diff --git a/lib/services/local/assistant.go b/lib/services/local/assistant.go
index ce2133152fcee..542cbe71107ae 100644
--- a/lib/services/local/assistant.go
+++ b/lib/services/local/assistant.go
@@ -219,6 +219,15 @@ func (s *AssistService) CreateAssistantMessage(ctx context.Context, req *assist.
 		return trace.BadParameter("missing conversation ID")
 	}
 
+	// Check if the conversation exists.
+	conversationKey := backend.Key(assistantConversationPrefix, req.Username, req.ConversationId)
+	if _, err := s.Get(ctx, conversationKey); err != nil {
+		if trace.IsNotFound(err) {
+			return trace.NotFound("conversation %q not found", req.ConversationId)
+		}
+		return trace.Wrap(err)
+	}
+
 	msg := req.GetMessage()
 	value, err := json.Marshal(msg)
 	if err != nil {
diff --git a/lib/services/local/assistant_test.go b/lib/services/local/assistant_test.go
index 5a5853bd8ec93..90fc4642d11ea 100644
--- a/lib/services/local/assistant_test.go
+++ b/lib/services/local/assistant_test.go
@@ -23,6 +23,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/google/uuid"
 	"github.com/jonboulle/clockwork"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/types/known/timestamppb"
@@ -140,4 +141,18 @@ func TestAssistantCRUD(t *testing.T) {
 		require.Equal(t, conversationID, conversations.Conversations[0].Id)
 		require.Equal(t, conversationResp.Id, conversations.Conversations[1].Id)
 	})
+
+	t.Run("refuse to add messages if conversion does not exist", func(t *testing.T) {
+		msg := &assist.CreateAssistantMessageRequest{
+			Username:       username,
+			ConversationId: uuid.New().String(),
+			Message: &assist.AssistantMessage{
+				CreatedTime: timestamppb.New(time.Now()),
+				Payload:     "foo",
+				Type:        "USER_MSG",
+			},
+		}
+		err := identity.CreateAssistantMessage(ctx, msg)
+		require.Error(t, err)
+	})
 }
diff --git a/lib/web/assistant_test.go b/lib/web/assistant_test.go
index 8f15b6a82bf51..87969a39f8621 100644
--- a/lib/web/assistant_test.go
+++ b/lib/web/assistant_test.go
@@ -17,6 +17,7 @@
 package web
 
 import (
+	"context"
 	"crypto/tls"
 	"encoding/json"
 	"fmt"
@@ -26,7 +27,6 @@ import (
 	"net/url"
 	"testing"
 
-	"github.com/google/uuid"
 	"github.com/gorilla/websocket"
 	"github.com/gravitational/roundtrip"
 	"github.com/gravitational/trace"
@@ -168,7 +168,13 @@ func Test_runAssistant(t *testing.T) {
 				tc.setup(t, s)
 			}
 
-			ws, err := s.makeAssistant(t, s.authPack(t, "foo"))
+			ctx := context.Background()
+			authPack := s.authPack(t, "foo")
+			// Create the conversation
+			conversationID := s.makeAssistConversation(t, ctx, authPack)
+
+			// Make WS client and start the conversation
+			ws, err := s.makeAssistant(t, authPack, conversationID)
 			require.NoError(t, err)
 			t.Cleanup(func() { require.NoError(t, ws.Close()) })
 
@@ -186,10 +192,26 @@ func Test_runAssistant(t *testing.T) {
 			tc.act(t, ws)
 		})
 	}
+}
+
+// makeAssistConversation creates a new assist conversation and returns its ID
+func (s *WebSuite) makeAssistConversation(t *testing.T, ctx context.Context, authPack *authPack) string {
+	clt := authPack.clt
+
+	resp, err := clt.PostJSON(ctx, clt.Endpoint("webapi", "assistant", "conversations"), nil)
+	require.NoError(t, err)
+
+	convResp := struct {
+		ConversationID string `json:"id"`
+	}{}
+	err = json.Unmarshal(resp.Bytes(), &convResp)
+	require.NoError(t, err)
 
+	return convResp.ConversationID
 }
 
-func (s *WebSuite) makeAssistant(t *testing.T, pack *authPack) (*websocket.Conn, error) {
+// makeAssistant creates a new assistant websocket connection.
+func (s *WebSuite) makeAssistant(t *testing.T, pack *authPack, conversationID string) (*websocket.Conn, error) {
 	u := url.URL{
 		Host:   s.url().Host,
 		Scheme: client.WSS,
@@ -197,7 +219,7 @@ func (s *WebSuite) makeAssistant(t *testing.T, pack *authPack) (*websocket.Conn,
 	}
 
 	q := u.Query()
-	q.Set("conversation_id", uuid.New().String())
+	q.Set("conversation_id", conversationID)
 	q.Set(roundtrip.AccessTokenQueryParam, pack.session.Token)
 	u.RawQuery = q.Encode()