docs(nextjs): Document the tunnel route handler and tunnel credentials

[s1gr1d]

Documents the tunnel changes in SDK 11.9.0. Merge this PR only after 11.9.0 is released.

• Next.js tunnelRoute: add handler mode (app/sentry-tunnel/route.ts). In this mode, the app forwards only the request body to Sentry. Rewrite mode now sends with credentials: "omit".
• Troubleshooting: list what a custom tunnel must do, add handleTunnelRequest, and add a new "Credentials on Tunnel Requests" entry.
• Shorter notes in options.mdx, the getting-started includes, the Next.js setup pages, and TanStack Start.

⚠️ First merge related SDK PRs: getsentry/sentry-javascript#25456, getsentry/sentry-javascript#25509, getsentry/sentry-javascript#25515

We might need to update the mentioned versions in the docs so it's up-to-date and correct.

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PRE-MERGE CHECKLIST

Make sure you've checked the following before merging your changes:

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

[vercel]

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Project	Deployment	Actions	Updated
sentry-docs	Ready	Preview	Oct 19, 2026 12:02pm UTC

1 Skipped Deployment

Project	Deployment	Actions	Updated
develop-docs	Ignored	Preview	Oct 19, 2026 12:02pm UTC

[cursor]

The plan checks page loads of the edited tunnel docs, client errors on those pages, and JavaScript docs 404s. An issue escalates when client errors rise or JavaScript 404s rise.

Services: sentry-docs.

Mention @change-monitor in a comment to update the plan.

Plan

What changed

Readers of the Next.js build-options page now see tunnel handler mode. JavaScript troubleshooting pages now say a tunnel must forward only the request body and must not send credentials in rewrite mode.

sentry-docs must deploy before this copy is live. The Vercel production project sentry-docs maps to that service in the deploy report workflow.

Risk

A render fault in the shared troubleshooting MDX can break every JavaScript troubleshooting page. A bad new link can send readers to a 404. A fault in the new Expandable or new code samples can throw in the browser.

Intended effect

Connected telemetry cannot show that the new copy is live. No metric increments when handler mode or handleTunnelRequest appears on the page. absent means the live pages still omit that copy after deploy. That HTML check is not in Sentry. See Not observable.

Page loads of the edited paths only show that the pages still open. They do not prove the new copy.

Regression watch

Watch client errors on the edited tunnel pages, JavaScript 404s, and page-load volume on those paths. Pageload failure rate on the JavaScript troubleshooting URLs is the availability signal. The shared troubleshooting MDX is the widest blast radius.

Known error titles in this baseline are TypeError: Cannot read properties of undefined (reading 'xS') and Event: Event \Event` (type=error) captured as promise rejection`. Treat a new title as a regression. Volume may be lower on weekends. The lower page-load bound is the weekend window.

Signal	Baseline	Window	Source	Rule
Client errors on tunnel docs URLs as a share of docs.page.load	2 errors and 69 page loads, 2.9%. 48h: 10 errors and 98 page loads, 10.2%	2026-10-18T11:30:00Z to 2026-10-19T11:30:00Z and 2026-10-17T11:30:00Z to 2026-10-19T11:30:00Z	Sentry org sentry project docs. Errors: environment:production http.url:[*/platforms/javascript/guides/nextjs/configuration/build*,*/platforms/javascript/troubleshooting*,*/platforms/javascript/guides/nextjs/manual-setup*]. Page loads: environment:production path:[/platforms/javascript/guides/nextjs/configuration/build/,/platforms/javascript/troubleshooting/,/platforms/javascript/guides/nextjs/manual-setup/] sum of docs.page.load	Hold the 24h error share at or below 10.2%. Escalate when the share rises above that band or a new error title appears
docs.page.not_found for platforms/javascript	20 events in 24h. 35 events in the prior 24h	2026-10-18T11:30:00Z to 2026-10-19T11:30:00Z and 2026-10-17T11:30:00Z to 2026-10-18T11:30:00Z	Sentry org sentry project docs. environment:production requested_path:"platforms/javascript" sum of docs.page.not_found	Hold in 20-35 events per 24h. Escalate when the count rises above 35
Combined docs.page.load on the edited tunnel paths	162 loads in 24h. 93 loads in the prior 24h	2026-10-18T11:30:00Z to 2026-10-19T11:30:00Z and 2026-10-17T11:30:00Z to 2026-10-18T11:30:00Z	Sentry org sentry project docs. environment:production path:[/platforms/javascript/guides/nextjs/,/platforms/javascript/guides/nextjs/manual-setup/,/platforms/javascript/guides/nextjs/configuration/build/,/platforms/javascript/troubleshooting/,/platforms/javascript/guides/nextjs/troubleshooting/] sum of docs.page.load	Hold in 93-162 loads per 24h. Escalate when volume falls below 93
Pageload failure rate on JavaScript troubleshooting URLs	0 failures in 13 sampled pageloads. p95 2765-3106ms	2026-10-18T11:30:00Z to 2026-10-19T11:30:00Z	Sentry org sentry project docs. span.op:pageload environment:production url.full:*platforms/javascript/troubleshooting* failure_rate()	Hold failure rate at 0. Escalate when failure rate rises above 0

If client errors rise, inspect Expandable, code tabs, and MDX render on the Next.js build-options and JavaScript troubleshooting pages.

Not observable

Live HTML of the edited pages. Sentry does not store page body text. The executor cannot confirm handler mode, handleTunnelRequest, or Credentials on Tunnel Requests from telemetry. Server 5xx sliced by docs path. Pageload spans use transaction /:path*?. Pageload failure rate has no prior-day sample. Soft-navigation web vitals are not in this baseline.

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