diff --git a/packages/router-core/src/path.ts b/packages/router-core/src/path.ts
index da410e9c7d..26b55b4b41 100644
--- a/packages/router-core/src/path.ts
+++ b/packages/router-core/src/path.ts
@@ -112,13 +112,25 @@ export function resolvePath({
   trailingSlash = 'never',
   cache,
 }: ResolvePathOptions) {
-  const isBase = to === '.'
-  const isAbsolute = to.startsWith('/')
+  if (to.includes('//')) {
+    to = cleanPath(to)
+  }
+
+  if (to.startsWith('/')) {
+    if (to.length === 1 || trailingSlash === 'preserve') {
+      return to
+    }
+    if (trailingSlash === 'always') {
+      return to.endsWith('/') ? to : `${to}/`
+    }
+    return to.endsWith('/') ? to.slice(0, -1) : to
+  }
 
+  const isBase = to === '.'
   let key
   if (cache) {
     // `trailingSlash` is static per router, so it doesn't need to be part of the cache key
-    key = isAbsolute ? to : isBase ? base : base + '\0' + to
+    key = isBase ? base : base + '\0' + to
     const cached = cache.get(key)
     if (cached) return cached
   }
@@ -126,9 +138,10 @@ export function resolvePath({
   let baseSegments: Array<string>
   if (isBase) {
     baseSegments = base.split('/')
-  } else if (isAbsolute) {
-    baseSegments = to.split('/')
   } else {
+    if (base.includes('//')) {
+      base = cleanPath(base)
+    }
     baseSegments = base.split('/')
     while (baseSegments.length > 1 && last(baseSegments) === '') {
       baseSegments.pop()
@@ -171,7 +184,8 @@ export function resolvePath({
     }
   }
 
-  const result = cleanPath(baseSegments.join('/')) || '/'
+  const joined = baseSegments.join('/')
+  const result = (isBase ? cleanPath(joined) : joined) || '/'
   if (key && cache) cache.set(key, result)
   return result
 }
diff --git a/packages/router-core/src/router.ts b/packages/router-core/src/router.ts
index b473339c70..372f87a22b 100644
--- a/packages/router-core/src/router.ts
+++ b/packages/router-core/src/router.ts
@@ -22,7 +22,6 @@ import {
   processRouteTree,
 } from './new-process-route-tree'
 import {
-  cleanPath,
   compileDecodeCharMap,
   interpolatePath,
   resolvePath,
@@ -1469,7 +1468,7 @@ export class RouterCore<
   resolvePathWithBase = (from: string, path: string) => {
     return resolvePath({
       base: from,
-      to: path.includes('//') ? cleanPath(path) : path,
+      to: path,
       trailingSlash: this.options.trailingSlash,
       cache: this.resolvePathCache,
     })
@@ -1892,28 +1891,19 @@ export class RouterCore<
         dest.unsafeRelative === 'path'
           ? currentLocation.pathname
           : (dest.from ?? lightweightResult[1 /* fullPath */])
-      const destTo = dest.to ? `${dest.to}` : undefined
 
       // From search should always use the current location
       const fromSearch = lightweightResult[2 /* search */]
       // Same with params. It can't hurt to provide as many as possible
-      const fromParams = Object.assign(
-        Object.create(null),
-        lightweightResult[3 /* params */],
-      )
+      const fromParams = lightweightResult[3 /* params */]
 
-      const isAbsoluteTo = destTo?.charCodeAt(0) === 47
-      const sourcePath = isAbsoluteTo
-        ? '/'
-        : this.resolvePathWithBase(defaultedFromPath, '.')
-
-      // Resolve the destination. Absolute destinations don't need the source path.
-      const nextTo = destTo
-        ? this.resolvePathWithBase(sourcePath, destTo)
-        : sourcePath
+      const nextTo = this.resolvePathWithBase(
+        defaultedFromPath,
+        dest.to ? `${dest.to}` : '.',
+      )
 
       // Resolve the next params
-      const nextParams = resolveNextParams(dest.params, fromParams)
+      let nextParams = resolveNextParams(dest.params, fromParams)
 
       const destRoute = this.routesByPath[
         trimPathRight(nextTo) as keyof typeof this.routesByPath
@@ -1945,6 +1935,9 @@ export class RouterCore<
           const fn =
             route.options.params?.stringify ?? route.options.stringifyParams
           if (fn) {
+            if (nextParams === fromParams) {
+              nextParams = Object.assign(Object.create(null), nextParams)
+            }
             try {
               Object.assign(nextParams, fn(nextParams))
             } catch {
@@ -2864,11 +2857,14 @@ function resolveNextParams(
   spec: unknown,
   base: Record<string, unknown>,
 ): Record<string, unknown> {
-  return spec === false || spec === null
-    ? Object.create(null)
-    : (spec ?? true) === true
-      ? base
-      : Object.assign(base, functionalUpdate(spec as any, base))
+  if (spec === false || spec === null) {
+    return Object.create(null)
+  }
+  if ((spec ?? true) === true) {
+    return base
+  }
+  const next = Object.assign(Object.create(null), base)
+  return Object.assign(next, functionalUpdate(spec as any, next))
 }
 
 function extractStrictParams(
diff --git a/packages/router-core/tests/build-location.test.ts b/packages/router-core/tests/build-location.test.ts
index 14ab661ff6..c6fa108ef8 100644
--- a/packages/router-core/tests/build-location.test.ts
+++ b/packages/router-core/tests/build-location.test.ts
@@ -1300,6 +1300,39 @@ describe('buildLocation - relative paths', () => {
     expect(location.pathname).toBe('/a/d')
   })
 
