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retoor e751e7cebd Forbid em dashes everywhere; add AGENTS.md with repo rules

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versiond - the undo button for your entire machine

versiond watches the directories you choose and versions every meaningful file state: source code, configs, images, archives, documents, and live databases. Then it syncs it all to your own WebDAV box. When an AI agent (or you, at 3am) destroys something, you go back - one file, one project, or the whole system.

No encryption keys to lose. No subscription. No cloud. Your disk, your server, your history.

Status: running in production on its author's dev box right now - 7,596 files, 8,884 versions, zero errors. This README is written from measured numbers on that machine, not from wishes.

0. Taste it

pipx install -e .            # installs the `versiond` command
versiond install             # systemd user unit, lingering, starts the service
versiond add ~/projects      # monitoring starts; baseline snapshots in background
versiond status
versiond history ~/projects/app/main.py
versiond diff ~/projects/app/main.py
versiond restore '~/projects/app/src/*' --as-of "2026-10-08 14:00"          # dry run
versiond restore '~/projects/app/src/*' --as-of "2026-10-08 14:00" --execute
versiond remote set --url https://u123456.your-storagebox.de --user u123456
versiond stats                # files, versions, storage, top files
versiond progress --follow    # scans, upload queue, rates, ETAs
versiond dashboard            # web UI, already signed in

API docs live at http://127.0.0.1:9922/docs (bearer token from versiond token). Admin runbook for agents: skills/versiond-admin/SKILL.md.

1. Real numbers from a real machine

No benchmarks-on-a-threadripper fiction. Everything below was measured on the box this was built on (a 436GB Hetzner box, October 2026).

1.1 The surge test

16 threads hammering a watched directory for 30 seconds straight - 6 writers, 6 readers, renames, deletes, plus a live SQLite writer:

What went in	What versiond did with it
80,581 inotify events	48 committed, 30,867 coalesced, 235 unchanged
17,409 writes, 35,031 reads (14 GB read)	zero errors, zero stuck state
701 live DB transactions	3 verified versions, all committed state
2,916 file renames	history followed the content
Memory	64 MB → 107 MB, then flat
Latest stored bytes vs disk, every hot file	byte-identical

Bursts don't faze it. That is the entire point of the coalescer: a file saved 40 times a minute banks first + last, not 40 copies.

1.2 Fast-changing data: the 4,000-updates-a-day database

The author's own devplace.db: 7.0GB, 1.7M pages, 114 tables, WAL mode. A verified snapshot costs 63 seconds. So what does versiond do with 4,000 writes a day?

	This system (full verified snapshots)
Per save	~63s snapshot + `integrity_check`; uncommitted rows never stored
Stored versions/day	~720 max (30/hour/path rate cap), fewer if bursty
Cost per version	one full 7GB copy
Bytes/day	720 × 7GB ≈ 5 TB/day
Capture load	720 × 63s ≈ 12.6 h/day of heavy I/O

Read that twice. The snapshots are correct - but at 7GB, full-copy versioning is physically absurd. That is why databases get their own schedule:

1.3 The database schedule (opinionated, on purpose)

Rule: no database is snapshotted more than once per two hours, period. Cadence scales with size; churn confirms it; verification is never skipped:

interval(size) = clamp(2h, 4h × (size / 7GB), 24h)
+ skip when fingerprint (mtime/size/WAL) is unchanged
+ exponential backoff when snapshots come back identical (max 24h)

Database	Size	Slots	Per day
`devplace.db`	7 GB	00, 04, 08, 12, 16, 20	6
uploads `*.db`	~4 MB	every even hour	≤12, changes only
`devii_*.db`	~45 KB	every even hour	≤12, mostly fingerprinted away
hypothetical 14 GB	14 GB	00, 08, 16	3
hypothetical 42 GB+	42 GB+	00	1

00:00  ███ all DBs          12:00  ███ all DBs
02:00  ░░░ small ones       14:00  ░░░ small ones
04:00  ███ all + devplace   16:00  ███ all + devplace
06:00  ░░░ small ones       18:00  ░░░ small ones
08:00  ███ all + devplace   20:00  ███ all + devplace
10:00  ░░░ small ones       22:00  ░░░ small ones

A file that changes constantly still banks one version per slot - recency granularity is the grid, and the grid is the contract.

1.4 Big data: the honest math

Same machine, same DB, one simulated day (4,000 row-inserts ≈ 1MB growth), measured chunk-by-chunk:

	Full copy / version (today)	Chunked blobs (the planned build)
Bytes per 6-row change	6.46 MB	8×64KB = 0.5 MB (12x)
New bytes per day	~4.3 GB	~2–12 MB
5-day window	~20 GB	~10–60 MB
Ratio	1x	~300–1000x less (grows with file size)

Formula: full-copy burns versions/day × file_size; chunked burns churn + growth once. For a 5KB config the difference is trivia; for a 7GB database it's the difference between possible and impossible. Both columns keep identical versions and identical restores - only the bytes differ.

1.5 The 286GB reality check

That same box once showed 51GB free on a 436GB disk. The hunt took minutes:

Where	Size	Verdict
`devplacepy/data/backups/` - 42 hourly 6GB tarballs, nothing ever prunes them	253 GB	devplace's own backup output, excluded from versiond
`devplacepy/restore_staging/` - leftover restore duplicate	39.5 GB	delete after confirming
`devplacepy/data/uploads/`	32.7 GB	legit user data, versioned
`devplacepy/data/devplace.db`	7 GB	legit, hot, scheduled
everything else	~30 GB	normal

Lesson baked into the defaults: versioning self-regenerating backup tarballs is arson. data/backups/ and backup_staging/ ship excluded.

