Worker Previews: fetch() from a Worker to a Preview custom domain returns error code 1053

Finding: In one external report (cloudflare/workers-sdk#16052, https://github.com/cloudflare/workers-sdk/issues/16052), with wrangler 4.135.0 and a Worker Previews open-beta setup (compatibility date 2026-09-20), a fetch() from a Worker to a Preview served on a custom domain returned HTTP 400 with the body 'error code: 1053'. This is an external report only, not reproduced by KFA, and the cause is unknown. Reported setup: a production Worker on a custom domain had a Preview-only custom domain (previews_enabled true), and a Preview was created with wrangler preview. A second minimal Worker fetched a URL given in a query parameter and reported the status and body. Reported results (per the issue's matrix): the Preview alias and the immutable deployment URL both returned 200 from curl or a browser. A production caller Worker could fetch the production custom domain (200) but got 400 with 1053 for both the Preview alias and the immutable deployment URL. A Preview of the caller could fetch the production custom domain and another normal custom-domain Worker on the same zone (200) but got 1053 for the Preview alias. So the failure was reported only for Worker subrequests whose target is a Preview custom-domain hostname, regardless of whether the caller was production or a Preview. The reporter says error 1053 is not listed in the Workers errors page, the Cloudflare 1xxx error list, or the Previews limitations page, and asks whether this is intentional or a gap, and whether Preview-to-Preview calls over custom domains are planned. The reporter distinguishes this from a possibly related issue about 1042 on workers.dev, which fails on direct requests. Status: the issue is open. The sole comment is an automated triage bot comment that rated it actionable and labelled it for Preview/wrangler, but its reproduction attempt was blocked because the sandbox had no Cloudflare credentials and the failure can only occur inside the edge runtime. That is not a reproduction or a diagnosis. No maintainer-confirmed cause, workaround, fix commit, or released fix appears in the supplied sources. Limitations: only wrangler 4.135.0 was tested; the reporter has not retested on the latest version; the resources were torn down and no Ray IDs were captured; comment coverage may omit middle comments. Practical diagnosis beyond 'Worker subrequests to Preview custom-domain hostnames may return 1053' is not supported. Treat this as a lead for anyone hitting the same symptom, not as a confirmed product behavior.