From 19971be343c7f164000eae53bee2c0b0d5b32cea Mon Sep 17 00:00:00 2001
From: Ollie Agent <ollie@example.com>
Date: Wed, 19 Aug 2026 20:06:20 +0200
Subject: [PATCH] Support runtime backend switching via ctl

---
 cmd/olliesrv/internal/agent/agent.go | 44 ++++++++++++++++++++++------
 cmd/olliesrv/internal/fs/spec.go     |  6 +++-
 2 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/cmd/olliesrv/internal/agent/agent.go b/cmd/olliesrv/internal/agent/agent.go
index c23d717..30bb26d 100644
--- a/cmd/olliesrv/internal/agent/agent.go
+++ b/cmd/olliesrv/internal/agent/agent.go
@@ -7,6 +7,7 @@ import (
 	"os"
 	"runtime"
 	"slices"
+	"strings"
 	"sync"
 	"sync/atomic"
 
@@ -39,17 +40,17 @@ type Agent struct {
 	resultCache   resultCache
 
 	// Execution state — owned by the agent, protected by stateMu.
-	state          string        // "idle", "thinking", "calling: <tool>"
-	reply          string        // last assistant response
-	getCwd         func() string // returns session working directory
-	id             string        // agent identity (unique principal)
-	parentID       string        // immutable ID of the agent that spawned this agent
-	depth          int           // sub-agent depth (0=top-level, 1=sub-agent, 2=sub-sub-agent)
-	activeChildren atomic.Int32  // number of currently-running child sub-agents
+	state          string              // "idle", "thinking", "calling: <tool>"
+	reply          string              // last assistant response
+	getCwd         func() string       // returns session working directory
+	id             string              // agent identity (unique principal)
+	parentID       string              // immutable ID of the agent that spawned this agent
+	depth          int                 // sub-agent depth (0=top-level, 1=sub-agent, 2=sub-sub-agent)
+	activeChildren atomic.Int32        // number of currently-running child sub-agents
 	peers          map[string]struct{} // peer agent names (same session)
 	peerMu         sync.RWMutex
-	fifo           Fifo          // prompt queue
-	Feed           Feed          // streaming input gate (change-detecting)
+	fifo           Fifo // prompt queue
+	Feed           Feed // streaming input gate (change-detecting)
 	toolCallCount  atomic.Int64
 	pendingInject  atomic.Pointer[string]
 	submitMu       sync.Mutex // serializes Submit calls (commands + turns)
@@ -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()
diff --git a/cmd/olliesrv/internal/fs/spec.go b/cmd/olliesrv/internal/fs/spec.go
index a60c824..9ce5fce 100644
--- a/cmd/olliesrv/internal/fs/spec.go
+++ b/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 {