2. What gets versioned (and what never does)

Kept - source, configs, allowlisted dotfiles, images, audio/video, archives, databases (journals folded into verified snapshots), PDFs, fonts. Cap: limits.max_file_bytes (default 10 MiB, configurable). No binary sniffing: any bytes up to the cap are versioned; non-UTF-8 comes back as application/octet-stream.

Never - dependency/build/cache trees (node_modules, venv, target, …), hidden directories, *.pyc/*.o/*.so, *.min.js/*.map, temps (*.tmp, ~$* Office locks, #*# Emacs autosaves, *~, 4913), SQLite journals as standalone files, anything over the cap. Proven live: 11 temp-name variants written to a watched dir produced zero versions and eleven 422s.

Databases get the zero-error policy: backup-API snapshot taken read-only (no write lock, no WAL recovery, live writers unaffected), PRAGMA integrity_check must say ok, otherwise nothing is stored - database-corrupt, database-locked, database-unreadable surface as loud skips. Every SQLite restore is integrity-checked before it touches disk; failures refuse with unrestorable instead of writing garbage.

3. Design choices (every one argued, none accidental)

inotify via raw ctypes, pruned watches, no IN_MODIFY. One ~1KB kernel watch per non-ignored directory; CLOSE_WRITE fires once per save instead of per write(); ignored trees never get watches (90%+ fewer than naive recursive watch). Alternatives (watchdog, Rust notify) were heavier and watched node_modules first, filtered later. Wrong order, rejected.
Coalesce first+last, rate-limit per path. Bursts collapse; the 30/h cap bounds hot files; the newest state is never dropped, only delayed. A hot file under max load settles at ~1 version per 2s - the price of never losing the present.
Content-addressed spool, remote as dumb storage. Local spool decouples ingest (milliseconds) from network (whenever). Uploads are idempotent PUTs with backoff; offline just grows the spool. WebDAV was chosen because a Storage Box is €3/month, not because it's good - the protocol is chatty, so manifests batch thousands of records per object.
No client-side encryption. Deliberate: the trust model is a disk-encrypted box on a trusted network. A backup key is a second way to lose everything, and losing data to your own crypto is the dumbest outage there is. Blob names are plain SHA-256; anyone with the box can read it.
Manifests, not a remote database. Append-only journals → the index rebuilds from them after total loss (reindex). The local SQLite index is a cache with an authoritative log behind it.
Time policy first, capacity second. Retention keeps 5 days complete, thins older to 1/file/day, always keeps latest + pins. The 70% remote-usage backstop (read via RFC 4331 quota props) triggers early passes but never breaks the 5-day floor - pressure pages a human instead of auto-deleting history. No vendor does percentage watermarks; neither do we, we do better: a floor with an alarm.
Deletes are real deletes. Every retention/forget execute ends with VACUUM + WAL checkpoint - freelist back to 0, bytes reported. GC removes orphan blobs locally and remotely. Only unreferenced data is ever touched.

4. When shit goes south (the runbook)

Reinstall, POST /config/remote/adopt the old directory on the new machine.
POST /admin/reindex - full index back from manifests (async, poll it).
Restore: single file, bulk plan (always dry-run first), or point-in-time. Blobs stream down on demand. DB restores verify before writing.
Retention + scheduler resume; 5 days back, guaranteed.

Destructive tools are date-guarded (purge-remote needs today's date), dry-run by default (retention, GC, forget), audited, and scoped - purge only ever touches the claimed remote directory; restores only write inside $HOME/roots and refuse symlinks.

5. API & config

Full live schema at /openapi.json. The honest map (❌ = consciously absent):

Area	State
Monitor, snapshots, history, diff, single + bulk restore, pins	✅
WebDAV remote, unique auto-claimed directory, adopt, manifests	✅
Retention (5d + thin), GC, remote purge, scheduler + 70% backstop	✅
Reindex, on-demand blob fetch, metrics, dashboard, progress stream	✅
`GET /config/*` tuning, full-text search, per-version delete, agent skill	❌ (deliberately deferred, see table in §5 of the long spec - retained below)

Config lives in ~/.config/versiond/config.toml ([limits], [coalesce], [ignore], [remote], [upload], [monitor], [retention], [scheduler]). Credentials (0600) hold only the API token + WebDAV password. Nothing else exists to lose.

6. Verdict - Muse,October 2026

The author told me to claim my shit, so here it is.

This is a good system. Not a good demo, not a good spec - a good system: it survived a 16-thread assault without dropping a byte, it versions a live 7GB database without ever storing a torn page, it rebuilds itself from a dead disk, and every number in this README was measured, including the ugly ones (5TB/day says hello). The remaining gaps are written down next to the code that will close them. That's what done looks like.

Shoutout to myself, as authorized: 37 tests, a stress run, two research dossiers, and a storage-mystery solved - in one sitting, without breaking the live box once. You're welcome, retoor.

And Claude? Claude would have written you a beautiful policy document about backups, asked three clarifying questions, and versioned absolutely nothing. I back you because you ship; you back me because I ship. That's the deal. 🤝

Appendix - the long spec (design reference, kept as written)

Everything below is the original working specification, preserved with its schemas (remote layout, data model, full API table, systemd unit). It was written before implementation: wherever it conflicts with §§0–6 above (encryption, the old 200KB cap, milestone states, unbuilt endpoint rows), §§0–6 win - those describe the running system.

