How I Fixed an SSI-Breaking Bug in NGINX Gateway Fabric

Introduction This bug found me, not the other way around. I was in the middle of migrating my team's infrastructure from NGINX Ingress Controller to NGINX Gateway Fabric when SSI (Server-Side Includes) stopped working. Subrequests were hitting the wrong backend paths, and pages that relied on SSI includes were silently broken. I could have worked around it, added a flag, patched a config, moved on. Instead, I decided to find where it was actually coming from and fix it at the source. This is the story of that bug, what caused it, and the one-line condition that fixed it. Background: What is NGINX Gateway Fabric? NGINX Gateway Fabric is a Kubernetes Gateway API implementation backed by NGINX. It translates Kubernetes HTTPRoute resources into NGINX configuration, handling routing, load balancing, and traffic management. It's the next-generation replacement for NGINX Ingress Controller, built around the Kubernetes Gateway API spec. When processing HTTP routes, it generates proxy_pass dire