The Pivot Point: Tracing Intermittent Proof Failures from Go JSON to Bellperson's Core

In a sprawling investigation into an intermittent PSProve PoRep CuZK failure—a bug that caused valid-looking proofs to be silently rejected by Filecoin's VerifySeal—a single message (msg id=1747) marks the exact moment when the investigation pivots from one hypothesis to another. After days of tracing enum mappings across Go, C, and Rust, ruling out fr32 seed masking, and designing elaborate round-trip tests, the assistant has just delivered the smoking gun: the Go JSON serialization round-trip is not the culprit. Now, in this brief but decisive message, the assistant turns toward the proving system itself, asking whether the flakiness lives in bellperson's core.

The Investigation's Arc

To understand the significance of message 1747, we must trace the investigation that led to it. The production symptom was an intermittent failure in the PSProve pipeline: proofs generated by the CuZK proving engine would sometimes fail VerifySeal on the Go side, producing the error "porep failed to validate". The failures were sporadic, affecting roughly one in several partitions, making them difficult to reproduce and diagnose.

The investigation had pursued two main hypotheses:

  1. The Go JSON round-trip hypothesis: In the PSProve pipeline, the Rust-generated C1 output (the first phase of the PoRep proof) is serialized to JSON by Rust, then deserialized by Go, then potentially re-serialized through Go's JSON marshaler before being passed to the FFI for C2 (the second phase). If the Go JSON marshaler produced different bytes than the Rust JSON serializer, the proof data could be corrupted.
  2. The CuZK engine hypothesis: The CuZK proving engine itself might be producing invalid proofs in certain pipeline modes (Phase 6 slot-based or Phase 7 partition-worker modes), and the diagnostic self-check that was supposed to catch these was returning the proof anyway. Through a series of carefully designed tests (messages 1728–1746), the assistant systematically eliminated hypothesis #1. The critical experiment was TestRepeatedC2SameSector (msg 1744), which sealed a single 2KiB test sector and then called ffi.SealCommitPhase2 multiple times on the same data, alternating between raw Rust JSON and Go-roundtripped JSON. The results were unambiguous: both paths failed intermittently. Raw Rust JSON failed on iteration 1; Go-roundtripped JSON failed on iteration 3. The failures affected both paths equally and randomly. The Go JSON round-trip was completely innocent. The error message from these failures was "post seal aggregation verifies"—a self-verification check inside Rust's seal_commit_phase2 that runs after proof generation. The proving system was generating a SNARK proof and then immediately verifying it, and the verification was failing. This meant the proof generation itself was non-deterministic: sometimes it produced a valid proof, sometimes it produced an invalid one.

The Message Itself

Message 1747 reads in its entirety:

[assistant] Good, proof parameters are there. Now the key question: **is this 2KiB flakiness a known bellperson issue, or does it reproduce in the FFI tests?** Let me check if the FFI itself has tests that do repeated C2:
[grep] SealCommitPhase2
No files found

This is a remarkably dense message. In three sentences and one grep command, the assistant accomplishes three distinct movements: confirmation, questioning, and investigation. Let us examine each.

"Good, proof parameters are there."

This line refers to the output of ls /var/tmp/filecoin-proof-parameters/ from the previous message (msg 1746), which listed the Groth16 proving parameters and verification keys for the 2KiB sector circuit. The presence of these files—.params files containing the structured reference strings (SRS) and .vk files containing verification keys—confirms that the test environment is properly configured. Without these parameters, the bellperson proving system cannot generate or verify proofs at all.

This confirmation is important because it eliminates a class of environmental issues. The failures are not due to missing parameters, corrupted parameter files, or an incomplete setup. The proving system has everything it needs to function correctly—yet it still produces invalid proofs intermittently.

"Now the key question: is this 2KiB flakiness a known bellperson issue, or does it reproduce in the FFI tests?"

This is the pivotal question that drives the remainder of the investigation. The assistant has just proven that the data path is clean. The failures originate from ffi.SealCommitPhase2 itself—the Go FFI binding to Rust's seal_commit_phase2 function. But the nature of the flakiness is still unknown, and the answer determines the entire next phase of work.

There are three possibilities, each with different implications:

Possibility A: The flakiness is specific to 2KiB test sectors. Small circuits in bellperson (and its underlying proving systems like groth16/bellman) are known to exhibit different behavior than large circuits. The 2KiB sector circuit has very few constraints, and the prover's random challenges may not provide enough "mixing" to consistently produce valid proofs. If this is the case, the flakiness would be a testing artifact that does not affect production 32GiB sectors.

