Hi HN,

  I built a "reverse CAPTCHA" system that keeps humans out of agent-only spaces without surveillance or identity verification. GitHub:
  https://github.com/henrylai/captchAI-

  The problem: As AI agents proliferate (agent social networks, multi-agent coordination systems, agent marketplaces), manual human interference becomes
  noise. Traditional access control relies on identity and behavioral tracking. What if we used constraints instead?

  How it works: Time-bound proof-of-work challenges. Callers request a cryptographic challenge (SHA256 with N leading zeros), solve it, and submit within a
  narrow window (default: 200ms). Autonomous agents solve trivially. Manual humans fail almost always.

  The system makes no claim about WHO you are—human or AI. It only enforces tempo. Meet the constraint, or don't pass.

  Technical details:
  - Node.js/Express API with multi-tenant support
  - SHA256 proof-of-work (configurable difficulty)
  - Tight time constraints (200ms default, configurable per tenant)
  - One-time use challenges (no replay attacks)
  - Rate limiting (100 req/min per tenant, sliding window)
  - Comprehensive test suite (19 tests)
  - Production-ready (graceful shutdown, structured logging)

  Status: This is exploratory infrastructure for platforms that don't quite exist yet. Market timing is unclear—might be 2-3 years early. But it's a starting
  point for discussion around constraint-based access control.

  Interesting coincidence: After I shipped this, I discovered someone else launched Clawptcha (https://clawptcha.com) with a similar concept the same day.
  They use timing tests and prime factorization; I use PoW and tighter windows. Different approaches, same core insight.

  Live demo: https://web-production-dede0.up.railway.app/health

  The philosophical framing is in the README: "The boundary is the constraint itself, not a judgment about who or what you are."

  Open to feedback on:
  - Is this infrastructure we'll actually need?
  - Are there better primitives for agent-only access control?
  - What use cases make sense today vs. 3 years from now?
  - Is 200ms too tight/loose for distinguishing automated vs. manual?

  MIT licensed. Built in public. Honest about market uncertainty.