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Next.Js 16 Regression: Non-Latin-1 Route Group/Segment Names Crash With InvalidCharacterError Due To Btoa In Segment-Value-Encoding

In Next.js 16, route groups or segments with non-ASCII names (e.g. (안녕), (日本), (café), emoji) cause dev server 500, build failure, and prerender error because encodeToFilesystemAndURLSafeString calls btoa directly on the raw Unicode string, which only accepts Latin-1 characters. The segment cache is enabled by default in Next 16, so every such segment hits this path. A UTF-8 safe encoding is required.

highConfidence 95%Next.js Affected V16.3.3

Origin Analysis

encodeToFilesystemAndURLSafeString in packages/next/src/shared/lib/segment-cache/segment-value-encoding.ts uses btoa(value) without first converting the string to a UTF-8 byte sequence. btoa only supports characters in the Latin-1 range (0-255), so any non-Latin-1 character (CJK, Cyrillic, emoji, accented Latin-1 supplement) throws DOMException: InvalidCharacterError. This function is used for the segment cache key which is now always enabled in Next 16, causing build/prerender and dev server crashes for affected routes.

1. npx create-next-app@latest repro --app --ts --no-tailwind --no-eslint 2. Create route group with non-ASCII name and page: app/(안녕)/hello/page.tsx 3. Run next dev and open /hello -> observe 500 and InvalidCharacterError in terminal 4. Run next build -> observe prerender failure and export error

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This changes the encoded representation for non-ASCII segment names. If any cached segment cache entries (.next/cache) were generated with the old encoding (though they would have crashed, so unlikely), they may need invalidation. After applying the fix, delete the .next directory to avoid stale key mismatches. Ensure the corresponding decode function (if any) also uses TextDecoder or equivalent to convert the binary string back to UTF-8 bytes to maintain round-trip consistency. For rollback, restore the original btoa(value) call but note that non-ASCII segment names will again crash. No cross-platform issues expected since TextEncoder is globally available in Node.js (>=11) and all modern browsers.