fix(router): apply trailingSlash config before basepath redirect comparison on SSR

[EduardF1]

Summary

Fixes #7291

When a router is configured with a custom �asepath (e.g. �asepath: "/preview"), visiting the bare basepath URL (/preview) on SSR triggers a spurious 308 redirect to /preview/ — even with railingSlash: "never" (the default).

Root cause

The SSR redirect check in �eforeLoad compares the incoming latestLocation.publicHref against
extLocation.publicHref rebuilt via �uildLocation:

if (this.latestLocation.publicHref !== nextLocation.publicHref) {
  throw redirect(...)
}

When a basepath rewrite is active, �uildLocation calls ^[xecuteRewriteOutput which runs
ewriteBasepath.output:

url.pathname = joinPaths(['/', trimmedBasepath, url.pathname])
// joinPaths(['/', 'preview', '/']) → '/preview/'

The rewrite output always produces a trailing slash (basepath + internal /), but the router's railingSlash option was never applied to the result. The fast path (no rewrite) correctly goes through
esolvePath which handles railingSlash, but the rewrite path didn't.

Result:

• incoming publicHref = /preview
• rebuilt publicHref = /preview/
• mismatch → 308 redirect (wrong)

Fix

After ^[xecuteRewriteOutput produces the rewritten URL inside �uildLocation, normalize the pathname component of publicHref according to his.options.trailingSlash:

• "never" (default): rimPathRight("/preview/") → /preview — matches incoming, no redirect ✅
• "always": ensure trailing slash present — correctly redirects /preview → /preview/ ✅
• "preserve": leave rewrite output unchanged (existing behaviour) ✅

The change is surgical: it only affects the publicHref used for the SSR canonicality check; href (internal routing) is unchanged.

Summary by CodeRabbit

Bug Fixes

• Fixed incorrect 308 redirects when using SSR with basepath rewriting and trailingSlash: 'never' configuration.

[github-actions]

Bundle Size Benchmarks

• Commit: b1c061aff918
• Measured at: 2026-05-09T23:47:22.218Z
• Baseline source: history:b1c061aff918
• Dashboard: bundle-size history

Scenario	Current (gzip)	Delta vs baseline	Initial gzip	Raw	Brotli	Trend
react-router.minimal	87.34 KiB	+47 B (+0.05%)	87.20 KiB	274.20 KiB	75.95 KiB	▁▁▁▁▁▁▁▁▁▆▆█
react-router.full	90.86 KiB	+40 B (+0.04%)	90.72 KiB	285.70 KiB	78.94 KiB	▁▁▁▁▁▁▁▁▁▆▆█
solid-router.minimal	35.56 KiB	+46 B (+0.13%)	35.43 KiB	106.48 KiB	31.97 KiB	▁▁▁▁▁▁▁▁▁▆▆█
solid-router.full	40.26 KiB	+35 B (+0.08%)	40.14 KiB	120.69 KiB	36.17 KiB	▁▁▁▁▁▁▁▁▁▆▇█
vue-router.minimal	53.33 KiB	+49 B (+0.09%)	53.20 KiB	151.63 KiB	47.97 KiB	▁▁▁▁▁▁▁▁▁▆▆█
vue-router.full	58.46 KiB	+46 B (+0.08%)	58.33 KiB	167.81 KiB	52.38 KiB	▁▁▁▁▁▁▁▁▁▆▆█
react-start.minimal	102.02 KiB	+47 B (+0.05%)	101.88 KiB	322.64 KiB	88.12 KiB	▁▁▁▁▁▁▁▁▃▇▇█
react-start.full	105.46 KiB	+57 B (+0.05%)	105.33 KiB	332.97 KiB	91.14 KiB	▁▁▁▁▁▁▁▁▃▇▇█
react-start.rsbuild.minimal	99.63 KiB	+39 B (+0.04%)	99.46 KiB	317.08 KiB	85.72 KiB	▁▁▁▁▁▁▁▁▃▇▇█
react-start.rsbuild.full	102.94 KiB	+46 B (+0.04%)	102.77 KiB	327.51 KiB	88.52 KiB	▁▁▁▁▁▁▁▁▃▇▇█
solid-start.minimal	49.65 KiB	+41 B (+0.08%)	49.52 KiB	152.60 KiB	43.78 KiB	▁▁▁▁▁▁▁▁▃▇▇█
solid-start.full	55.44 KiB	+42 B (+0.07%)	55.31 KiB	169.51 KiB	48.84 KiB	▁▁▁▁▁▁▁▁▃▇▇█

Current gzip tracks all emitted client JS chunks. Initial gzip tracks only the entry/import graph. Trend sparkline is historical current gzip ending with this PR measurement; lower is better.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 35d2a84d-fc5e-4122-9cf7-ec05bace980c

📥 Commits

Reviewing files that changed from the base of the PR and between b1c061a and 00fc9a4.

📒 Files selected for processing (2)

• .changeset/fix-ssr-bare-basepath-redirect.md
• packages/router-core/src/router.ts

---

📝 Walkthrough

Walkthrough

This PR fixes SSR redirect behavior for bare basepath URLs. When a basepath is configured and SSR rewrites the URL to the same origin, the rewritten pathname is now normalized according to the router's trailingSlash option before constructing publicHref, preventing spurious redirects to slash-appended paths.

Changes

SSR Basepath Redirect Fix

Layer / File(s)	Summary
Release Documentation .changeset/fix-ssr-bare-basepath-redirect.md	Changeset documenting patch release for @tanstack/router-core that applies trailingSlash normalization to rewritten basepath URLs before redirect comparison.
Router Pathname Normalization packages/router-core/src/router.ts	When this.rewrite is active and rewritten origin matches router origin, the publicHref now normalizes the rewritten pathname based on this.options.trailingSlash (trims for 'never', adds for 'always') before concatenating search and hash.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A basepath bare was trailing slashed,
When SSR redirects it brashly flashed,
Now trailingSlash rules the rewritten way,
No phantom / to ruin the day! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: applying trailingSlash config before basepath redirect comparison on SSR.
Linked Issues check	✅ Passed	The PR correctly implements the fix described in issue #7291 by normalizing the rewritten pathname according to trailingSlash before the SSR canonicality check.
Out of Scope Changes check	✅ Passed	All changes directly address the core issue: applying trailingSlash normalization to publicHref in buildLocation and documenting the fix in a changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[schiller-manuel]

we will look into this, just a lot of things to do

[EduardF1]

Thanks @schiller-manuel, appreciated — no rush. Happy to adjust anything once you get a chance to review.

[EduardF1]

Summary

Applies trailingSlash normalization before the SSR basepath redirect comparison, preventing an infinite redirect loop when the configured trailing slash policy differs from the incoming URL.

Status

• CI: ✅ All checks passing (Benchmark PR, labeler, Bundle Size, CodeRabbit)
• Changeset included
• No unresolved review threads
• Ready for review

@schiller-manuel @SeanCassiere @tannerlinsley — happy to adjust anything if needed. Thanks!

[EduardF1]

Hi team, CI is green. Would love a review when you get a chance!