The Byte in the Gap: Tracing a PSProve CuZK Failure Through the Serialization Boundary

Introduction

In a deep investigation spanning multiple sub-sessions of an opencode coding session, an AI assistant and a user were locked in a battle with one of the most elusive classes of software bugs: a serialization round-trip inconsistency that caused a GPU-accelerated proof system to produce invalid proofs, while the identical data path through the CPU-based FFI worked flawlessly. The bug manifested in the PSProve variant of Proof-of-Replication (PoRep) in the Filecoin storage protocol, specifically when proofs were routed through the CuZK GPU proving engine. Message [msg 1679] captures a pivotal moment in this investigation — the moment when the assistant, prompted by a sharp question from the user, re-examined the fundamental assumption that had guided the entire inquiry, and pivoted toward a more precise analysis of the serialization boundary between Go and Rust.

The Context: A Bug at the Intersection of Two Worlds

To understand the significance of [msg 1679], one must first appreciate the architecture at play. The system had two parallel paths for generating SNARK proofs for Filecoin's Proof-of-Replication (PoRep) mechanism:

  1. The FFI (Foreign Function Interface) path: Go code called Rust functions directly via C bindings. This path worked correctly for all proof types, including PSProve.
  2. The CuZK path: Go code serialized proof data as JSON, sent it over gRPC to a Rust-based GPU proving service, which generated a SNARK and returned it. This path worked for standard PoRep but failed verification for PSProve proofs. The failure mode was specific: CuZK would successfully generate a SNARK from PSProve data, but when that SNARK was verified by ffi.VerifySeal(), it would fail. The same data, passed through the FFI path directly (bypassing CuZK), would produce a valid SNARK that passed verification. The assistant had spent extensive effort ruling out obvious causes: enum mappings were identical across Go, C, and Rust; JSON struct fields matched perfectly; prover_id derivation was identical; Rust dependency versions were aligned. Every structural analysis said the two paths should produce the same result. Yet they didn't.

The User's Pivotal Question

Message [msg 1679] is the assistant's response to a question from the user in [msg 1675]:

"Do we know why PSProve with filecoin-ffi / non-cuzk works fine? Are we correctly serializing the main json payload into the second b64 c1out json like we do in sdr task_porep?"

This question cuts to the heart of the matter. The user is asking two things simultaneously:

The Assistant's Response: A Moment of Clarification

In [msg 1679], the assistant lays out the comparison between the two paths with crystalline clarity:

"Good. In the normal path (task_porep.go), it calls p.sc.PoRepSnarkCuzk() which: 1. Calls GeneratePoRepVanillaProof() → gets raw Rust JSON bytes 2. Calls wrapC1Output(vproof, sectorNum, sectorSize) → wraps with base64 Phase1Out 3. Sends to cuzk

>

Now look at the PSProve path (task_prove.go:264): 1. Has request *proof.Commit1OutRaw (already a Go struct) 2. vproof = json.Marshal(request) → Go re-serialized JSON 3. Wraps inline with the same c1OutputWrapper structure"

The assistant then articulates the critical question:

"The critical question you're asking is: does json.Marshal(request) produce JSON that's compatible with what seal_commit_phase2() / the cuzk Rust code expects as SealCommitPhase1Output?"

This is the moment of reframing. The assistant had been focused on structural parity — do the fields match, do the enum values align — but the user's question forces a shift to byte-level compatibility. The difference between the two paths is not just structural but procedural: in the normal path, the inner JSON is produced by Rust's serde_json serializer; in the PSProve path, it's produced by Go's encoding/json after a round-trip through Go structs.

The Key Insight: FFI Works, So JSON Is Valid

The assistant then makes a crucial observation:

"The FFI non-cuzk path works because ffi.SealCommitPhase2(vproof, sectorNum, minerID) is the same Rust function seal_commit_phase2 — and it successfully deserializes the Go-produced JSON. So we know Go's JSON IS valid for Rust deserialization."

This is both reassuring and confounding. The Go-produced JSON is valid enough for the Rust FFI function to deserialize and produce a valid SNARK. But when the same bytes are wrapped in the CuZK envelope, deserialized by the same Rust library code, and fed to the same seal_commit_phase2 function, the result is invalid.

This narrows the problem space dramatically. The issue cannot be in the inner JSON content itself — if it were, the FFI path would also fail. The issue must be in:

The Spawned Investigation: Tracing the Rust FFI Path

The assistant then spawns a task to investigate the Rust FFI path for seal_commit_phase2:

"[task] {\"description\":\"Check Rust seal_commit_phase2 FFI vs cuzk\",\"prompt\":\"I need to understand exactly how the Rust seal_commit_phase2 function works in the FFI path vs the cuzk path...\"}"

This task represents a deliberate methodological choice. Rather than continuing to trace Go code or compare struct definitions — which had already been exhausted — the assistant decides to examine the Rust side of the boundary in detail. The hypothesis is that there might be a subtle difference in how the FFI entry point and the CuZK service entry point call seal_commit_phase2, even though they ultimately invoke the same core function.

