diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 000000000..ba5fe55bc
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,41 @@
+# Agent guidelines
+
+Coriolis migrates VMs between clouds. Cloud support lives in separate
+provider plugins (often private). Target Python 3.10 and 3.12.
+
+## Coriolis terminology
+
+- **Minion**: temporary worker VM for disk transfer and os-morphing
+  (guest prep: network, packages, drivers). **Minion pools** reuse them.
+  Some source providers skip minions and read disks directly.
+- **Transfer**: creates destination volumes and copies disk data.
+  Re-run a transfer as a new **execution** to pick up later changes.
+  Most providers can do that incrementally.
+- **Deployment**: creates the destination VM from a completed transfer.
+- **Replica** vs **migration** (`transfer.scenario`: `replica` /
+  `live_migration`) is a licensing split, not two engines. Replicas can
+  be re-executed and re-deployed even after license fulfillment.
+
+Users configure source/destination **endpoints** (credentials) and
+**environment options** (transfer and resulting VM settings).
+
+## Working in this repo
+
+- Use `.tox/py3/bin/` (`stestr`, `ruff`); it has project deps. Ignore
+  `.mypy_cache`, `.ruff_cache`, `.tox`.
+- Follow ruff (`tox.ini`, `ruff.toml`).
+- Public methods need docstrings (subclasses may inherit). Use type
+  hints when the type is known.
+- Do not add helpers for trivial checks such as
+  `server.power_status == "RUNNING"`; keep those inline.
+- Do not strip still-relevant inline comments.
+- If regenerating a file, replace its contents; do not append duplicates.
+- Empty `__init__.py` files must not have license headers.
+
+## Tests
+
+- Prefer `@mock.patch` / `@mock.patch.object` over `with mock.patch`.
+- For multiple mock calls, use `assert_has_calls`. For a single call,
+  `assert_called_once_with` is fine. Match nearby tests.
+- Integration tests use Docker providers; external cloud providers are
+  optional.
diff --git a/coriolis/providers/backup_writers.py b/coriolis/providers/backup_writers.py
index 1c7e34737..8c20f4790 100644
--- a/coriolis/providers/backup_writers.py
+++ b/coriolis/providers/backup_writers.py
@@ -257,7 +257,16 @@ def truncate(self, size):
         pass
 
     @abc.abstractmethod
-    def write(self, data):
+    def write(self, data, encoding=None, uncompressed_size=None):
+        """Write the given data chunk.
+
+        Use the `seek` method to specify the write offset.
+
+        :param data: data chunk to write (raw bytes)
+        :param encoding: a data encoding format supported by the backup writer.
+        :param uncompressed_size: the uncompressed size in case of
+            compressed chunks.
+        """
         pass
 
     @abc.abstractmethod
@@ -319,7 +328,19 @@ def seek(self, pos):
     def truncate(self, size):
         self._file.truncate(size)
 
-    def write(self, data):
+    def write(self, data, encoding=None, uncompressed_size=None):
+        """Write the given data chunk.
+
+        Use the `seek` method to specify the write offset.
+
+        :param data: data chunk to write (raw bytes)
+        :param encoding: unsupported, this backup writer only accepts raw chunks.
+        :param uncompressed_size: unsupported.
+        """
+        if encoding:
+            raise exception.InvalidInput(
+                f"The file backup writer does not support {encoding} encoded chunks."
+            )
         self._file.write(data)
 
     def close(self):
@@ -463,7 +484,15 @@ def _encoder(self):
                 self._enc_q.task_done()
         LOG.debug("Backup encoder stopped.")
 
-    def write(self, data):
+    def write(self, data, encoding=None, uncompressed_size=None):
+        """Write the given data chunk.
+
+        Use the `seek` method to specify the write offset.
+
+        :param data: data chunk to write (raw bytes)
+        :param encoding: unsupported, this backup writer only accepts raw chunks.
+        :param uncompressed_size: unsupported.
+        """
         if self._closing:
             raise exception.CoriolisException("Attempted to write to a closed writer.")
 
@@ -472,6 +501,11 @@ def write(self, data):
                 "Failed to write data. See log for details."
             ) from self._exception
 
+        if encoding:
+            raise exception.InvalidInput(
+                f"The file backup writer does not support {encoding} encoded chunks."
+            )
+
         payload = {
             "offset": self._offset,
             "data": data,
@@ -785,6 +819,10 @@ def _sender(self):
             if payload.get("encoding", None):
                 enc = copy.copy(payload["encoding"])
                 headers["content-encoding"] = enc
+            if payload.get("uncompressed_size") is not None:
+                headers["X-Uncompressed-Content-Length"] = str(
+                    payload["uncompressed_size"]
+                )
 
             @utils.retry_on_error()
             def send():
@@ -831,7 +869,18 @@ def send():
         LOG.debug("Backup sender stopped.")
 
