[chaliy]

What changed

• New python_diff_fuzz target (features python,cpython): builds small programs from a grammar of the Monty-supported subset, runs each through the Monty fork and the CPython wasm runtime, and fails when stdout differs. Each statement is wrapped in try/except printing the exception type, so error types are compared too. Runs that hit a limit are skipped (the runtimes enforce limits differently). Nightly in fuzz.yml, build-checked in fuzz-check.
• Fix: python_fuzz never actually ran Monty. It didn't set BASHKIT_ALLOW_INPROCESS_PYTHON, so every input hit the opt-in refusal message. Both targets now opt in and assert python3 really runs, so this can't silently regress.

Why

Crash-only fuzzing can't see wrong results. We now own the Monty fork and re-sync it with upstream; a differential oracle catches semantic regressions from local edits and re-syncs.

Before / After

python_fuzz, before (debug print of each run):

DBG exit=1 err="python3: in-process Python disabled by default for security; set BASHKIT_ALLOW_INPROCESS_PYTHON=1 to enable"

After: 309k execs in 326 s locally, coverage 26,976 edges (vs ~9.8k before), 0 crashes.

python_diff_fuzz, ~9.2k programs locally. One divergence class found, documented and excluded from the grammar:

c.append(len(0))   # c = 0
monty:   TypeError        (evaluates the argument first)
cpython: AttributeError   (looks up c.append first)

No other divergences.

Risk

• Low: fuzz targets and CI wiring only. The diff target runs ~7 exec/s (CPython startup per program), fine for the 5-minute nightly slot.

Checklist

• Tests added or updated
• Backward compatibility considered (n/a)

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	b97f346	Commit Preview URL Branch Preview URL	Nov 07 2026, 04:31 AM

[bashkit-pr-reviewer]

Differential fuzz design is sound (grammar avoids SyntaxError-hash noise, both runtimes metered, refusal fails loudly). One oracle gap below; otherwise no blockers.

[bashkit-pr-reviewer]

Review verdict: approve — rebase only, no new findings

Head b97f346 is a rebase onto new main (449075a); the PR contents are unchanged from the previously reviewed head: python_diff_fuzz.rs still 379 lines with the OverflowError-compared fix and KNOWN GAPS docs, python_fuzz.rs canary intact, same 8 files +423/−1.

Re-verified against the new base: all APIs the targets depend on are present with unchanged semantics — BashBuilder::python_with_limits / cpython_with_limits (with max_duration/max_memory/max_output), ExecResult::err refusal path with the "disabled by default" message on stderr, testing::fuzz_init/fuzz_exec. Prior findings all resolved, nothing outstanding.

Not built/run (no cargo in this shell); compile covered by the ci.yml build line.