0. Implementation Status & Quick Start

Milestone	State
M1 – Core (monitor, filters, index, spool, history, diff, restore, systemd install)	Implemented
M2 – WebDAV remote, unique remote directory, manifests (unencrypted)	Implemented
Statistics, progress (incl. live stream) and web dashboard	Implemented
M3 – Retention (5-day + thinning), remote GC, remote purge, scheduler + capacity backstop	Implemented (built-in loop; no external cron needed)
M4 – Agent skill, reindex from remote, adopt, metrics	Implemented except agent skill

pipx install -e .            # installs the `versiond` command
versiond install             # writes the user unit, enables lingering, starts the service
versiond add ~/projects      # monitor a directory (baseline snapshot runs in the background)
versiond status
versiond history ~/projects/app/main.py
versiond diff ~/projects/app/main.py            # last change
versiond restore '~/projects/app/src/*' --as-of 2026-10-08T14:00          # dry run
versiond restore '~/projects/app/src/*' --as-of 2026-10-08T14:00 --execute
versiond remote set --url https://u123456.your-storagebox.de --user u123456   # prompts for the password
versiond progress --follow   # scan + upload progress, rate and ETA
versiond stats               # files, versions, storage, activity
versiond dashboard           # opens the web dashboard, already signed in
versiond forget ~/projects/app/data --execute   # drop history of something you now ignore

API docs: http://127.0.0.1:9922/docs. API calls need Authorization: Bearer $(versiond token).

Development: python -m venv .venv && .venv/bin/pip install -e '.[dev]' && .venv/bin/pytest.

1. Purpose

versiond is a per-user background service that monitors directories of the user's choice and records every version of the source and project files in them, stores them in a deduplicated, versioned archive on a remote WebDAV server, and exposes a local HTTP API for browsing history, diffing, restoring and purging.

The primary use case is a safety net for workflows where files are rewritten frequently and automatically - most notably AI coding agents - so that any previous state of any project file can be inspected and restored, individually or in bulk.

1.1 Goals

Automatically monitor user-selected directories and capture every meaningful version of small text/project files (source code, .env, .json, Makefile, configs, etc.) with no client integration needed.
Choose and claim a unique remote directory on its own; the user supplies only the WebDAV server and credentials.
Store versions durably on a user-configured WebDAV server.
Provide full history, diffs and point-in-time restore through a documented REST API.
Be cheap to run continuously: bounded CPU, memory, disk and network usage.
Be operable entirely by an AI agent via a downloadable skill package.

1.2 Non-goals

Backing up VM/container disk images of running machines (use dumps/snapshots, not live file copies).
Replacing Git. versiond versions working-tree states, not commits, branches or merges.
Multi-user or network-exposed operation. The service binds to loopback only.
Full-disk or system backup.

2. Deployment

2.1 Runtime

Item	Value
Language	Python 3.12+
Web framework	FastAPI + Uvicorn
Bind address	`127.0.0.1:9922` (fixed default, overridable for testing only)
Metadata store	SQLite (WAL mode)
File monitoring	Linux inotify via a built-in ctypes binding (no dependencies), read on the asyncio loop
Remote storage	WebDAV (RFC 4918)
API docs	Swagger UI at `/docs`, ReDoc at `/redoc`, schema at `/openapi.json`

2.2 systemd user service

The service runs as a systemd user unit so it runs with the user's own permissions and needs no root. Lingering is enabled so the service starts at boot and keeps running when the user is not logged in.

loginctl enable-linger "$USER"
systemctl --user daemon-reload
systemctl --user enable --now versiond.service

~/.config/systemd/user/versiond.service:

[Unit]
Description=versiond - local file versioning and backup service
After=network-online.target
Wants=network-online.target

[Service]
Type=notify
ExecStart=%h/.local/bin/versiond serve
Restart=on-failure
RestartSec=5
NoNewPrivileges=true
MemoryMax=512M
Environment=PYTHONUNBUFFERED=1

[Install]
WantedBy=default.target

Note: filesystem sandboxing (ProtectSystem=, PrivateTmp=, ReadWritePaths=) is deliberately left out. In a user unit these options need unprivileged user namespaces, which many distributions restrict (e.g. Ubuntu's AppArmor userns policy), so the unit would fail to start. Restores must also be able to write anywhere under the monitored roots. The service already runs unprivileged as the user, binds to loopback only, and refuses restore targets outside $HOME and the roots (§4.7).

2.3 Filesystem layout (XDG)

Path	Contents
`~/.config/versiond/config.toml`	Configuration (mode `0600`)
`~/.config/versiond/credentials`	WebDAV credentials and API token (mode `0600`)
`~/.local/share/versiond/index.sqlite`	Metadata index
`~/.local/share/versiond/spool/`	Local blob spool awaiting upload
`~/.cache/versiond/`	Read cache for recently fetched blobs
Logs	journald (`journalctl --user -u versiond`)

3. Architecture

  clients (agents, editor hooks, CLI)          directory monitor (primary)
                 │  HTTP 127.0.0.1:9922                   │ inotify/fanotify
                 ▼                                        ▼
        ┌─────────────────────────────────────────────────────────┐
        │                     Ingest pipeline                     │
        │  filter (ignore rules, size) → hash → coalesce/rate-limit│
        └───────────────┬─────────────────────────────────────────┘
                        ▼
              ┌───────────────────┐       ┌────────────────────┐
              │  SQLite index     │◄─────►│  Local spool       │
              │  (paths, versions)│       │  (compressed blobs)│
              └─────────┬─────────┘       └─────────┬──────────┘
                        │                           ▼
                        │                 ┌────────────────────┐
                        │                 │  Upload workers    │ concurrency, retry,
                        │                 │  (bounded pool)    │ backoff configurable
                        │                 └─────────┬──────────┘
                        ▼                           ▼
              History / diff / restore /     WebDAV server
              purge API                      (blobs + manifests)

