diff --git a/lib/morph-cli.rb b/lib/morph-cli.rb
index ddadb4d..9fc0bd4 100644
--- a/lib/morph-cli.rb
+++ b/lib/morph-cli.rb
@@ -6,6 +6,7 @@
 require 'tempfile'
 require 'fileutils'
 require 'filesize'
+require 'zlib'
 require 'faraday'
 require 'faraday/multipart'
 require 'minitar'
@@ -42,7 +43,7 @@ def self.execute(directory, _development, env_config)
     connection.post("/run") do |req|
       req.body = {
         api_key: env_config[:api_key],
-        code: Faraday::Multipart::FilePart.new(file, "application/octet-stream")
+        code: Faraday::Multipart::FilePart.new(file, "application/gzip")
       }
       req.options.timeout = timeout
       req.options.on_data = proc do |chunk, _overall_received_bytes, env|
@@ -106,21 +107,22 @@ def self.in_directory(directory)
     FileUtils.cd(cwd)
   end
 
-  # Packs the given paths (relative to directory) into a tar file and returns
-  # an open, rewound file handle ready for upload.
+  # Packs the given paths (relative to directory) into a gzip-compressed tar
+  # file and returns an open, rewound file handle ready for upload.
   def self.create_tar(directory, paths)
-    tempfile = Tempfile.new(["morph", ".tar"])
+    tempfile = Tempfile.new(["morph", ".tar.gz"])
     tempfile.binmode
 
     in_directory(directory) do
-      output = Minitar::Output.new(tempfile)
-      paths.each do |entry|
-        Minitar.pack_file(entry, output)
-      end
+      gzip = Zlib::GzipWriter.new(tempfile)
+      output = Minitar::Output.new(gzip)
+      paths.each { |entry| Minitar.pack_file(entry, output) }
     ensure
-      # Writes the tar trailer and flushes without closing the underlying
-      # tempfile, so the returned handle stays open for reading.
+      # Closing the tar writer writes the tar trailer; finishing (not
+      # closing) the gzip stream writes the gzip trailer, leaving the
+      # underlying tempfile handle open for reading.
       output&.tar&.close
+      gzip&.finish
     end
 
     tempfile.flush
diff --git a/spec/morph_cli_spec.rb b/spec/morph_cli_spec.rb
index 66111de..79b25f5 100644
--- a/spec/morph_cli_spec.rb
+++ b/spec/morph_cli_spec.rb
@@ -30,7 +30,7 @@ def with_scraper_directory
       end
     end
 
-    it "posts the API key and the code as multipart form data" do
+    it "posts the API key and the gzipped code as multipart form data" do
       stub_request(:post, "https://morph.io/run").to_return(status: 200, body: "")
 
       with_scraper_directory do |dir|
@@ -41,7 +41,7 @@ def with_scraper_directory
       expect(WebMock).to(have_requested(:post, "https://morph.io/run").with do |req|
         req.headers["Content-Type"].start_with?("multipart/form-data") &&
           req.body.include?("secret-key") &&
-          req.body.include?("scraper.rb")
+          req.body.b.include?("\x1f\x8b".b) # gzip magic bytes
       end)
     end
 
@@ -122,7 +122,7 @@ def with_scraper_directory
   end
 
   describe ".create_tar" do
-    it "packs the given paths into a readable tar" do
+    it "packs the given paths into a readable gzip-compressed tar" do
       Dir.mktmpdir do |dir|
         File.write(File.join(dir, "scraper.rb"), "puts 'hi'\n")
         FileUtils.mkdir_p(File.join(dir, "lib"))
@@ -132,13 +132,25 @@ def with_scraper_directory
         tar = described_class.create_tar(dir, paths)
 
         names = []
-        Minitar::Input.open(tar.path) do |input|
-          input.each { |entry| names << entry.full_name }
+        Zlib::GzipReader.open(tar.path) do |gzip|
+          Minitar::Input.open(gzip) do |input|
+            input.each { |entry| names << entry.full_name }
+          end
         end
         expect(names).to contain_exactly("scraper.rb", "lib/helper.rb")
       end
     end
 
+    it "compresses the tar" do
+      Dir.mktmpdir do |dir|
+        File.write(File.join(dir, "scraper.rb"), "a" * 100_000)
+
+        tar = described_class.create_tar(dir, described_class.all_paths(dir))
+
+        expect(File.size(tar.path)).to be < 100_000
+      end
+    end
+
     it "returns an open file handle ready for reading" do
       Dir.mktmpdir do |dir|
         File.write(File.join(dir, "scraper.rb"), "puts 'hi'\n")
@@ -147,7 +159,7 @@ def with_scraper_directory
 
         expect(tar).not_to be_closed
         expect(tar.pos).to eq(0)
-        expect(tar.read).to include("scraper.rb")
+        expect(tar.read(2)).to eq("\x1f\x8b".b) # gzip magic bytes
       end
     end
   end