diff --git a/helpers/txnid.go b/helpers/txnid.go
new file mode 100644
index 00000000..b24cd2bc
--- /dev/null
+++ b/helpers/txnid.go
@@ -0,0 +1,16 @@
+package helpers
+
+import (
+	"fmt"
+	"sync/atomic"
+	"time"
+)
+
+var txnCounter atomic.Int64
+var start = time.Now().Unix()
+
+// GetTxnID generates a unique transaction ID for use with endpoints like /send/{eventType/{txnId}.
+// The prefix is only for debugging purposes.
+func GetTxnID(prefix string) (txnID string) {
+	return fmt.Sprintf("%s-%d-%d", prefix, start, txnCounter.Add(1))
+}
diff --git a/tests/csapi/invalid_test.go b/tests/csapi/invalid_test.go
index 7d59cb6d..697ae832 100644
--- a/tests/csapi/invalid_test.go
+++ b/tests/csapi/invalid_test.go
@@ -35,7 +35,7 @@ func TestJson(t *testing.T) {
 			}
 
 			for _, testCase := range testCases {
-				res := alice.Do(t, "POST", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "complement.dummy"}, client.WithJSONBody(t, testCase))
+				res := alice.Do(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "complement.dummy", helpers.GetTxnID("TestJson-InvalidNum")}, client.WithJSONBody(t, testCase))
 
 				must.MatchResponse(t, res, match.HTTPResponse{
 					StatusCode: 400,
@@ -57,7 +57,7 @@ func TestJson(t *testing.T) {
 			}
 
 			for _, testCase := range testCases {
-				res := alice.Do(t, "POST", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "complement.dummy"}, client.WithJSONBody(t, testCase))
+				res := alice.Do(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "complement.dummy", helpers.GetTxnID("TestJson-InvalidVal")}, client.WithJSONBody(t, testCase))
 
 				must.MatchResponse(t, res, match.HTTPResponse{
 					StatusCode: 400,
diff --git a/tests/csapi/room_messages_test.go b/tests/csapi/room_messages_test.go
index 4900c063..b21aa55a 100644
--- a/tests/csapi/room_messages_test.go
+++ b/tests/csapi/room_messages_test.go
@@ -22,7 +22,6 @@ import (
 	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
-// sytest: POST /rooms/:room_id/send/:event_type sends a message
 // sytest: GET /rooms/:room_id/messages returns a message
 func TestSendAndFetchMessage(t *testing.T) {
 	runtime.SkipIf(t, runtime.Dendrite) // flakey
@@ -37,8 +36,8 @@ func TestSendAndFetchMessage(t *testing.T) {
 
 	_, token := alice.MustSync(t, client.SyncReq{})
 
-	// first use the non-txn endpoint
-	alice.MustDo(t, "POST", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message"}, client.WithJSONBody(t, map[string]interface{}{
+	// first use the send endpoint
+	alice.MustDo(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", helpers.GetTxnID("TestSendAndFetchMessage")}, client.WithJSONBody(t, map[string]interface{}{
 		"msgtype": "m.text",
 		"body":    testMessage,
 	}))
diff --git a/tests/room_timestamp_to_event_test.go b/tests/room_timestamp_to_event_test.go
index f6de01cc..a63b0947 100644
--- a/tests/room_timestamp_to_event_test.go
+++ b/tests/room_timestamp_to_event_test.go
@@ -11,7 +11,6 @@ import (
 	"fmt"
 	"net/url"
 	"strconv"
-	"sync/atomic"
 	"testing"
 	"time"
 
@@ -253,16 +252,6 @@ type eventTime struct {
 	AfterTimestamp  time.Time
 }
 
-var txnCounter int64 = 0
-
-func getTxnID(prefix string) (txnID string) {
-	txnId := fmt.Sprintf("%s-%d", prefix, atomic.LoadInt64(&txnCounter))
-
-	atomic.AddInt64(&txnCounter, 1)
-
-	return txnId
-}
-
 func createTestRoom(t *testing.T, c *client.CSAPI) (roomID string, eventA, eventB *eventTime) {
 	t.Helper()
 
@@ -305,7 +294,7 @@ func sendMessageWithTimestamp(t *testing.T, as *client.CSAPI, c *client.CSAPI, r
 	//
 	// We can't use as.SendEventSynced(...) because application services can't use
 	// the /sync API.
-	sendRes := as.Do(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", getTxnID("sendMessageWithTimestamp-txn")}, client.WithContentType("application/json"), client.WithJSONBody(t, map[string]interface{}{
+	sendRes := as.Do(t, "PUT", []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", helpers.GetTxnID("sendMessageWithTimestamp-txn")}, client.WithContentType("application/json"), client.WithJSONBody(t, map[string]interface{}{
 		"body":    message,
 		"msgtype": "m.text",
 	}), client.WithQueries(url.Values{