[BUG] kagent invoke fails with "cannot unmarshal object into []*errordetails.Typed" for agents without SupportedInterfaces

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📋 Prerequisites

• I have searched the existing issues to avoid creating a duplicate
• By submitting this issue, you agree to follow our Code of Conduct
• I am using the latest version of the software
• I have tried to clear cache/cookies or used incognito mode (if ui-related)
• I can consistently reproduce this issue

🎯 Affected Service(s)

Multiple services / System-wide issue

🚦 Impact/Severity

Blocker

🐛 Bug Description

Bug Description

kagent invoke fails with a JSON-RPC internal error for all declarative agents. The controller selects the wrong A2A transport version when an agent's card does not advertise SupportedInterfaces, causing a JSON unmarshalling panic when the agent returns any error response.

🔄 Steps To Reproduce

1. Install kagent 0.10.0-beta7
2. Create a declarative agent via kubectl apply
3. Run:

   kagent invoke --agent <agent-name> --task "hello"
   

🤔 Expected Behavior

The agent responds to the task normally, same as when invoked via the UI.

📱 Actual Behavior

ERROR client/client.go:302  JSON-RPC error in SSE event for request : jsonrpc error -32603:
failed to decode response: json: cannot unmarshal object into Go struct field
Error.error.data of type []*errordetails.Typed

The invocation always fails. The UI works fine for the same agent.

💻 Environment

• kagent version: 0.10.0-beta7
• Controller image: ghcr.io/kagent-dev/kagent/controller:0.10.0-beta7
• Kubernetes: GKE
• OS: macOS

🔧 CLI Bug Report

No response

🔍 Additional Context

Possible Root Cause

File: go/core/internal/a2a/a2a_registrar.go
Function: filterInterfacesByVersion

The controller registers a v0.3 compat transport (a2av0.NewJSONRPCTransport) to talk to agent pods, but it is only selected when the agent card advertises SupportedInterfaces with ProtocolVersion: "0.3". Declarative agents created via kubectl apply do not advertise SupportedInterfaces, so filterInterfacesByVersion returns unversioned interfaces and the v1 transport is selected instead.

When the agent pod returns any error (e.g. a 400 from the upstream LLM), the v1 transport tries to decode error.data as []*errordetails.Typed (array), but the agent pod running the v0.3 wire format sends it as a JSON object {}. Unmarshalling fails and the controller returns a -32603 internal error to the CLI.

Workaround

Port-forward directly to the agent pod (bypassing the controller) and call it via curl:

kubectl port-forward -n kagent svc/<agent-name> 9091:8080

curl -s -X POST http://localhost:9091 \
  -H "Content-Type: application/json" \
  -d "{
    \"jsonrpc\":\"2.0\",
    \"method\":\"message/send\",
    \"params\":{
      \"message\":{
        \"messageId\":\"msg-1\",
        \"role\":\"user\",
        \"parts\":[{\"kind\":\"text\",\"text\":\"your task here\"}]
      }
    },
    \"id\":1
  }" | jq '.result.artifacts[0].parts[0].text'

📋 Logs

📷 Screenshots

No response

🙋 Are you willing to contribute?

• I am willing to submit a PR to fix this issue