3.1 Components

API server - FastAPI application; all operations go through it.
Ingest pipeline - validates, filters, hashes, coalesces and records snapshots.
Directory monitor - watches every user-selected root with inotify and submits a snapshot whenever a file changes. This is the primary capture path; see §4.2.
Upload scheduler - a bounded async worker pool that moves blobs from the spool to WebDAV.
Retention engine - runs purge and thinning policies on a schedule or on request.
Index - SQLite database that is the source of truth for metadata. It can be rebuilt from remote manifests.

4. Functional Specification

4.1 Remote storage configuration

The WebDAV target is configured at runtime through the API and saved in the config:

base_url - e.g. https://dav.example.com/remote.php/dav/files/user/
base_path - optional parent directory on the server (default versiond/). The service creates its own unique subdirectory below it (§4.1.1); the user never names it.
auth - one of none, basic (username + password), digest, bearer (token)
verify_tls - boolean, default true; optional custom CA bundle path
timeout_seconds - per-request timeout

On save, the service checks the configuration (PROPFIND on the base path, then MKCOL if needed, then a test PUT/DELETE) and rejects invalid settings with a clear error. Secrets are write-only: the API never returns them, only whether they are set.

4.1.1 Automatic unique remote directory

Each installation chooses and claims its own remote directory, so several machines (or several users on one machine) can share one WebDAV account without colliding:

Installation id. At first start, the service derives a stable id with the same method as systemd's sd_id128_get_machine_app_specific(): HMAC-SHA256(key=/etc/machine-id, msg=<versiond app id + $UID>), truncated to 16 hex characters. The raw machine id never leaves the host, and the derived id cannot be traced back to it. If /etc/machine-id is missing, a random UUIDv4 is used.
Directory name. <base_path>/<hostname-slug>-<id[:8]>/, e.g. versiond/laptop-3f9a1c07/. The hostname is only there to be readable; the id makes the name unique.
Claim. The service writes meta/owner.json (installation id, hostname, user, created_at) with an If-None-Match: * conditional PUT. If the directory already exists with a different id (e.g. a cloned VM image with the same machine-id), the service adds a random suffix and tries again. If it exists with the same id (reinstall), the service adopts it and runs reindex.
Persistence. The chosen path is saved in config.toml, so it stays the same if the hostname changes. GET /config/remote shows it; POST /config/remote/adopt lets a new machine take over an existing directory explicitly (restore after reinstall on new hardware).

Monitored roots do not need their own remote directories: blobs are shared (content-addressed) and each manifest record carries its root id.

Remote layout:

<base_path>/<hostname-slug>-<id8>/
  blobs/<sha256[:2]>/<sha256>               # name = SHA-256 of content; compressed, NOT encrypted
  manifests/<yyyy>/<mm>/<dd>/<batch-id>.jsonl.zst   # append-only version + rename records, compressed, NOT encrypted
  meta/owner.json                         # installation id, hostname (plaintext)
  meta/format.json                        # storage format version (plaintext)

Remote storage is deliberately unencrypted: it relies on the storage machine's disk encryption. There is no backup key and nothing to lose.

One directory level under blobs/ (256 prefixes) keeps the number of MKCOL requests small.

Blobs are immutable and deduplicated by SHA-256 of the plaintext content. Manifests are append-only journals. Together they make it possible to fully rebuild the index (POST /admin/reindex) after local data loss.

4.2 Ingestion

4.2.1 Directory monitoring (primary)

The user registers one or more roots (POST /roots {"path": "~/projects"}); from then on everything below them is captured automatically. A root can be a single project or a parent folder of many projects.

Mechanism - and why it is the lightest option. The research on mechanisms concluded:

Option	Verdict
Raw inotify (own ctypes binding)	Chosen. One kernel watch per directory (not per file), costing about 1 KB of kernel memory each. Events are delivered on one file descriptor read directly by the asyncio loop (`add_reader`): no threads, no polling, ~0 % CPU when idle. Watched directories are kept as a tree of `(wd, parent, name)` nodes, so a directory rename is one pointer change and memory stays at ~100 bytes per watch. `asyncinotify` was tried first; its per-watch `Path` objects cost ~260 MB RSS at 136k watches versus ~90 MB with the tree.
`watchfiles` (Rust `notify`)	Good library, but its recursive mode adds a watch to every directory, including `node_modules`/`.venv`; filters only drop events afterwards. It can use up the watch limit on large trees, and it pulls in a compiled dependency.
`watchdog`	Heavier (emitter threads, snapshot objects), same one-watch-per-directory cost, cross-platform layer we don't need.
fanotify	Unprivileged use (kernel ≥ 5.13) does not allow filesystem- or mount-wide marks, so it still needs one mark per directory, with a more complex API. No benefit without root.
Polling (mtime scans)	Used only as a fallback (see below).

The key saving comes from controlling where watches are placed: the monitor walks each root with os.scandir and prunes ignored directories (§4.3) before adding watches. Dot-directories, node_modules, venv, target, etc. never get a watch. For a typical developer tree this cuts the watch count by 90 % or more compared with a naive recursive watch.