+  test('unsafe path-relative navigation ignores repeated slash segments', async () => {
+    const rootRoute = new BaseRootRoute({})
+    const aRoute = new BaseRoute({
+      getParentRoute: () => rootRoute,
+      path: '/a',
+    })
+    const bRoute = new BaseRoute({
+      getParentRoute: () => aRoute,
+      path: '/b',
+    })
+    const cRoute = new BaseRoute({
+      getParentRoute: () => rootRoute,
+      path: '/c',
+    })
+    const routeTree = rootRoute.addChildren([
+      aRoute.addChildren([bRoute]),
+      cRoute,
+    ])
+    const router = createTestRouter({
+      routeTree,
+      history: createMemoryHistory({ initialEntries: ['/a//b'] }),
+    })
+
+    await router.load()
+
+    expect(
+      router.buildLocation({
+        to: '../../c',
+        unsafeRelative: 'path',
+      }).pathname,
+    ).toBe('/c')
+  })
+
   test('over-root traversal stays rooted for javascript-like segments', async () => {
     const rootRoute = new BaseRootRoute({})
     const indexRoute = new BaseRoute({
@@ -1613,6 +1646,37 @@ describe('buildLocation - params edge cases', () => {
     expect(location.pathname).toBe('/users/000042')
   })
 
+  test('params.stringify should not mutate current params', async () => {
+    const rootRoute = new BaseRootRoute({})
+    const userRoute = new BaseRoute({
+      getParentRoute: () => rootRoute,
+      path: '/users/$userId',
+      params: {
+        parse: ({ userId }: { userId: string }) => ({
+          userId: parseInt(userId, 10),
+        }),
+        stringify: (params: { userId: number }) => {
+          const userId = params.userId
+          params.userId = 999
+          return { userId: String(userId).padStart(6, '0') }
+        },
+      },
+    })
+
+    const routeTree = rootRoute.addChildren([userRoute])
+    const router = createTestRouter({
+      routeTree,
+      history: createMemoryHistory({ initialEntries: ['/users/000123'] }),
+    })
+
+    await router.load()
+
+    expect(router.buildLocation({ to: '/users/$userId' }).pathname).toBe(
+      '/users/000123',
+    )
+    expect(router.state.matches.at(-1)?.params).toEqual({ userId: 123 })
+  })
+
   test('params.stringify should run for params.parse route templates', async () => {
     const rootRoute = new BaseRootRoute({})
     const languageRoute = new BaseRoute({
diff --git a/packages/router-core/tests/path.test.ts b/packages/router-core/tests/path.test.ts
index 658bbcaa38..49c8f022e7 100644
--- a/packages/router-core/tests/path.test.ts
+++ b/packages/router-core/tests/path.test.ts
@@ -109,6 +109,8 @@ describe('resolvePath', () => {
     ['/a/b/c', '../..', '/a'],
     ['/a/b/c', '../../..', '/'],
     ['/a/b/c/', '../../..', '/'],
+    ['/a//b', '../../c', '/c'],
+    ['/a///b', '../c', '/a/c'],
     ['/', '../javascript:alert(1)', '/javascript:alert(1)'],
     ['/posts', '../../data:text/html,test', '/data:text/html,test'],
   ])('resolves correctly', (a, b, eq) => {
@@ -123,6 +125,10 @@ describe('resolvePath', () => {
     })
   })
 
+  it('normalizes repeated slashes when resolving the base path', () => {
+    expect(resolvePath({ base: '/a//b', to: '.' })).toBe('/a/b')
+  })
+
   describe('trailingSlash', () => {
     describe(`'always'`, () => {
       it('keeps trailing slash', () => {
@@ -184,6 +190,17 @@ describe('resolvePath', () => {
         ).toBe('/a/b/c/d')
       })
     })
+
+    it.each([
+      ['always', '/a//b', '/a/b/'],
+      ['never', '/a//b///', '/a/b'],
+      ['preserve', '/a//b///', '/a/b/'],
+    ] as const)(
+      "normalizes repeated slashes with trailingSlash '%s'",
+      (trailingSlash, to, expected) => {
+        expect(resolvePath({ base: '/', to, trailingSlash })).toBe(expected)
+      },
+    )
   })
 
   describe.each([{ base: '/' }, { base: '/nested' }])(