From Grok:

Iran is the primary external supplier of missiles and missile technology to the Houthis (Ansar Allah).

Iran’s Islamic Revolutionary Guard Corps (IRGC), particularly its Quds Force, provides ballistic and cruise missiles (or components/kits based on Iranian designs), along with training, technical advisors, guidance systems, warheads, and related support. This includes systems derived from Iranian models such as variants of the Qiam, Shahab, Kheibar Shekan, and anti-ship missiles like the Ghadir.

Key evidence includes:

• Multiple large interdictions of Iranian-origin weapons shipments (including hundreds of tons of missiles, components, seekers, and warheads) by Yemeni forces and others, with Iranian markings and documentation. alhurra.com +1
• UN Panel of Experts reports concluding that the Houthis lack the independent capacity for complex systems like advanced ballistic and cruise missiles and that their capabilities stem from Iranian (and allied) transfers of materiel, technology, and training. Similarities to Iranian systems are well-documented. apnews.com +1
• Analyses by groups like Conflict Armament Research showing Iranian missile designations on components recovered in seizures, even as many dual-use parts come from commercial global sources.

The Houthis also assemble and produce some systems locally using Iranian designs, kits, expertise, and captured legacy Yemeni stockpiles (e.g., earlier Scud-type missiles). Dual-use commercial components (electronics, materials) frequently originate from China and other countries via supply chains, while groups like Hezbollah and Iraqi militias have provided additional training and support. Longer-range and more advanced systems remain heavily dependent on external (primarily Iranian) input.

Iran officially denies supplying missiles or weapons to the Houthis, stating it offers only political support and that the group develops its own capabilities. These denials are contradicted by the pattern of interdictions, technical matching of components, and assessments from UN experts, Western governments, and independent arms researchers.