serc, an Astral experiment, compiles Python bytecode in Rust with code written by Codex

serc is an experimental compiler that turns Python source into CPython bytecode (.pyc files), written in Rust and published under the astral-experiments GitHub organisation. It is dual-licensed Apache-2.0 and MIT. The repository dates from 24 September and is drawing attention this week.

What it claims

The README says serc produces byte-for-byte identical output to CPython on a large corpus of real code, including Django, Airflow and Transformers, across CPython 3.12, 3.13, 3.14 and 3.15 and optimisation levels 0, 1 and 2, while compiling 3 to 4 times faster than CPython’s compileall. It also warns, plainly, that serc is experimental and “may emit incorrect bytecode in some cases”.

The other headline is how it was built. The README says it is human-edited, but “all code changes, PR summaries, and additional documentation were authored entirely by GPT in Codex”. The repo has about 1,280 commits, and an agents/prompts folder sits in the tree.

Trying it

cargo install serc-cli --locked   # needs Rust 1.96 or later
serc compile hello.py             # writes __pycache__/hello.cpython-314.pyc
serc compile --check path/to/package

It compiles in parallel across all CPUs by default; --jobs sets a limit. The current crate is serc-cli 0.1.11.

Read the benchmark closely

The benchmark table compares serc 0.1.9 with CPython 3.14.5 on a 32-vCPU AMD EPYC VM. Transformers (2,876 files) took 832 ms with CPython and 288 ms with serc; Django took 358 ms and 94 ms. Note that CPython ran with 16 workers and serc with 32, so part of the gap may come from parallelism rather than the compiler itself.

Why this matters

Byte-identical output against CPython is an unusually strict test, which is what makes this a good case study for agent-written Rust: every change can be checked against a reference instead of judged by eye. It is the same pattern as Claude-written ports run against shadow traffic and the Quake port checked by agents. If you want to try it, run serc compile --check on your own package and compare the output with CPython’s before trusting it anywhere.