When Proofs Fail: A Deep Dive into Debugging Midnight Proof Server Errors

Your contract compiles. The witness generates correctly. But proveTx hangs for five minutes and throws an error you've never seen before. Welcome to the world of ZK proof generation — where the gap between "should work" and "does work" is measured in docker logs, wire format mismatches, and version drift.

I've spent the last few months debugging proof server failures across multiple Midnight dApp projects. Most tutorials tell you to "check docker logs" and leave it at that. This one goes deeper — into the SDK source code, the HTTP protocol between your app and the proof server, and the exact failure modes that cause proofs to silently reject or timeout.

---

How the Proof Server Actually Works (The Architecture You Need to Understand)

Before debugging, you need to understand the request flow. The Midnight SDK doesn't use a single /prove-tx endpoint. Instead, it breaks proving into two HTTP calls against the proof server:

Your dApp
    │
    │ httpClientProofProvider(url, zkConfigProvider)
    │
    ├─→ POST /check   (validates circuit preimage)
    │       ↓
    │    Returns constraint check results
    │
    └─→ POST /prove   (generates the actual ZK proof)
            ↓
         Returns proof bytes

From the httpClientProofProvider source code, here's the critical part:

const retryOptions = {
  retries: 3,
  retryDelay: (attempt: number) => 2 ** attempt * 1_000,  // 2s, 4s, 8s
  retryOn: [500, 503]
};

The SDK automatically retries on HTTP 500 and 503 errors with exponential backoff. If you see a request taking 14+ seconds and then failing, that's not your code — that's three retries timing out.