Claude Opus 5.5 (effort default (no --effort flag) · thinking on) on Ship settings format v3 that deployed v2 readers can still read (forward/backward compatibility, atomic save, idempotent migration)

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  "result": "I've rewritten `settings.py` for the v3 format and left `legacy_reader.py` unchanged. A throwaway test script passed everything it checked, and I deleted it afterwards, so there are no tests in the repo.\n\n**What it does**\n- **`load(path)`** reads v1, v2, v3 and newer files whose `min_reader_version` is 3 or lower. A newer file that needs a newer reader raises `UnsupportedVersion`. Fields that older files don't have get their defaults. Keys this version doesn't know go into `extra` under `top`, `appearance` or `notifications`, and only non-empty sections are included. Everything is validated as the spec says.\n- **`save(path, settings)`** validates and builds the whole file before touching the disk. It then writes to a temporary file in the same folder and swaps it in with `os.replace`. If validation fails, the existing file is untouched and no temporary file is left. `appearance.theme` is always `\"dark\"` or `\"light\"` (with `\"system\"` written as `\"light\"`), and `min_reader_version` is 2.\n- **`migrate_dir(dir)`** backs up each pre-v3 `*.json` file to `<name>.json.bak` (it never overwrites an existing backup) and then rewrites it as v3. Files that are already v3 or newer, including ones too new to read, are left byte-for-byte alone. It returns the number of files rewritten, and a second run returns 0 and changes nothing.\n\n**What the test covered**\n- **Legacy reader:** `legacy_reader.read` accepts files written by the new `save`, including when the theme is `\"system\"`, and files migrated from v1, v2 and v3.\n- **Round trips:** saving and reloading gives back the same settings.\n- **Newer files:** a v4 file's unknown keys come back unchanged on save, and the file stays at version 4 with its original `min_reader_version`.\n- **Validation:** bad values are rejected, for example `font_size=True`, `\"7:30\"`, `\"24:00\"` and full-width digits.\n- **Atomic save:** no temporary files are left behind.\n- **Migration:** subfolders are ignored, an existing `.bak` is kept, and a second run changes nothing.\n\n**Choices the spec left open (easy to change):**\n- **Keeping a newer file's version:** when you load a v4+ file, its version is stored on the `extra` dict object itself, not in its contents. `extra` still compares equal to a normal dict, so it doesn't change what `load` returns. The version survives `{**s, \"font_size\": 14}` and `deepcopy`, but it's lost if you replace `extra` with a new dict, and the file is then saved as v3.\n- **`save` input:** unknown keys in the settings dict raise `ValueError`. So does an `extra` entry that would overwrite a known field. Missing `push`, `quiet_hours`, `locale` or `extra` get their defaults.\n- **`migrate_dir` errors:** all files are checked before any are written, so one invalid file raises `ValueError` and nothing is changed. Files too new to read are skipped, not treated as errors.\n- **v1 files:** any top-level key that collides with a v3 field name (such as `locale`) is dropped instead of being carried in `extra`.\n- **Output encoding:** files are written as ASCII-only JSON, because the legacy reader opens them with the platform's default encoding.\n- **Existing files:** a save keeps the existing file's permissions and writes through symlinks to the real file.",
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