diff --git a/influxdata-plugin-utils/tests/test_write.py b/influxdata-plugin-utils/tests/test_write.py
new file mode 100644
index 0000000..4f0e080
--- /dev/null
+++ b/influxdata-plugin-utils/tests/test_write.py
@@ -0,0 +1,177 @@
+"""Tests for influxdata_plugin_utils.write."""
+
+import pytest
+
+from influxdata_plugin_utils.write import BatchLines, write_data
+
+
+class FakeLineBuilder:
+    def __init__(self, built):
+        self.built = built
+
+    def build(self):
+        return self.built
+
+
+class FakeInfluxDB:
+    def __init__(self, *, failures_before_success=0):
+        self.calls = []
+        self.failures_before_success = failures_before_success
+
+    def _record(self, method, *args, **kwargs):
+        self.calls.append((method, args, kwargs))
+        if self.failures_before_success > 0:
+            self.failures_before_success -= 1
+            raise RuntimeError("temporary write failure")
+
+    def write(self, payload):
+        self._record("write", payload)
+
+    def write_to_db(self, database, payload):
+        self._record("write_to_db", database, payload)
+
+    def write_sync(self, payload, *, no_sync):
+        self._record("write_sync", payload, no_sync=no_sync)
+
+    def write_sync_to_db(self, database, payload, *, no_sync):
+        self._record("write_sync_to_db", database, payload, no_sync=no_sync)
+
+
+def _lines(*built):
+    return [FakeLineBuilder(line) for line in built]
+
+
+def test_write_data_batches_to_default_buffered_write():
+    influxdb3_local = FakeInfluxDB()
+
+    write_data(influxdb3_local, _lines("cpu value=1", "cpu value=2"))
+
+    assert len(influxdb3_local.calls) == 1
+    method, args, kwargs = influxdb3_local.calls[0]
+    assert method == "write"
+    assert kwargs == {}
+    payload = args[0]
+    assert isinstance(payload, BatchLines)
+    assert payload.build() == "cpu value=1\ncpu value=2"
+
+
+def test_write_data_batches_to_cross_database_buffered_write():
+    influxdb3_local = FakeInfluxDB()
+
+    write_data(
+        influxdb3_local,
+        _lines("cpu value=1", "cpu value=2"),
+        database="target_db",
+    )
+
+    assert len(influxdb3_local.calls) == 1
+    method, args, kwargs = influxdb3_local.calls[0]
+    assert method == "write_to_db"
+    assert args[0] == "target_db"
+    assert kwargs == {}
+    payload = args[1]
+    assert isinstance(payload, BatchLines)
+    assert payload.build() == "cpu value=1\ncpu value=2"
+
+
+def test_write_data_preserves_empty_database_for_engine_validation():
+    influxdb3_local = FakeInfluxDB()
+
+    write_data(influxdb3_local, _lines("cpu value=1"), database="")
+
+    method, args, kwargs = influxdb3_local.calls[0]
+    assert method == "write_to_db"
+    assert args[0] == ""
+    assert kwargs == {}
+
+
+def test_write_data_unbatched_default_sync_write():
+    influxdb3_local = FakeInfluxDB()
+
+    write_data(
+        influxdb3_local,
+        _lines("cpu value=1", "cpu value=2"),
+        batch=False,
+        no_sync=False,
+    )
+
+    assert [call[0] for call in influxdb3_local.calls] == [
+        "write_sync",
+        "write_sync",
+    ]
+    assert [call[1][0].build() for call in influxdb3_local.calls] == [
+        "cpu value=1",
+        "cpu value=2",
+    ]
+    assert [call[2] for call in influxdb3_local.calls] == [
+        {"no_sync": False},
+        {"no_sync": False},
+    ]
+
+
+def test_write_data_unbatched_cross_database_sync_write():
+    influxdb3_local = FakeInfluxDB()
+
+    write_data(
+        influxdb3_local,
+        _lines("cpu value=1", "cpu value=2"),
+        batch=False,
+        no_sync=True,
+        database="target_db",
+    )
+
+    assert [call[0] for call in influxdb3_local.calls] == [
+        "write_sync_to_db",
+        "write_sync_to_db",
+    ]
+    assert [call[1][0] for call in influxdb3_local.calls] == [
+        "target_db",
+        "target_db",
+    ]
+    assert [call[1][1].build() for call in influxdb3_local.calls] == [
+        "cpu value=1",
+        "cpu value=2",
+    ]
+    assert [call[2] for call in influxdb3_local.calls] == [
+        {"no_sync": True},
+        {"no_sync": True},
+    ]
+
+
+def test_write_data_retries_selected_cross_database_sync_writer(monkeypatch):
+    influxdb3_local = FakeInfluxDB(failures_before_success=1)
+    sleeps = []
+    monkeypatch.setattr("influxdata_plugin_utils.write.random.uniform", lambda a, b: 0)
+    monkeypatch.setattr("influxdata_plugin_utils.write.time.sleep", sleeps.append)
+
+    write_data(
+        influxdb3_local,
+        _lines("cpu value=1"),
+        no_sync=True,
+        database="target_db",
+        base_delay=0.5,
+    )
+
+    assert [call[0] for call in influxdb3_local.calls] == [
+        "write_sync_to_db",
+        "write_sync_to_db",
+    ]
+    assert [call[1][0] for call in influxdb3_local.calls] == [
+        "target_db",
+        "target_db",
+    ]
+    assert sleeps == [0.5]
+
+
+def test_write_data_raises_after_sync_retries_exhausted(monkeypatch):
+    influxdb3_local = FakeInfluxDB(failures_before_success=2)
+    monkeypatch.setattr("influxdata_plugin_utils.write.random.uniform", lambda a, b: 0)
+    monkeypatch.setattr("influxdata_plugin_utils.write.time.sleep", lambda delay: None)
+
+    with pytest.raises(RuntimeError, match="temporary write failure"):
+        write_data(influxdb3_local, _lines("cpu value=1"), retries=1, no_sync=True)
+
+    assert [call[0] for call in influxdb3_local.calls] == [
+        "write_sync",
+        "write_sync",
+    ]