fix(router-core): buildLocation should interpolatePath w/ stringified params for skipRouteOnParseError routes

packages/router-core/src/new-process-route-tree.ts

@@ -986,7 +986,7 @@ function extractParams<T extends RouteLike>(
   ]
 }
 
-function buildRouteBranch<T extends RouteLike>(route: T) {
+export function buildRouteBranch<T extends RouteLike>(route: T) {
   const list = [route]
   while (route.parentRoute) {
     route = route.parentRoute as T

packages/router-core/src/router.ts

@@ -12,6 +12,7 @@ import {
   replaceEqualDeep,
 } from './utils'
 import {
+  buildRouteBranch,
   findFlatMatch,
   findRouteMatch,
   findSingleMatch,
@@ -1775,17 +1776,47 @@ export class RouterCore<
                 functionalUpdate(dest.params as any, fromParams),
               )
 
+      // Apply stringify BEFORE interpolating to ensure route matching works with skipRouteOnParseError.params: true
+      // We look up the route by its template path and apply stringify functions from the route branch before interpolation
+      const trimmedNextTo = trimPathRight(nextTo)
+      const targetRoute = this.routesByPath[trimmedNextTo]
+      let prestringifiedParams: Record<string, unknown> | null = null
+      if (targetRoute && Object.keys(nextParams).length > 0) {
+        const routeBranch = buildRouteBranch<AnyRoute>(targetRoute)
+        if (
+          routeBranch.some(
+            (route) => route.options.skipRouteOnParseError?.params,
+          )
+        ) {
+          prestringifiedParams = { ...nextParams }
+          for (const route of routeBranch) {
+            const fn =
+              route.options.params?.stringify ?? route.options.stringifyParams
+            if (fn) {
+              Object.assign(prestringifiedParams!, fn(prestringifiedParams))
+            }
+          }
+        }
+      }
+
       // Interpolate the path first to get the actual resolved path, then match against that
       const interpolatedNextTo = interpolatePath({
         path: nextTo,
         params: nextParams,
+        decoder: this.pathParamsDecoder,
       }).interpolatedPath
 
       // Use lightweight getMatchedRoutes instead of matchRoutesInternal
       // This avoids creating full match objects (AbortController, ControlledPromise, etc.)
       // which are expensive and not needed for buildLocation
       const destMatchResult = this.getMatchedRoutes(interpolatedNextTo)
       let destRoutes = destMatchResult.matchedRoutes
+      if (
+        !destMatchResult.foundRoute ||
+        destMatchResult.foundRoute.fullPath !== trimmedNextTo
+      ) {
+        prestringifiedParams = null
+      }
 
       // Compute globalNotFoundRouteId using the same logic as matchRoutesInternal
       const isGlobalNotFound = destMatchResult.foundRoute
@@ -1806,12 +1837,14 @@ export class RouterCore<
       }
 
       // If there are any params, we need to stringify them
-      if (Object.keys(nextParams).length > 0) {
+      let stringifiedParams = prestringifiedParams
+      if (!stringifiedParams && Object.keys(nextParams).length > 0) {
+        stringifiedParams = nextParams
         for (const route of destRoutes) {
           const fn =
             route.options.params?.stringify ?? route.options.stringifyParams
           if (fn) {
-            Object.assign(nextParams, fn(nextParams))
+            Object.assign(stringifiedParams!, fn(stringifiedParams))
           }
         }
       }
@@ -1821,11 +1854,13 @@ export class RouterCore<
           // This preserves the original parameter syntax including optional parameters
           nextTo
         : decodePath(
-            interpolatePath({
-              path: nextTo,
-              params: nextParams,
-              decoder: this.pathParamsDecoder,
-            }).interpolatedPath,
+            prestringifiedParams
+              ? interpolatedNextTo
+              : interpolatePath({
+                  path: nextTo,
+                  params: stringifiedParams ?? {},
+                  decoder: this.pathParamsDecoder,
+                }).interpolatedPath,
           )
 
       // Resolve the next search

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

@@ -0,0 +1,102 @@
+import { describe, expect, test } from 'vitest'
+import { createMemoryHistory } from '@tanstack/history'
+import { BaseRootRoute, BaseRoute, RouterCore } from '../src'
+
+describe('buildLocation', () => {
+  describe('#6490 - skipRouteOnParseError respects params.stringify', () => {
+    test('skipRouteOnParseError is true', () => {
+      const rootRoute = new BaseRootRoute({})
+      const langRoute = new BaseRoute({
+        getParentRoute: () => rootRoute,
+        path: '/$lang',
+        skipRouteOnParseError: {
+          params: true,
+        },
+        params: {
+          parse: (rawParams) => {
+            if (rawParams.lang === 'en') {
+              return { lang: 'en-US' }
+            }
+
+            if (rawParams.lang === 'pl') {
+              return { lang: 'pl-PL' }
+            }
+
+            throw new Error('Invalid language')
+          },
+          stringify: (params) => {
+            if (params.lang === 'en-US') {
+              return { lang: 'en' }
+            }
+
+            if (params.lang === 'pl-PL') {
+              return { lang: 'pl' }
+            }
+
+            return params
+          },
+        },
+      })
+      const routeTree = rootRoute.addChildren([langRoute])
+
+      const router = new RouterCore({
+        routeTree,
+        history: createMemoryHistory(),
+      })
+
+      const location = router.buildLocation({
+        to: '/$lang',
+        params: { lang: 'en-US' },
+      })
+
+      expect(location.pathname).toBe('/en')
+    })
+    test('skipRouteOnParseError is false', () => {
+      const rootRoute = new BaseRootRoute({})
+      const langRoute = new BaseRoute({
+        getParentRoute: () => rootRoute,
+        path: '/$lang',
+        skipRouteOnParseError: {
+          params: false,
+        },
+        params: {
+          parse: (rawParams) => {
+            if (rawParams.lang === 'en') {
+              return { lang: 'en-US' }
+            }
+
+            if (rawParams.lang === 'pl') {
+              return { lang: 'pl-PL' }
+            }
+
+            throw new Error('Invalid language')
+          },
+          stringify: (params) => {
+            if (params.lang === 'en-US') {
+              return { lang: 'en' }
+            }
+
+            if (params.lang === 'pl-PL') {
+              return { lang: 'pl' }
+            }
+
+            return params
+          },
+        },
+      })
+      const routeTree = rootRoute.addChildren([langRoute])
+
+      const router = new RouterCore({
+        routeTree,
+        history: createMemoryHistory(),
+      })
+
+      const location = router.buildLocation({
+        to: '/$lang',
+        params: { lang: 'en-US' },
+      })
+
+      expect(location.pathname).toBe('/en')
+    })
+  })
+})