Here we report the polyoxometalate (POM) condensation or transformation reactions mediated by adaptive coordination-assembled molecular flasks. Addition of Na₂SiO₃ to the (Mo₆O₁₉)₂⊂1·(NO₃)₈ complex containing the Lindquist-type clusters as guests leads to the formation of a new (SiMo₁₂O₄₀)⊂2·(NO₃)₈ host-guest complex, where the in-situ generated Keggin-type cluster served as trigger for the host transformation from cage 1 to isomeric bowl 2. Conversion from 1 to 2 driven by the in-situ condensation was found 27.5-fold faster than the direct templation with independently prepared SiMo₁₂O₄₀^4-. As a comparison, cage 1 was noticed to bind only one W₆O₁₉²⁻ cluster in its cavity, and the formation of (W₁₀O₃₂)⊂2·(NO₃)₈as the main product and (SiW₁₂O₄₀)⊂2·(NO₃)₈as the minor host-guest complex was observed when it was used for the above condensation reaction, highlighting the crucial role of encapsulation during the cavity-confined POMs transformations. The reaction processes and the final structure of all the new host-guest complexes have been investigated by NMR, ESI-TOF-MS and SCXRD. Our findings not only showcase a unique example of inorganic-reaction-driven responsive supramolecular system, but also provide a new approach for the preparation of functional POMs⊂cage composite materials.