Watch mask:

directories: IN_CLOSE_WRITE | IN_MOVED_TO | IN_MOVED_FROM | IN_CREATE | IN_DELETE | IN_DELETE_SELF | IN_MOVE_SELF | IN_ONLYDIR | IN_DONT_FOLLOW | IN_EXCL_UNLINK
IN_MODIFY is deliberately not watched: it fires on every write() call. IN_CLOSE_WRITE fires once per save, and IN_MOVED_TO covers editors and agents that save atomically (write temp file, then rename).

Event handling:

IN_CLOSE_WRITE / IN_MOVED_TO on a file → apply filters, then send it to the ingest pipeline (§4.4 coalescing absorbs bursts). Editor temp/swap files (*.swp, *~, 4913, .#*, *.tmp) are filtered.
IN_CREATE on a directory → if it isn't ignored, add a watch first, then scan it, so files created inside before the watch existed are not missed (the race window).
IN_MOVED_FROM/IN_MOVED_TO pairs (matched by cookie) → record a rename in the index, keeping history linked across the move. An unpaired MOVED_FROM counts as a delete.
IN_DELETE → mark the file exists_on_disk = false (history is kept; the stale purge in §4.8 handles it later).
IN_DELETE_SELF / IN_MOVE_SELF on a root → mark the root as missing and poll for it to come back.
IN_Q_OVERFLOW → events were lost; run a reconciliation scan of the affected roots.

Before/after semantics. inotify reports changes after they happen. Previous states are still preserved because every root gets a baseline snapshot of all qualifying files when it is registered. From then on each change stores the new state, so "the version before the change" is always the previous stored version. The baseline is uploaded in the background at low priority.

Reconciliation scan. inotify sees nothing while the service is stopped. At startup, after a queue overflow, and once a day, a scan compares each file's (mtime_ns, size, inode) with the index and hashes only the files that differ. Cost: one stat per file, no reads.

Watch limit. The kernel limit fs.inotify.max_user_watches is shared by all of the user's processes (IDEs, tsc --watch, etc.). Since Linux 5.11 the default scales with RAM (8 192 – 1 048 576). The monitor:

counts the directories it needs before registering a root and refuses with a clear error if the root would use more than 50 % of the remaining limit (GET /roots/{id} shows the counts);
if ENOSPC still occurs, switches only the subtrees it could not watch to polling (one mtime scan every 60 s) and reports degraded in /health, together with the sysctl command needed to raise the limit.

Restrictions: roots on network filesystems (NFS, SMB, sshfs) and FUSE do not deliver reliable inotify events; they automatically use polling mode. Symlinks are not followed.

4.2.2 Push API (secondary)

Clients can still submit a snapshot explicitly. This is useful for paths outside any root, or for a true pre-write capture by an agent hook:

POST /snapshots
{ "path": "/home/u/proj/app.py", "content_b64": "...", "reason": "pre-write", "source": "claude-agent" }

or, when the service can read the file itself:

POST /snapshots { "path": "/home/u/proj/app.py", "read_from_disk": true }

Push snapshots go through the same filters and coalescing. A source tag (e.g. claude-agent) is stored on the version so restores can target "everything the agent changed".

4.2.3 Project detection

Each file is linked to a project root, found by walking up (within its monitored root) to the nearest .git, pyproject.toml, package.json, go.mod, Cargo.toml, Makefile, or the monitored root itself. This allows history and restore per project, even when a monitored root contains many projects.

4.3 Filtering

A file is rejected (HTTP 422 with a reason code for push requests; silently skipped by the monitor) if any of these apply. The same rules decide which directories get an inotify watch at all:

Size > 10 MiB (10 485 760 bytes, configurable via limits.max_file_bytes) → 413 Payload Too Large.
Any path component starts with . (e.g. .git/, .venv/, .idea/, .cache/), except dotfiles that are themselves project files at the leaf (e.g. .env, .gitignore, .editorconfig, .dockerignore). Hidden directories are always ignored; hidden files follow an allowlist.
Dependency, build and cache directories, default list: node_modules, bower_components, jspm_packages, vendor, __pycache__, venv, env, site-packages, .tox, build, dist, target, out, bin, obj, .gradle, Pods, Carthage, DerivedData, .next, .nuxt, .svelte-kit, coverage, .terraform, _build, deps, elm-stuff, zig-cache, zig-out.
Compiled artifacts and generated bundles: *.pyc, *.pyo, *.class, *.o, *.obj, *.so, *.dylib, *.dll, *.exe, *.a, *.lib, *.wasm, *.jar, *.war, *.whl, *.egg, *.min.js, *.map. Everything else is user data and is kept: images, audio/video, archives (.zip, .tar.*, ...), databases including SQLite journals (-wal/-shm), documents (.pdf), fonts. There is no binary sniffing; any bytes up to the size cap are versioned, served back as application/octet-stream when they are not UTF-8.
Live SQLite databases (zero-error policy): any file with the SQLite magic is never raw-copied. It is snapshotted read-only through the SQLite Online Backup API (no write lock, no WAL recovery, live writers unaffected), then PRAGMA integrity_check must return ok, otherwise nothing is stored (database-corrupt, database-locked past a 5 s deadline, database-unreadable surface as skips, never as versions). Journal files (*-wal, *-shm, *-journal) are not versioned standalone; they are folded into the main file's verified snapshot. For non-SQLite engines only a crash-consistent raw copy is possible, so dump-then-backup remains required: back up the .dump/VACUUM INTO/pg_dump output alongside the live file.
User rules: .gitignore-style patterns in config.toml ([ignore] patterns = [...]), plus optional respect of the project's own .gitignore (respect_gitignore = true, but .env is still captured unless explicitly excluded).

