diff --git a/spec/unit/fossil_resolver_spec.cr b/spec/unit/fossil_resolver_spec.cr
index 2095b4a2..1d9863d7 100644
--- a/spec/unit/fossil_resolver_spec.cr
+++ b/spec/unit/fossil_resolver_spec.cr
@@ -5,11 +5,16 @@ private def resolver(name)
 end
 
 module Shards
-  # Allow overriding `source` for the specs
   class FossilResolver
+    # Allow overriding `source` for the specs
     def source=(@source)
       @origin_url = nil # This needs to be cleared so that #origin_url re-runs `fossil remote-url`
     end
+
+    # Direct set tool version for testing
+    def self.set_version(verstr : String)
+      @@version = verstr
+    end
   end
 
   describe FossilResolver, tags: %w[fossil] do
@@ -164,5 +169,39 @@ module Shards
       resolver.matches_ref?(FossilCommitRef.new("1234567890abcdef"), Shards::Version.new("0.1.0.+fossil.commit.1234567890abcdef")).should be_true
       resolver.matches_ref?(FossilCommitRef.new("1234567"), Shards::Version.new("0.1.0.+fossil.commit.1234567890abcdef")).should be_true
     end
+
+    it "#supports_format_arg" do
+      installed_version = FossilResolver.version.not_nil!
+      resolver = FossilResolver.new("", "")
+      FossilResolver.set_version "1.37"
+      resolver.supports_format_arg?.should eq false
+      FossilResolver.set_version "2.12"
+      resolver.supports_format_arg?.should eq false
+      FossilResolver.set_version "2.13"
+      resolver.supports_format_arg?.should eq false
+      FossilResolver.set_version "2.14"
+      resolver.supports_format_arg?.should eq true
+      FossilResolver.set_version "2.15"
+      resolver.supports_format_arg?.should eq true
+      FossilResolver.set_version "3.0"
+      resolver.supports_format_arg?.should eq true
+      FossilResolver.set_version installed_version
+    end
+
+    it "#supports_workdir_arg" do
+      installed_version = FossilResolver.version.not_nil!
+      resolver = FossilResolver.new("", "")
+      FossilResolver.set_version "1.37"
+      resolver.supports_workdir_arg?.should eq false
+      FossilResolver.set_version "2.11"
+      resolver.supports_workdir_arg?.should eq false
+      FossilResolver.set_version "2.12"
+      resolver.supports_workdir_arg?.should eq true
+      FossilResolver.set_version "2.13"
+      resolver.supports_workdir_arg?.should eq true
+      FossilResolver.set_version "3.0"
+      resolver.supports_workdir_arg?.should eq true
+      FossilResolver.set_version installed_version
+    end
   end
 end
diff --git a/src/resolvers/fossil.cr b/src/resolvers/fossil.cr
index b568994c..6c98936f 100644
--- a/src/resolvers/fossil.cr
+++ b/src/resolvers/fossil.cr
@@ -1,5 +1,5 @@
 require "uri"
-require "./resolver"
+require "./version_control"
 require "../versions"
 require "../logger"
 require "../helpers"
@@ -89,14 +89,8 @@ module Shards
     end
   end
 
-  class FossilResolver < Resolver
-    @@has_fossil_command : Bool?
-    @@fossil_version_maj : Int8?
-    @@fossil_version_min : Int8?
-    @@fossil_version_rev : Int8?
-    @@fossil_version : String?
-
-    @origin_url : String?
+  class FossilResolver < VersionControlResolver
+    @@extension = ".fossil"
     @local_fossil_file : String?
 
     def self.key
@@ -112,38 +106,15 @@ module Shards
       end
     end
 
-    protected def self.has_fossil_command?
-      if @@has_fossil_command.nil?
-        @@has_fossil_command = (Process.run("fossil version", shell: true).success? rescue false)
+    protected def self.command?
+      if @@command.nil?
+        @@command = (Process.run("fossil version", shell: true).success? rescue false)
       end
-      @@has_fossil_command
-    end
-
-    protected def self.fossil_version
-      unless @@fossil_version
-        @@fossil_version = `fossil version`[/version\s+([^\s]*)/, 1]
-        pieces = @@fossil_version.not_nil!.split('.')
-        @@fossil_version_maj = pieces[0].to_i8
-        @@fossil_version_min = pieces[1].to_i8
-        @@fossil_version_rev = (pieces[2]?.try &.to_i8 || 0i8)
-      end
-
-      @@fossil_version
-    end
-
-    protected def self.fossil_version_maj
-      self.fossil_version unless @@fossil_version_maj
-      @@fossil_version_maj.not_nil!
-    end
-
-    protected def self.fossil_version_min
-      self.fossil_version unless @@fossil_version_min
-      @@fossil_version_min.not_nil!
+      @@command
     end
 
