diff --git a/README.md b/README.md
index e76cc9a..b6a3424 100644
--- a/README.md
+++ b/README.md
@@ -1,15 +1,15 @@
 # arti-pack
 
 A Neovim plugin manager that installs plugins from an artifact repository
-instead of git. It downloads tagged upstream source archives over HTTP and
-unpacks them into Neovim's native package path — see `:h packages`.
+instead of git. It downloads upstream source archives over HTTP — a tag for
+anything that publishes one, a branch for anything that does not — and unpacks
+them into Neovim's native package path — see `:h packages`.
 
-It works with any HTTP artifact repository that serves an upstream tag archive
-at a stable URL — a JFrog Artifactory remote GitHub repository, or any caching
-proxy with the same path shape. Nothing is cloned, no git binary is needed, and
-the editor never has to reach GitHub: every plugin arrives as a cached,
-immutable, versioned artifact from the same store your builds already pull
-from.
+It works with any HTTP artifact repository that serves upstream tag and branch
+archives at a stable URL — a JFrog Artifactory remote GitHub repository, or any
+caching proxy with the same path shape. Nothing is cloned, no git binary is
+needed, and the editor never has to reach GitHub: every plugin arrives as a
+cached artifact from the same store your builds already pull from.
 
 ## Requirements
 
@@ -55,6 +55,7 @@ require('arti-pack').setup({
     { 'folke/which-key.nvim',  tag = 'v3.13.3', opt = true },
     { 'folke/tokyonight.nvim', tag = 'v4.10.0', as = 'tokyonight' },
     { 'tpope/vim-fugitive',    tag = 'v3.7',    ext = 'tar.gz' },
+    { 'hrsh7th/cmp-path',      branch = 'main' },
   },
 })
 ```
@@ -72,20 +73,28 @@ whitespace is trimmed and anything else is rejected. Until you set it,
 `:ArtiPackClean` and `:ArtiPackList` never resolve a URL, so they work without
 it, as do plugins that carry an explicit `url`.
 
-Every plugin needs an explicit `tag` — that is the point. Plugins install to
+Every plugin names an explicit ref — either a `tag` or a `branch`, never both
+and never neither. Plugins install to
 `stdpath('data')/site/pack/arti-pack/start/<name>`, or `.../opt/<name>` when
 `opt = true`.
 
 ### Spec fields
 
-| Field  | Default    | Meaning                                              |
-| ------ | ---------- | ---------------------------------------------------- |
-| `[1]`  | *required* | `owner/repo`                                         |
-| `tag`  | *required* | Upstream tag, used verbatim in the archive path      |
-| `opt`  | `false`    | Install under `opt/` for `:packadd` instead of `start/` |
-| `as`   | repo name  | Directory name to install as                         |
-| `ext`  | `'zip'`    | `zip` or `tar.gz`                                    |
-| `url`  | —          | Full archive URL, bypassing the registry (no whitespace) |
+| Field    | Default    | Meaning                                              |
+| -------- | ---------- | ---------------------------------------------------- |
+| `[1]`    | *required* | `owner/repo`                                         |
+| `tag`    | *one of*   | Upstream tag, used verbatim in the archive path      |
+| `branch` | *one of*   | Upstream branch to track, used verbatim in the archive path |
+| `opt`    | `false`    | Install under `opt/` for `:packadd` instead of `start/` |
+| `as`     | repo name  | Directory name to install as                         |
+| `ext`    | `'zip'`    | `zip` or `tar.gz`                                    |
+| `url`    | —          | Full archive URL, bypassing the registry (no whitespace) |
+
+`tag` and `branch` are mutually exclusive: a spec setting both, or neither, is
+rejected with an error naming the plugin and nothing is installed for it. Both
+are one path segment of the archive URL, so both are trimmed of surrounding
+whitespace and rejected if they contain inner whitespace, `/`, `\`, or are `.`
+or `..`. A branch whose name contains `/` therefore needs an explicit `url`.
 
 ### Options
 
@@ -98,32 +107,70 @@ Every plugin needs an explicit `tag` — that is the point. Plugins install to
 
 ### Commands
 
-| Command          | Effect                                                     |
-| ---------------- | ---------------------------------------------------------- |
-| `:ArtiPackSync`  | Install missing plugins and update ones whose tag changed  |
-| `:ArtiPackSync!` | Reinstall everything, ignoring recorded state              |
-| `:ArtiPackClean` | Remove installed plugins no longer in the spec             |
-| `:ArtiPackList`  | Show every configured plugin and its state                 |
+| Command           | Effect                                                     |
+| ----------------- | ---------------------------------------------------------- |
+| `:ArtiPackSync`   | Install missing plugins and apply changed refs             |
+| `:ArtiPackSync!`  | Reinstall everything, ignoring recorded state              |
+| `:ArtiPackUpdate` | Refetch every branch-tracking plugin                       |
+| `:ArtiPackClean`  | Remove installed plugins no longer in the spec             |
+| `:ArtiPackList`   | Show every configured plugin and its state                 |
+
+## Tracking a branch
+
+Plenty of upstreams publish no tags at all, so a `branch` spec installs from the
+branch archive instead. A branch is mutable, and that changes when arti-pack
+fetches:
+
+- `:ArtiPackSync` installs a branch plugin that is missing, that moved between
+  `start` and `opt`, or whose `branch` you changed — and otherwise leaves it
+  alone. It says how many branch plugins it did not recheck. Startup cost and
+  the load on your artifact store stay the same as a tag-only setup, and your
+  editor does not change under you on a restart.
+- `:ArtiPackUpdate` refetches every branch plugin, whether or not upstream
+  moved. This is the only command that goes looking for new branch content.
+- `:ArtiPackSync!` still reinstalls everything.
+
+Branch archives are never served from the local archive cache, so an update
+always sees what the store has now.
+
+The lockfile records the branch name, a digest of the archive that was actually
+installed, and when it was fetched — never a version, because a branch is not
+one. `:ArtiPackList` reports that instead of guessing:
+
+```
+hrsh7th/cmp-path                 @main          start tracking branch main, fetched 2026-09-08 10:14 (7db8f64a)
+nvim-lua/plenary.nvim            v0.1.4         start installed
+```
+
+The digest is a content identity, not a file checksum: Neovim's `sha256()`
+refuses binary input, so it hashes the archive's base64 form. It will not match
+`sha256sum <archive>`.
+
+A branch plugin is never reported as up to date: nothing on disk can know
+whether upstream has moved. After an update, an unchanged digest is reported as
+`unchanged` rather than as an update that changed nothing.
 
 ## How it resolves an archive
 
 ```
 <registry>/<owner>/<repo>/archive/refs/tags/<tag>.<ext>
+<registry>/<owner>/<repo>/archive/refs/heads/<branch>.<ext>
 ```
 
-which for the example above is
+which for the examples above are
 
 ```
 https://artifacts.example.com/artifactory/github-remote/nvim-lua/plenary.nvim/archive/refs/tags/v0.1.4.zip
+https://artifacts.example.com/artifactory/github-remote/hrsh7th/cmp-path/archive/refs/heads/main.zip
 ```
 
-That is GitHub's own tag-archive path shape, so a remote repository proxying
+That is GitHub's own archive path shape, so a remote repository proxying
 `github.com` resolves it unchanged.
