act container "exited with code 137" (OOM) running Actions locally

Exit code 137 means the container received SIGKILL (128 + 9). During act runs this is almost always the out-of-memory killer: the job needed more RAM than Docker Desktop was allotted.

What this error means

A step or the whole job stops with "exited with code 137", often right after a memory-heavy build or test, with no application error printed.

act

[CI/build] container for step exited with code 137
Error: The process '/usr/bin/docker' failed with exit code 137

Common causes

Docker Desktop memory limit is too low

Local Docker caps memory (often 2 to 4 GB by default). A build that fits GitHub-hosted 7 GB runners is killed locally.

A single step allocates more than available

A test suite or compiler spikes past the container limit and the OOM killer terminates the process.

How to fix it

Raise Docker Desktop memory

Open Docker Desktop Settings, Resources, and increase the memory limit.
Apply and restart Docker.
Re-run act; the container now has headroom.

Reduce the step memory footprint

Limit parallelism in the memory-heavy step so it stays under the container ceiling.

Terminal

# run tests with fewer workers to cut peak memory
pytest -n 2

How to prevent it

Give Docker Desktop enough memory to mirror the 7 GB hosted runner.
Cap parallelism in memory-heavy steps for local runs.
Remember local resources differ from GitHub-hosted runners.