Support runtime backend switching via ctl

cmd/olliesrv/internal/agent/agent.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"runtime"
 	"slices"
+	"strings"
 	"sync"
 	"sync/atomic"
 
@@ -298,6 +299,31 @@ func (ag *Agent) ModelName() string {
 	return ag.runtime.Backend.Model()
 }
 
+// SwitchBackend replaces the active backend by name. The new backend is fully
+// constructed before the runtime is changed, so a failed switch leaves the
+// current backend usable. Backend changes are only safe between turns.
+func (ag *Agent) SwitchBackend(name string) error {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		return fmt.Errorf("backend requires a name")
+	}
+	if ag.IsRunning() {
+		return fmt.Errorf("cannot switch backend while agent is running")
+	}
+	if ag.newBackend == nil {
+		return fmt.Errorf("backend switching unavailable")
+	}
+	be, err := ag.newBackend(name)
+	if err != nil {
+		return fmt.Errorf("backend %q: %w", name, err)
+	}
+	ag.runtime.Backend = be
+	ag.save()
+	ag.flush()
+	ag.notifyChange()
+	return nil
+}
+
 // State returns the agent's current execution state.
 func (ag *Agent) State() string {
 	ag.stateMu.RLock()

cmd/olliesrv/internal/fs/spec.go

@@ -1083,7 +1083,11 @@ func buildAgentChildren(a *agent.Agent, s *session.Session) []virtfs.FsNodeDecl
 							}
 							return nil, nil
 						}
-						return nil, fmt.Errorf("backend switching not supported via ctl; use cfg")
+						name := strings.Join(args, " ")
+						if err := a.SwitchBackend(name); err != nil {
+							return nil, err
+						}
+						return []byte(name + "\n"), nil
 					},
 					"name": func(args []string) ([]byte, error) {
 						if len(args) == 0 {