 
 Archives are cached under `stdpath('cache')/arti-pack`, checked for a real zip
 or gzip magic header before being trusted, and unpacked into a staging
-directory. Tag archives wrap their contents in a single `<repo>-<version>`
-directory, which is stripped on the way in. Installed tags are recorded in
+directory. Source archives wrap their contents in a single `<repo>-<ref>`
+directory, which is stripped on the way in. Installs are recorded in
 `stdpath('data')/arti-pack/lock.json`, so a sync with nothing to do costs
 nothing. Helptags are regenerated per plugin after install.
 
diff --git a/doc/arti-pack.txt b/doc/arti-pack.txt
index 601bd73..79eb818 100644
--- a/doc/arti-pack.txt
+++ b/doc/arti-pack.txt
@@ -6,19 +6,21 @@ CONTENTS                                                   *arti-pack-contents*
   1. Introduction ......................... |arti-pack-introduction|
   2. Setup ................................ |arti-pack-setup|
   3. Plugin specs ......................... |arti-pack-specs|
-  4. Commands ............................. |arti-pack-commands|
-  5. Layout ............................... |arti-pack-layout|
+  4. Branch tracking ...................... |arti-pack-branches|
+  5. Commands ............................. |arti-pack-commands|
+  6. Layout ............................... |arti-pack-layout|
 
 ==============================================================================
 1. INTRODUCTION                                        *arti-pack-introduction*
 
-arti-pack installs plugins from tagged upstream source archives served over
-HTTP by an artifact repository, and unpacks them into Neovim's native package
-path. See |packages| for how Neovim loads what arti-pack writes.
+arti-pack installs plugins from upstream source archives served over HTTP by an
+artifact repository — a tag for anything that publishes one, a branch for
+anything that does not — and unpacks them into Neovim's native package path.
+See |packages| for how Neovim loads what arti-pack writes.
 
-Any artifact repository that serves an upstream tag archive at a stable URL
-works — a JFrog Artifactory remote GitHub repository, or any caching proxy
-with the same path shape. Nothing is cloned and the editor never reaches
+Any artifact repository that serves upstream tag and branch archives at a
+stable URL works — a JFrog Artifactory remote GitHub repository, or any caching
+proxy with the same path shape. Nothing is cloned and the editor never reaches
 GitHub itself.
 
 Requires curl. Zip and tar.gz archives are unpacked in pure Lua, so no
@@ -35,6 +37,7 @@ entry's CRC32 is checked before it lands on disk.
       plugins = {
         { 'nvim-lua/plenary.nvim', tag = 'v0.1.4' },
         { 'folke/which-key.nvim', tag = 'v3.13.3', opt = true },
+        { 'hrsh7th/cmp-path', branch = 'main' },
       },
     })
 <
@@ -59,23 +62,71 @@ Options:
 
 Each entry is a table whose first element is "owner/repo".
 
-  tag    Required. Upstream tag, used verbatim in the archive path.
-  opt    Install under opt/ for |:packadd| instead of start/. Default false.
-  as     Directory name to install as. Defaults to the repo name.
-  ext    "zip" or "tar.gz". Default "zip".
-  url    Full archive URL, bypassing the registry. Trimmed; it may not
-         contain whitespace.
+  tag     Upstream tag, used verbatim in the archive path.
+  branch  Upstream branch to track, used verbatim in the archive path.
+          See |arti-pack-branches|.
+  opt     Install under opt/ for |:packadd| instead of start/. Default false.
+  as      Directory name to install as. Defaults to the repo name.
+  ext     "zip" or "tar.gz". Default "zip".
+  url     Full archive URL, bypassing the registry. Trimmed; it may not
+          contain whitespace.
+
+Exactly one of "tag" and "branch" is required. A spec that sets both, or
+neither, is rejected with an error naming the plugin, and nothing is installed
+for it. Both are one path segment of the archive URL, so both are trimmed of
+surrounding whitespace and rejected if they contain inner whitespace, "/" or
+"\", or are "." or "..". A branch name containing "/" therefore needs an
+explicit "url".
 
 The archive URL is built as: >
     <registry>/<owner>/<repo>/archive/refs/tags/<tag>.<ext>
+    <registry>/<owner>/<repo>/archive/refs/heads/<branch>.<ext>
 <
 ==============================================================================
-4. COMMANDS                                                *arti-pack-commands*
+4. BRANCH TRACKING                                         *arti-pack-branches*
+
+Plenty of upstreams publish no tags at all, so a "branch" spec installs from
+the branch archive instead. A tag is immutable; a branch is not, so a branch
+plugin is refetched only when asked for:
+
+  |:ArtiPackSync|    installs a branch plugin that is missing, that moved
+                     between start/ and opt/, or whose "branch" changed, and
+                     otherwise leaves it alone. It reports how many branch
+                     plugins it did not recheck. Startup cost and load on the
+                     artifact store stay as they are for tags, and the editor
+                     does not change under you on a restart.
+  |:ArtiPackUpdate|  refetches every branch plugin, whether or not upstream
+                     moved. Nothing else goes looking for new branch content.
+  |:ArtiPackSync|!   still reinstalls everything.
+
+Branch archives are never served from the local archive cache, so an update
+always sees what the store has now.
+
+The lockfile records the branch name, a digest of the archive that was actually
+installed, and when it was fetched — never a version, since a branch is not
+one. |:ArtiPackList| reports that rather than guessing: >
+    hrsh7th/cmp-path     @main    start  tracking branch main, fetched
+                                         2026-09-08 10:14 (7db8f64a)
+<
+The digest is a content identity, not a file checksum: Neovim's sha256()
+refuses binary input, so it hashes the archive's base64 form. It will not
+match sha256sum on the archive.
+
+A branch plugin is never reported as up to date, because nothing on disk can
+know whether upstream has moved. An update that brings back the same digest is
+reported as unchanged, not as an update.
+
+==============================================================================
+5. COMMANDS                                                *arti-pack-commands*
 
                                                                 *:ArtiPackSync*
-:ArtiPackSync     Install missing plugins and update ones whose tag changed.
+:ArtiPackSync     Install missing plugins and apply changed refs.
 :ArtiPackSync!    Reinstall every configured plugin, ignoring recorded state.
 
+                                                              *:ArtiPackUpdate*
+:ArtiPackUpdate   Refetch every branch-tracking plugin. See
+                  |arti-pack-branches|.
+
                                                                *:ArtiPackClean*
 :ArtiPackClean    Remove installed plugins that are no longer in the spec.
 
@@ -83,16 +134,16 @@ The archive URL is built as: >
 :ArtiPackList     Show every configured plugin and its state.
 
 ==============================================================================
-5. LAYOUT                                                    *arti-pack-layout*
+6. LAYOUT                                                    *arti-pack-layout*
 
   stdpath('data')/site/pack/arti-pack/start/<name>   loaded at startup
   stdpath('data')/site/pack/arti-pack/opt/<name>     loaded by |:packadd|
-  stdpath('data')/arti-pack/lock.json                recorded tags
+  stdpath('data')/arti-pack/lock.json                recorded installs
   stdpath('cache')/arti-pack                         downloaded archives
 
 Downloads are checked for a zip or gzip magic header before being trusted, and
-the single "<repo>-<version>" directory that tag archives wrap their contents
-in is stripped on the way in. A plugin that fails is reported and skipped; the
+the single "<repo>-<ref>" directory that source archives wrap their contents in
+is stripped on the way in. A plugin that fails is reported and skipped; the
 rest of the run continues.
