feat: wire cascade stop into handoff pipeline

- HandoffTool wraps context with cancel, registers run in RunRegistry
- Deregisters on completion (normal or error) via defer
- Propagates ParentRunKey to nested handoffs for correct tree structure
- AgentLoop creates shared RunRegistry, passes to all HandoffTools

pkg/agent/loop.go

@@ -39,6 +39,7 @@ type AgentLoop struct {
 	blackboards    sync.Map // sessionKey -> *multiagent.Blackboard
 	fallback       *providers.FallbackChain
 	channelManager *channels.Manager
+	runRegistry    *multiagent.RunRegistry // tracks active handoff/spawn runs for cascade stop
 }
 
 // processOptions configures how a message is processed
@@ -55,9 +56,10 @@ type processOptions struct {
 
 func NewAgentLoop(cfg *config.Config, msgBus *bus.MessageBus, provider providers.LLMProvider) *AgentLoop {
 	registry := NewAgentRegistry(cfg, provider)
+	runRegistry := multiagent.NewRunRegistry()
 
 	// Register shared tools to all agents
-	registerSharedTools(cfg, msgBus, registry, provider)
+	registerSharedTools(cfg, msgBus, registry, provider, runRegistry)
 
 	// Set up shared fallback chain
 	cooldown := providers.NewCooldownTracker()
@@ -77,6 +79,7 @@ func NewAgentLoop(cfg *config.Config, msgBus *bus.MessageBus, provider providers
 		state:       stateManager,
 		summarizing: sync.Map{},
 		fallback:    fallbackChain,
+		runRegistry: runRegistry,
 	}
 }
 
@@ -122,7 +125,7 @@ func (r *registryResolver) ListAgents() []multiagent.AgentInfo {
 }
 
 // registerSharedTools registers tools that are shared across all agents (web, message, spawn).
-func registerSharedTools(cfg *config.Config, msgBus *bus.MessageBus, registry *AgentRegistry, provider providers.LLMProvider) {
+func registerSharedTools(cfg *config.Config, msgBus *bus.MessageBus, registry *AgentRegistry, provider providers.LLMProvider, runReg *multiagent.RunRegistry) {
 	for _, agentID := range registry.ListAgentIDs() {
 		agent, ok := registry.GetAgent(agentID)
 		if !ok {
@@ -197,6 +200,7 @@ func registerSharedTools(cfg *config.Config, msgBus *bus.MessageBus, registry *A
 				}
 				return registry.CanSpawnSubagent(currentAgentIDForHandoff, to)
 			}))
+			handoffTool.SetRunRegistry(runReg, "")
 			agent.Tools.Register(handoffTool)
 
 			// List agents tool: discover available agents

pkg/multiagent/handoff.go

@@ -65,6 +65,7 @@ type HandoffRequest struct {
 	Depth        int               // current depth level (0 = top-level)
 	Visited      []string          // agent IDs already in the call chain
 	MaxDepth     int               // max allowed depth (0 = use DefaultMaxHandoffDepth)
+	ParentRunKey string            // parent run session key for cascade stop tracking
 }
 
 // HandoffResult contains the outcome of a handoff execution.
@@ -139,6 +140,7 @@ func ExecuteHandoff(ctx context.Context, resolver AgentResolver, board *Blackboa
 				ht.depth = req.Depth + 1
 				ht.visited = newVisited
 				ht.maxDepth = maxDepth
+				ht.parentSessionKey = req.ParentRunKey
 			}
 		}
 	}

pkg/multiagent/handoff_tool.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/sipeed/picoclaw/pkg/tools"
 )
@@ -19,6 +20,8 @@ type HandoffTool struct {
 	visited          []string         // agent IDs already in the call chain
 	maxDepth         int              // max allowed depth (0 = use DefaultMaxHandoffDepth)
 	allowlistChecker AllowlistChecker // optional; nil = allow all
+	registry         *RunRegistry     // optional; nil = no run tracking
+	parentSessionKey string           // session key of the parent run
 }
 
 // NewHandoffTool creates a handoff tool bound to a specific source agent.
@@ -101,6 +104,12 @@ func (t *HandoffTool) SetAllowlistChecker(checker AllowlistChecker) {
 	t.allowlistChecker = checker
 }
 
+// SetRunRegistry sets the registry for tracking active runs (cascade cancellation).
+func (t *HandoffTool) SetRunRegistry(registry *RunRegistry, parentSessionKey string) {
+	t.registry = registry
+	t.parentSessionKey = parentSessionKey
+}
+
 // SetContext updates the origin channel and chat ID for handoff routing.
 func (t *HandoffTool) SetContext(channel, chatID string) {
 	t.originChannel = channel
@@ -143,7 +152,25 @@ func (t *HandoffTool) Execute(ctx context.Context, args map[string]any) *tools.T
 		}
 	}
 
-	result := ExecuteHandoff(ctx, t.resolver, t.board, HandoffRequest{
+	// Create cancellable context for cascade stop support.
+	// If the parent context is cancelled, this handoff is also cancelled.
+	childCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	// Register this run in the registry for cascade cancellation.
+	childSessionKey := fmt.Sprintf("handoff:%s:%s:%d:%d", t.fromAgentID, agentID, t.depth, time.Now().UnixNano())
+	if t.registry != nil {
+		t.registry.Register(&ActiveRun{
+			SessionKey: childSessionKey,
+			AgentID:    agentID,
+			ParentKey:  t.parentSessionKey,
+			Cancel:     cancel,
+			StartedAt:  time.Now(),
+		})
+		defer t.registry.Deregister(childSessionKey)
+	}
+
+	result := ExecuteHandoff(childCtx, t.resolver, t.board, HandoffRequest{
 		FromAgentID:  t.fromAgentID,
 		ToAgentID:    agentID,
 		Task:         task,
@@ -151,6 +178,7 @@ func (t *HandoffTool) Execute(ctx context.Context, args map[string]any) *tools.T
 		Depth:        t.depth,
 		Visited:      t.visited,
 		MaxDepth:     t.maxDepth,
+		ParentRunKey: childSessionKey,
 	}, t.originChannel, t.originChatID)
 
 	if !result.Success {