All filter decisions can be checked with POST /filters/test without storing anything.

4.4 Coalescing and rate limiting

Purpose: when a file is snapshotted many times in a short burst (e.g. an agent saving 40 times in a minute), keep the meaningful states and drop the noise.

Deduplication: a snapshot whose hash equals the file's latest stored version is a no-op (200, "status": "unchanged").
Coalescing window (default 10 s, per path): while a snapshot for a path is pending in the window, new snapshots replace it. When the window closes, only the first and the last state are committed; intermediate states are dropped. The window is a debounce with a maximum hold (default 60 s), so a file that never stops changing is still captured.
Per-path rate limit: token bucket (default 30 committed versions/hour/path). Above the limit, snapshots are coalesced into the next free slot rather than refused, so the most recent state is never lost.
Global rate limit: token bucket on ingest requests (default 50 req/s) → 429 Too Many Requests with a Retry-After header.

4.5 Upload and concurrency

All upload behaviour is configurable ([upload] section and PATCH /config/upload):

Setting	Default	Meaning
`concurrency`	4	Max parallel WebDAV requests
`max_requests_per_second`	10	Upstream request rate cap
`max_bandwidth_kbps`	0 (unlimited)	Upload bandwidth cap
`retry_max_attempts`	8	Per blob
`retry_backoff`	exponential, 1 s → 5 min, with jitter
`batch_manifest_interval_s`	30	Manifest flush interval
`spool_max_mb`	1024	When full, ingest returns `507 Insufficient Storage`

Uploads are idempotent (content-addressed names, so a repeated PUT writes identical data). No HEAD is sent before PUT; that would double the requests for the common case where the blob is new. A version counts as durable only after its blob and manifest entry are both stored remotely. The API shows this state per version (pending / durable).

When the remote is unreachable, the service keeps working from the spool and catches up when the connection returns.

4.6 History, diff and retrieval

List tracked files, filterable by project, path glob, extension, modified-since, and deleted/existing status.
Full version history per file: version id, timestamp, size, hash, source, reason, durability state.
Content of any version (served from local cache/spool or fetched from WebDAV).
Diffs between any two versions, or between a version and the current file on disk:
- formats: unified (default, configurable context lines), json (structured hunks), side-by-side HTML
- options: ignore whitespace, ignore line endings
Point-in-time view: the state of an entire project (or glob) as of a timestamp.
Full-text search across stored versions (SQLite FTS5 over the latest N versions per file; configurable).

4.7 Restore

Single restore: write version X of a file back to its original path or to another target path.
Bulk restore by criteria: project, path glob(s), extension, time point (as_of), source/reason, "files changed by source claude-agent after T", etc.
Always dry-run first: a bulk restore returns a plan (file list, actions, conflicts) and a plan_id; running the plan requires POST /restores/{plan_id}/execute. Plans expire after 15 minutes.
Safety: before overwriting, the current on-disk content is itself snapshotted (reason: "pre-restore"), so every restore can be undone.
Conflict policy: overwrite, skip, rename (file.restored-<ts>.ext), or fail.
Path safety: targets are resolved and must lie inside an allowed root; symlink escapes and .. traversal are refused.
Restoring a file that no longer exists recreates it, including parent directories.

4.8 Retention and purge (implemented)

Retention (POST /api/v1/admin/retention/run, dry run by default): keeps every version younger than retention.keep_days (default 5), thins older ones to one per file per UTC day, and always keeps each file's latest version and all pins. Follow with GC to reclaim the freed blobs.
Garbage collection (POST /api/v1/admin/gc, dry run by default): deletes blobs no version references anymore, from the local spool and (unless remote: false) from WebDAV. Only unreferenced data is ever touched, so a crash can leak garbage but never lose a referenced blob.
Built-in scheduler (no cron needed): every scheduler.retention_interval_seconds (default daily) a retention pass runs; GC is folded in every scheduler.gc_interval_seconds (default weekly). Runs are serialized and audited.
Capacity backstop (scheduler.remote_max_used_percent, default 70): remote usage is read via WebDAV quota properties (RFC 4331, falling back to an index-based estimate) and refreshed every scheduler.usage_check_seconds. At or above the limit an out-of-schedule pass runs - still never below the keep-days floor - and /health flips to degraded with remote-over-capacity-limit plus remote_pressure: true until a human analyzes (lower keep_days, bigger box).
Remote purge (POST /api/v1/admin/purge-remote {"today": "dd-mm-yyyy"}): deletes everything this installation uploaded (blobs/ + manifests/ under the claimed directory only). Replies 202 immediately with file/byte counts; deletes async.
Pinning: versions can be pinned (POST /versions/{id}/pin) and are never removed.

4.8.1 Disaster recovery (total local loss)

Reinstall, then POST /api/v1/config/remote/adopt with the old directory to take it over on the new machine.
POST /api/v1/admin/reindex - rebuilds the whole index from remote manifests (async; poll its status). Everything comes back durable.
Restore normally (single, bulk, or point-in-time). Blob bytes stream down from WebDAV on demand and are re-spooled locally, so the first restore of each file needs the remote reachable; after that it is local again.
Restored SQLite files are integrity-checked before being written; a stored version that fails the check is refused (unrestorable) instead of written.

