    await router.load()
+
+    const location = router.buildLocation({
+      to: '/$postType/$postId/download',
+      params: { postType: 'article', postId: '1' },
+    })
+
+    expect(location.pathname).toBe('/articles/1/download')
+  })
+
   test('params.stringify should use the exact route template over path matching priority', async () => {
     const rootRoute = new BaseRootRoute({})
     const dollarRoute = new BaseRoute({