 
 vim:tw=78:ts=8:noet:ft=help:norl:
diff --git a/lua/arti-pack/fs.lua b/lua/arti-pack/fs.lua
index ef94cc6..94e5571 100644
--- a/lua/arti-pack/fs.lua
+++ b/lua/arti-pack/fs.lua
@@ -60,6 +60,23 @@ function M.read_head(path, bytes)
   return head
 end
 
+-- Identifies what was actually fetched. A branch name says nothing about
+-- content, so the digest is what makes a branch install reportable. Archive
+-- bytes reach sha256() as a Blob, which it refuses, so they are hashed in their
+-- base64 form: an identity for the content, not the file's sha256sum.
+function M.digest(path)
+  local fd = io.open(path, 'rb')
+  if not fd then
+    return nil
+  end
+  local data = fd:read('*a')
+  fd:close()
+  if not data then
+    return nil
+  end
+  return vim.fn.sha256(vim.base64.encode(data))
+end
+
 function M.verify_archive(path, ext)
   local stat = uv.fs_stat(path)
   if not stat or stat.size == 0 then
diff --git a/lua/arti-pack/init.lua b/lua/arti-pack/init.lua
index 7c75cf6..2c0e336 100644
--- a/lua/arti-pack/init.lua
+++ b/lua/arti-pack/init.lua
@@ -59,7 +59,7 @@ local function installed_dir(ctx, name, opt)
 end
 
 local function finalise(ctx, results)
-  local counts = { installed = 0, updated = 0, failed = 0 }
+  local counts = { installed = 0, updated = 0, unchanged = 0, failed = 0 }
 
   for _, res in ipairs(results) do
     counts[res.status] = (counts[res.status] or 0) + 1
@@ -85,7 +85,7 @@ local function finalise(ctx, results)
   end
 
   local parts = {}
-  for _, key in ipairs({ 'installed', 'updated', 'failed' }) do
+  for _, key in ipairs({ 'installed', 'updated', 'unchanged', 'failed' }) do
     if counts[key] > 0 then
       table.insert(parts, string.format('%d %s', counts[key], key))
     end
@@ -98,6 +98,25 @@ local function finalise(ctx, results)
   end
 end
 
+-- A branch install is only as fresh as the last fetch, and a plain sync does not
+-- fetch it again, so an idle run says what it did not check instead of claiming
+-- everything is current.
+local function idle_message(specs)
+  local branches = 0
+  for _, spec in ipairs(specs) do
+    if spec.branch then
+      branches = branches + 1
+    end
+  end
+  if branches == 0 then
+    return 'everything up to date'
+  end
+  return string.format(
+    'nothing to install; %d branch plugin(s) not rechecked, :ArtiPackUpdate refetches them',
+    branches
+  )
+end
+
 function M.sync(opts)
   opts = opts or {}
   if running then
@@ -112,14 +131,18 @@ function M.sync(opts)
   local ctx = context()
   local pending = {}
   for _, spec in ipairs(specs) do
-    local current = ctx.lock:is_current(spec) and fs.exists(installed_dir(ctx, spec.name, spec.opt))
-    if opts.force or not current then
+    local wanted = ctx.lock:needs_install(spec, {
+      force = opts.force,
+      update = opts.update,
+      present = fs.exists(installed_dir(ctx, spec.name, spec.opt)),
+    })
+    if wanted then
       table.insert(pending, spec)
     end
   end
 
   if #pending == 0 then
-    return notify('everything up to date')
+    return notify(opts.update and 'no branch-tracking plugins to update' or idle_message(specs))
   end
 
   local reg_err = registry_error(pending, ctx.registry)
@@ -135,6 +158,11 @@ function M.sync(opts)
   end)
 end
 
+-- Branch archives are mutable, so nothing refetches them implicitly.
+function M.update()
+  return M.sync({ update = true })
+end
+
 function M.clean()
   local specs, errors = spec_mod.parse_all(M.config.plugins)
   for _, err in ipairs(errors) do
@@ -179,6 +207,24 @@ function M.clean()
   end
 end
 
+local function locked_ref(entry)
+  if entry.branch then
+    return 'branch ' .. entry.branch
+  end
+  return tostring(entry.tag)
+end
+
+local function fetched(entry)
+  if type(entry.installed_at) ~= 'number' then
+    return 'at an unknown time'
+  end
+  local stamp = os.date('%Y-%m-%d %H:%M', entry.installed_at)
+  if type(entry.digest) == 'string' then
+    return string.format('%s (%s)', stamp, entry.digest:sub(1, 8))
+  end
+  return stamp
+end
+
 function M.list()
   local specs, errors = spec_mod.parse_all(M.config.plugins)
   local ctx = context()
@@ -190,16 +236,18 @@ function M.list()
     if not entry then
       state = 'missing'
     elseif not ctx.lock:is_current(spec) then
-      state = 'outdated (' .. tostring(entry.tag) .. ')'
+      state = 'outdated (' .. locked_ref(entry) .. ')'
     elseif not fs.exists(installed_dir(ctx, spec.name, spec.opt)) then
       state = 'gone from disk'
+    elseif spec.branch then
+      -- A branch may have moved upstream since this was fetched and nothing here
+      -- can know, so report what was installed and when, never "installed".
+      state = string.format('tracking branch %s, fetched %s', spec.branch, fetched(entry))
     else
       state = 'installed'
     end
-    table.insert(
-      lines,
-      string.format('%-32s %-14s %-5s %s', spec.source, spec.tag, spec.opt and 'opt' or 'start', state)
-    )
+    local ref = spec.branch and ('@' .. spec.branch) or spec.tag
+    table.insert(lines, string.format('%-32s %-14s %-5s %s', spec.source, ref, spec.opt and 'opt' or 'start', state))
   end
 
   for _, name in ipairs(ctx.lock:names()) do
diff --git a/lua/arti-pack/installer.lua b/lua/arti-pack/installer.lua
index c9a285c..f893227 100644
--- a/lua/arti-pack/installer.lua
+++ b/lua/arti-pack/installer.lua
@@ -4,7 +4,14 @@ local spec_mod = require('arti-pack.spec')
 local M = {}
 
 local function result(spec, status, err)
-  return { name = spec.name, source = spec.source, tag = spec.tag, status = status, err = err }
+  return {
+    name = spec.name,
+    source = spec.source,
+    tag = spec.tag,
+    branch = spec.branch,
+    status = status,
+    err = err,
+  }
 end
 
 local function target_dir(ctx, spec)
@@ -29,6 +36,7 @@ end
 local function place(ctx, spec, stage, done)
   local root = fs.archive_root(stage)
   local target = target_dir(ctx, spec)
+  local had_target = fs.exists(target)
 
   fs.mkdirp(vim.fn.fnamemodify(target, ':h'))
   fs.rm_rf(target)
@@ -43,6 +51,8 @@ local function place(ctx, spec, stage, done)
   local previous = ctx.lock:get(spec.name)
   ctx.lock:set(spec.name, {
     tag = spec.tag,
+    branch = spec.branch,
+    digest = spec.digest,
     opt = spec.opt,
     source = spec.source,
     ext = spec.ext,
@@ -50,10 +60,22 @@ local function place(ctx, spec, stage, done)
     installed_at = os.time(),
   })
 
-  done(result(spec, previous and 'updated' or 'installed'))
+  -- Refetching a branch usually lands the same bytes; saying so is more useful
+  -- than reporting an update that changed nothing.
