diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 2f6b7fef..edca5d4d 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -38,7 +38,7 @@ jobs:
       matrix:
         include:
           - homeserver: Synapse
-            tags: synapse_blacklist msc3083 msc3787 faster_joins
+            tags: synapse_blacklist msc3083 msc3787 msc3874 faster_joins
             env: "COMPLEMENT_SHARE_ENV_PREFIX=PASS_ PASS_SYNAPSE_COMPLEMENT_DATABASE=sqlite"
             timeout: 20m
 
diff --git a/tests/csapi/room_messages_relation_filter_test.go b/tests/csapi/room_messages_relation_filter_test.go
new file mode 100644
index 00000000..225894bb
--- /dev/null
+++ b/tests/csapi/room_messages_relation_filter_test.go
@@ -0,0 +1,172 @@
+//go:build msc3874
+// +build msc3874
+
+package csapi_tests
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/tidwall/gjson"
+
+	"github.com/matrix-org/complement/internal/b"
+	"github.com/matrix-org/complement/internal/client"
+	"github.com/matrix-org/complement/internal/match"
+	"github.com/matrix-org/complement/internal/must"
+	"github.com/matrix-org/complement/runtime"
+)
+
+func TestFilterMessagesByRelType(t *testing.T) {
+	runtime.SkipIf(t, runtime.Dendrite) // flakey
+	deployment := Deploy(t, b.BlueprintAlice)
+	defer deployment.Destroy(t)
+
+	alice := deployment.Client(t, "hs1", "@alice:hs1")
+
+	roomID := alice.CreateRoom(t, map[string]interface{}{"preset": "public_chat"})
+
+	// A token which only has the messages of interest after it.
+	beforeToken := alice.MustSyncUntil(t, client.SyncReq{})
+
+	// Send messages with different relations.
+	res := alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-1"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "Message without a relation",
+	}))
+	rootEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	res = alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-2"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "Threaded Reply",
+		"m.relates_to": map[string]interface{}{
+			"event_id": rootEventID,
+			"rel_type": "m.thread",
+		},
+	}))
+	threadEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	res = alice.MustDoFunc(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-3"}, client.WithJSONBody(t, map[string]interface{}{
+		"msgtype": "m.text",
+		"body":    "Reference Reply",
+		"m.relates_to": map[string]interface{}{
+			"event_id": rootEventID,
+			"rel_type": "m.reference",
+		},
+	}))
+	referenceEventID := client.GetJSONFieldStr(t, client.ParseJSON(t, res), "event_id")
+
+	// sync until the server has processed it
+	alice.MustSyncUntil(t, client.SyncReq{Since: beforeToken}, client.SyncTimelineHas(roomID, func(r gjson.Result) bool {
+		return r.Get("type").Str == "m.room.message" && r.Get("event_id").Str == referenceEventID
+	}))
+
+	// Filter to only threaded events.
+	queryParams := url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.rel_types" : ["m.thread"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				threadEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Filtering to only references.
+	queryParams = url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.rel_types" : ["m.reference"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				referenceEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Filter to references or threads.
+	queryParams = url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.rel_types" : ["m.thread", "m.reference"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				threadEventID, referenceEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Filter to not threaded events.
+	queryParams = url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.not_rel_types" : ["m.thread"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				rootEventID, referenceEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Filtering to not references.
+	queryParams = url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.not_rel_types" : ["m.reference"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				rootEventID, threadEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+	// Filter to not references or threads.
+	queryParams = url.Values{}
+	queryParams.Set("dir", "f")
+	queryParams.Set("from", beforeToken)
+	queryParams.Set("filter", `{ "org.matrix.msc3874.not_rel_types" : ["m.thread", "m.reference"] }`)
+	res = alice.MustDoFunc(t, "GET", []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}, client.WithQueries(queryParams))
+	must.MatchResponse(t, res, match.HTTPResponse{
+		StatusCode: http.StatusOK,
+		JSON: []match.JSON{
+			match.JSONCheckOff("chunk", []interface{}{
+				rootEventID,
+			}, func(r gjson.Result) interface{} {
+				return r.Get("event_id").Str
+			}, nil,
+			),
+		},
+	})
+
+}