The Missing PCE: A Remote Investigation into Non-Deterministic Proof Failures

Introduction

In the course of debugging a complex proving pipeline for zero-knowledge proofs, a single investigative message can pivot an entire debugging session from one hypothesis to another. Message 245 in this opencode conversation represents precisely such a pivot point. In this message, the assistant—an AI coding agent—executes two remote shell commands on a calibnet host to inspect the deployed binary and check for a critical cache file. The results reveal a startling discrepancy: the Pre-Compiled Constraint Evaluator (PCE) file for PoRep 32GiB proofs, which the system claims to be using, does not actually exist on disk. This discovery fundamentally reframes the debugging effort and illustrates the detective work required when distributed proving systems exhibit non-deterministic failures.

The Message

The assistant's message consists of two bash commands executed via SSH on the remote host 10.1.16.218, followed by their output:

[assistant] The startup logs have rotated out. Let me check with more history and also look at the cuzk binary version:
[bash] ssh 10.1.16.218 'ls -la /usr/local/bin/cuzk && /usr/local/bin/cuzk --version 2>&1 || true; stat /usr/local/bin/cuzk 2>&1 | grep -i modif'
-rwxr-xr-x 1 root root 27902992 Mar  1 20:54 /usr/local/bin/cuzk
error: unexpected argument '--version' found

Usage: cuzk [OPTIONS]

For more information, try '--help'.
Modify: 2026-03-01 20:54:23.740835066 +0000

[bash] ssh 10.1.16.218 'ls -la /data/zk/params/pce-porep-32g.bin 2>&1; stat /data/zk/params/pce-porep-32g.bin 2>&1 | grep -i modif'
ls: cannot access '/data/zk/params/pce-porep-32g.bin': No such file or directory

The first command confirms that the cuzk binary exists at /usr/local/bin/cuzk, was last modified on March 1st at 20:54 UTC, and weighs approximately 28 MB. The --version flag is not supported, so the exact build cannot be identified from the binary alone. The second command checks for the PCE cache file at the expected path and returns a definitive negative: the file does not exist.

Context and Motivation

To understand why this message was written, we must trace the debugging arc that preceded it. The user had deployed a set of fixes to a remote calibnet host—fixes that harmonized constraint system types (WitnessCS, RecordingCS, ProvingAssignment) to resolve a WindowPoSt crash caused by mismatched input counts during PCE extraction. After deployment, the user reported that PoRep proofs were failing with a random, non-deterministic pattern: on one run, 7 out of 10 partitions would validate; on a retry with the same data, only 1 out of 10 would validate. This pattern of random partition invalidity was deeply suspicious because it did not look like a deterministic synthesis bug—it looked like a data race, a stale cache, or a randomness issue in the GPU proving path.

The assistant had been working through a systematic investigation. Earlier messages in the conversation (messages 232–244) show the assistant examining journalctl logs, reading the service configuration, inspecting the synthesis pipeline code, and searching for how random values r_s and s_s are generated for each partition. The startup logs had rotated out, preventing the assistant from seeing the initial PCE loading messages. This forced a direct inspection of the filesystem.

The assistant's reasoning, visible in the preamble to the bash commands, is explicit: "The startup logs have rotated out. Let me check with more history and also look at the cuzk binary version." This reveals a methodical approach—when one information source is exhausted (journalctl history), the assistant pivots to another (direct filesystem inspection).

The Investigation Process

The message reveals a two-pronged investigation strategy. The first command checks the binary itself: its size, modification time, and whether it supports a --version flag. The modification time of March 1st at 20:54 is significant because it tells the assistant that the binary was built before the most recent code changes (which included the WitnessCS::new() fix and the protobuf-compiler documentation updates). This confirms the user's suspicion that the host is running a "slightly stale build." The absence of a --version flag means the assistant cannot map the binary to a specific git commit, but the modification timestamp provides a rough proxy.

The second command is the critical one. The assistant checks for the PCE file at /data/zk/params/pce-porep-32g.bin—the path where the PCE cache for 32 GiB PoRep proofs should reside. The result is unambiguous: "No such file or directory." This is a bombshell discovery. Throughout the journalctl logs, the system repeatedly logs "using PCE fast path for synthesis circuit_id=porep-32g" for every partition. Yet the PCE file that should back this fast path does not exist on disk.

Assumptions and Their Implications

The assistant made several assumptions in crafting this message. First, it assumed that the PCE file would be stored at the standard path /data/zk/params/pce-porep-32g.bin, consistent with the parameter cache directory configured in /etc/cuzk.toml (param_cache = "/data/zk/params"). This assumption was reasonable given the configuration file, but it may have been incorrect if the PCE was stored elsewhere or if the naming convention differed.

Second, the assistant assumed that checking the binary's modification time would provide useful provenance information. This was correct—the March 1st timestamp confirmed the binary was built before the latest fixes.

