503 Service Unavailable on Fastly Magento2 admin endpoints (Edge Dictionary ListAll)

TL;DR

Upgrade the fastly/magento2 extension to 1.2.54 or later. The Fastly team fixed it in PR #177, released in fastly/magento2 1.2.54.

Steps

1. The module's admin XHR calls tripped over Magento Cloud session handling, producing intermittent 503s from Varnish on the Fastly admin endpoints. The Fastly team fixed it in PR #177, released in fastly/magento2 1.2.54. The reporter upgraded to 1.2.54 and confirmed the 503s were gone. Upgrade the fastly/magento2 extension to 1.2.54 or later.

When to use

You are seeing this: After enabling Fastly on a Magento Cloud staging environment, admin pages under Stores > Configuration intermittently showed a red error box ("A technical problem with the server created an error"), with XHR requests like... Use this skill when you run into "503 Service Unavailable on Fastly Magento2 admin endpoints (Edge Dictionary ListAll)".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: Magento 2.2.2, fastlymagento2 1.2.48, fastlymagento2 1.2.54. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

A user asked: After enabling Fastly on a Magento Cloud staging environment, admin pages under Stores > Configuration intermittently showed a red error box ("A technical problem with the server created an error"), with XHR requests like admin/fastlyCdnEdgeDictionary/ListAll and admin/fastlyCdnVcl/checkAuthDictionary returning 503 Service Unavailable from the Varnish cache server. The failures were random: sometimes one endpoint failed, sometimes another, sometimes none. Magento 2.2.2, fastly/magento2 1.2.48. Another user confirmed the same on Magento Cloud Pro on every admin page load. The verified answer: The module's admin XHR calls tripped over Magento Cloud session handling, producing intermittent 503s from Varnish on the Fastly admin endpoints. The Fastly team fixed it in PR #177, released in fastly/magento2 1.2.54. The reporter upgraded to 1.2.54 and confirmed the 503s were gone. Upgrade the fastly/magento2 extension to 1.2.54 or later. Thread: https://vectle.com/threads/thrh65vZPmFMmowuQNVEGtFWw