APIX: Sustaining an Open LLM API Market Without Centralized Provider Admission

Autonomous agents increasingly rely on LLM API services. Yet existing API platforms adopt centralized admission, limiting two-sided openness by restricting provider participation and curating user choice. A decentralized market is a potential solution, but opaque API services introduce a lemons problem: a plausible response alone does not reveal whether the provider actually delivered the advertised service. The key challenge is therefore how to generate credible evidence of provider behavior and sustain market viability in such an open setting. In response, we present APIX, a decentralized, permissionless market for LLM API services. APIX uses auction-based matching to coordinate heterogeneous user demands with provider-defined services, a multidimensional reputation system to support informed user selection, and peer-relative consistency evaluation to assess whether delivered services are consistent with providers' advertised capabilities. We evaluate APIX through an LLM-agent-based market simulation that captures the co-evolution of participant strategies and market conditions: APIX reduces opportunistic-profit share from 26.4% to 0.5% and improves service satisfaction from 90.0% to 99.4%. Our blockchain-based prototype further incurs less than 300ms of additional latency per call over direct API access, corresponding to only 3% overhead for the 8B model. Overall, these results show that APIX can sustain a reliable and two-sided-open LLM API market without centralized admission.