diff --git a/README.md b/README.md
index 6045ac8..ca06e00 100644
--- a/README.md
+++ b/README.md
@@ -47,6 +47,10 @@ Key features:
 rebar3 compile
 ```
 
+To work on erlang_python itself (tests, CI matrix, FreeBSD VM, how to add
+a NIF, an `erlang.*` function, an option, a suite or a guide), see
+[docs/contributing.md](docs/contributing.md).
+
 ## Quick Start
 
 ### Erlang
diff --git a/docs/contributing.md b/docs/contributing.md
new file mode 100644
index 0000000..825c7e7
--- /dev/null
+++ b/docs/contributing.md
@@ -0,0 +1,210 @@
+# Contributing
+
+How to build, test and extend erlang_python. Read this before your first
+change; each recipe lists every file a change of that kind touches, so
+nothing is left half done. Design background is in
+[architecture](architecture.md), file ownership in [code map](code-map.md),
+message shapes in [protocols](protocols.md).
+
+## Build
+
+You need OTP 27 or later, Python 3.12 or later with headers, CMake and a C
+compiler.
+
+```sh
+rebar3 compile              # runs do_cmake.sh / do_build.sh, builds priv/py_nif.so
+PYTHON_CONFIG=python3.13-config rebar3 compile   # pick an interpreter
+```
+
+Notes:
+
+- `c_src/py_nif.c` includes the other `.c` files; there is no per-file
+  compile. A change in any `c_src` file rebuilds the one NIF.
+- The interpreter used at build time is the one embedded at run time.
+  `python3-config` on `PATH` is the default.
+- `rm -rf _build` when switching Python versions or build flags; CMake
+  caches the interpreter.
+
+## Test
+
+```sh
+rebar3 ct --readable=compact                       # everything
+rebar3 ct --suite test/py_isolated_SUITE           # one suite
+rebar3 ct --suite test/py_context_SUITE --case test_call   # one case
+rebar3 dialyzer && rebar3 xref                     # required before a PR
+make lint-docs                                     # snippets in README and docs/
+```
+
+Notes:
+
+- `test/test.config` turns memory limits on for the whole run; it is
+  applied automatically.
+- The `test` profile pulls `iommap` for the shared memory suites; users
+  add it to their own deps.
+- Free-threaded Python: build and run with `PYTHON_GIL=0` and a `3.13t`
+  interpreter; cases that need the GIL skip themselves.
+- ASan: configure CMake by hand, then let rebar3 pick the objects up:
+
+```sh
+rm -rf _build && mkdir -p _build/cmake && cd _build/cmake
+cmake ../../c_src -DENABLE_ASAN=ON -DENABLE_UBSAN=ON && cmake --build . && cd ../..
+rebar3 compile
+LD_PRELOAD=$(gcc -print-file-name=libasan.so) ASAN_OPTIONS=detect_leaks=0 rebar3 ct
+```
+
+CI runs the matrix in `.github/workflows/ci.yml`: OTP 27 to 29 with Python
+3.12 to 3.14 on Ubuntu and macOS, FreeBSD 14 through `vmactions/freebsd-vm`,
+free-threaded 3.13t, and ASan. A PR is merged when all of them are green.
