feat(sessions): let a session message steer the running turn

sessions_message.go

@@ -61,6 +61,12 @@ type SessionMessageRequest struct {
 	// agent answer through its reply tool only; check the ack to see whether
 	// the server accepted it.
 	DeliveryMode string `json:"delivery_mode,omitempty"`
+	// Steering asks the session's running turn to take this message in at its
+	// next step instead of queueing it for a turn of its own. Its answer then
+	// comes from that running turn, so read it from the session's event stream
+	// rather than only from this message's own invocation. Ignored when no turn
+	// is running.
+	Steering bool `json:"steering,omitempty"`
 	// TeamID binds a new session to a team (0 = account scope).
 	TeamID int64 `json:"team_id,omitempty"`
 	// EnvironmentKind selects the execution environment: "cloud" or "byoc".

sessions_message_test.go

@@ -35,6 +35,9 @@ func TestSessionsWriteMessageSendsRequestAndDecodesAck(t *testing.T) {
 		if _, ok := body["team_id"]; ok {
 			t.Errorf("zero team_id must be omitted: %v", body)
 		}
+		if _, ok := body["steering"]; ok {
+			t.Errorf("false steering must be omitted: %v", body)
+		}
 		w.WriteHeader(http.StatusAccepted)
 		_, _ = io.WriteString(w, `{"request_id":"r1","data":{"session_id":"s1","invocation_id":"i1","root_invocation_id":"i1","delivery_mode":"reply_only"}}`)
 	})
@@ -58,6 +61,23 @@ func TestSessionsWriteMessageSendsRequestAndDecodesAck(t *testing.T) {
 	}
 }
 
+func TestSessionsWriteMessageSendsSteering(t *testing.T) {
+	c := newTestClient(t, func(w http.ResponseWriter, r *http.Request) {
+		var body map[string]any
+		if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+			t.Fatal(err)
+		}
+		if body["steering"] != true {
+			t.Errorf("steering = %v, want true", body["steering"])
+		}
+		w.WriteHeader(http.StatusAccepted)
+		_, _ = io.WriteString(w, `{"request_id":"r1","data":{"session_id":"s1","invocation_id":"i2","root_invocation_id":"i2"}}`)
+	})
+	if _, _, err := c.Sessions.WriteMessage(context.Background(), &SessionMessageRequest{SessionID: "s1", Query: "and now?", Steering: true}); err != nil {
+		t.Fatalf("WriteMessage: %v", err)
+	}
+}
+
 func TestSessionsWriteMessageSurfacesAPIError(t *testing.T) {
 	c := newTestClient(t, func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusBadRequest)