Relax v0.3 response deserialization status.message missing 'kind' and 'messageId'

[darrelmiller]

Summary

When deserializing v0.3 task responses, A2A.V0_3.AgentMessage requires kind and messageId properties to be present. However, real-world v0.3 agents return status.message without these fields, causing System.Text.Json to throw "missing required properties including: 'kind', 'messageId'".

Root Cause

The v0.3.0 specification was inconsistent in its requirements due to discrepancies between the .proto schema and the JSON Schema. Some agents implemented against one schema and not the other, resulting in status.message payloads like:

{
  "role": "agent",
  "parts": [{"kind": "text", "text": "..."}]
}

which omit the kind: "message" discriminator and messageId that the .NET SDK's AgentMessage type requires.

Failing Payload

{
  "jsonrpc": "2.0",
  "id": "1",
  "result": {
    "id": "63b2861e-...",
    "contextId": "2c727c41-...",
    "kind": "task",
    "status": {
      "state": "completed",
      "message": {
        "role": "agent",
        "parts": [{"kind": "text", "text": "Skill completed."}]
      },
      "timestamp": "2026-05-17T15:59:43.401Z"
    },
    "artifacts": [...]
  }
}

The "kind": "task" discriminator on the outer response is handled correctly (deserialized as AgentTask). The failure is deeper AgentTask.Status.Message tries to deserialize into AgentMessage which requires kind and messageId.

Suggested Fix

Relax the AgentMessage deserialization in the v0.3 models to:

1. Make kind optional (default to "message" if absent)
2. Make messageId optional (generate a UUID if absent)

This would allow the SDK to handle payloads from agents that implemented against either the .proto or JSON Schema definition of v0.3, without breaking agents that do include these fields.

Affected Packages

• A2A.V0_3 (1.0.0-preview2)
• A2A.V0_3Compat (1.0.0-preview2) conversion to v1 types never runs because deserialization fails first

Reproduction

Use a2a-ask v1.3.0 with --a2a-version 0.3 against any v0.3 agent that returns status.message without kind/messageId:

a2a-ask send <v0.3-agent-url> -m "test" --a2a-version 0.3