+
+### FreeBSD by hand
+
+CI covers FreeBSD, but the isolated mode (procctl, no cgroups, no
+`/dev/shm`) is easier to debug in a local VM. On an Apple Silicon Mac:
+
+```sh
+# 1. image (arm64 on Apple Silicon, amd64 elsewhere)
+curl -O https://download.freebsd.org/releases/VM-IMAGES/14.1-RELEASE/aarch64/Latest/FreeBSD-14.1-RELEASE-arm64-aarch64.qcow2.xz
+xz -d FreeBSD-14.1-RELEASE-arm64-aarch64.qcow2.xz
+qemu-img resize FreeBSD-14.1-RELEASE-arm64-aarch64.qcow2 +20G
+
+# 2. boot headless, ssh on host port 2222
+qemu-system-aarch64 -M virt -accel hvf -cpu host -m 4096 -smp 4 \
+  -bios /opt/homebrew/share/qemu/edk2-aarch64-code.fd \
+  -drive file=FreeBSD-14.1-RELEASE-arm64-aarch64.qcow2,if=virtio,format=qcow2 \
+  -netdev user,id=n0,hostfwd=tcp::2222-:22 -device virtio-net-pci,netdev=n0 \
+  -nographic -serial mon:stdio
+```
+
+On the console, log in as `root` (no password), set one, and enable ssh:
+
+```sh
+passwd
+sysrc sshd_enable=YES
+sed -i '' 's/^#PermitRootLogin no/PermitRootLogin yes/' /etc/ssh/sshd_config
+service sshd start
+pkg install -y erlang-runtime28 python313 py313-numpy cmake gmake git
+fetch -o /root/rebar3 https://github.com/erlang/rebar3/releases/download/3.25.0/rebar3
+chmod +x /root/rebar3
+```
+
+Then from the host, ship the tree and run:
+
+```sh
+git archive --format=tgz -o /tmp/ep.tgz HEAD
+scp -P 2222 /tmp/ep.tgz root@127.0.0.1:/root/
+ssh -p 2222 root@127.0.0.1 'rm -rf ep && mkdir ep && tar -C ep -xzf ep.tgz && cd ep \
+  && export PATH=/usr/local/lib/erlang28/bin:$PATH PYTHON_CONFIG=python3.13-config \
+  && /root/rebar3 compile && /root/rebar3 ct --readable=compact'
+```
+
+`erlang-runtime28` installs outside the default `PATH`; `pkg info -l
+erlang-runtime28 | grep bin/erl` shows where. Use `-accel kvm` and the
+amd64 image on Linux hosts.
+
+## Recipes
+
+Every recipe ends with the same three steps: a test case, `rebar3
+dialyzer && rebar3 xref`, and an entry in `CHANGELOG.md` under the
+unreleased version.
+
+### Add a NIF
+
+1. Implement `static ERL_NIF_TERM nif_x(ErlNifEnv*, int, const ERL_NIF_TERM[])`
+   in the `c_src` file that owns the area (see `c_src/README.md`).
+2. Add `{"x", Arity, nif_x, Flags}` to `nif_funcs[]` at the end of
+   `c_src/py_nif.c`. `Flags` is `ERL_NIF_DIRTY_JOB_CPU_BOUND` or
+   `ERL_NIF_DIRTY_JOB_IO_BOUND` when the NIF can block or run Python, `0`
+   otherwise.
+3. Add the stub, its `-spec` and a `@doc` to `src/py_nif.erl`, and the
+   export.
+4. Follow the rules in `c_src/README.md`: only the context thread touches
+   a context's Python objects; release the GIL around every blocking
+   wait; respect the lock order on `py_context_t`.
+5. Test it from a suite through the Erlang API that uses it, not through
+   `py_nif` directly, unless it is a test helper.
+
+### Add an `erlang.*` function for Python
+
+The `erlang` module has three implementations that must agree:
+
+1. Embedded modes, C: add `erlang_x_impl` and its entry in the method
+   table of `c_src/py_callback.c` (`PyMethodDef erlang_methods`). Names
+   starting with `_` are internal helpers for the Python package.
+2. Embedded modes, Python: if the function is written in Python, add it to
+   `priv/_erlang_impl/__init__.py` and to `__all__`; the C module copies
+   it onto `erlang` at import (see the bootstrap code near the end of
+   `py_callback.c`).
+3. Isolated mode: add it to `install_erlang_module` in
+   `priv/_erlang_impl/_isolated.py`. Anything the child cannot support
+   goes through `_not_supported(name)` so the user gets a clear
+   `RuntimeError`, never a silent difference.
+4. Document it in the guide of its area and in `README.md` (API
+   reference section), and add the row to `test/coverage_audit.md`.
+5. Test it in a cross-mode suite so it runs in `worker` and `isolated`
+   groups.
+
+### Add a context option
+
+Options arrive as the map given to `py_context:new/1`. Pool contexts are
+started by `py_context_sup:start_context/2` with the mode only, so an
+option that must apply pool-wide also needs `py_context_router` and the
+supervisor to carry it.
