diff --git a/docs/contributing/core.md b/docs/contributing/core.md
index e8a4e40..fc020ab 100644
--- a/docs/contributing/core.md
+++ b/docs/contributing/core.md
@@ -1 +1,122 @@
 # Contributing to Core
+
+`foreignthon-core` is the transpiler engine, CLI, and pack loader. Contributions that improve correctness, performance, or usability are welcome.
+
+---
+
+## Repository
+
+[https://git.keshavanand.net/foreign-thon/foreignthon-core](https://git.keshavanand.net/foreign-thon/foreignthon-core)
+
+---
+
+## Dev setup
+
+```bash
+git clone https://git.keshavanand.net/foreign-thon/foreignthon-core
+cd foreignthon-core
+
+python -m venv .venv
+source .venv/bin/activate          # Windows: .venv\Scripts\activate
+
+pip install -e ".[dev]"            # installs foreignthon + pytest + ruff
+```
+
+Verify the install:
+
+```bash
+fpy --version
+pytest
+```
+
+---
+
+## Running tests
+
+```bash
+pytest                  # all tests
+pytest -v               # verbose
+pytest -k postfix       # filter by name
+```
+
+Tests live in `tests/test_engine.py`. They use a local `test_pack.json` fixture (a copy of the Spanish pack) so no installed language pack is needed.
+
+---
+
+## Code style
+
+ForeignThon uses [Ruff](https://docs.astral.sh/ruff/) for linting and formatting:
+
+```bash
+ruff check src/         # lint
+ruff format src/        # format
+```
+
+The CI gate runs both. A PR that fails either will not be merged.
+
+Line length is 88. Import order follows the `I` rule set (isort-compatible).
+
+---
+
+## Project layout
+
+```
+foreignthon-core/
+├── src/
+│   └── foreignthon/
+│       ├── __init__.py      # version
+│       ├── cli.py           # click commands (fpy)
+│       ├── errors.py        # bilingual error hook
+│       ├── pack.py          # pack discovery and validation
+│       ├── template.json    # canonical list of all pack keys
+│       └── transpiler.py    # tokenizer-based engine
+└── tests/
+    ├── test_engine.py
+    └── test_pack.json       # Spanish fixture (no install needed)
+```
+
+---
+
+## Adding new keywords or builtins
+
+`template.json` is the **single source of truth** for the full set of keywords, builtins, exceptions, and stdlib names that all language packs must cover.
+
+To add a new entry (e.g. a new builtin):
+
+1. Add it to `template.json` with the English value as the default.
+2. Add a matching entry to `test_pack.json` (Spanish values).
+3. Update `_build_mapping()` in `transpiler.py` if a new section is needed.
+4. Bump `foreignthon-core` version.
+5. Announce to language pack maintainers so they can update their packs.
+
+Do not add entries to `template.json` that are specific to one language pack.
+
+---
+
+## Adding a new CLI command
+
+All commands are defined in `cli.py` using [Click](https://click.palletsprojects.com/).
+
+- Add your command as a function decorated with `@main.command()`.
+- Include a docstring — Click uses it as the `--help` text.
+- Add `CONTEXT_SETTINGS` to every command for consistent `-h` support.
+- Write tests in `tests/test_engine.py` or a new test file.
+
+---
+
+## Submitting a PR
+
+1. Fork the repository on Gitea.
+2. Create a branch: `git checkout -b feature/my-change`.
+3. Make your change, add or update tests, run `pytest` and `ruff check`.
+4. Open a PR with a clear description of the problem and the fix.
+
+For large changes (new features, engine behaviour changes), open an issue first to discuss before writing code.
+
+---
+
+## What not to change
+
+- **`template.json`** keys should only grow, never shrink — removing a key is a breaking change for all existing language packs.
+- The tokenizer-based approach in `transpiler.py` is intentional. Do not replace it with a regex or AST-based approach without a very strong reason — the tokenizer correctly handles strings, comments, and f-strings without any special casing.
+- The bilingual error format (`[XX] ForeignName: msg` / `[EN] EnglishName: msg`) is part of the public interface. Do not change the format without a major version bump.