Prevent infinite recursion when agents delegate to each other as tools

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/errors.go

@@ -25,6 +25,10 @@ type (
 		MaxTurns int
 	}
 
+	MaxToolDepthExceededError struct {
+		MaxDepth int
+	}
+
 	InputGuardrailTrippedError struct {
 		Guardrail string
 		Message   string
@@ -79,6 +83,10 @@ func (e *MaxTurnsExceededError) Error() string {
 	return fmt.Sprintf("agent exceeded maximum number of turns (%d)", e.MaxTurns)
 }
 
+func (e *MaxToolDepthExceededError) Error() string {
+	return fmt.Sprintf("agent-tool delegation exceeded maximum depth (%d)", e.MaxDepth)
+}
+
 func (e *InputGuardrailTrippedError) Error() string {
 	return fmt.Sprintf("input guardrail %q tripped: %s", e.Guardrail, e.Message)
 }