+
+1. Embedded modes: read it where the context is set up in
+   `src/py_context.erl` (`init/4` and the helpers it calls; `memory_limit`,
+   `preload`, `owner` and `start_timeout` are the existing examples).
+   If the C side needs it, pass it to `nif_context_create` and extend the
+   options parsing there.
+2. Isolated mode: read it in `src/py_isolated.erl` (`start_child/1`
+   assembles the child command line and environment; `rlimits`, `cgroup`,
+   `python`, `restart`, `max_restarts`, `kill_after` are the examples) and,
+   if the child must know, add it to the `{init, Opts}` request handled by
+   `_init` in `priv/_erlang_impl/_isolated.py`.
+3. If the option makes no sense in one mode, reject it there with an
+   error that names the option rather than ignoring it.
+4. Document it in the options table of the relevant guide
+   (`docs/isolated.md`, `docs/workers.md`, `docs/memory.md`) and test both
+   the effect and the rejection.
+
+### Add a test suite
+
+1. Name it `test/py_<area>_SUITE.erl`; Python helpers go in
+   `test/py_test_<area>.py` and are imported after
+   `sys.path.insert(0, TestDir)` (see `py_reentrant_SUITE` for the
+   pattern).
+2. Start and stop the application in `init_per_suite` /
+   `end_per_suite`; the default pool is `py:start_contexts/0`.
+3. Behaviour every mode must share runs in groups: `groups/0` with
+   `worker` and `isolated`, contexts created with
+   `py_context:new(#{mode => Mode})` from `init_per_group`
+   (`py_isolated_SUITE` shows the layout).
+4. Skip, do not fail, when a platform or interpreter cannot run a case:
+   `{skip, Reason}` with the reason a human can act on.
+5. Add the suite to the table in `docs/code-map.md` and the cases that
+   cover a documented API to `test/coverage_audit.md`.
+6. Cases that measure time or memory print their numbers with `ct:pal`
+   and assert only on invariants, never on absolute timings.
+
+### Add or change a guide
+
+1. Create `docs/<name>.md` in the task-oriented form: one paragraph on
+   what it is and when you need it, then steps with code, then short
+   notes. Second person, Erlang snippets, no hype.
+2. Every Erlang snippet must call real exports at the right arity and
+   every Python snippet must parse: `make lint-docs` checks both.
+   `<!-- skip-lint -->` above a fence exempts it; say why in the prose.
+3. Register the page in both `extras` lists of `rebar.config` (the flat
+   list and the grouped one) so `rebar3 ex_doc` builds it, and link it
+   from the guide or README section that leads to it.
+4. If the page documents a new API, add its rows to
+   `test/coverage_audit.md`.
+
+## Before opening a pull request
+
+- `rebar3 ct`, `rebar3 dialyzer`, `rebar3 xref`, `make lint-docs` all
+  clean locally.
+- `CHANGELOG.md` updated under the unreleased version: `Added`, `Changed`,
+  `Removed` or `Fixed`. Removing a public function is a major version.
+- The PR text says what the change intends and which path it takes; the
+  diff already lists the files.
+- One squashed commit per PR.
diff --git a/rebar.config b/rebar.config
index 6e23e77..7be76e5 100644
--- a/rebar.config
+++ b/rebar.config
@@ -81,6 +81,7 @@
         <<"docs/glossary.md">>,
         <<"docs/protocols.md">>,
         <<"docs/state-machines.md">>,
+        <<"docs/contributing.md">>,
         <<"docs/preload.md">>,
         <<"docs/owngil_internals.md">>,
         <<"docs/event_loop_architecture.md">>
@@ -121,6 +122,7 @@
             <<"docs/glossary.md">>,
             <<"docs/protocols.md">>,
             <<"docs/state-machines.md">>,
+            <<"docs/contributing.md">>,
             <<"docs/preload.md">>,
             <<"docs/owngil_internals.md">>,
             <<"docs/event_loop_architecture.md">>