diff --git a/AGENTS.md b/AGENTS.md
index 80bb887a2..d1c837079 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -137,6 +137,13 @@ Key recurring tasks (via `config/recurring.yml`):
 - Search records denormalized for performance
 - Models in `app/models/search/`
 
+### Imports and exports
+
+Allow people to move between OSS and SAAS Fizzy instances:
+- Exports/Imports can be wirtten to/read from local or S3 storage depending on the config of the instance (both myst be supported)
+- Must be able to handle very large ZIP files (500+GB)
+- Models in `app/models/account/data_transfer/`, `app/models/zip_file`
+
 ## Tools
 
 ### Chrome MCP (Local Dev)
diff --git a/app/models/zip_file.rb b/app/models/zip_file.rb
index 2f135fd8d..8c8299bc9 100644
--- a/app/models/zip_file.rb
+++ b/app/models/zip_file.rb
@@ -39,9 +39,15 @@ class ZipFile
         )
 
         writer = Writer.new
-        service.upload(blob.key, writer.io) do |io|
-          writer.stream_to(io)
+
+        # Use S3's upload_stream directly for write-based streaming.
+        # ActiveStorage's upload method expects a read-based IO, but ZipKit
+        # needs a write-based stream. The TransferManager's upload_stream
+        # yields a writable IO that we can stream directly to.
+        service.send(:upload_stream, key: blob.key, part_size: 100.megabytes) do |write_stream|
+          writer.stream_to(write_stream)
           yield writer
+          writer.close
         end
 
         blob.update!(byte_size: writer.byte_size, checksum: writer.checksum)
diff --git a/app/models/zip_file/writer.rb b/app/models/zip_file/writer.rb
index 8e9665563..8291c4948 100644
--- a/app/models/zip_file/writer.rb
+++ b/app/models/zip_file/writer.rb
@@ -9,10 +9,6 @@ class ZipFile::Writer
     @digest = Digest::MD5.new
   end
 
-  def io
-    @io ||= ZipFile::Writer::IO.new(self)
-  end
-
   def stream_to(io)
     @output_io = io
   end
diff --git a/app/models/zip_file/writer/io.rb b/app/models/zip_file/writer/io.rb
deleted file mode 100644
index 451dd0d07..000000000
--- a/app/models/zip_file/writer/io.rb
+++ /dev/null
@@ -1,18 +0,0 @@
-class ZipFile::Writer::IO
-  def initialize(writer)
-    @writer = writer
-  end
-
-  def write(data)
-    @writer.write(data)
-  end
-
-  def <<(data)
-    write(data)
-    self
-  end
-
-  def size
-    @writer.byte_size
-  end
-end
diff --git a/test/models/zip_file_test.rb b/test/models/zip_file_test.rb
index 257b9adf4..42db30f9e 100644
--- a/test/models/zip_file_test.rb
+++ b/test/models/zip_file_test.rb
@@ -1,13 +1,6 @@
 require "test_helper"
 
 class ZipFileTest < ActiveSupport::TestCase
-  test "writer io exposes size for S3 multipart upload detection" do
-    writer = ZipFile::Writer.new
-
-    assert_respond_to writer.io, :size
-    assert_equal writer.byte_size, writer.io.size
-  end
-
   test "writer adds files with content" do
     tempfile = Tempfile.new([ "test", ".zip" ])
     tempfile.binmode