Weights in aigatewayroutes set to 0 affects translation

[aabchoo]

Description:

We encountered an issue where setting weight of one backendRef to 0 results in translation failing in other backendRefs within that rule.

Ie

  apiVersion: aigateway.envoyproxy.io/v1alpha1
  kind: AIGatewayRoute
  metadata:
    name: my-route
    namespace: default
  spec:
    rules:
    - matches:
      - headers:
        - name: ai-model
          value: gpt-4
      backendRefs:
      - name: gcp-backend-1
        weight: 0                  # ← Weight 0!
      - name: gcp-backend-2
        weight: 100

Seems the issue stems from how envoy gateway evaluates httproutes w/ weight 0.

ai-gateway/internal/extensionserver/post_translate_modify.go

Lines 210 to 214 in 0a3ada4

	if len(cluster.LoadAssignment.Endpoints) != len(httpRouteRule.BackendRefs) {
	s.log.Info("LoadAssignment endpoints length does not match backend refs length",
	"cluster_name", cluster.Name, "endpoints_length", len(cluster.LoadAssignment.Endpoints), "backend_refs_length", len(httpRouteRule.BackendRefs))
	return nil
	}

There is a mismatch where cluster.LoadAssignment.Endpoints == 1 and httpRouteRule.BackendRefs == 2. This results in the metadata not being set and to the upstream filter returning early.

I want to say it's related to https://github.com/envoyproxy/gateway/blob/main/internal/gatewayapi/route.go#L1746-L1749

Repro steps:

1. Create a routing rule w/ 2 backend refs which requires translation
2. Set one of the weights to 0
3. Query route

Logs:

2025-12-16T21:16:51Z INFO envoy-gateway-extension-server LoadAssignment endpoints length does not match backend refs length {"cluster_name": "httproute/gateway/gcp.backend/rule/0", "endpoints_length": 1, "backend_refs_length": 2}

Solution idea:

Create a count for backendRef that do not have weights equal to 0.

[yuzisun]

@mathetake do you think above check is necessary ? currently if the weight is set to 0 the entire route is completed busted.

[mathetake]

not strictly necessary if we change the logic below

[mathetake]

but weight zero means that there won't be any request made to that backend. We can enforce weight >0 CEL validation?

[aabchoo]

Setting the weight to 0 is a valid use case as sometimes you just want to temporarily disable a route+it's valid for envoy gateway so we should keep it consistent. I prefer if we change the logic below + update the conditional

[mathetake]

cool, it shouldn't be that hard to fix the logic so feel free to take it up

[johnugeorge]

Setting the weight to 0 is a valid use case as sometimes you just want to temporarily disable a route+it's valid for envoy gateway so we should keep it consistent. I prefer if we change the logic below + update the conditional

This is a valid usecase