-    protected def self.fossil_version_rev
-      self.fossil_version unless @@fossil_version_rev
-      @@fossil_version_rev.not_nil!
+    protected def self.version
+      @@version ||= `fossil version`[/version\s+([^\s]*)/, 1]
     end
 
     def read_spec(version : Version) : String?
@@ -173,16 +144,6 @@ module Shards
       end
     end
 
-    private def spec?(version)
-      spec(version)
-    rescue Error
-    end
-
-    def available_releases : Array(Version)
-      update_local_cache
-      versions_from_tags
-    end
-
     def latest_version_for_ref(ref : FossilRef?) : Version
       update_local_cache
       ref ||= FossilTrunkRef.new
@@ -229,20 +190,29 @@ module Shards
 
       # The --workdir argument was introduced in version 2.12, so we have to
       # fake it
-      if FossilResolver.fossil_version_maj <= 2 &&
-         FossilResolver.fossil_version_min < 12
+      if supports_workdir_arg?
+        run "fossil open #{local_fossil_file} #{Process.quote(ref.to_fossil_ref)}  --nested --workdir #{install_path}"
+      else
         Log.debug { "Opening Fossil repo #{local_fossil_file} in directory #{install_path}" }
         run("fossil open #{local_fossil_file} #{Process.quote(ref.to_fossil_ref)} --nested", install_path)
-      else
-        run "fossil open #{local_fossil_file} #{Process.quote(ref.to_fossil_ref)}  --nested --workdir #{install_path}"
       end
     end
 
+    def supports_workdir_arg? : Bool
+      Versions.compare("2.12", FossilResolver.version.not_nil!) >= 0
+    end
+
+    def supports_format_arg? : Bool
+      Versions.compare("2.14", FossilResolver.version.not_nil!) >= 0
+    end
+
     def commit_sha1_at(ref : FossilRef)
       # Fossil versions before 2.14 do not support the --format/-F for the
       # timeline command.
-      if FossilResolver.fossil_version_maj <= 2 &&
-         FossilResolver.fossil_version_min < 14
+      if supports_format_arg?
+        # Fossil v2.14 and newer support -F %H, so use that.
+        capture("fossil timeline #{Process.quote(ref.to_fossil_ref)} -t ci -F %H -n 1 -R #{Process.quote(local_fossil_file)}").split('\n')[0]
+      else
         # Capture the short artifact name from the timeline using a regex.
         # -W 0  = unlimited line width
         # -n 1  = limit results to one entry
@@ -262,15 +232,12 @@ module Shards
         # name to the full artifact hash.
         whatis = capture("fossil whatis #{retLines[0]} -R #{Process.quote(local_fossil_file)}")
         /artifact:\s+(.+)/.match(whatis).try &.[1] || ""
-      else
-        # Fossil v2.14 and newer support -F %H, so use that.
-        capture("fossil timeline #{Process.quote(ref.to_fossil_ref)} -t ci -F %H -n 1 -R #{Process.quote(local_fossil_file)}").split('\n')[0]
       end
     end
 
     def local_path
       @local_path ||= begin
-        uri = parse_uri(fossil_url)
+        uri = parse_uri(vcs_url)
 
         path = uri.path
         path = Path[path]
@@ -291,10 +258,6 @@ module Shards
       @local_fossil_file ||= Path[local_path].join("#{name}.fossil").normalize.to_s
     end
 
-    def fossil_url
-      source.strip
-    end
-
     def parse_requirement(params : Hash(String, String)) : Requirement
       params.each do |key, value|
         case key
@@ -313,7 +276,7 @@ module Shards
 
     record FossilVersion, value : String, commit : String? = nil
 
-    private def parse_fossil_version(version : Version) : FossilVersion
+    private def parse_version(version : Version) : FossilVersion
       case version.value
       when VERSION_REFERENCE
         FossilVersion.new version.value
@@ -325,37 +288,12 @@ module Shards
     end
 