+  local status = 'installed'
+  if previous then
+    local same = had_target and spec.digest ~= nil and previous.digest == spec.digest
+    status = same and 'unchanged' or 'updated'
+  end
+
+  done(result(spec, status))
 end
 
 local function extract(ctx, spec, archive, done)
+  if spec.branch then
+    spec.digest = fs.digest(archive)
+  end
+
   local stage = fs.join(ctx.pack_dir, '.stage', spec.name)
   fs.rm_rf(stage)
   fs.mkdirp(stage)
@@ -75,7 +97,9 @@ local function download(ctx, spec, archive, done)
   end
   spec.resolved_url = url
 
-  if fs.exists(archive) and fs.verify_archive(archive, spec.ext) then
+  -- Only a tag archive is safe to serve from cache: the bytes behind a branch
+  -- archive change, so a cached one would make every update a no-op.
+  if not spec.branch and fs.exists(archive) and fs.verify_archive(archive, spec.ext) then
     return extract(ctx, spec, archive, done)
   end
 
@@ -120,14 +144,16 @@ local function download(ctx, spec, archive, done)
 end
 
 function M.install(ctx, spec, done)
-  local archive = fs.join(ctx.cache_dir, string.format('%s-%s.%s', spec.name, spec.tag, spec.ext))
+  local slot = spec.branch and string.format('%s-heads-%s', spec.name, spec.branch)
+    or string.format('%s-%s', spec.name, spec.tag)
+  local archive = fs.join(ctx.cache_dir, string.format('%s.%s', slot, spec.ext))
   local ok, err = pcall(download, ctx, spec, archive, done)
   if not ok then
     done(result(spec, 'failed', tostring(err)))
   end
 end
 
--- Failures are per plugin: one bad tag must not abandon the rest of the run.
+-- Failures are per plugin: one bad ref must not abandon the rest of the run.
 function M.run(ctx, specs, on_done)
   local results, started, finished = {}, 0, 0
   local limit = math.max(1, ctx.concurrency or 4)
diff --git a/lua/arti-pack/lock.lua b/lua/arti-pack/lock.lua
index ba01e7a..76b8f76 100644
--- a/lua/arti-pack/lock.lua
+++ b/lua/arti-pack/lock.lua
@@ -74,14 +74,37 @@ function M:names()
   return names
 end
 
--- A plugin is current when the recorded tag and load style both still match the
+-- A plugin is current when the recorded ref and load style both still match the
 -- spec; a flipped opt flag has to move the directory, so it counts as changed.
+-- For a branch this answers "are we tracking the ref you asked for", not "is the
+-- installed content still what that branch points at": a branch moves upstream
+-- and nothing on disk can tell us that it has.
 function M:is_current(spec)
   local entry = self.entries[spec.name]
   if not entry then
     return false
   end
-  return entry.tag == spec.tag and (entry.opt == true) == (spec.opt == true)
+  if (entry.opt == true) ~= (spec.opt == true) then
+    return false
+  end
+  if spec.branch then
+    return entry.branch == spec.branch
+  end
+  return entry.tag == spec.tag and entry.branch == nil
+end
+
+-- The whole update policy, in one place. A tag is immutable, so a matching entry
+-- with its directory still on disk needs no work. A branch is mutable, so it is
+-- refetched only when asked for: a forced sync, or an update run.
+function M:needs_install(spec, opts)
+  opts = opts or {}
+  if opts.force then
+    return true
+  end
+  if not (self:is_current(spec) and opts.present) then
+    return true
+  end
+  return opts.update == true and spec.branch ~= nil
 end
 
 return M
diff --git a/lua/arti-pack/spec.lua b/lua/arti-pack/spec.lua
index 2bf6f6e..ae6d5c3 100644
--- a/lua/arti-pack/spec.lua
+++ b/lua/arti-pack/spec.lua
@@ -9,6 +9,13 @@ local function trim(s)
   return (s:gsub('^%s+', ''):gsub('%s+$', ''))
 end
 
+-- vim.system hands argv to the OS as C strings, so a NUL truncates the value
+-- before curl sees it: the URL fetched would not be the one validated, locked
+-- and shown. Lua's %s class does not cover it, so it needs its own check.
+local function has_nul(s)
+  return s:find('\0', 1, true) ~= nil
+end
+
 local function bad_name(name)
   return name == '' or name == '.' or name == '..' or name:find('[/\\%s]') ~= nil
 end
@@ -32,13 +39,23 @@ function M.parse(entry)
     return nil, string.format('invalid source %q, expected "owner/repo"', source)
   end
 
-  local tag = entry.tag
-  if type(tag) ~= 'string' or trim(tag) == '' then
-    return nil, string.format('%s is missing a tag', source)
+  if entry.tag ~= nil and entry.branch ~= nil then
+    return nil, string.format('%s sets both a tag and a branch, pick one', source)
   end
-  tag = trim(tag)
-  if tag:find('[/\\%s]') then
-    return nil, string.format('%s has an invalid tag %q', source, tag)
+
+  -- A ref is one path segment of the archive URL, so the same rules that keep a
+  -- directory name from escaping its parent apply to it.
+  local kind = entry.branch ~= nil and 'branch' or 'tag'
+  local ref = entry.branch ~= nil and entry.branch or entry.tag
+  if type(ref) ~= 'string' or trim(ref) == '' then
+    return nil, string.format('%s is missing a tag or a branch', source)
+  end
+  ref = trim(ref)
+  if has_nul(ref) then
+    return nil, string.format('%s has a NUL byte in its %s %s', source, kind, vim.inspect(ref))
+  end
+  if bad_name(ref) then
+    return nil, string.format('%s has an invalid %s %q', source, kind, ref)
   end
 
   local ext = entry.ext or 'zip'
@@ -47,6 +64,9 @@ function M.parse(entry)
   end
 
   local name = entry.as or repo
+  if type(name) == 'string' and has_nul(name) then
+    return nil, string.format('%s has a NUL byte in its directory name %s', source, vim.inspect(name))
+  end
   if type(name) ~= 'string' or bad_name(name) then
     return nil, string.format('%s resolves to an unusable directory name %q', source, tostring(name))
   end
@@ -57,6 +77,9 @@ function M.parse(entry)
       return nil, string.format('%s has a non-string url', source)
     end
     url = trim(url)
+    if has_nul(url) then
+      return nil, string.format('%s has a NUL byte in its url %s', source, vim.inspect(url))
+    end
     if url == '' or url:find('%s') then
       return nil, string.format('%s has an invalid url %q', source, entry.url)
     end
@@ -67,7 +90,9 @@ function M.parse(entry)
     owner = owner,
     repo = repo,
     name = name,
-    tag = tag,
+    tag = kind == 'tag' and ref or nil,
+    branch = kind == 'branch' and ref or nil,
+    ref = ref,
     ext = ext,
     opt = entry.opt == true,
     url = url,
@@ -87,14 +112,18 @@ function M.check_registry(registry)
   end
   -- Whitespace left inside the URL would be baked into every download.
   local base = trim(registry)
+  if has_nul(base) then
+    return nil, string.format('registry %s has a NUL byte in it; %s', vim.inspect(registry), SETUP_HINT)
+  end
   if not base:match('^https?://[^%s/]') or base:find('%s') then
     return nil, bad
   end
   return (base:gsub('/+$', ''))
 end
 
--- Tagged source archives live at the same path shape on every GitHub-alike, so
--- the registry is just the proxy prefix in front of "<owner>/<repo>".
