Buying a finance API suite vs building the math yourself
Buy when you want correct, audited finance math now and your edge is the agent, not the quant library. Build when the calculations are your core IP or you have unusual requirements the suite doesn't cover.
Build the math in-house, Use the Hermes Plant suite
Build the math in-house
Building gives full control and no per-call cost, and makes sense when the quant library is your differentiator. The hidden cost is real: IRR solvers, Black-Scholes with a correct normal CDF, waterfalls with catch-up edge cases, and bond duration are each easy to get subtly wrong, need a test suite, and must be hosted and maintained.
Use the Hermes Plant suite
The suite gives deterministic, test-backed endpoints for cashflow returns, options, waterfalls, bonds, and portfolio risk, callable per request over x402 with no infrastructure to run. You pay a few cents per call and ship the agent today. The tradeoff is a dependency and a per-call fee — worth it when the math is table stakes, not your moat.
Recommendation
Buy when you want correct, audited finance math now and your edge is the agent, not the quant library. Build when the calculations are your core IP or you have unusual requirements the suite doesn't cover.
How to combine
Most teams let the LLM orchestrate and route the actual finance math to deterministic endpoints, paying per call over x402. Start with the agent services page and the OpenAPI spec to wire the first endpoint into your agent.