diff --git a/coordinator/tests/unit/test_remote_write_compression.py b/coordinator/tests/unit/test_remote_write_compression.py
new file mode 100644
index 0000000..1fee668
--- /dev/null
+++ b/coordinator/tests/unit/test_remote_write_compression.py
@@ -0,0 +1,125 @@
+# Copyright 2026 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+"""Feature: compression of the alert rules received over receive-remote-write.
+
+The coordinator advertises the alert rule encodings it can read, so that requirers know
+they may compress large rule sets, and ingests the rules in either encoding.
+"""
+
+import json
+
+import pytest
+import yaml
+from charms.prometheus_k8s.v1.prometheus_remote_write import (
+    ALERT_RULES_ENCODINGS_KEY,
+    ALERT_RULES_KEY,
+    JSON_ENCODING,
+    LZMA_ENCODING,
+)
+from cosl.utils import LZMABase64
+from scenario import Container, Exec, Relation, State
+
+ALERT_RULES = {
+    "groups": [
+        {
+            "name": "compressed-remote-write-group",
+            "rules": [
+                {
+                    "alert": "CompressedRuleFiring",
+                    "expr": 'sum(rate({job="valid"}[5m])) > 0',
+                    "for": "1m",
+                    "labels": {"severity": "warning"},
+                    "annotations": {"summary": "compressed"},
+                }
+            ],
+        }
+    ]
+}
+
+METADATA = json.dumps(
+    {
+        "model": "test",
+        "model_uuid": "20ce8299-3634-4bef-8bd8-5ace6c8816b4",
+        "application": "remote-write-compressed",
+        "charm_name": "remote-write-compressed-charm",
+    }
+)
+
+
+def _nginx_container_for_alert_rules():
+    return Container(
+        "nginx",
+        can_connect=True,
+        execs={
+            Exec(["mimirtool", "rules", "sync"], return_code=0),
+            Exec(["update-ca-certificates", "--fresh"], return_code=0),
+            Exec(["nginx", "-s", "reload"], return_code=0),
+        },
+    )
+
+
+def _written_group_names(context, state_out):
+    fs = state_out.get_container("nginx").get_filesystem(context)
+    rules_dir = fs.joinpath("etc", "mimir-alerts", "rules")
+    if not rules_dir.exists():
+        return set()
+
+    written_group_names = set()
+    for rule_file in sorted(path for path in rules_dir.iterdir() if path.is_file()):
+        written_rules = yaml.safe_load(rule_file.read_text())
+        for group in written_rules["groups"]:
+            written_group_names.add(group["name"])
+    return written_group_names
+
+
+@pytest.mark.parametrize(
+    "published",
+    [
+        pytest.param(json.dumps(ALERT_RULES), id="plain_json"),
+        pytest.param(LZMABase64.compress(json.dumps(ALERT_RULES)), id="compressed"),
+    ],
+)
+def test_alert_rules_are_ingested_in_either_encoding(
+    context, s3, all_worker, nginx_prometheus_exporter_container, published
+):
+    # GIVEN a requirer that published its alert rules, compressed or not
+    remote_write_relation = Relation(
+        "receive-remote-write",
+        remote_app_name="remote-write-compressed",
+        remote_app_data={ALERT_RULES_KEY: published, "scrape_metadata": METADATA},
+    )
+    state_in = State(
+        leader=True,
+        relations=[s3, all_worker, remote_write_relation],
+        containers=[_nginx_container_for_alert_rules(), nginx_prometheus_exporter_container],
+    )
+
+    # WHEN the relation changed event is processed
+    state_out = context.run(context.on.relation_changed(remote_write_relation), state_in)
+
+    # THEN the rules are written to disk regardless of the encoding they arrived in
+    assert _written_group_names(context, state_out) == {"compressed-remote-write-group"}
+
+
+def test_coordinator_advertises_the_encodings_it_can_read(
+    context, s3, all_worker, nginx_prometheus_exporter_container
+):
+    # GIVEN a requirer related over receive-remote-write
+    remote_write_relation = Relation(
+        "receive-remote-write", remote_app_name="remote-write-compressed"
+    )
+    state_in = State(
+        leader=True,
+        relations=[s3, all_worker, remote_write_relation],
+        containers=[_nginx_container_for_alert_rules(), nginx_prometheus_exporter_container],
+    )
+
+    # WHEN the relation joined event is processed
+    state_out = context.run(context.on.relation_joined(remote_write_relation), state_in)
+
+    # THEN requirers are told that compressed alert rules are supported
+    advertised = state_out.get_relation(remote_write_relation.id).local_app_data[
+        ALERT_RULES_ENCODINGS_KEY
+    ]
+    assert json.loads(advertised) == [LZMA_ENCODING, JSON_ENCODING]