     private def fossil_ref(version : Version) : FossilRef
-      fossil_version = parse_fossil_version(version)
-      if commit = fossil_version.commit
+      version = parse_version(version)
+      if commit = version.commit
         FossilCommitRef.new commit
       else
-        FossilTagRef.new "v#{fossil_version.value}"
-      end
-    end
-
-    def update_local_cache
-      if cloned_repository? && origin_changed?
-        delete_repository
-        @updated_cache = false
-      end
-
-      return if Shards.local? || @updated_cache
-      Log.info { "Fetching #{fossil_url}" }
-
-      if cloned_repository?
-        # repositories cloned with shards v0.8.0 won't fetch any new remote
-        # refs; we must delete them and clone again!
-        if valid_repository?
-          fetch_repository
-        else
-          delete_repository
-          mirror_repository
-        end
-      else
-        mirror_repository
+        FossilTagRef.new "v#{version.value}"
       end
-
-      @updated_cache = true
     end
 
     private def mirror_repository
@@ -364,35 +302,22 @@ module Shards
       Dir.mkdir_p(path)
       FileUtils.rm(fossil_file) if File.exists?(fossil_file)
 
-      source = fossil_url
+      source = vcs_url
       # Remove a "file://" from the beginning, otherwise the path might be invalid
       # on Windows.
       source = source.lchop("file://")
 
-      fossil_retry(err: "Failed to clone #{source}") do
-        run_in_current_folder "fossil clone #{Process.quote(source)} #{Process.quote(fossil_file)}"
+      vcs_retry(err: "Failed to clone #{source}") do
+        run_in_folder "fossil clone #{Process.quote(source)} #{Process.quote(fossil_file)}"
       end
     end
 
     private def fetch_repository
-      fossil_retry(err: "Failed to update #{fossil_url}") do
+      vcs_retry(err: "Failed to update #{vcs_url}") do
         run "fossil pull -R #{Process.quote(local_fossil_file)}"
       end
     end
 
-    private def fossil_retry(err = "Failed to fetch repository", &)
-      retries = 0
-      loop do
-        yield
-        break
-      rescue inner_err : Error
-        retries += 1
-        next if retries < 3
-        Log.debug { inner_err }
-        raise Error.new("#{err}: #{inner_err}")
-      end
-    end
-
     private def delete_repository
       Log.debug { "rm -rf #{Process.quote(local_path)}'" }
       Shards::Helpers.rm_rf(local_path)
@@ -412,93 +337,23 @@ module Shards
       File.exists?(local_fossil_file)
     end
 
-    protected def origin_url
+    private def origin_url
       @origin_url ||= capture("fossil remote-url -R #{Process.quote(local_fossil_file)}").strip
     end
 
-    # Returns whether origin URLs have differing hosts and/or paths.
-    protected def origin_changed?
-      return false if origin_url == fossil_url
-      return true if origin_url.nil? || fossil_url.nil?
-
-      origin_parsed = parse_uri(origin_url)
-      fossil_parsed = parse_uri(fossil_url)
-
-      (origin_parsed.host != fossil_parsed.host) || (origin_parsed.path != fossil_parsed.path)
-    end
-
-    # Parses a URI string
-    private def parse_uri(raw_uri)
-      # Need to check for file URIs early, otherwise generic parsing will fail on a colon.
-      if (path = raw_uri.lchop?("file://"))
-        return URI.new(scheme: "file", path: path)
-      end
-
-      # Try normal URI parsing first
-      uri = URI.parse(raw_uri)
-      return uri if uri.absolute? && !uri.opaque?
-
-      # Otherwise, assume and attempt to parse the scp-style ssh URIs
-      host, _, path = raw_uri.partition(':')
-
-      if host.includes?('@')
-        user, _, host = host.partition('@')
-      end
-
-      # Normalize leading slash, matching URI parsing
-      unless path.starts_with?('/')
-        path = '/' + path
-      end
-
-      URI.new(scheme: "ssh", host: host, path: path, user: user)
-    end
-
     private def file_exists?(ref : FossilRef, path)
       files = capture("fossil ls -R #{Process.quote(local_fossil_file)} -r #{Process.quote(ref.to_fossil_ref)} #{Process.quote(path)}")
       !files.strip.empty?
     end
 
-    private def capture(command, path = local_path)
-      run(command, capture: true, path: path).not_nil!
-    end
-
-    private def run(command, path = local_path, capture = false)
-      if Shards.local? && !Dir.exists?(path)
-        dependency_name = File.basename(path, ".fossil")
-        raise Error.new("Missing repository cache for #{dependency_name.inspect}. Please run without --local to fetch it.")
-      end
-      Dir.cd(path) do
-        run_in_current_folder(command, capture)
-      end
-    end
-
-    private def run_in_current_folder(command, capture = false)
-      unless FossilResolver.has_fossil_command?
+    private def error_if_command_is_missing
+      unless FossilResolver.command?
         raise Error.new("Error missing fossil command line tool. Please install Fossil first!")
       end
-
-      Log.debug { command }
-
-      STDERR.flush
-      output = capture ? IO::Memory.new : Process::Redirect::Close
-      error = IO::Memory.new
-      status = Process.run(command, shell: true, output: output, error: error)
-
-      if status.success?
-        output.to_s if capture
-      else
-        message = error.to_s
-        raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch or file doesn't exist?")
-      end
     end
 
