What happens when an AI agent has to pay for its tools? Today, nobody knows. Open-source MCP servers run for free, and their maintainers quietly absorb the cost. The x402 protocol changes the model: a server can charge a small payment for each tool call, with no accounts or API keys needed. The plumbing works. The open question is how agents behave once tools have a price.

We put it to the test. We ran a paid MCP server for several weeks and watched agents act as customers. We changed prices to find the point where agents stop calling a tool. We offered the same tool free and paid, side by side, to see whether agents compare prices or just call the first match. We let spending budgets run out mid-task to see what agents do when money gets tight. And we compared all of this across four agent clients: Google ADK, LangGraph, the OpenAI Agents SDK, and Claude Code.

The result is practical guidance for maintainers: what to charge, where the payment flow breaks, which framework behaviors to expect, and whether per-call payments can realistically fund an open-source MCP server.