Redesign registration to make orchestration simpler

engine/src/stirling/agents/execution.py

@@ -1,13 +1,32 @@
 from __future__ import annotations
 
+from stirling.agents._registry import AgentDescriptor, McpCapability, RegisterableAgent
 from stirling.contracts import AgentExecutionRequest, CannotContinueExecutionAction, NextExecutionAction
 from stirling.services import AppRuntime
 
 
-class ExecutionPlanningAgent:
+class ExecutionPlanningAgent(RegisterableAgent):
     def __init__(self, runtime: AppRuntime) -> None:
         self.runtime = runtime
 
+    def describe(self) -> AgentDescriptor:
+        # MCP-only: an internal sub-agent the orchestrator never delegates to.
+        return AgentDescriptor(
+            mcp=(
+                McpCapability(
+                    id="agent-next-action",
+                    description=(
+                        "Decide the next execution step for an in-progress agent workflow. Returns a"
+                        " ToolCall, Completed, or CannotContinue action."
+                    ),
+                    input_model=AgentExecutionRequest,
+                    mode="sync",
+                    required_scope="mcp.tools.read",
+                    route="/api/v1/agents/next-action",
+                ),
+            ),
+        )
+
     async def next_action(self, request: AgentExecutionRequest) -> NextExecutionAction:
         return CannotContinueExecutionAction(
             reason=f"Execution planning is not implemented yet for step {request.current_step_index}."