[Next.js] HTTP localhost repeats development-browser handshake three times

ピン留めされたコミットにリンクされている再現手順から始め、その .env.example のセットアップに従い、bun run build の後に bun run test:repro -- --repeat-each=5 を実行します。/v1/client/handshake リクエスト、307 レスポンス、および __clerk_db_jwt Cookie の挙動を調査します。完了条件は、コールド状態のゲストによる / へのナビゲーションで handshake が最大 1 回だけ発生し、それでも公開ページに到達することです。

Preliminary Checks

• I have reviewed the relevant Clerk documentation, including the SDK development terminology for handshakes and development-instance cookies: https://clerk.com/docs/guides/development/sdk-development/terminology
• I have searched existing clerk/javascript issues for DevBrowserMissing, repeated localhost handshakes, __clerk_db_jwt, and SameSite/Secure combinations.
• No email or Discord support request was opened as part of preparing this report.
• This is a bug report directly related to Clerk's JavaScript packages, not a general support question.

Reproduction

https://github.com/jakvbs/clerk-localhost-handshake-repro/tree/0f56bb5332dbde96ed3911c6b220e2b4c6b5ac7c

Publishable key

pk_test_aGVscGVkLW95c3Rlci01Ny5jbGVyay5hY2NvdW50cy5kZXYk

Description

A cold guest navigation to a public page on plain HTTP localhost repeatedly enters Clerk's development-browser handshake path. The reproduction is a fresh, standalone Next.js App Router application containing only ClerkProvider, clerkMiddleware, one public page, and a Playwright measurement. It has no application business logic, database, login flow, testing tokens, or mocks.

Steps to reproduce:

1. Clone the reproduction at the exact commit linked above.

2. Copy .env.example to .env.local and provide the included development instance's publishable and secret keys locally. The secret key is not committed or included in this report.

3. Install and build:

   bun install --frozen-lockfile
   bunx playwright install chromium
   bun run build
   

4. Run five fresh-browser-context repetitions:

   bun run test:repro -- --repeat-each=5
   

The command is expected to exit with code 1 when the bug reproduces because the final assertion requires at most one handshake; the page itself still reaches HTTP 200.

Expected behavior:

One cold guest navigation to / should reach the public page with at most one /v1/client/handshake document request.

Actual behavior:

The problem reproduced in 5/5 fresh Chromium contexts. Every single page.goto("/") produced:

• exactly 3 document requests whose pathname was /v1/client/handshake;
• 9 HTTP 307 document responses;
• a final HTTP 200 response on /.

The Node-observed redirect-prefix durations were 1087, 729, 704, 748, and 710 ms (median 729 ms). The corresponding navigation-to-load durations were 1287, 934, 890, 940, and 902 ms (median 934 ms). These are observations from this machine, not a latency benchmark or a claim that any particular change guarantees an improvement.

The responses set __clerk_db_jwt with SameSite=None and without the Secure attribute. The missing Secure attribute is a measured response fact. A browser refusing to persist or return that cookie and thereby causing the repeated handshake is a causal hypothesis, not established by this reproduction. Likewise, this report does not claim that HTTPS would fix the issue.

The test records only sanitized URL origins/pathnames, statuses, Clerk header names, cookie names/attribute names, request counts, and monotonic timings. Raw Playwright traces remain ignored and are not published.

Environment

System:
  OS: macOS 26.6.2
  CPU: (8) arm64 Apple M2
Binaries:
  Node: 24.14.1
  npm: 11.19.1
  bun: 1.4.0
Browsers installed on the host:
  Chrome: 152.0.7977.83
  Edge: 153.0.4234.32
  Firefox: 155.0.1
  Safari: 26.6.2
Browser used by the verified Playwright run:
  Chromium: 151.0.7922.34
npmPackages (pinned to match the application where the behavior was observed; not claimed to be latest):
  @clerk/nextjs: 7.7.6
  @clerk/backend override: 3.16.6
  @clerk/react override: 6.14.3
  @clerk/shared override: 4.29.1
  @playwright/test: 1.62.1
  next: 16.3.1
  react: 0.0.0-experimental-eafeac09-20260819
  react-dom: 0.0.0-experimental-eafeac09-20260819
  typescript: 7.0.2