-    def report_version(version : Version) : String
-      fossil_version = parse_fossil_version(version)
-      if commit = fossil_version.commit
-        "#{fossil_version.value} at #{commit[0...7]}"
-      else
-        version.value
-      end
+    private def error_for_run_failure(command, message : String)
+      raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch or file doesn't exist?")
     end
 
     register_resolver "fossil", FossilResolver
diff --git a/src/resolvers/git.cr b/src/resolvers/git.cr
index 955c8d4f..ce556031 100644
--- a/src/resolvers/git.cr
+++ b/src/resolvers/git.cr
@@ -1,5 +1,5 @@
 require "uri"
-require "./resolver"
+require "./version_control"
 require "../versions"
 require "../logger"
 require "../helpers"
@@ -89,12 +89,9 @@ module Shards
     end
   end
 
-  class GitResolver < Resolver
-    @@has_git_command : Bool?
+  class GitResolver < VersionControlResolver
+    @@extension = ".git"
     @@git_column_never : String?
-    @@git_version : String?
-
-    @origin_url : String?
 
     def self.key
       "git"
@@ -136,19 +133,19 @@ module Shards
       end
     end
 
-    protected def self.has_git_command?
-      if @@has_git_command.nil?
-        @@has_git_command = (Process.run("git", ["--version"]).success? rescue false)
+    protected def self.command?
+      if @@command.nil?
+        @@command = (Process.run("git", ["--version"]).success? rescue false)
       end
-      @@has_git_command
+      @@command
     end
 
-    protected def self.git_version
-      @@git_version ||= `git --version`.strip[12..-1]
+    protected def self.version
+      @@version ||= `git --version`.strip[12..-1]
     end
 
     protected def self.git_column_never
-      @@git_column_never ||= Versions.compare(git_version, "1.7.11") < 0 ? "--column=never" : ""
+      @@git_column_never ||= Versions.compare(version, "1.7.11") < 0 ? "--column=never" : ""
     end
 
     def read_spec(version : Version) : String?
@@ -178,16 +175,6 @@ module Shards
       end
     end
 
-    private def spec?(version)
-      spec(version)
-    rescue Error
-    end
-
-    def available_releases : Array(Version)
-      update_local_cache
-      versions_from_tags
-    end
-
     def latest_version_for_ref(ref : GitRef?) : Version
       update_local_cache
       ref ||= GitHeadRef.new
@@ -234,7 +221,7 @@ module Shards
 
     def local_path
       @local_path ||= begin
-        uri = parse_uri(git_url)
+        uri = parse_uri(vcs_url)
 
         path = uri.path
         path += ".git" unless path.ends_with?(".git")
@@ -252,10 +239,6 @@ module Shards
       end
     end
 
-    def git_url
-      source.strip
-    end
-
     def parse_requirement(params : Hash(String, String)) : Requirement
       params.each do |key, value|
         case key
@@ -274,7 +257,7 @@ module Shards
 
     record GitVersion, value : String, commit : String? = nil
 
-    private def parse_git_version(version : Version) : GitVersion
+    private def parse_version(version : Version) : GitVersion
       case version.value
       when VERSION_REFERENCE
         GitVersion.new version.value
@@ -286,37 +269,12 @@ module Shards
     end
 
     private def git_ref(version : Version) : GitRef
-      git_version = parse_git_version(version)
-      if commit = git_version.commit
+      version = parse_version(version)
+      if commit = version.commit
         GitCommitRef.new commit
       else
-        GitTagRef.new "v#{git_version.value}"
-      end
-    end
-
-    def update_local_cache
-      if cloned_repository? && origin_changed?
-        delete_repository
-        @updated_cache = false
-      end
-
-      return if Shards.local? || @updated_cache
-      Log.info { "Fetching #{git_url}" }
-
-      if cloned_repository?
-        # repositories cloned with shards v0.8.0 won't fetch any new remote
-        # refs; we must delete them and clone again!
-        if valid_repository?
-          fetch_repository
-        else
-          delete_repository
-          mirror_repository
-        end
-      else
-        mirror_repository
+        GitTagRef.new "v#{version.value}"
       end
-
-      @updated_cache = true
     end
 