     @utils.retry_on_error()
-    def write(self, data):
+    def write(self, data, encoding=None, uncompressed_size=None):
+        """Write the given data chunk.
+
+        Use the `seek` method to specify the write offset.
+
+        :param data: data chunk to write (raw bytes)
+        :param encoding: the encoding of the data chunk, one of the following:
+            * compression algorithms: fastlz, deflate, gzip, zlib
+            * None: raw data, no encoding (default)
+        :param uncompressed_size: the uncompressed size in case of
+            compressed chunks. Required for fastlz.
+        """
         if self._closing:
             raise exception.CoriolisException("Attempted to write to a closed writer.")
         if self._exception:
@@ -841,7 +890,27 @@ def write(self, data):
             "offset": self._offset,
             "data": data,
         }
-        self._comp_q.put(payload)
+        if encoding is None:
+            self._comp_q.put(payload)
+        elif encoding in ("fastlz", "gzip", "zlib", "deflate"):
+            # The payload is already compressed, skip the compressor
+            # queue, use the sender queue directly.
+            payload["encoding"] = encoding
+            payload["chunk"] = data
+            if encoding == "fastlz":
+                if uncompressed_size is None:
+                    raise exception.InvalidInput(
+                        "fastlz without explicit uncompressed size."
+                    )
+                payload["uncompressed_size"] = uncompressed_size
+            self._sender_q.put(payload)
+        elif encoding == "incompressible":
+            # The caller determined that the chunk is incompressible,
+            # skip the compression queue.
+            payload["chunk"] = data
+            self._sender_q.put(payload)
+        else:
+            raise exception.InvalidInput("Unsupported write encoding: %s" % encoding)
         self._offset += len(data)
 
     def _wait_for_queues(self):
diff --git a/coriolis/resources/bin/coriolis-writer b/coriolis/resources/bin/coriolis-writer
index ea9f62458..7a6035fac 100755
Binary files a/coriolis/resources/bin/coriolis-writer and b/coriolis/resources/bin/coriolis-writer differ
diff --git a/coriolis/tests/providers/test_backup_writers.py b/coriolis/tests/providers/test_backup_writers.py
index 10c470ed8..913299e72 100644
--- a/coriolis/tests/providers/test_backup_writers.py
+++ b/coriolis/tests/providers/test_backup_writers.py
@@ -282,6 +282,15 @@ def test_write(self):
         self.writer.write(mock.sentinel.data)
         self.writer._file.write.assert_called_once_with(mock.sentinel.data)
 
+    def test_write_with_encoding(self):
+        self.assertRaises(
+            exception.InvalidInput,
+            self.writer.write,
+            mock.sentinel.data,
+            encoding='gzip',
+        )
+        self.writer._file.write.assert_not_called()
+
     @mock.patch('os.system')
     def test_close(self, mock_system):
         self.writer.close()
@@ -479,6 +488,19 @@ def test_write(self):
         self.assertEqual(self.writer._offset, len('test_data'))
         self.assertEqual(self.writer._msg_id, 1)
 
+    def test_write_with_encoding(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._enc_q = self._enc_queue
+
+        self.assertRaises(
+            exception.InvalidInput,
+            self.writer.write,
+            'test_data',
+            encoding='gzip',
+        )
+        self._enc_queue.put.assert_not_called()
+
     def test_write_with_closing(self):
         self.writer._closing = True
         self.assertRaises(
@@ -1055,6 +1077,44 @@ def test__sender(self, mock_conf, mock_uri, mock_ensure_session):
         self.assertEqual(self.writer._write_error, False)
         self.assertIsInstance(self.writer._exception, BaseException)
 
