It is possible to make Integrated Development Environments supporting formal methods that can be as flexible as the support for dynamic programming languages. This paper contributes with a demonstration employing different support environments for the Vienna Development Method Specification Language (VDMSL) and design by contract for visual programming language. This includes ViennaTalk developed on top of Pharo 4 providing Smalltalk-styled LIVE browsers, VDM-SL interpreters, Smalltalk code generators, UI prototyping environments and a prototypeWeb API server to enable rigorous and flexible modeling during exploratory phases of software development. ViennaTalk uses the Slot mechanism in Pharo to test invariant assertions on instance variables in Smalltalk objects generated from VDM-SL specifications. In addition, we present a plugin named Assertch for Phratch, a scratch-clone visual programming environment on top of Pharo 4, that provides assertion blocks for designing and debugging a series of blocks. Both ViennaTalk and Assertch combine flexible live modeling or coding while still supporting rigorous checking. ViennaTalk has been evaluated by experienced professional engineers of VDM-SL while Assertch has been evaluated by undergraduate students of computer science. ViennaTalk and Assertch both demonstrate that Pharo and its contemporary features support rigorous modeling in formal specification languages as well as flexible prototyping in Smalltalk.