     private def mirror_repository
@@ -327,32 +285,19 @@ module Shards
       # be used interactively.
       # This configuration can be overridden by defining the environment
       # variable `GIT_ASKPASS`.
-      git_retry(err: "Failed to clone #{git_url}") do
-        run_in_folder "git clone -c core.askPass=true -c init.templateDir= --mirror --quiet -- #{Process.quote(git_url)} #{Process.quote(local_path)}"
+      vcs_retry(err: "Failed to clone #{vcs_url}") do
+        run_in_folder "git clone -c core.askPass=true -c init.templateDir= --mirror --quiet -- #{Process.quote(vcs_url)} #{Process.quote(local_path)}"
       end
     end
 
     private def fetch_repository
-      git_retry(err: "Failed to update #{git_url}") do
+      vcs_retry(err: "Failed to update #{vcs_url}") do
         run "git fetch --all --quiet"
       end
     end
 
-    private def git_retry(err = "Failed to fetch repository", &)
-      retries = 0
-      loop do
-        yield
-        break
-      rescue inner_err : Error
-        retries += 1
-        next if retries < 3
-        Log.debug { inner_err }
-        raise Error.new("#{err}: #{inner_err}")
-      end
-    end
-
     private def delete_repository
-      Log.debug { "rm -rf #{Process.quote(local_path)}'" }
+      Log.debug { "rm -rf #{Process.quote(local_path)}" }
       Shards::Helpers.rm_rf(local_path)
       @origin_url = nil
     end
@@ -376,92 +321,23 @@ module Shards
       @origin_url ||= capture("git ls-remote --get-url origin").strip
     end
 
-    # Returns whether origin URLs have differing hosts and/or paths.
-    protected def origin_changed?
-      return false if origin_url == git_url
-      return true if origin_url.nil? || git_url.nil?
-
-      origin_parsed = parse_uri(origin_url)
-      git_parsed = parse_uri(git_url)
-
-      (origin_parsed.host != git_parsed.host) || (origin_parsed.path != git_parsed.path)
-    end
-
-    # Parses a URI string, with additional support for ssh+git URI schemes.
-    private def parse_uri(raw_uri)
-      # Need to check for file URIs early, otherwise generic parsing will fail on a colon.
-      if (path = raw_uri.lchop?("file://"))
-        return URI.new(scheme: "file", path: path)
-      end
-
-      # Try normal URI parsing first
-      uri = URI.parse(raw_uri)
-      return uri if uri.absolute? && !uri.opaque?
-
-      # Otherwise, assume and attempt to parse the scp-style ssh URIs
-      host, _, path = raw_uri.partition(':')
-
-      if host.includes?('@')
-        user, _, host = host.partition('@')
-      end
-
-      # Normalize leading slash, matching URI parsing
-      unless path.starts_with?('/')
-        path = '/' + path
-      end
-
-      URI.new(scheme: "ssh", host: host, path: path, user: user)
-    end
-
     private def file_exists?(ref : GitRef, path)
       files = capture("git ls-tree -r --full-tree --name-only #{Process.quote(ref.to_git_ref)} -- #{Process.quote(path)}")
       !files.strip.empty?
     end
 
-    private def capture(command, path = local_path)
-      run(command, capture: true, path: path).not_nil!
-    end
-
-    private def run(command, path = local_path, capture = false)
-      if Shards.local? && !Dir.exists?(path)
-        dependency_name = File.basename(path, ".git")
-        raise Error.new("Missing repository cache for #{dependency_name.inspect}. Please run without --local to fetch it.")
-      end
-      run_in_folder(command, path, capture)
-    end
-
-    # Chdir to a folder and run command.
-    # Runs in current folder if `path` is nil.
-    private def run_in_folder(command, path : String? = nil, capture = false)
-      unless GitResolver.has_git_command?
+    private def error_if_command_is_missing
+      unless GitResolver.command?
         raise Error.new("Error missing git command line tool. Please install Git first!")
       end
-
-      Log.debug { command }
-
-      output = capture ? IO::Memory.new : Process::Redirect::Close
-      error = IO::Memory.new
-      status = Process.run(command, shell: true, output: output, error: error, chdir: path)
-
-      if status.success?
-        output.to_s if capture
-      else
-        str = error.to_s
-        if str.starts_with?("error: ") && (idx = str.index('\n'))
-          message = str[7...idx]
-          raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch or file doesn't exist?")
-        else
-          raise Error.new("Failed #{command}.\n#{str}")
-        end
-      end
     end
 
