BUG: A2A dispatch uses tasks/send (v0.2) — incompatible with nullclaw A2A v0.3.0 message/send #42

Bug: A2A dispatch uses tasks/send (v0.2) — incompatible with nullclaw A2A v0.3.0 message/send

Summary

buildA2aRequestBody builds the A2A JSON-RPC request with method: "tasks/send" and message parts using the type discriminator. Current nullclaw (src/a2a.zig) implements A2A v0.3.0, which requires:

• method message/send (not tasks/send)
• parts discriminated by kind (not type)
• message object with kind: "message" and a messageId

Dispatching a step to an A2A worker (e.g. nullclaw with an a2a worker) fails protocol validation on the worker side.

Root cause

src/dispatch.zig:407-441:

const parts = [_]struct {
    type: []const u8,   // v0.2 discriminator
    text: []const u8,
}{ .{ .type = "text", .text = prompt } };

const message = .{ .role = "user", .parts = parts[0..] };  // no kind, no messageId

return std.json.Stringify.valueAlloc(allocator, .{
    .jsonrpc = "2.0",
    .id = context_id,
    .method = "tasks/send",   // v0.2 method
    .params = params,
}, .{});

Reproduction

1. Register an A2A worker (nullclaw instance exposing the a2a endpoint).
2. Run a workflow with a step whose worker_tags select the A2A worker.
3. Dispatch fails / response cannot be parsed — the worker rejects tasks/send and the part/message shape.

Expected behavior

Request body per A2A v0.3.0:

{
  "jsonrpc": "2.0",
  "id": "<context_id>",
  "method": "message/send",
  "params": {
    "message": {
      "role": "user",
      "parts": [{ "kind": "text", "text": "..." }],
      "kind": "message",
      "messageId": "msg-<context_id>"
    },
    "configuration": { "contextId": "<context_id>" }
  }
}

Context/session semantics unchanged: same context_id → same conversation.