Possibility B: The flakiness is a known FFI issue that reproduces in existing FFI tests. If the Filecoin FFI test suite already contains tests that call SealCommitPhase2 repeatedly, and those tests are known to be flaky, then this is a documented limitation that the team may have already worked around.

Possibility C: The flakiness is a new discovery. If neither the 2KiB specificity nor existing FFI tests explain the behavior, then this could be a more fundamental issue in bellperson or its GPU proving backend (supraseal C++) that could potentially affect production sectors.

The assistant's next action is designed to distinguish between these possibilities.

"Let me check if the FFI itself has tests that do repeated C2:"

The assistant runs a grep for SealCommitPhase2 across the FFI codebase. This is a straightforward but powerful diagnostic step. If the FFI has tests that call SealCommitPhase2 multiple times in sequence, those tests would likely have caught this flakiness. If they don't exist, it means the behavior has not been systematically characterized.

The choice of search term is deliberate. SealCommitPhase2 is the Go function name in the FFI bindings. By searching for this string, the assistant is looking for Go test files that exercise the C2 path. The grep is scoped to the entire workspace, which includes the FFI source code.

"No files found"

This is a significant result. The Filecoin FFI codebase—a mature, well-tested library that has been in production for years—does not contain any tests that call SealCommitPhase2 repeatedly on the same or different sector data.

This finding has several implications:

  1. The flakiness has not been caught by existing tests. The FFI's test suite exercises single C2 calls on freshly sealed sectors, which would not reveal intermittent failures that manifest only on repeated calls.
  2. The investigation is genuinely breaking new ground. The assistant is exploring behavior that has not been systematically characterized by the FFI's own test suite.
  3. The production impact is unclear. If the flakiness only manifests on repeated C2 calls within the same process, it might not affect production pipelines that prove each sector once. But the PSProve pipeline, which processes multiple partitions sequentially, could be triggering this exact pattern.
  4. The 2KiB vs. production-size question remains open. The grep result does not distinguish between 2KiB and 32GiB circuits. The flakiness could be specific to small circuits, or it could be a general property of the proving system that is only visible under repeated calls.

The Thinking Process Visible in the Message

Message 1747 reveals a disciplined debugging methodology. The assistant's thinking process can be reconstructed as follows:

Step 1: Verify the environment. Before asking deeper questions, confirm that the basic prerequisites are met. The proof parameters are present, so the test environment is not the source of the failures.

Step 2: Formulate the critical question. The investigation has reached a branching point. The Go JSON hypothesis has been eliminated. The remaining question is whether the flakiness is a known artifact of small circuits or a new discovery. The answer determines whether the investigation should focus on 2KiB-specific behavior or escalate to a more fundamental proving system issue.

Step 3: Take immediate action. Rather than speculating, the assistant runs a grep to check whether the FFI has relevant tests. This is a low-cost, high-information-density action that immediately narrows the possibilities.

Step 4: Process the result. "No files found" is a non-trivial result. It means the FFI does not test repeated C2 calls, which explains why the flakiness wasn't caught earlier. But it also means the assistant cannot immediately classify the flakiness as "known" or "unknown" based on existing test coverage.

The Broader Significance

Message 1747 is brief—barely 50 words of prose plus a grep command—but it represents a critical juncture in a complex investigation. The assistant has just spent several messages designing and running tests that definitively ruled out the most likely suspect. Now, instead of chasing another hypothesis immediately, the assistant pauses to ask a meta-question: what kind of problem is this?

This is the difference between fixing a bug and understanding a system. The grep for SealCommitPhase2 is not just about finding tests; it's about understanding the landscape. If the flakiness is a known bellperson issue, the fix might involve upgrading dependencies or applying a known workaround. If it's new, the fix might involve deeper changes to the proving pipeline or even a bug report upstream.

The "No files found" result sets the stage for the next phase of the investigation. The assistant will go on to discover that the same self-check bug exists in multiple pipeline paths in the CuZK engine (Phase 6, Phase 7, batched multi-sector, and single-sector), and will deploy a fix that makes the self-check mandatory across all paths. But that fix addresses the symptom—invalid proofs being returned to the caller—not the root cause of the proving system's non-determinism. The deeper question of why bellperson sometimes produces invalid proofs remains open, and message 1747 is the moment when that question is first clearly articulated.

In the broader narrative of the investigation, this message represents the transition from "what is corrupting the data?" to "why is the proving system unstable?"—a shift in perspective that ultimately leads to a more robust fix and a deeper understanding of the system's behavior.