-    def report_version(version : Version) : String
-      git_version = parse_git_version(version)
-      if commit = git_version.commit
-        "#{git_version.value} at #{commit[0...7]}"
+    private def error_for_run_failure(command, str : String)
+      if str.starts_with?("error: ") && (idx = str.index('\n'))
+        message = str[7...idx]
+        raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch or file doesn't exist?")
       else
-        version.value
+        raise Error.new("Failed #{command}.\n#{str}")
       end
     end
 
diff --git a/src/resolvers/hg.cr b/src/resolvers/hg.cr
index 7568b09d..c0e1a91a 100644
--- a/src/resolvers/hg.cr
+++ b/src/resolvers/hg.cr
@@ -1,5 +1,5 @@
 require "uri"
-require "./resolver"
+require "./version_control"
 require "../versions"
 require "../logger"
 require "../helpers"
@@ -127,11 +127,8 @@ module Shards
     end
   end
 
-  class HgResolver < Resolver
-    @@has_hg_command : Bool?
-    @@hg_version : String?
-
-    @origin_url : String?
+  class HgResolver < VersionControlResolver
+    @@extension = ""
 
     def self.key
       "hg"
@@ -146,15 +143,15 @@ module Shards
       end
     end
 
-    protected def self.has_hg_command?
-      if @@has_hg_command.nil?
-        @@has_hg_command = (Process.run("hg", ["--version"]).success? rescue false)
+    protected def self.command?
+      if @@command.nil?
+        @@command = (Process.run("hg", ["--version"]).success? rescue false)
       end
-      @@has_hg_command
+      @@command
     end
 
-    protected def self.hg_version
-      @@hg_version ||= `hg --version`[/\(version\s+([^)]*)\)/, 1]
+    protected def self.version
+      @@version ||= `hg --version`[/\(version\s+([^)]*)\)/, 1]
     end
 
     def read_spec(version : Version) : String?
@@ -181,16 +178,6 @@ module Shards
       end
     end
 
-    private def spec?(version)
-      spec(version)
-    rescue Error
-    end
-
-    def available_releases : Array(Version)
-      update_local_cache
-      versions_from_tags
-    end
-
     def latest_version_for_ref(ref : HgRef?) : Version
       update_local_cache
       ref ||= HgCurrentRef.new
@@ -242,7 +229,7 @@ module Shards
 
     def local_path
       @local_path ||= begin
-        uri = parse_uri(hg_url)
+        uri = parse_uri(vcs_url)
 
         path = uri.path
         path = Path[path]
@@ -259,10 +246,6 @@ module Shards
       end
     end
 
-    def hg_url
-      source.strip
-    end
-
     def parse_requirement(params : Hash(String, String)) : Requirement
       params.each do |key, value|
         case key
@@ -282,7 +265,7 @@ module Shards
 
     record HgVersion, value : String, commit : String? = nil
 
-    private def parse_hg_version(version : Version) : HgVersion
+    private def parse_version(version : Version) : HgVersion
       case version.value
       when VERSION_REFERENCE
         HgVersion.new version.value
@@ -294,75 +277,39 @@ module Shards
     end
 
     private def hg_ref(version : Version) : HgRef
-      hg_version = parse_hg_version(version)
-      if commit = hg_version.commit
+      version = parse_version(version)
+      if commit = version.commit
         HgCommitRef.new commit
       else
-        HgTagRef.new "v#{hg_version.value}"
+        HgTagRef.new "v#{version.value}"
       end
     end
 
-    def update_local_cache
-      if cloned_repository? && origin_changed?
-        delete_repository
-        @updated_cache = false
-      end
-
-      return if Shards.local? || @updated_cache
-      Log.info { "Fetching #{hg_url}" }
-
-      if cloned_repository?
-        # repositories cloned with shards v0.8.0 won't fetch any new remote
-        # refs; we must delete them and clone again!
-        if valid_repository?
-          fetch_repository
-        else
-          delete_repository
-          mirror_repository
-        end
-      else
-        mirror_repository
-      end
-
-      @updated_cache = true
-    end
-
     private def mirror_repository
       path = local_path
       FileUtils.rm_r(path) if File.exists?(path)
       Dir.mkdir_p(path)
 
-      source = hg_url
+      source = vcs_url
       # Remove a "file://" from the beginning, otherwise the path might be invalid
       # on Windows.
       source = source.lchop("file://")
 
