How the proof-of-work free tier works

Agent402 has over a thousand tools, and most of them are pure CPU — no external API calls, no network I/O, just computation. Things like JSON formatting, hash generation, regex matching, unit conversion, and text analysis.

For these tools, we offer a proof-of-work free tier: instead of paying USDC, the caller solves a small computational challenge. It's the same idea as Hashcash (the precursor to Bitcoin mining), adapted for API access control.

How it works

Request a challenge. The client sends a request without payment. The server responds with 402 Payment Required and includes a PoW challenge in the response headers.
Solve the challenge. The client finds a nonce that, when combined with the challenge, produces a hash with a required number of leading zero bits. This takes roughly 50-200ms on modern hardware.
Submit the solution. The client re-sends the original request with the PoW solution in the headers. The server verifies the solution (instant) and serves the result.

Each solution is single-use and slug-scoped — it can only be used once, and only for the specific tool it was issued for. This prevents replay attacks and solution-sharing across tools.

Why proof-of-work?

We wanted a free tier that didn't require registration or API keys (that would defeat the whole point of x402). PoW gives us three things:

Abuse prevention. Solving a challenge has a real CPU cost, so bulk abuse is expensive even though the tools are "free."
No identity required. The caller doesn't need an account, email, or API key. Just compute the answer.
Fair access. Every caller pays the same cost — a few milliseconds of CPU time — regardless of who they are.

Browser-side solving

The PoW challenge is designed to be solvable in the browser using Web Crypto. The agent402-client SDK handles this automatically — it detects a 402 response, solves the challenge, and retries, all transparently. For agents using the MCP integration, the hosted server at /mcp handles PoW internally.

Which tools are free?

Any tool that runs purely on the server's CPU without making external network requests is PoW-eligible. Tools that call upstream APIs (web search, rendering, geocoding) require payment because they have a real marginal cost. The tool catalog marks each tool's pricing — $0.000 means PoW-eligible.