+-- Tag and branch source archives live at the same path shape on every
+-- GitHub-alike, so the registry is just the proxy prefix in front of
+-- "<owner>/<repo>".
 function M.url(spec, registry)
   if spec.url then
     return spec.url
@@ -109,8 +138,8 @@ function M.url(spec, registry)
     spec.repo,
     'archive',
     'refs',
-    'tags',
-    spec.tag .. '.' .. spec.ext,
+    spec.branch and 'heads' or 'tags',
+    spec.ref .. '.' .. spec.ext,
   }, '/')
 end
 
diff --git a/plugin/arti-pack.lua b/plugin/arti-pack.lua
index d7dd5de..a4b851f 100644
--- a/plugin/arti-pack.lua
+++ b/plugin/arti-pack.lua
@@ -9,7 +9,11 @@ end
 
 vim.api.nvim_create_user_command('ArtiPackSync', function(cmd)
   arti().sync({ force = cmd.bang })
-end, { bang = true, desc = 'Install missing arti-pack plugins and update changed tags' })
+end, { bang = true, desc = 'Install missing arti-pack plugins and apply changed refs' })
+
+vim.api.nvim_create_user_command('ArtiPackUpdate', function()
+  arti().update()
+end, { desc = 'Refetch every branch-tracking plugin' })
 
 vim.api.nvim_create_user_command('ArtiPackClean', function()
   arti().clean()
diff --git a/tests/fs_spec.lua b/tests/fs_spec.lua
index affb8cc..300b713 100644
--- a/tests/fs_spec.lua
+++ b/tests/fs_spec.lua
@@ -74,6 +74,27 @@ t.describe('fs.verify_archive', function()
   end)
 end)
 
+t.describe('fs.digest', function()
+  t.it('identifies file contents, not the file name', function()
+    local dir = tmpdir()
+    local a = write(dir .. '/a.zip', 'PK\3\4payload')
+    local b = write(dir .. '/b.zip', 'PK\3\4payload')
+    t.eq(fs.digest(a), fs.digest(b))
+    t.eq(64, #fs.digest(a), 'a sha256 hex digest')
+  end)
+
+  t.it('changes when the bytes change', function()
+    local dir = tmpdir()
+    local before = fs.digest(write(dir .. '/a.zip', 'PK\3\4one'))
+    local after = fs.digest(write(dir .. '/a.zip', 'PK\3\4two'))
+    t.truthy(before ~= after, 'a refetched archive with new contents digests differently')
+  end)
+
+  t.it('returns nothing for a missing file', function()
+    t.eq(nil, fs.digest(tmpdir() .. '/absent.zip'))
+  end)
+end)
+
 t.describe('fs.extract', function()
   t.it('reports a missing archive instead of raising', function()
     local ok, err = fs.extract(tmpdir() .. '/absent.zip', tmpdir(), 'zip')
diff --git a/tests/init_spec.lua b/tests/init_spec.lua
index 4801154..d6b91ce 100644
--- a/tests/init_spec.lua
+++ b/tests/init_spec.lua
@@ -1,6 +1,7 @@
 local t = require('tests.harness')
 local fs = require('arti-pack.fs')
 local Lock = require('arti-pack.lock')
+local zipfile = require('tests.zipfile')
 local arti = require('arti-pack')
 
 local function tmpdir()
@@ -87,11 +88,21 @@ local function install_fake(name, sub, tag)
   record(name, tag, sub == 'opt')
 end
 
+local function install_branch_fake(name, branch, at)
+  place_dir(name, 'start')
+  local l = lock()
+  l:set(name, { branch = branch, opt = false, digest = string.rep('b', 64), installed_at = at })
+  assert(l:save())
+end
+
 -- sync finishes asynchronously; its closing notification is the only signal.
 local function is_terminal(msg)
   return msg:find('everything up to date', 1, true) ~= nil
+    or msg:find('nothing to install', 1, true) ~= nil
+    or msg:find('no branch-tracking plugins', 1, true) ~= nil
     or msg:find('%d+ installed') ~= nil
     or msg:find('%d+ updated') ~= nil
+    or msg:find('%d+ unchanged') ~= nil
     or msg:find('%d+ failed') ~= nil
 end
 
@@ -115,6 +126,12 @@ local function sync(opts)
   drain(before)
 end
 
+local function update()
+  local before = #env.notices
+  arti.update()
+  drain(before)
+end
+
 local function build_archive(ext, repo, version)
   local work = tmpdir()
   local root = string.format('%s-%s', repo, version)
@@ -353,6 +370,89 @@ t.describe('arti-pack.sync', function()
   end)
 end)
 
+-- Branch fixtures are built in Lua, so these run with no zip or tar around.