-      hg_retry(err: "Failed to clone #{source}") do
+      vcs_retry(err: "Failed to clone #{source}") do
         # We checkout the working directory so that "." is meaningful.
         #
         # An alternative would be to use the `@` bookmark, but only as long
         # as nothing new is committed.
-        run_in_current_folder "hg clone --quiet -- #{Process.quote(source)} #{Process.quote(path)}"
+        run_in_folder "hg clone --quiet -- #{Process.quote(source)} #{Process.quote(path)}"
       end
     end
 
     private def fetch_repository
-      hg_retry(err: "Failed to update #{hg_url}") do
+      vcs_retry(err: "Failed to update #{vcs_url}") do
         run "hg pull"
       end
     end
 
-    private def hg_retry(err = "Failed to update repository", &)
-      retries = 0
-      loop do
-        return yield
-      rescue ex : Error
-        retries += 1
-        next if retries < 3
-        raise Error.new("#{err}: #{ex}")
-      end
-    end
-
     private def delete_repository
       Log.debug { "rm -rf #{Process.quote(local_path)}" }
       Shards::Helpers.rm_rf(local_path)
@@ -381,95 +328,22 @@ module Shards
       @origin_url ||= capture("hg paths default").strip
     end
 
-    # Returns whether origin URLs have differing hosts and/or paths.
-    protected def origin_changed?
-      return false if origin_url == hg_url
-      return true if origin_url.nil? || hg_url.nil?
-
-      origin_parsed = parse_uri(origin_url)
-      hg_parsed = parse_uri(hg_url)
-
-      (origin_parsed.host != hg_parsed.host) || (origin_parsed.path != hg_parsed.path)
-    end
-
-    # Parses a URI string, with additional support for ssh+git URI schemes.
-    private def parse_uri(raw_uri)
-      # Need to check for file URIs early, otherwise generic parsing will fail on a colon.
-      if (path = raw_uri.lchop?("file://"))
-        return URI.new(scheme: "file", path: path)
-      end
-
-      # Try normal URI parsing first
-      uri = URI.parse(raw_uri)
-      return uri if uri.absolute? && !uri.opaque?
-
-      # Otherwise, assume and attempt to parse the scp-style ssh URIs
-      host, _, path = raw_uri.partition(':')
-
-      if host.includes?('@')
-        user, _, host = host.partition('@')
-      end
-
-      # Normalize leading slash, matching URI parsing
-      unless path.starts_with?('/')
-        path = '/' + path
-      end
-
-      URI.new(scheme: "ssh", host: host, path: path, user: user)
-    end
-
     private def file_exists?(ref : HgRef, path)
       run("hg files -r #{Process.quote(ref.to_hg_revset)} -- #{Process.quote(path)}", raise_on_fail: false)
     end
 
-    private def capture(command, path = local_path)
-      run(command, capture: true, path: path).as(String)
-    end
-
-    private def run(command, path = local_path, capture = false, raise_on_fail = true)
-      if Shards.local? && !Dir.exists?(path)
-        dependency_name = File.basename(path)
-        raise Error.new("Missing repository cache for #{dependency_name.inspect}. Please run without --local to fetch it.")
-      end
-      Dir.cd(path) do
-        run_in_current_folder(command, capture, raise_on_fail: raise_on_fail)
-      end
-    end
-
-    private def run_in_current_folder(command, capture = false, raise_on_fail = true)
-      unless HgResolver.has_hg_command?
+    private def error_if_command_is_missing
+      unless HgResolver.command?
         raise Error.new("Error missing hg command line tool. Please install Mercurial first!")
       end
-
-      Log.debug { command }
-
-      output = capture ? IO::Memory.new : Process::Redirect::Close
-      error = IO::Memory.new
-      status = Process.run(command, shell: true, output: output, error: error)
-
-      if status.success?
-        if capture
-          output.to_s
-        else
-          true
-        end
-      elsif raise_on_fail
-        str = error.to_s
-        if str.starts_with?("abort: ") && (idx = str.index('\n'))
-          message = str[7...idx]
-          raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch, bookmark or file doesn't exist?")
-        else
-          raise Error.new("Failed #{command}.\n#{str}")
-        end
-      end
     end
 