+    @mock.patch.object(backup_writers.HTTPBackupWriterImpl, '_ensure_session')
+    @mock.patch.object(backup_writers.HTTPBackupWriterImpl, '_uri')
+    @mock.patch.object(backup_writers, 'CONF')
+    def test__sender_with_uncompressed_size(
+        self, mock_conf, mock_uri, mock_ensure_session
+    ):
+        self.writer._session = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        self.writer._sender_q.get.return_value = {
+            "offset": mock.sentinel.offset,
+            "chunk": 'compressed_data',
+            "encoding": "fastlz",
+            "uncompressed_size": 4096,
+        }
+
+        mock_response = mock.MagicMock()
+        mock_response.status_code = 200
+        mock_response.content = "OK"
+        mock_response.raise_for_status.side_effect = [None, BaseException()]
+        self.writer._session.post.return_value = mock_response
+
+        with self.assertLogs('coriolis.providers.backup_writers', level=logging.ERROR):
+            self.assertRaises(BaseException, self.writer._sender)
+
+        expected_headers = {
+            "X-Write-Offset": str(mock.sentinel.offset),
+            "X-Client-Token": self.writer._id,
+            "content-encoding": "fastlz",
+            "X-Uncompressed-Content-Length": "4096",
+        }
+
+        self.writer._session.post.assert_called_with(
+            mock_uri,
+            headers=expected_headers,
+            data='compressed_data',
+            timeout=mock_conf.default_requests_timeout,
+        )
+
     @mock.patch("time.sleep")
     @mock.patch.object(backup_writers.HTTPBackupWriterImpl, '_ensure_session')
     @mock.patch.object(backup_writers.HTTPBackupWriterImpl, '_uri')
@@ -1112,8 +1172,117 @@ def test_write(self):
             "data": 'test_data',
         }
         self.writer._comp_q.put.assert_called_once_with(expected_payload)
+        self.writer._sender_q.put.assert_not_called()
+        self.assertEqual(self.writer._offset, len('test_data'))
+
+    def test_write_with_precompressed_encoding(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._comp_q = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        original_write = testutils.get_wrapped_function(self.writer.write)
+
+        for encoding in ('gzip', 'zlib', 'deflate'):
+            self.writer._offset = 0
+            self.writer._comp_q.reset_mock()
+            self.writer._sender_q.reset_mock()
+
+            original_write(self.writer, 'compressed', encoding=encoding)
+
+            expected_payload = {
+                "offset": 0,
+                "data": 'compressed',
+                "encoding": encoding,
+                "chunk": 'compressed',
+            }
+            self.writer._sender_q.put.assert_called_once_with(expected_payload)
+            self.writer._comp_q.put.assert_not_called()
+            self.assertEqual(self.writer._offset, len('compressed'))
+
+    def test_write_with_fastlz_encoding(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._comp_q = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        self.writer._offset = 0
+
+        original_write = testutils.get_wrapped_function(self.writer.write)
+
+        original_write(
+            self.writer, 'compressed', encoding='fastlz', uncompressed_size=4096
+        )
+
+        expected_payload = {
+            "offset": 0,
+            "data": 'compressed',
+            "encoding": 'fastlz',
+            "chunk": 'compressed',
+            "uncompressed_size": 4096,
+        }
+        self.writer._sender_q.put.assert_called_once_with(expected_payload)
+        self.writer._comp_q.put.assert_not_called()
+        self.assertEqual(self.writer._offset, len('compressed'))
+
+    def test_write_with_fastlz_encoding_missing_uncompressed_size(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._comp_q = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        self.writer._offset = 0
+
+        original_write = testutils.get_wrapped_function(self.writer.write)
+
+        self.assertRaises(
+            exception.InvalidInput,
+            original_write,
+            self.writer,
+            'compressed',
+            encoding='fastlz',
+        )
+        self.writer._sender_q.put.assert_not_called()
+        self.writer._comp_q.put.assert_not_called()
+        self.assertEqual(self.writer._offset, 0)
+
+    def test_write_with_incompressible_encoding(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._comp_q = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        self.writer._offset = 0
+
+        original_write = testutils.get_wrapped_function(self.writer.write)
+
+        original_write(self.writer, 'test_data', encoding='incompressible')
+
+        expected_payload = {
+            "offset": 0,
+            "data": 'test_data',
+            "chunk": 'test_data',
+        }
+        self.writer._sender_q.put.assert_called_once_with(expected_payload)
+        self.writer._comp_q.put.assert_not_called()
         self.assertEqual(self.writer._offset, len('test_data'))
 
+    def test_write_with_unsupported_encoding(self):
+        self.writer._closing = False
+        self.writer._exception = None
+        self.writer._comp_q = mock.MagicMock()
+        self.writer._sender_q = mock.MagicMock()
+        self.writer._offset = 0
+
+        original_write = testutils.get_wrapped_function(self.writer.write)
+
+        self.assertRaises(
+            exception.InvalidInput,
+            original_write,
+            self.writer,
+            'test_data',
+            encoding='lz4',
+        )
+        self.writer._sender_q.put.assert_not_called()
+        self.writer._comp_q.put.assert_not_called()
+        self.assertEqual(self.writer._offset, 0)
+
     def test_write_with_closing(self):
         self.writer._closing = True