`[_]` escape for leading underscore in path is dropped when route has a pathless layout ancestor (virtual file routes)

[dmarcey]

Which project does this relate to?

Router

Describe the bug

When a route's path uses the documented [_] escape to keep a literal leading underscore in the URL, the escape is silently ignored if any ancestor is a pathless layout (a route whose path starts with _). The leading underscore is stripped from the generated fullPath.

This reproduces with virtual file routes; I haven't tested file-based routing but the offending code (inferFullPath / removeUnderscoresWithEscape) is shared.

Complete minimal reproducer

https://stackblitz.com/github/dmarcey/tanstack-router-underscore-reproducer

Steps to Reproduce the Bug

// routes.ts
import {layout, rootRoute, route} from '@tanstack/virtual-file-routes'

export const routes = rootRoute('root.tsx', [
  // ✅ Works: '/[_]foo' → URL '/_foo'
  route('/[_]foo', 'foo.tsx'),

  // ❌ Broken: '/[_]bar' under a pathless layout → URL '/bar'
  layout('_layout', 'layout.tsx', [
    route('/[_]bar', 'bar.tsx'),
  ]),
])

After running the generator, the routeTree.gen.ts FileRoutesByFullPath includes '/bar' for bar.tsx instead of the expected '/_bar'.

1. Configure virtual file routes as above.
2. Run the router generator (@tanstack/router-generator 1.166.41).
3. Inspect the generated fullPath / path entries — the underscore is stripped from any [_]… segment that lives under a pathless layout.

I also reproduced this directly against the published utils:

import * as utils from '@tanstack/router-generator/dist/esm/utils.js'

// Top-level (correct)
utils.inferFullPath({ routePath: '/_foo', originalRoutePath: '/[_]foo' })
// → '/_foo'  ✅

// Nested under pathless layout (incorrect)
utils.inferFullPath({ routePath: '/_layout/_foo', originalRoutePath: '/_layout/[_]foo' })
// → '/foo'  ❌  (expected '/_foo')

If you're using the app in the stackblitz, you can observe that:

1. There is a type error when trying to define a <Link /> with a to for _nested.
2. Hard navigating directly to /_nested fails.

Expected behavior

'/[_]foo' should produce '/_foo' regardless of whether the route is nested under a pathless layout.

Screenshots or Videos

No response

Platform

• @tanstack/router-generator: 1.166.41
• @tanstack/virtual-file-routes: 1.161.7
• Node: 24.8.0
• OS: Ubuntu 24.04

Additional context

Here's my best guess at the underlying problem:

In inferFullPath (packages/router-generator/src/utils.ts):

const fullPath = removeGroups(
  removeUnderscoresWithEscape(
    removeLayoutSegmentsWithEscape(routeNode.routePath, routeNode.originalRoutePath),
    routeNode.originalRoutePath, // not adjusted to match
  ),
)

removeLayoutSegmentsWithEscape filters segments out of routePath, but removeUnderscoresWithEscape is then handed the unmodified originalRoutePath and uses segment array indices to look up the escape state:

return routeSegments.map((segment, i) => {
  const originalSegment = originalSegments[i] || ''
  const leadingEscaped = hasEscapedLeadingUnderscore(originalSegment)
  // …
  if (result.startsWith('_') && !leadingEscaped) result = result.slice(1)
})

After the layout segment is dropped from routeSegments, the indexes no longer align with originalSegments, so hasEscapedLeadingUnderscore is checked against the wrong segment and returns false.

A fix would be to either (a) also strip the corresponding original segments inside removeLayoutSegmentsWithEscape and pass the trimmed originalRoutePath along, or (b) align segments by content rather than by index.