A2A JSON-RPC endpoint

A2A Protocol v1.0 JSON-RPC 2.0 transport endpoint. The request body is a JSON-RPC 2.0 envelope { jsonrpc: "2.0", id?, method, params } dispatched on method. Method names are the 1.0 PascalCase forms; the legacy 0.3 slash-strings are still accepted as tolerant aliases:

• SendMessage (alias message/send) — params: { message, configuration?, metadata? }; returns an envelope whose result is { task }.
• SendStreamingMessage (alias message/stream) — params: same; returns a text/event-stream of A2A 1.0 StreamResponse events.
• GetTask (alias tasks/get) — params: { id | taskId, historyLength? }; returns a Task envelope.
• ListTasks — params: { pageSize?, pageToken?, status?, includeArtifacts? }; returns { tasks, nextPageToken, pageSize, totalSize } (cursor-paginated).
• CancelTask (alias tasks/cancel) — params: { id | taskId }; returns a Task envelope.
• SubscribeToTask (alias tasks/resubscribe) — params: { id }; returns a text/event-stream of A2A 1.0 StreamResponse events. Each streaming data: line is a JSON-RPC response carrying a StreamResponse oneof (statusUpdate / artifactUpdate); errors stream as a JSON-RPC error envelope (google.rpc.Status form). The body is validated by the handler (not by request-schema middleware) so JSON-RPC error envelopes (-32700, METHOD_NOT_FOUND, INVALID_PARAMS) are preserved. Authentication is an A2A surface key (Authorization: Bearer a2a_xxx or X-API-Key header) or a Clerk JWT (dashboard testing).