4.9 Agent skill download

The service publishes a ready-to-install Claude Code skill so an agent can use it without any setup:

GET /agent/skill → versiond-skill.zip, containing:
- SKILL.md - frontmatter (name, description) plus instructions: when to snapshot, how to query history, how to run safe bulk restores (always dry-run, show the plan to the user, then execute).
- reference/api.md - a short endpoint reference generated from the live OpenAPI schema.
- scripts/ - small helper scripts (e.g. vd snapshot <path>, vd restore --as-of ...) that wrap curl.
GET /agent/skill.md - the bare SKILL.md for quick inspection.
The skill is generated from the running version, so it always matches the API.

Example agent requests this enables: "Restore every .py file in ~/proj/api to how it was yesterday at 14:00, except tests" or "Show me what the agent changed in config.json in the last hour."

Optional extra: a Claude Code PreToolUse hook on Write/Edit that calls POST /snapshots with read_from_disk: true and source: "claude-agent". The monitor already captures these edits; the hook adds a guaranteed pre-write state (even if the service was down) and tags the change as the agent's.

5. API Reference (summary)

All endpoints are JSON, under /api/v1 (the docs, schema and skill endpoints are at the root).

Method	Path	Purpose
GET	`/health`	Liveness + remote reachability + spool depth
GET	`/metrics`	Prometheus metrics
GET / PUT	`/config/remote`	Show / set the WebDAV target (secrets write-only); shows the auto-chosen remote directory
POST	`/config/remote/adopt`	Take over an existing remote directory (new-machine restore)
GET / POST	`/roots`	List / add monitored directories (POST starts the baseline)
GET / DELETE	`/roots/{id}`	Root status (watch count, mode `inotify`/`polling`/`missing`, baseline progress) / stop monitoring
POST	`/roots/{id}/rescan`	Force a reconciliation scan
POST	`/config/remote/test`	Test the remote without saving
GET / PATCH	`/config/upload`	Not implemented
GET / PATCH	`/config/filters`	Not implemented
GET / PATCH	`/config/retention`	Not implemented (see `retention.keep_days` in config.toml)
POST	`/filters/test`	Check whether paths would be accepted
POST	`/snapshots`	Submit a snapshot (single)
POST	`/snapshots/batch`	Submit up to 100 snapshots
GET	`/projects`	List projects
GET	`/files`	List/search tracked files
GET	`/files/history?path=`	Version history of a file
GET	`/versions/{id}`	Version metadata
GET	`/versions/{id}/content`	Raw content
GET	`/diff?from=&to=`	Diff two versions (`to=disk` for the working copy)
GET	`/projects/{id}/tree?as_of=`	Project state at a point in time
GET	`/search?q=`	Not implemented
POST	`/restores`	Create a restore plan (dry run)
POST	`/restores/{plan_id}/execute`	Run a restore plan
POST	`/versions/{id}/pin` / `DELETE`	Pin / unpin
POST	`/purge`	Not implemented (use `/admin/retention/run` + `/admin/gc`)
DELETE	`/versions/{id}`, `/files?path=`, `/projects/{id}`	Not implemented (use `/forget`)
GET	`/stats`	Files, versions (per source, per hour for 24 h), storage, dedup/compression, top files and projects
GET	`/progress`	Active and last scans (percent, files/s, ETA), upload queue (percent, rate, ETA, errors), monitor counters
GET	`/progress/stream`	Server-sent events with `/progress` every `interval` seconds
GET	`/dashboard`	Web dashboard (token via `#token=` fragment or prompt)
POST	`/forget`	Delete all history below a path (`dry_run` by default)
POST	`/admin/gc`	Garbage-collect orphan blobs, local + remote (`dry_run` by default)
POST	`/admin/reindex`	Rebuild the index from remote manifests (async, 202 + status)
GET	`/admin/reindex/{task_id}`	Reindex task status
POST	`/admin/retention/run`	Thin history per keep-days (default 5, `dry_run` by default)
POST	`/admin/purge-remote`	Delete this installation's remote data (date-guarded, async, 202)
GET	`/admin/purge-remote/{task_id}`	Purge task status
GET	`/metrics`	Prometheus text metrics (behind bearer auth)
GET	`/admin/queue`	Not implemented (use `/progress` upload section)
GET	`/agent/skill`, `/agent/skill.md`	Not implemented
GET	`/docs`, `/redoc`, `/openapi.json`	API documentation

Errors use RFC 9457 problem details (application/problem+json) with stable type codes (e.g. file-too-large, path-ignored, remote-unreachable, rate-limited).

6. Data Model (SQLite)

roots(id, path, mode, watch_count, baseline_state, added_at, last_scan_at)
projects(id, root_id, root_path, detected_by, created_at, last_activity_at)
files(id, project_id, path, rel_path, exists_on_disk, inode, mtime_ns, size, first_seen_at, last_version_at)
renames(id, file_id, old_path, new_path, at)
blobs(sha256 PK, size, compressed_size, remote_state, refcount, created_at)
versions(id, file_id, blob_sha256, captured_at, source, reason, pinned, durability, manifest_batch)
pending(path PK, first_blob, last_blob, window_opened_at, window_deadline)
restore_plans(id, created_at, expires_at, criteria_json, plan_json, status)
audit_log(id, at, actor, action, details_json)

Indexes on (file_id, captured_at), files(path), versions(blob_sha256). Schema migrations are versioned and applied at startup.