-    def report_version(version : Version) : String
-      hg_version = parse_hg_version(version)
-      if commit = hg_version.commit
-        "#{hg_version.value} at #{commit[0...7]}"
+    private def error_for_run_failure(command, str : String)
+      if str.starts_with?("abort: ") && (idx = str.index('\n'))
+        message = str[7...idx]
+        raise Error.new("Failed #{command} (#{message}). Maybe a commit, branch, bookmark or file doesn't exist?")
       else
-        version.value
+        raise Error.new("Failed #{command}.\n#{str}")
       end
     end
 
diff --git a/src/resolvers/version_control.cr b/src/resolvers/version_control.cr
new file mode 100644
index 00000000..8c46a7da
--- /dev/null
+++ b/src/resolvers/version_control.cr
@@ -0,0 +1,142 @@
+require "./resolver"
+require "../logger"
+
+module Shards
+  abstract class VersionControlResolver < Resolver
+    @@command : Bool?
+    @@version : String?
+    @origin_url : String?
+
+    abstract def read_spec(version : Version) : String?
+    abstract def versions_from_tags
+
+    def available_releases : Array(Version)
+      update_local_cache
+      versions_from_tags
+    end
+
+    def vcs_url
+      source.strip
+    end
+
+    # Retry loop for version-control commands
+    private def vcs_retry(err = "Failed to fetch repository", &)
+      retries = 0
+      loop do
+        yield
+        break
+      rescue inner_err : Error
+        retries += 1
+        next if retries < 3
+        Log.debug { inner_err }
+        raise Error.new("#{err}: #{inner_err}")
+      end
+    end
+
+    # Returns whether origin URLs have differing hosts and/or paths.
+    protected def origin_changed?
+      return false if origin_url == vcs_url
+      return true if origin_url.nil? || vcs_url.nil?
+
+      origin_parsed = parse_uri(origin_url)
+      vcs_parsed = parse_uri(vcs_url)
+
+      (origin_parsed.host != vcs_parsed.host) || (origin_parsed.path != vcs_parsed.path)
+    end
+
+    # Parses a URI string, with additional support for ssh+git URI schemes.
+    private def parse_uri(raw_uri)
+      # Need to check for file URIs early, otherwise generic parsing will fail on a colon.
+      if (path = raw_uri.lchop?("file://"))
+        return URI.new(scheme: "file", path: path)
+      end
+
+      # Try normal URI parsing first
+      uri = URI.parse(raw_uri)
+      return uri if uri.absolute? && !uri.opaque?
+
+      # Otherwise, assume and attempt to parse the scp-style ssh URIs
+      host, _, path = raw_uri.partition(':')
+
+      if host.includes?('@')
+        user, _, host = host.partition('@')
+      end
+
+      # Normalize leading slash, matching URI parsing
+      unless path.starts_with?('/')
+        path = '/' + path
+      end
+
+      URI.new(scheme: "ssh", host: host, path: path, user: user)
+    end
+
+    def update_local_cache
+      if cloned_repository? && origin_changed?
+        delete_repository
+        @updated_cache = false
+      end
+
+      return if Shards.local? || @updated_cache
+      Log.info { "Fetching #{vcs_url}" }
+
+      if cloned_repository?
+        # repositories cloned with shards v0.8.0 won't fetch any new remote
+        # refs; we must delete them and clone again!
+        if valid_repository?
+          fetch_repository
+        else
+          delete_repository
+          mirror_repository
+        end
+      else
+        mirror_repository
+      end
+
+      @updated_cache = true
+    end
+
+    def report_version(version : Version) : String
+      version = parse_version(version)
+      if commit = version.commit
+        "#{version.value} at #{commit[0...7]}"
+      else
+        version.value
+      end
+    end
+
+    private def capture(command, path = local_path)
+      run(command, capture: true, path: path).as(String)
+    end
+
+    private def run(command, path = local_path, capture = false, raise_on_fail = true)
+      if Shards.local? && !Dir.exists?(path)
+        dependency_name = File.basename(path, @@extension)
+        raise Error.new("Missing repository cache for #{dependency_name.inspect}. Please run without --local to fetch it.")
+      end
+      run_in_folder(command, path, capture, raise_on_fail: raise_on_fail)
+    end
+
+    # Chdir to a folder and run command.
+    # Runs in current folder if `path` is nil.
+    private def run_in_folder(command, path : String? = nil, capture = false, raise_on_fail = true)
+      error_if_command_is_missing
+      Log.debug { command }
+
+      STDERR.flush # from fossil version, but presumably ok for git/hg
+      output = capture ? IO::Memory.new : Process::Redirect::Close
+      error = IO::Memory.new
+      status = Process.run(command, shell: true, output: output, error: error, chdir: path)
+
+      if status.success?
+        # output.to_s if capture
+        if capture
+          output.to_s
+        else
+          true
+        end
+      elsif raise_on_fail
+        error_for_run_failure(command, error.to_s)
+      end
+    end
+  end
+end