feat: enforce handoff allowlist with default-open policy

Add AllowlistChecker interface and wire it into HandoffTool to control
which agents can delegate to which. Default behavior is open (allow all)
when no subagents config is present; enforces allow_agents list when
configured via CanSpawnSubagent.

pkg/agent/loop.go

@@ -181,7 +181,23 @@ func registerSharedTools(cfg *config.Config, msgBus *bus.MessageBus, registry *A
 			agent.Tools.Register(multiagent.NewBlackboardTool(placeholderBoard, agentID))
 
 			// Handoff tool: delegate tasks to other agents
-			agent.Tools.Register(multiagent.NewHandoffTool(resolver, placeholderBoard, agentID))
+			handoffTool := multiagent.NewHandoffTool(resolver, placeholderBoard, agentID)
+
+			// Allowlist checker: default-open when no subagents config,
+			// enforces allow_agents when configured.
+			currentAgentIDForHandoff := agentID
+			handoffTool.SetAllowlistChecker(multiagent.AllowlistCheckerFunc(func(from, to string) bool {
+				parent, ok := registry.GetAgent(from)
+				if !ok {
+					return false
+				}
+				// Default open: if no allowlist configured, allow all handoffs
+				if parent.Subagents == nil || parent.Subagents.AllowAgents == nil {
+					return true
+				}
+				return registry.CanSpawnSubagent(currentAgentIDForHandoff, to)
+			}))
+			agent.Tools.Register(handoffTool)
 
 			// List agents tool: discover available agents
 			agent.Tools.Register(multiagent.NewListAgentsTool(resolver))

pkg/multiagent/handoff.go

@@ -16,6 +16,18 @@ type AgentResolver interface {
 	ListAgents() []AgentInfo
 }
 
+// AllowlistChecker determines whether a handoff from one agent to another is allowed.
+type AllowlistChecker interface {
+	CanHandoff(fromAgentID, toAgentID string) bool
+}
+
+// AllowlistCheckerFunc adapts a function to the AllowlistChecker interface.
+type AllowlistCheckerFunc func(fromAgentID, toAgentID string) bool
+
+func (f AllowlistCheckerFunc) CanHandoff(fromAgentID, toAgentID string) bool {
+	return f(fromAgentID, toAgentID)
+}
+
 // AgentInfo is a minimal view of an agent for handoff purposes,
 // decoupled from the full AgentInstance to avoid circular imports.
 type AgentInfo struct {

pkg/multiagent/handoff_tool.go

@@ -18,6 +18,7 @@ type HandoffTool struct {
 	depth            int              // current handoff depth (0 = top-level)
 	visited          []string         // agent IDs already in the call chain
 	maxDepth         int              // max allowed depth (0 = use DefaultMaxHandoffDepth)
+	allowlistChecker AllowlistChecker // optional; nil = allow all
 }
 
 // NewHandoffTool creates a handoff tool bound to a specific source agent.
@@ -94,6 +95,12 @@ func (t *HandoffTool) SetBoard(board *Blackboard) {
 	t.board = board
 }
 
+// SetAllowlistChecker sets an optional checker that controls which agents
+// can be handed off to. If nil, all handoffs are allowed.
+func (t *HandoffTool) SetAllowlistChecker(checker AllowlistChecker) {
+	t.allowlistChecker = checker
+}
+
 // SetContext updates the origin channel and chat ID for handoff routing.
 func (t *HandoffTool) SetContext(channel, chatID string) {
 	t.originChannel = channel
@@ -122,6 +129,11 @@ func (t *HandoffTool) Execute(ctx context.Context, args map[string]any) *tools.T
 		return tools.ErrorResult("agent_id or capability is required")
 	}
 
+	// Allowlist check: if a checker is set and it denies the handoff, block it.
+	if t.allowlistChecker != nil && !t.allowlistChecker.CanHandoff(t.fromAgentID, agentID) {
+		return tools.ErrorResult(fmt.Sprintf("handoff from %q to %q not allowed by policy", t.fromAgentID, agentID))
+	}
+
 	// Parse optional context map
 	var contextMap map[string]string
 	if ctxRaw, ok := args["context"].(map[string]any); ok {