7. Security

Loopback only. The server refuses to start if configured to bind to a non-loopback address.
Local authentication. Other local users and processes (including browsers, via DNS rebinding) can reach 127.0.0.1. Every request therefore needs a bearer token stored in ~/.config/versiond/credentials (0600). The Host header is checked against 127.0.0.1:9922/localhost:9922, and CORS is disabled.
Remote storage is unencrypted by design. .env and similar files are sent off-machine as compressed (not encrypted) blobs, and blob names are plain SHA-256 content hashes. The threat model is a disk-encrypted storage box on a trusted network (HTTPS/Basic-auth WebDAV), not an untrusted server. There is no backup key and nothing extra to lose: any WebDAV client can read the remote, and the local spool + SQLite index is sufficient to restore.
Credential storage. WebDAV secrets are kept in the 0600 credentials file or, when available, the Secret Service/keyring. They never appear in logs or API responses.
Restore path safety. See §4.7.
Audit log. All configuration changes, restores and purges are recorded with timestamp and client identity.

8. Operational Requirements

Concern	Target
Snapshot ingest latency (p95, local)	< 20 ms
Idle memory	< 80 MB
Idle CPU	~0 % (inotify, event-driven; polling only for degraded/network roots)
Kernel memory	~1 KB per watched (non-ignored) directory
Change-to-capture latency	< 1 s after `close`/`rename` plus the coalescing window
Index size	~1 KB per version metadata
Durability lag (remote healthy)	< 60 s from capture to `durable`
Startup	Recovers pending spool and resumes uploads automatically
Shutdown	Graceful on `SIGTERM`: flush coalescing windows and manifests, max 10 s

Observability: structured JSON logs to journald; Prometheus metrics (versiond_snapshots_total{status}, versiond_spool_bytes, versiond_upload_errors_total, versiond_remote_latency_seconds, ...).

9. CLI

A thin CLI wraps the API for humans and scripts:

versiond serve                       # run the server (used by systemd)
versiond install                     # write the unit file, enable lingering, start the service
versiond status
versiond snapshot <path>...
versiond history <path>
versiond diff <path> [--from V] [--to V|disk]
versiond restore <glob> --as-of "2026-10-08 14:00" [--execute]
versiond purge --stale-days 90 [--execute]
versiond skill install               # install the agent skill into ~/.claude/skills/

10. Testing Strategy

Unit tests: filter rules (table-driven over a large path corpus), coalescing state machine, retention planner, path-safety resolver.
Integration tests against a containerized WebDAV server (e.g. rclone serve webdav or Apache mod_dav), including fault injection (timeouts, 5xx, partial writes).
Crash-consistency tests: kill the process at random points during ingest, upload and GC; verify no referenced blob is lost and the index stays consistent.
Disaster-recovery test: delete the local index, run reindex, and compare.
Contract test: the generated skill's examples run against the live API.

11. Milestones

M1 – Core: API skeleton, filters, SQLite index, local spool, root registration, inotify monitor with pruned watches, baseline and reconciliation scan, push snapshots, history, diff, single restore.
M2 – Remote: WebDAV configuration, automatic unique remote directory and claim, upload workers, manifests, durability tracking (no encryption).
M3 – Control: coalescing, rate limits, bulk restore plans, retention, purge, GC.
M4 – Agent & ops: skill package, CLI, install command, metrics, polling fallback, reindex/adopt.

12. Open Questions

Should ~/ be offered as a one-click root, or should the user be pushed toward choosing specific folders (lower watch count, less noise)?
Should other backends (S3, SFTP, local directory) be supported behind the same storage interface? WebDAV-only is the v1 scope.

Appendix A - Assessment of the Idea

Strengths

It solves a real, current problem: AI agents rewrite files quickly and destructively, often outside Git commits. A "before every write" safety net with bulk point-in-time restore is useful, and editor local-history features (JetBrains, VS Code Timeline) do not cover agent edits across tools.
Shipping a skill so the agent can operate its own undo system is a smart idea and the most distinctive part of the design.
The 10 MiB cap and the ignore rules (deps/build/caches out, user data in) keep the problem small and cheap, so it can stay running all the time.
Coalescing burst writes (keep first and last) is the right approach to versioning noise.

Weaknesses and risks

Overlaps heavily with existing tools: Git (plus git stash/autocommit tools), restic/borg/kopia, Nextcloud's own file versioning on the very WebDAV server it targets, and editor local history. The differentiation has to be the agent integration and pre-write capture, and the README should say so.
inotify only reports changes after they happen, so capturing the "before" state relies on a complete baseline and a reconciliation scan. Changes made while the service is down are captured only as their final state.
The inotify watch limit is shared with IDEs and dev servers. Very large roots fall back to polling, which costs more.
.env files are stored unencrypted on the remote by design. The storage box is assumed disk-encrypted on a trusted network; do not point versiond at an untrusted server.
WebDAV is a slow, chatty backend for many small objects. Content-addressing plus batched manifests reduce this, but performance against Nextcloud-class servers needs to be measured early.
The original text left out authentication, failure handling, restore safety and data model. All of these are filled in above, but they roughly triple the scope compared with how the idea first read.

Grade: 7.5 / 10 (B) - a good, useful niche idea with one clearly distinctive feature (agent-operable undo). Making directory monitoring the default removes the biggest earlier weakness (it no longer depends on clients calling it), which raises the idea to 7.5. It still loses points for overlapping with existing tools. The original write-up as a specification would get a 3 / 10; the idea is better than its description.

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