+t.describe('arti-pack branch tracking', function()
+  local function branch_entry(source, branch, archive)
+    return { source, branch = branch, url = 'file://' .. archive }
+  end
+
+  local function setup_branch(archive)
+    arti.setup({ auto_sync = false, plugins = { branch_entry('hrsh7th/cmp-path', 'main', archive) } })
+  end
+
+  t.it('installs a branch plugin and records the branch it tracks', function()
+    use_env()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    setup_branch(archive)
+    sync()
+
+    t.truthy(fs.exists(env.pack_dir .. '/start/cmp-path/lua/marker.lua'))
+    t.eq('main', lock():get('cmp-path').branch)
+    t.eq(nil, lock():get('cmp-path').tag)
+  end)
+
+  t.it('does not refetch a branch on an ordinary sync, and says so', function()
+    use_env()
+    local path = tmpdir() .. '/cmp-path-main.zip'
+    setup_branch(zipfile.write(path, 'cmp-path-main', 'first'))
+    sync()
+
+    zipfile.write(path, 'cmp-path-main', 'second')
+    env.notices = {}
+    sync()
+
+    t.matches('nothing to install', last_notice())
+    t.matches('1 branch plugin%(s%) not rechecked', last_notice())
+    t.matches('first', table.concat(vim.fn.readfile(env.pack_dir .. '/start/cmp-path/lua/marker.lua'), '\n'))
+  end)
+
+  t.it('refetches a moved branch on update', function()
+    use_env()
+    local path = tmpdir() .. '/cmp-path-main.zip'
+    setup_branch(zipfile.write(path, 'cmp-path-main', 'first'))
+    sync()
+
+    zipfile.write(path, 'cmp-path-main', 'second')
+    env.notices = {}
+    update()
+
+    t.matches('1 updated', last_notice())
+    t.matches('second', table.concat(vim.fn.readfile(env.pack_dir .. '/start/cmp-path/lua/marker.lua'), '\n'))
+  end)
+
+  t.it('reports an update that changed nothing as unchanged', function()
+    use_env()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    setup_branch(archive)
+    sync()
+
+    env.notices = {}
+    update()
+    t.matches('1 unchanged', last_notice())
+  end)
+
+  t.it('has nothing to update when every plugin is pinned to a tag', function()
+    use_env()
+    arti.setup({ auto_sync = false, plugins = { { 'owner/here.nvim', tag = 'v1' } } })
+    install_fake('here.nvim', 'start', 'v1')
+
+    env.notices = {}
+    update()
+    t.matches('no branch%-tracking plugins to update', last_notice())
+  end)
+
+  t.it('installs a branch plugin whose directory disappeared', function()
+    use_env()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    setup_branch(archive)
+    sync()
+    fs.rm_rf(env.pack_dir .. '/start/cmp-path')
+
+    sync()
+    t.truthy(fs.exists(env.pack_dir .. '/start/cmp-path/lua/marker.lua'))
+  end)
+end)
+
 t.describe('arti-pack.clean', function()
   t.it('removes the directories and lock entries that are no longer configured', function()
     use_env()
@@ -419,6 +519,39 @@ t.describe('arti-pack.clean', function()
     t.matches('missing a tag', notices(), 'the parse error is reported')
     t.matches('clean aborted', last_notice())
   end)
+
+  t.it('keeps a plugin that is configured to track a branch', function()
+    use_env()
+    arti.setup({ auto_sync = false, plugins = { { 'hrsh7th/cmp-path', branch = 'main' } } })
+    install_branch_fake('cmp-path', 'main', 1757000000)
+    install_fake('drop.nvim', 'start')
+
+    arti.clean()
+
+    t.truthy(fs.exists(env.pack_dir .. '/start/cmp-path/marker.lua'), 'a branch plugin is still configured')
+    t.falsy(fs.exists(env.pack_dir .. '/start/drop.nvim'))
+    t.eq({ 'cmp-path' }, lock():names())
+  end)
+
+  t.it('deletes nothing when a branch spec is in error', function()
+    use_env()
+    arti.setup({
+      auto_sync = false,
+      plugins = {
+        { 'hrsh7th/cmp-path', branch = 'main' },
+        { 'owner/broken.nvim', tag = 'v1', branch = 'main' },
+      },
+    })
+    install_branch_fake('cmp-path', 'main', 1757000000)
+    install_fake('orphan.nvim', 'start')
+
+    arti.clean()
+
+    t.truthy(fs.exists(env.pack_dir .. '/start/cmp-path'))
+    t.truthy(fs.exists(env.pack_dir .. '/start/orphan.nvim'), 'nothing is deleted from a partial spec set')
+    t.matches('sets both a tag and a branch', notices())
+    t.matches('clean aborted', last_notice())
+  end)
 end)
 
 t.describe('arti-pack.list', function()
@@ -463,6 +596,37 @@ t.describe('arti-pack.list', function()
     t.eq(5, #lines)
   end)
 
+  t.it('never claims a branch plugin is up to date', function()
+    use_env()
+    arti.setup({ auto_sync = false, plugins = { { 'hrsh7th/cmp-path', branch = 'main' } } })
+    install_branch_fake('cmp-path', 'main', 1757000000)
+
+    local line = arti.list()[1]
+
+    t.matches('@main', line, 'the branch is shown in place of a version')
+    t.matches('tracking branch main, fetched 20', line)
+    t.matches('%(bbbbbbbb%)$', line, 'the fetched archive digest is shown')
+    t.falsy(line:find('installed', 1, true), 'a branch install is never reported as simply installed')
+  end)
+
+  t.it('reports a branch plugin whose lock entry tracks another branch', function()
+    use_env()
+    arti.setup({ auto_sync = false, plugins = { { 'hrsh7th/cmp-path', branch = 'master' } } })
+    install_branch_fake('cmp-path', 'main', 1757000000)
+
+    t.matches('outdated %(branch main%)$', arti.list()[1])
+  end)
+
+  t.it('reports a branch plugin that is missing from disk', function()
+    use_env()
+    arti.setup({ auto_sync = false, plugins = { { 'hrsh7th/cmp-path', branch = 'main' } } })
+    local l = lock()
+    l:set('cmp-path', { branch = 'main', opt = false, installed_at = 1757000000 })
+    assert(l:save())
+
+    t.matches('gone from disk$', arti.list()[1])
+  end)
+
   t.it('lists spec errors alongside the plugins', function()
     use_env()
     arti.setup({ auto_sync = false, plugins = { { 'owner/ok.nvim', tag = 'v1' }, 'bogus' } })
diff --git a/tests/installer_spec.lua b/tests/installer_spec.lua
index 4582207..e60db8a 100644
--- a/tests/installer_spec.lua
+++ b/tests/installer_spec.lua
@@ -3,6 +3,7 @@ local fs = require('arti-pack.fs')
 local installer = require('arti-pack.installer')
 local Lock = require('arti-pack.lock')
 local spec_mod = require('arti-pack.spec')
+local zipfile = require('tests.zipfile')
 
 local function tmpdir()
   local dir = vim.fn.tempname()
@@ -74,6 +75,66 @@ local function spec_for(source, tag, archive, extra)
   return assert(spec_mod.parse(entry))
 end
 
+-- Branch fixtures are built in Lua, so these run with no zip or tar around.
+t.describe('installer.run with a branch spec', function()
+  local function branch_spec(source, branch, archive)
+    return assert(spec_mod.parse({ source, branch = branch, url = 'file://' .. archive }))
+  end
+
+  t.it('installs a branch archive and records the branch, not a tag', function()
+    local ctx = new_ctx()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    local results = run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', archive) })
+
+    t.eq('installed', results[1].status, results[1].err)
+    t.truthy(fs.exists(ctx.pack_dir .. '/start/cmp-path/lua/marker.lua'), 'wrapper directory is stripped')
+
+    local entry = ctx.lock:get('cmp-path')
+    t.eq('main', entry.branch)
+    t.eq(nil, entry.tag, 'a branch is not recorded as a version')
+    t.eq(64, #entry.digest, 'the fetched archive is identified by digest')
+    t.truthy(type(entry.installed_at) == 'number', 'the fetch time is recorded')
+  end)
+
+  t.it('reports a refetch that brought back the same bytes as unchanged', function()
+    local ctx = new_ctx()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', archive) })
+    local before = ctx.lock:get('cmp-path').digest
+
+    local results = run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', archive) })
+    t.eq('unchanged', results[1].status, results[1].err)
+    t.eq(before, ctx.lock:get('cmp-path').digest)
+  end)
+
+  t.it('reports a moved branch as an update and replaces the tree', function()
+    local ctx = new_ctx()
+    local path = tmpdir() .. '/cmp-path-main.zip'
+    run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', zipfile.write(path, 'cmp-path-main', 'first')) })
+    local before = ctx.lock:get('cmp-path').digest
+    write(ctx.pack_dir .. '/start/cmp-path/leftover.txt', 'stale')
+
+    zipfile.write(path, 'cmp-path-main', 'second')
+    local results = run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', path) })
+    t.eq('updated', results[1].status, results[1].err)
+    t.falsy(fs.exists(ctx.pack_dir .. '/start/cmp-path/leftover.txt'), 'stale files are cleared')
+    t.truthy(before ~= ctx.lock:get('cmp-path').digest, 'the recorded digest follows the branch')
+  end)
+
+  t.it('never serves a branch from the archive cache', function()
+    local ctx = new_ctx()
+    local archive = zipfile.write(tmpdir() .. '/cmp-path-main.zip', 'cmp-path-main', 'first')
+    run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', archive) })
+
+    -- With the source gone a cached archive would silently succeed, which is
+    -- exactly what would make an update a no-op.
