[Bug]: a2a-java client sends message with empty task ID and context ID

[jmesnil]

What happened?

While investigating errors reported by https://darrelmiller.github.io/agentbin/index.html, I found out that messages sent by the A2A Java client contains empty taskId and contextId.
They look like (verbatim):

curl -sN -X POST https://agentbin.greensmoke-1163cb63.eastus.azurecontainerapps.io/spec \
    -H "Content-Type: application/json" \
    -H "Accept: text/event-stream" \
    -d '{
  "jsonrpc": "2.0",
  "id": "839ffa13-d966-462c-88a2-136805189343",
  "method": "SendStreamingMessage",
  "params": {
    "tenant": "",
    "message": {
      "messageId": "'$(uuidgen | tr '[:upper:]' '[:lower:]')'",
      "taskId": "",
      "contextId": "",
      "role": "ROLE_USER",
      "parts": [
        {
          "text": "task-lifecycle process this",
          "metadata": {},
          "filename": "",
          "mediaType": ""
        }
      ],
      "metadata": {},
      "extensions": [],
      "referenceTaskIds": []
    },
    "configuration": {
      "acceptedOutputModes": [],
      "returnImmediately": false
    },
    "metadata": {}
  }
}'

I think the .Net remote agent should reject these messages (or ignore the empty taskId) but it replies with responses such as

data: {"jsonrpc":"2.0","id":"839ffa13-d966-462c-88a2-136805189343","result":{"task":{"id":"","contextId":"","status":{"state":"TASK_STATE_SUBMITTED","timestamp":"2026-03-26T12:06:00.4665426+00:00"}}}}

which are not deserialised by the a2a-java SDK due to the empty task.id.

The a2a-java client should omit optional fields that are not set when sending messages.
If I remove the taskId: "" field from the reproducer, the .Net agent replies with a generated taskId as expected.

Relevant log output

Code of Conduct

• I agree to follow this project's Code of Conduct