Third, the assistant implicitly assumed that the PCE should exist on disk if the system is using it. This assumption is more nuanced. The system could be generating the PCE in memory on each startup without persisting it to disk, or it could be loading it from a different path. However, the fact that the system logs "using PCE fast path" without any preceding "PCE loaded from disk" or "PCE generated" log lines suggests that either the PCE loading log was in the rotated-out startup logs, or the PCE is being generated silently.

Input Knowledge Required

To fully understand this message, one needs knowledge of several domains:

  1. The CuZK proving system architecture: Understanding that PCE (Pre-Compiled Constraint Evaluator) is a cache of evaluated constraints that speeds up synthesis by avoiding re-evaluation of the circuit structure on every proof run.
  2. The deployment topology: The remote host 10.1.16.218 is a calibnet node running the cuzk daemon as a systemd service, with GPU proving enabled and a partitioned pipeline for PoRep proofs.
  3. The parameter cache directory structure: The configuration at /etc/cuzk.toml specifies param_cache = "/data/zk/params", which is where PCE files and verification keys are stored.
  4. The debugging context: The random partition invalidity pattern (7/10 valid on one run, 1/10 on retry) is the central mystery, and the PCE's existence is a key variable in solving it.
  5. The previous code changes: The assistant had just fixed a WitnessCS::new() pre-allocation issue that caused input count mismatches between WitnessCS and ProvingAssignment. This fix was deployed to the remote host, but the binary timestamp suggests it may not include this fix.

Output Knowledge Created

This message produces several critical pieces of knowledge:

  1. The binary is stale: The March 1st modification timestamp confirms the binary was built before the latest WitnessCS::new() fix, meaning the deployed code may still have the pre-allocation bug for any proof type that uses WitnessCS::new() in a way that triggers the mismatch.
  2. The PCE file is missing: The file pce-porep-32g.bin does not exist at the expected path. This immediately raises questions: Is the PCE being generated in memory but not persisted? Is it stored elsewhere? Was it deleted? Was the PCE generation failing silently?
  3. A new hypothesis emerges: The missing PCE file suggests that the random partition failures might be caused by the system falling back to a different synthesis path, or by the PCE being regenerated inconsistently across runs. If the PCE is generated fresh on each daemon restart and stored only in memory, then a service restart between proof attempts could cause the PCE to be regenerated with different parameters, leading to non-deterministic behavior.
  4. The investigation must pivot: Rather than focusing on the WitnessCS::new() fix or the GPU proving pipeline, the assistant must now investigate why the PCE file is missing and whether the PCE is being generated correctly at all.

The Thinking Process

The assistant's thinking process is visible in the structure of the message itself. The preamble—"The startup logs have rotated out. Let me check with more history and also look at the cuzk binary version"—reveals a problem-solving strategy: when one line of inquiry is blocked (rotated logs), pivot to another (direct filesystem inspection).

The choice of two parallel commands is also revealing. The first command checks the binary version (provenance), and the second checks the PCE file (state). This is a classic debugging pattern: verify what code is running, then verify what data it has access to. The assistant is systematically ruling out hypotheses.

The error handling in the first command is notable: --version 2>&1 || true. The || true ensures that the SSH command doesn't fail if --version returns a non-zero exit code. This is a practical consideration for remote command execution—it prevents a single failed subcommand from aborting the entire SSH session.

The second command uses a redundant check: both ls -la and stat are called, with error output redirected to stdout via 2>&1. This ensures that even if ls fails (which it does, with "cannot access"), the error message is captured, and stat is still attempted. This redundancy is a defensive programming technique that maximizes the information returned from a single remote command.

Broader Significance

This message is a turning point in the debugging session. Before it, the assistant was investigating whether the random partition failures were caused by a GPU pipeline race condition or a randomness issue in r_s/s_s generation. After it, the focus shifts to the missing PCE file. The non-deterministic failure pattern now has a plausible explanation: if the PCE is generated in memory on daemon startup and lost on restart, then different runs of the proving pipeline might be using different PCE states, leading to inconsistent results.

The message also highlights a common challenge in distributed systems debugging: the gap between what a system claims to be doing (logging "using PCE fast path") and what the filesystem actually contains (no PCE file). This gap is where bugs hide. The assistant's willingness to verify assumptions by directly inspecting the remote filesystem, rather than trusting the logs alone, is a hallmark of rigorous debugging.

Moreover, the message demonstrates the importance of provenance tracking in deployed systems. The inability to get a version string from the binary (--version not supported) means the assistant must rely on indirect evidence (modification timestamps) to determine what code is running. This is a fragility that the development team might want to address by adding proper versioning to the binary.

Conclusion

Message 245 is a masterclass in remote debugging methodology. Faced with rotated logs and a non-deterministic failure pattern, the assistant pivots to direct filesystem inspection and makes a critical discovery: the PCE file that the system claims to be using does not exist on disk. This finding reframes the investigation from a GPU pipeline race condition to a PCE persistence problem, opening a new line of inquiry that will ultimately lead to the root cause. The message demonstrates the value of systematic hypothesis testing, defensive remote command construction, and the willingness to verify assumptions against ground truth.