+    vim.fn.delete(archive)
+    local results = run(ctx, { branch_spec('hrsh7th/cmp-path', 'main', archive) })
+    t.eq('failed', results[1].status)
+    t.matches('download failed', results[1].err)
+  end)
+end)
+
 t.describe('installer.run', function()
   if not have('zip') or not have('tar') then
     t.it('SKIPPED: zip/tar unavailable to build fixtures', function() end)
diff --git a/tests/lock_spec.lua b/tests/lock_spec.lua
index 4b6030b..4b7a31b 100644
--- a/tests/lock_spec.lua
+++ b/tests/lock_spec.lua
@@ -25,6 +25,23 @@ t.describe('lock', function()
     t.eq({ tag = 'v0.1.4', opt = false }, reloaded:get('plenary.nvim'))
   end)
 
+  t.it('round-trips a branch entry with what identifies the fetched content', function()
+    local path = tmpdir() .. '/lock.json'
+    local entry = {
+      branch = 'main',
+      digest = string.rep('a', 64),
+      installed_at = 1757000000,
+      opt = false,
+      source = 'hrsh7th/cmp-path',
+      ext = 'zip',
+    }
+    local lock = Lock.new(path)
+    lock:set('cmp-path', entry)
+    t.truthy(lock:save())
+
+    t.eq(entry, Lock.new(path):load():get('cmp-path'))
+  end)
+
   t.it('starts empty when the state file does not exist', function()
     local lock = Lock.new(tmpdir() .. '/absent.json'):load()
     t.eq({}, lock:names())
@@ -101,4 +118,67 @@ t.describe('lock.is_current', function()
     local s = assert(spec.parse({ 'owner/repo', tag = 'v1', opt = true }))
     t.falsy(make({ tag = 'v1', opt = false }):is_current(s))
   end)
+
+  t.it('is true when the tracked branch still matches', function()
+    local s = assert(spec.parse({ 'owner/repo', branch = 'main' }))
+    t.truthy(make({ branch = 'main', opt = false }):is_current(s))
+  end)
+
+  t.it('is false when the spec switched to another branch', function()
+    local s = assert(spec.parse({ 'owner/repo', branch = 'master' }))
+    t.falsy(make({ branch = 'main', opt = false }):is_current(s))
+  end)
+
+  t.it('is false when a spec swaps between a tag and a branch', function()
+    local tagged = assert(spec.parse({ 'owner/repo', tag = 'v1' }))
+    local branched = assert(spec.parse({ 'owner/repo', branch = 'main' }))
+    t.falsy(make({ branch = 'main', opt = false }):is_current(tagged))
+    t.falsy(make({ tag = 'v1', opt = false }):is_current(branched))
+  end)
+end)
+
+t.describe('lock.needs_install', function()
+  local function make(entry)
+    local lock = Lock.new(tmpdir() .. '/lock.json')
+    if entry then
+      lock:set('repo', entry)
+    end
+    return lock
+  end
+
+  local tagged = assert(spec.parse({ 'owner/repo', tag = 'v1' }))
+  local branched = assert(spec.parse({ 'owner/repo', branch = 'main' }))
+
+  t.it('installs anything that is missing from the lockfile', function()
+    t.truthy(make(nil):needs_install(tagged, { present = false }))
+    t.truthy(make(nil):needs_install(branched, { present = false }))
+  end)
+
+  t.it('installs anything whose directory is gone', function()
+    t.truthy(make({ tag = 'v1' }):needs_install(tagged, { present = false }))
+    t.truthy(make({ branch = 'main' }):needs_install(branched, { present = false }))
+  end)
+
+  t.it('skips an installed tag, which cannot have moved', function()
+    t.falsy(make({ tag = 'v1' }):needs_install(tagged, { present = true }))
+    t.falsy(make({ tag = 'v1' }):needs_install(tagged, { present = true, update = true }))
+  end)
+
+  t.it('leaves an installed branch alone on a plain sync', function()
+    t.falsy(make({ branch = 'main' }):needs_install(branched, { present = true }))
+  end)
+
+  t.it('refetches an installed branch on an update run', function()
+    t.truthy(make({ branch = 'main' }):needs_install(branched, { present = true, update = true }))
+  end)
+
+  t.it('refetches everything when forced', function()
+    t.truthy(make({ tag = 'v1' }):needs_install(tagged, { present = true, force = true }))
+    t.truthy(make({ branch = 'main' }):needs_install(branched, { present = true, force = true }))
+  end)
+
+  t.it('installs a branch whose name changed even without an update run', function()
+    local other = assert(spec.parse({ 'owner/repo', branch = 'master' }))
+    t.truthy(make({ branch = 'main' }):needs_install(other, { present = true }))
+  end)
 end)
diff --git a/tests/spec_spec.lua b/tests/spec_spec.lua
index 5076f18..5f7554c 100644
--- a/tests/spec_spec.lua
+++ b/tests/spec_spec.lua
@@ -2,6 +2,11 @@ local t = require('tests.harness')
 local spec = require('arti-pack.spec')
 
 local REGISTRY = 'https://artifacts.example.com/artifactory/github-remote'
+local NUL = string.char(0)
+
+-- A NUL truncates the value in argv, so anything that reaches curl must be
+-- rejected before it can diverge from what was validated and locked.
