extract-fuzzer-repro

name	extract-fuzzer-repro
description	Reproduce an AST-fuzzer failure locally from its CI seed. Use when a CI fuzzer test (e.g. `pass_vs_prev`) fails and you need a local Noir project to debug. Prefer `just fuzz-repro <seed>` over hand-copying the AST out of the logs.

Reproduce an AST-fuzzer failure locally

The AST fuzzers (pass_vs_prev, acir_vs_brillig, min_vs_full, orig_vs_morph, comptime_vs_brillig_direct, comptime_vs_brillig_nargo, valid_after_pass) are seeded. The fastest and most reliable reproduction is to regenerate the failing program from its seed with just fuzz-repro, which sets the env vars, runs the target, prints the failing AST, and — given an output directory — writes a runnable nargo project for you. This avoids copying a possibly-truncated AST out of the CI log by hand.

1. Get the seed and target from CI

From the GitHub Actions URL (.../runs/RUN_ID/job/JOB_ID), fetch the log and pull out the seed and the failing test name:

gh api repos/noir-lang/noir/actions/jobs/JOB_ID/logs 2>&1 | tee fuzzer_logs.txt

# Seed: printed on failure as `Seed: 0x...` and `NOIR_AST_FUZZER_SEED=0x...`
grep -oE '(NOIR_AST_FUZZER_SEED=|Seed:[[:space:]]*)0x[0-9a-fA-F]+' fuzzer_logs.txt \
  | grep -oE '0x[0-9a-fA-F]+' | sort -u

# Target: the nextest FAIL line, `targets::<target>::tests::fuzz_with_arbtest`
grep -oE 'targets::[a-z_]+::tests::fuzz_with_arbtest' fuzzer_logs.txt | sort -u

CI's .github/scripts/extract-fuzz-seeds.sh produces the same seeds.txt and a seeds-by-test.tsv (test↔seed pairs) from a captured run log, if you have that.

2. Check out the commit CI ran on

The seed→program mapping is deterministic for a given compiler revision but can drift as the AST generator changes. Reproduce on the same commit the fuzzer ran on:

git fetch origin <ci-sha> && git checkout <ci-sha>

If the fuzzer ran on an up-to-date master, your current master is usually fine.

3. Reproduce with just fuzz-repro

# <target> is optional — omit it to try every target until one reproduces.
# <out-dir> makes the harness also emit a runnable nargo project.
just fuzz-repro 0xc63ed07b00100000 pass_vs_prev ./repro

This prints the failing AST (and, for a comparison failure, the ABI inputs) and writes ./repro/{Nargo.toml, src/main.nr, Prover.toml}. When a target compares more than one program (orig_vs_morph) each is written under ast_1/, ast_2/, sharing the same Prover.toml. The env vars it sets — NOIR_AST_FUZZER_SEED, NOIR_AST_FUZZER_EMIT_PROJECT, NOIR_AST_FUZZER_SHOW_SSA, RUST_LOG=debug — are documented in tooling/ast_fuzzer/README.md.

4. Verify the reproduction

cd repro
nargo execute        # add feature flags (e.g. -Zenums for match) if the program/log needs them

This should reproduce the failure. For pass_vs_prev (a semantic-preservation failure) continue with the bisect-ssa-pass skill using this project.

Fallback: copy the AST from the log

If the seed no longer reproduces (e.g. the generator drifted and you cannot check out the original commit), the failing AST and inputs are still printed verbatim in the log. Look for the --- AST: (or --- Failing AST:) and --- Inputs: / ABI Inputs: sections, copy the AST into src/main.nr and the inputs into Prover.toml, then verify as above.

Note in both cases: for the compiled targets the rendered main.nr is best-effort Noir (the exact string the fuzzer uses to build integration tests) and may need small tweaks to parse; the comptime targets emit exact Noir source, and the Prover.toml inputs are always exact.