fix(docs): Handle missing MDX source files correctly in Edge runtime

[sentry]

DESCRIBE YOUR PR

This PR addresses an issue where requests for non-existent documentation pages (e.g., /docs/changelog) resulted in a 500 Internal Server Error instead of a 404 Not Found.

The root cause was identified in app/[[...path]]/page.tsx. The isExpectedError check, which is intended to gracefully catch 'Failed to find a valid source file' errors thrown when an MDX file is missing, used e instanceof Error. In Edge runtime environments, Error objects crossing VM context boundaries cause instanceof Error to return false. This prevented the error from being recognized as an expected condition, leading to an unhandled exception and a 500 response.

The fix updates the error checking logic in both the main docs and developer docs catch blocks. Instead of relying on instanceof Error, it now checks typeof (e as any)?.message === 'string' && (e as any).message.includes('Failed to find a valid source file'). This ensures the error is correctly identified in Edge environments, allowing the application to return a proper 404 response.

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LEGAL BOILERPLATE

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EXTRA RESOURCES

• Sentry Docs contributor guide

Fixes DOCS-9WN

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}

[vercel]

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview	Jul 27, 2026 10:53pm
sentry-docs	Ready	Preview	Jul 27, 2026 10:53pm

[sfanahata]

Previous fix was clearly broken, so this does need repair. The proposed fix would work, but it is flimsy, and a bit bulky. Creating a new PR to address what looks like a larger issue.

1. Use of as any causes repetition, and could be replaced with a type guard function.
2. String matching 'Failed to find a valid source file' will fail the moment the string is changed. It looks like a .code property should be catching these as 404s, but something deeper is going on. It would be more ideal if the LLM could question one layer down as to why the thing is failing in the first place.

• Is this a problem Seer could ever fix?
  • Yes, I believe with more context and a stronger LLM, a more ideal fix would have been presented.
• Is this a good use of Seer — the kind of problem where a good LLM fix would actually be useful?
  • Yes, this would be a good use of Seer. It's a concrete, knowable fix that can easily be researched and implemented. It also addresses 400k instances, which is significant!