The task spawns a subagent session to read the relevant Rust source files and compare the two code paths. This is a good example of how the assistant uses the task tool to parallelize investigation — it can spawn focused sub-investigations while maintaining the main thread of reasoning.

Assumptions and Their Validity

This message rests on several assumptions, some explicit and some implicit:

Explicit assumption: The Go-re-serialized JSON is valid for Rust deserialization because the FFI path succeeds. This is logically sound — if the same bytes produce a valid SNARK in one context, they should be structurally valid in another. However, the assumption does not account for environmental differences in how the bytes are handled (e.g., encoding issues in the gRPC transport, or differences in how the CuZK Rust server initializes its state).

Implicit assumption: The c1OutputWrapper structure and wrapping process are identical between the normal path and the PSProve path. The assistant states that the PSProve path "wraps inline with the same c1OutputWrapper structure," but this is worth verifying — are the field names, types, and serialization tags truly identical?

Implicit assumption: The CuZK Rust server's deserialization of the outer envelope is lossless. The assistant assumes that base64 encoding/decoding is symmetric, but there could be edge cases with padding, whitespace, or character encoding that cause subtle differences.

Implicit assumption: The seal_commit_phase2 function behaves identically when called from FFI vs when called from the CuZK service. This is likely true at the Rust level, but the CuZK service might pass different parameters (e.g., prover_id or sector_id derived differently from the gRPC request fields vs from the inner JSON).

Input Knowledge Required

To fully understand [msg 1679], one needs:

  1. Knowledge of the Filecoin proof architecture: Understanding that PoRep has multiple variants (standard, PSProve, SnapDeals), each using different circuit parameters and registered proof types.
  2. Knowledge of the CuZK system: Understanding that CuZK is a GPU-accelerated proving service that communicates via gRPC, accepting JSON-encoded proof data and returning SNARK proofs.
  3. Knowledge of Go-Rust serialization boundaries: Understanding that Go's encoding/json and Rust's serde_json have different default behaviors for field ordering, numeric types, and byte array encoding. Go encodes []byte as base64 by default, while Rust's serde might use different representations depending on the type.
  4. Knowledge of the codebase structure: Understanding that task_porep.go handles standard PoRep while task_prove.go handles PSProve, and that cuzk_funcs.go contains the shared CuZK client code.
  5. Knowledge of the previous investigation: Understanding that the assistant had already ruled out enum mappings, struct field definitions, prover_id derivation, and dependency version mismatches.

Output Knowledge Created

This message creates several important outputs:

  1. A clear problem statement: The bug is narrowed to the serialization boundary between Go's json.Marshal and the CuZK Rust server's serde_json::from_slice, specifically in the context of the PSProve path.
  2. A falsified hypothesis: The hypothesis that the Go-re-serialized JSON is somehow "wrong" for Rust deserialization is falsified by the working FFI path. The JSON is valid; the issue is elsewhere.
  3. A new investigative direction: The assistant pivots from comparing Go-side struct definitions to examining the Rust-side code paths, specifically comparing how the FFI entry point and the CuZK service entry point invoke seal_commit_phase2.
  4. A methodological precedent: The use of a spawned task to investigate the Rust code path demonstrates a systematic approach to cross-language debugging.

The Thinking Process Visible in the Message

The reasoning in [msg 1679] follows a clear arc:

  1. Recap: The assistant restates the two code paths, establishing the baseline for comparison.
  2. Reframing: The user's question causes the assistant to reframe the problem from "what's different about the data?" to "what's different about the wrapping?"
  3. Elimination: The FFI path's success eliminates the inner JSON content as the root cause, forcing the investigation toward the wrapping/transport layer.
  4. Deepening: Rather than stopping at this conclusion, the assistant immediately spawns a deeper investigation into the Rust side, recognizing that the answer likely lies in how the CuZK service processes the wrapped data differently from the FFI call. The message also reveals the assistant's debugging philosophy: when faced with a bug that defies structural analysis, the correct approach is to follow the bytes — trace the exact data flow through each transformation, comparing inputs and outputs at each stage. The assistant cannot run code in plan mode, so it uses code reading and task spawning as proxies for runtime analysis.

Conclusion

Message [msg 1679] represents a turning point in a complex debugging session. The user's incisive question forced a re-examination of a key assumption, and the assistant's response demonstrates the disciplined reasoning required to debug cross-language serialization issues. By recognizing that the FFI path's success with the same bytes ruled out the inner JSON as the culprit, the assistant narrowed the search to the wrapping layer and the CuZK service's deserialization logic. The subsequent task investigation would reveal the exact mechanism of the failure, but this message captures the moment when the investigation found its true direction — not in the data itself, but in the boundary across which it traveled.