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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Common Commands
+
+```bash
+# Run all tests (no network required)
+python test_suite.py
+
+# Check Python version and dependencies
+python check_deps.py
+
+# Full pipeline (parse → compare → fetch → link)
+python run.py
+
+# Parse + compare only (no download, no linking)
+python run.py --skip-fetch --skip-link
+
+# Diagnose mod folder name / steam_id issues
+python check_names.py
+python check_names.py --fix --fix-ids
+```
+
+There is no build step, linter config, or package install beyond `pip install -r requirements.txt`.
+
+## Architecture
+
+### Package vs CLI layer
+
+`arma_modlist_tools/` is a pure library — no I/O side effects, no `sys.exit`, no `print`. All CLI scripts (`run.py`, `fetch_mods.py`, `link_mods.py`, etc.) sit at the project root and call into the package. New functionality goes in the package first, then a CLI script wraps it.
+
+### Data flow
+
+```
+modlist_html/*.html
+    └─ parser.parse_modlist_dir()
+           └─ compare.compare_presets()
+                  └─ comparison.json  ←─ source of truth for groups + mod identity
+                         ├─ fetcher.build_server_index()  ←─ Caddy JSON API
+                         │      └─ fetcher.find_mod_folder()  (steam_id first, name fallback)
+                         │             └─ downloads/{group}/@ModName/
+                         │                    └─ linker.link_group()
+                         │                           └─ arma_dir/@ModName  (junction/symlink)
+                         └─ reporter.build_missing_report()  →  missing_report.json
+```
+
+### Group naming convention
+
+- `"shared"` — mods present in **all** compared presets
+- `"<preset_name>"` — mods unique to one preset (key from `comparison["unique"]`)
+
+This group label is stored in `missing_report.json` per-mod so `sync_missing.py` knows where to place newly available mods without re-reading `comparison.json`.
+
+### Server index structure
+
+`build_server_index()` returns:
+```python
+{
+    "by_steam_id": {"450814997": "https://server/@cba_a3/"},  # primary lookup
+    "by_name":     {"cbaa3":     "https://server/@cba_a3/"},  # normalized fallback
+    "folders":     [...]                                       # raw Caddy listing
+}
+```
+
+`_normalize_name` strips `@`, lowercases, removes all non-alphanumeric: `"@CBA_A3"` → `"cbaa3"`. Used in both the index builder and every lookup.
+
+### Junction / symlink critical rules
+
+**Detection:** `os.path.islink()` returns `False` for Windows junctions. Always use `_is_junction()` from `linker.py`, which checks `st_file_attributes & 0x400` (`FILE_ATTRIBUTE_REPARSE_POINT`) on Windows.
+
+**Removal:** Use `os.rmdir()` on Windows and `os.unlink()` on Linux. **Never** `shutil.rmtree()` — it follows the junction and deletes the target mod files.
+
+**Creation:** `cmd /c mklink /J <link> <target>` on Windows, `os.symlink()` on Linux.
+
+### check_names.py classification (two-pass)
+
+Pass 1 collects raw `(server_name, local_steam_id)` for every disk folder.
+Pass 2 builds `ok_disk_names` — the set of disk names that already match the server exactly. Any MISMATCH whose proposed server name is in `ok_disk_names` is reclassified as `ID_COLLISION` (the local `meta.cpp` has a wrong `publishedid` that belongs to a different mod). This prevents false rename suggestions caused by shared/duplicate steam IDs on the server.
+
+`--fix-ids` corrects `meta.cpp` using steam IDs from `comparison.json` (sourced from Steam Workshop URLs in the HTML presets) as the authoritative source.
+
+## Python Version Compatibility
+
+Minimum is Python **3.9**. All files that use `X | Y` union type annotations **must** have `from __future__ import annotations` as the first import. Without it, the `|` syntax raises `TypeError` at runtime on Python < 3.10. Every module in `arma_modlist_tools/` already has it; any new CLI script you add must include it too.
+
+## Test Suite
+
+`test_suite.py` uses a custom harness (no pytest/unittest dependency). Structure:
+
+```python
+group("section name")   # prints header
+test("description", callable)  # runs fn, catches exceptions, tracks pass/fail
+skip("description", "reason")  # marks skipped
+```
+
+Tests that exercise the linker use `tempfile.TemporaryDirectory()` — never the real `arma_dir`. Tests that would require network calls mock `list_mod_files` with `unittest.mock.patch`.
+
+## Key Files Not in Git
+
+- `config.json` — credentials + paths (copy from `config.template.json`)
+- `downloads/` — downloaded mod files, can be several GB
+- `modlist_json/` — generated JSON output
+
+The `.html` preset files in `modlist_html/` **are** tracked as example inputs.