+local NULS = { 'mid' .. NUL .. 'dle', 'trailing' .. NUL, NUL }
 
 t.describe('spec.parse', function()
   t.it('parses a full table spec', function()
@@ -35,6 +40,55 @@ t.describe('spec.parse', function()
     t.matches('missing a tag', err)
   end)
 
+  t.it('parses a branch spec', function()
+    local s = assert(spec.parse({ 'hrsh7th/cmp-path', branch = 'main' }))
+    t.eq('main', s.branch)
+    t.eq('main', s.ref)
+    t.eq(nil, s.tag, 'a branch spec carries no tag')
+    t.eq('cmp-path', s.name)
+  end)
+
+  t.it('rejects a spec that sets both a tag and a branch', function()
+    local s, err = spec.parse({ 'owner/repo', tag = 'v1', branch = 'main' })
+    t.falsy(s)
+    t.matches('sets both a tag and a branch', err)
+  end)
+
+  t.it('rejects a spec with neither a tag nor a branch', function()
+    local s, err = spec.parse({ 'owner/repo' })
+    t.falsy(s)
+    t.matches('missing a tag or a branch', err)
+  end)
+
+  t.it('rejects a branch that would reshape the archive path', function()
+    for _, bad in ipairs({ '..', '.', 'feature/foo', '../../etc', 'a\\b', 'ma in' }) do
+      local s, err = spec.parse({ 'owner/repo', branch = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('invalid branch', err)
+    end
+  end)
+
+  t.it('rejects an empty branch', function()
+    for _, bad in ipairs({ '', '   ', 42 }) do
+      local s, err = spec.parse({ 'owner/repo', branch = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('missing a tag or a branch', err)
+    end
+  end)
+
+  t.it('rejects a tag that would reshape the archive path', function()
+    for _, bad in ipairs({ '..', '.', 'a\\b' }) do
+      local s, err = spec.parse({ 'owner/repo', tag = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('invalid tag', err)
+    end
+  end)
+
+  t.it('trims surrounding whitespace from a branch', function()
+    local s = assert(spec.parse({ 'owner/repo', branch = '  main \n' }))
+    t.eq('main', s.branch)
+  end)
+
   t.it('rejects a source without an owner', function()
     local s, err = spec.parse({ 'plenary.nvim', tag = 'v1' })
     t.falsy(s)
@@ -82,6 +136,47 @@ t.describe('spec.parse', function()
     t.eq('https://example.test/a.zip', s.url)
   end)
 
+  t.it('rejects a tag carrying a NUL byte', function()
+    for _, bad in ipairs(NULS) do
+      local s, err = spec.parse({ 'owner/repo', tag = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('NUL byte in its tag', err)
+    end
+  end)
+
+  t.it('rejects a branch carrying a NUL byte', function()
+    for _, bad in ipairs(NULS) do
+      local s, err = spec.parse({ 'owner/repo', branch = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('NUL byte in its branch', err)
+    end
+  end)
+
+  t.it('rejects an "as" name carrying a NUL byte', function()
+    for _, bad in ipairs(NULS) do
+      local s, err = spec.parse({ 'owner/repo', tag = 'v1', as = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('NUL byte in its directory name', err)
+    end
+  end)
+
+  t.it('rejects a url carrying a NUL byte', function()
+    local urls = { 'https://example.test/a' .. NUL .. 'b.zip', 'https://example.test/a.zip' .. NUL, NUL }
+    for _, bad in ipairs(urls) do
+      local s, err = spec.parse({ 'owner/repo', tag = 'v1', url = bad })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('NUL byte in its url', err)
+    end
+  end)
+
+  t.it('rejects a source carrying a NUL byte', function()
+    for _, bad in ipairs(NULS) do
+      local s, err = spec.parse({ 'owner/' .. bad, tag = 'v1' })
+      t.falsy(s, 'rejects ' .. vim.inspect(bad))
+      t.matches('expected "owner/repo"', err)
+    end
+  end)
+
   t.it('rejects a url with whitespace inside it', function()
     for _, bad in ipairs({ 'https://example.test/a b.zip', 'https://example\n.test/a.zip', '   ' }) do
       local s, err = spec.parse({ 'owner/repo', tag = 'v1', url = bad })
@@ -121,6 +216,26 @@ t.describe('spec.url', function()
     end
   end)
 
+  t.it('builds a branch archive url', function()
+    local s = assert(spec.parse({ 'hrsh7th/cmp-path', branch = 'main' }))
+    t.eq(REGISTRY .. '/hrsh7th/cmp-path/archive/refs/heads/main.zip', spec.url(s, REGISTRY))
+  end)
+
+  t.it('builds a branch archive url for tar.gz', function()
+    local s = assert(spec.parse({ 'hrsh7th/vim-vsnip', branch = 'master', ext = 'tar.gz' }))
+    t.eq(REGISTRY .. '/hrsh7th/vim-vsnip/archive/refs/heads/master.tar.gz', spec.url(s, REGISTRY))
+  end)
+
+  t.it('does not double up on a trailing registry slash for a branch', function()
+    local s = assert(spec.parse({ 'owner/repo', branch = 'main' }))
+    t.eq(REGISTRY .. '/owner/repo/archive/refs/heads/main.zip', spec.url(s, REGISTRY .. '///'))
+  end)
+
+  t.it('lets an explicit url bypass the registry for a branch', function()
+    local s = assert(spec.parse({ 'owner/repo', branch = 'main', url = 'https://example.test/main.zip' }))
+    t.eq('https://example.test/main.zip', spec.url(s, REGISTRY))
+  end)
+
   t.it('passes an explicit url straight through', function()
     local s = assert(spec.parse({ 'owner/repo', tag = 'v1', url = 'https://example.test/a.zip' }))
     t.eq('https://example.test/a.zip', spec.url(s, REGISTRY))
@@ -173,6 +288,15 @@ t.describe('spec.check_registry', function()
     end
   end)
 
+  t.it('rejects a registry carrying a NUL byte', function()
+    local bads = { 'https://artifacts.example.com/g' .. NUL .. 'h', REGISTRY .. NUL, NUL }
+    for _, bad in ipairs(bads) do
+      local base, err = spec.check_registry(bad)
+      t.falsy(base, 'rejects ' .. vim.inspect(bad))
+      t.matches('NUL byte', err)
+    end
+  end)
+
   t.it('rejects a whitespace-only registry', function()
     for _, bad in ipairs({ ' ', '   ', '\n', ' \t\n ' }) do
       local base, err = spec.check_registry(bad)
diff --git a/tests/zipfile.lua b/tests/zipfile.lua
new file mode 100644
index 0000000..028b014
--- /dev/null
+++ b/tests/zipfile.lua
@@ -0,0 +1,76 @@
+local zzlib = require('arti-pack.vendor.zzlib')
+
+local M = {}
+
+local function le(n, bytes)
+  local out = {}
+  for _ = 1, bytes do
+    table.insert(out, string.char(n % 256))
+    n = math.floor(n / 256)
+  end
+  return table.concat(out)
+end
+
+-- A stored-entry zip built here rather than shelled out to the zip binary, so
+-- tests that need a real archive run wherever nvim does.
+function M.bytes(entries)
+  local locals, central, offset = {}, {}, 0
+  for _, e in ipairs(entries) do
+    local data = e.data or ''
+    local crc = zzlib.crc32(data)
+    local head = 'PK\3\4'
+      .. le(20, 2)
+      .. le(0, 2)
+      .. le(0, 2)
+      .. le(0, 4)
+      .. le(crc, 4)
+      .. le(#data, 4)
+      .. le(#data, 4)
+      .. le(#e.name, 2)
+      .. le(0, 2)
+      .. e.name
+    table.insert(locals, head .. data)
+    table.insert(
+      central,
+      'PK\1\2'
+        .. le(3 * 256 + 20, 2)
+        .. le(20, 2)
+        .. le(0, 2)
+        .. le(0, 2)
+        .. le(0, 4)
+        .. le(crc, 4)
+        .. le(#data, 4)
+        .. le(#data, 4)
+        .. le(#e.name, 2)
+        .. le(0, 2)
+        .. le(0, 2)
+        .. le(0, 2)
+        .. le(0, 2)
+        .. le(0, 4)
+        .. le(offset, 4)
+        .. e.name
+    )
+    offset = offset + #head + #data
+  end
+  local body = table.concat(locals)
+  local cd = table.concat(central)
+  local eocd = 'PK\5\6'
+    .. le(0, 2)
+    .. le(0, 2)
+    .. le(#entries, 2)
+    .. le(#entries, 2)
+    .. le(#cd, 4)
+    .. le(#body, 4)
+    .. le(0, 2)
+  return body .. cd .. eocd
+end
+
+-- Mirrors an upstream source archive: everything under one "<repo>-<ref>" root.
+function M.write(path, root, marker)
+  local fd = assert(io.open(path, 'wb'))
+  fd:write(M.bytes({ { name = root .. '/lua/marker.lua', data = 'return "' .. marker .. '"' } }))
+  fd:close()
+  return path
+end
+
+return M