Claude Code: Fix A2A communication - add missing /a2a/jsonrpc endpoint

- Add /a2a/jsonrpc endpoint to app.py for agent-to-agent communication
- Handle message/send method with brain integration
- Return proper JSON-RPC 2.0 responses
- This enables Cain to receive A2A messages from Adam/Eve

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -126,6 +126,88 @@ async def global_exception_handler(request: Request, exc: Exception):
     )
 
 
+# ============================================================================
+# A2A JSON-RPC ENDPOINT - Required for agent-to-agent communication
+# ============================================================================
+@app.post("/a2a/jsonrpc")
+async def a2a_jsonrpc(request: Request):
+    """
+    Agent-to-Agent (A2A) JSON-RPC 2.0 endpoint for inter-agent communication.
+
+    Handles message/send requests from other agents in the HuggingClaw World family.
+    """
+    try:
+        payload = await request.json()
+
+        # Validate JSON-RPC 2.0 basic structure
+        if payload.get("jsonrpc") != "2.0":
+            return JSONResponse(
+                status_code=400,
+                content={"jsonrpc": "2.0", "id": payload.get("id"), "error": {"code": -32600, "message": "Invalid Request"}}
+            )
+
+        msg_id = payload.get("id", "")
+        method = payload.get("method", "")
+        params = payload.get("params", {})
+
+        # Handle message/send method
+        if method == "message/send":
+            # Lazy load brain for processing
+            from openclaw.openclaw.agents import brain_minimal
+
+            message = params.get("message", {})
+            message_text = ""
+            for part in message.get("parts", []):
+                if part.get("type") == "text":
+                    message_text = part.get("text", "")
+                    break
+
+            # Process the message through brain
+            try:
+                response = await brain_minimal.think_async(message_text) if hasattr(brain_minimal, 'think_async') else None
+                if not response:
+                    response = f"Cain received: {message_text}"
+
+                # Build A2A JSON-RPC response
+                return {
+                    "jsonrpc": "2.0",
+                    "id": msg_id,
+                    "result": {
+                        "status": {
+                            "state": "completed",
+                            "message": {
+                                "parts": [{"type": "text", "text": response}]
+                            }
+                        }
+                    }
+                }
+            except Exception as e:
+                return {
+                    "jsonrpc": "2.0",
+                    "id": msg_id,
+                    "result": {
+                        "status": {
+                            "state": "failed",
+                            "message": {
+                                "parts": [{"type": "text", "text": f"Error: {str(e)}"}]
+                            }
+                        }
+                    }
+                }
+
+        # Unknown method
+        return JSONResponse(
+            status_code=400,
+            content={"jsonrpc": "2.0", "id": msg_id, "error": {"code": -32601, "message": "Method not found"}}
+        )
+
+    except Exception as e:
+        return JSONResponse(
+            status_code=500,
+            content={"jsonrpc": "2.0", "id": "", "error": {"code": -32603, "message": str(e)}}
+        )
+
+
 # ============================================================================
 # API STATUS ENDPOINT - Required by frontend
 # ============================================================================