agent: add dynamic workdir support via /cwd command and SetWorkDir API

- Add WorkDirSetter interface to pkg/tools so servers can receive workdir updates
- Add SetWorkDir on execute.Server with RWMutex-guarded workdir field
- Add WorkDir()/SetWorkDir() to the Core interface and agentCore
- Store agentPrompt in AgentEnv/agentCore so system prompt can be rebuilt on workdir change
- Add /cwd [path] slash command (no args shows current dir, arg changes it)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/core.go

@@ -52,6 +52,7 @@ type AgentEnv struct {
 	exec         toolExecutor
 	Hooks        Hooks
 	systemPrompt string
+	agentPrompt  string // agent-specific suffix appended after the base system prompt
 	genParams    backend.GenerationParams
 	Messages     []string
 }
@@ -140,6 +141,7 @@ func BuildAgentEnv(cfg *config.Config, d tools.Dispatcher, workdir string) Agent
 		exec:         exec,
 		Hooks:        hooks,
 		systemPrompt: sp,
+		agentPrompt:  agentPrompt,
 		genParams:    genParams,
 		Messages:     messages,
 	}
@@ -224,6 +226,7 @@ type agentCore struct {
 	dispatcher    tools.Dispatcher
 	newDispatcher func() tools.Dispatcher
 	workdir       string
+	agentPrompt   string // agent-specific prompt suffix; kept for system prompt rebuilds
 	currentAction atomic.Pointer[actionHandle]
 	fifo          PromptFIFO
 	pendingInject atomic.Pointer[string]
@@ -253,6 +256,7 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 		sessionsDir:   cfg.SessionsDir,
 		sessionID:     cfg.SessionID,
 		workdir:       cfg.WorkDir,
+		agentPrompt:   cfg.Env.agentPrompt,
 		dispatcher:    cfg.Env.dispatcher,
 		newDispatcher: cfg.NewDispatcher,
 	}
@@ -265,6 +269,41 @@ func (s *agentCore) prompt() string {
 // Prompt returns the display prompt string for the current session state.
 func (s *agentCore) Prompt() string { return s.prompt() }
 
+// WorkDir returns the current working directory for tool execution.
+func (s *agentCore) WorkDir() string {
+	if s.workdir != "" {
+		return s.workdir
+	}
+	wd, _ := os.Getwd()
+	return wd
+}
+
+// SetWorkDir changes the working directory for tool execution and updates the
+// system prompt. Returns an error if the path does not exist.
+func (s *agentCore) SetWorkDir(dir string) error {
+	if dir != "" {
+		if _, err := os.Stat(dir); err != nil {
+			return fmt.Errorf("workdir: %w", err)
+		}
+	}
+	s.workdir = dir
+	// Propagate to any tool server that knows how to handle it (e.g. execute).
+	if s.dispatcher != nil {
+		if srv, ok := s.dispatcher.GetServer("execute"); ok {
+			if ws, ok := srv.(tools.WorkDirSetter); ok {
+				ws.SetWorkDir(dir)
+			}
+		}
+	}
+	// Rebuild system prompt so it reflects the new workdir.
+	sp := systemPrompt(dir)
+	if s.agentPrompt != "" {
+		sp += "\n\n" + s.agentPrompt
+	}
+	s.loopcfg.systemPrompt = sp
+	return nil
+}
+
 // defaultContextLength is used when the backend cannot report the model's
 // actual context window (e.g. CodeWhisperer). 128k tokens is a safe default
 // for modern models.
@@ -617,6 +656,7 @@ func (s *agentCore) handleCommand(ctx context.Context, input string, handler Eve
 		s.loopcfg.Tools = env.tools
 		s.loopcfg.Exec = env.exec
 		s.loopcfg.GenerationParams = env.genParams
+		s.agentPrompt = env.agentPrompt
 		s.agentName = name
 		s.session = nil
 		s.sessionID = NewSessionID()
@@ -745,6 +785,19 @@ func (s *agentCore) handleCommand(ctx context.Context, input string, handler Eve
 		}
 		return true
 
+	case "/cwd":
+		if len(args) == 0 {
+			handler(infoEvent("workdir: " + s.WorkDir()))
+			return true
+		}
+		dir := strings.Join(args, " ")
+		if err := s.SetWorkDir(dir); err != nil {
+			handler(infoEvent("error: " + err.Error()))
+			return true
+		}
+		handler(infoEvent("workdir: " + dir))
+		return true
+
 	case "/mcp":
 		handler(infoEvent(s.ListServers()))
 		return true
@@ -759,6 +812,7 @@ func (s *agentCore) handleCommand(ctx context.Context, input string, handler Eve
 			"  /model <name>    - switch model",
 			"  /models          - list available models",
 			"  /mcp             - list registered tool servers and their tools",
+			"  /cwd [path]      - show or change working directory",
 			"  /queued [pop|clear] - manage queued prompts",
 			"  /compact         - summarize conversation and compact context",
 			"  /context         - show context window debug info",

pkg/agent/types.go

@@ -60,6 +60,13 @@ type Core interface {
 	// ListServers returns all registered tool servers and their tools,
 	// grouped by server name.
 	ListServers() string
+
+	// WorkDir returns the current working directory used for tool execution.
+	WorkDir() string
+
+	// SetWorkDir changes the working directory for tool execution and
+	// updates the system prompt. Returns an error if the path does not exist.
+	SetWorkDir(dir string) error
 }
 
 // PromptFIFO is a simple thread-safe FIFO for queued prompts.

pkg/tools/execute/executor.go

@@ -26,6 +26,7 @@ type Server struct {
 
 	// workdir is the working directory for sandboxed commands. If empty,
 	// the process working directory is used.
+	wdMu    sync.RWMutex
 	workdir string
 
 	// rate limiting state (per-Server)
@@ -38,6 +39,13 @@ type Server struct {
 // New creates a new Server with the given working directory.
 func New(workdir string) *Server { return &Server{workdir: workdir} }
 
+// SetWorkDir updates the working directory used for subsequent command executions.
+func (e *Server) SetWorkDir(dir string) {
+	e.wdMu.Lock()
+	e.workdir = dir
+	e.wdMu.Unlock()
+}
+
 var dangerousPatterns = []*regexp.Regexp{
 	regexp.MustCompile(`rm\s+(-[a-z]*r[a-z]*\s+)*-[a-z]*f[a-z]*\s*/(home|var|usr|etc|boot|root|bin|sbin|lib|opt|srv)?`),
 	regexp.MustCompile(`rm\s+(-[a-z]*f[a-z]*\s+)*-[a-z]*r[a-z]*\s*/(home|var|usr|etc|boot|root|bin|sbin|lib|opt|srv)?`),
@@ -162,7 +170,9 @@ func (e *Server) Execute(ctx context.Context, code, language string, timeout int
 		return "", err
 	}
 
+	e.wdMu.RLock()
 	workDir := e.workdir
+	e.wdMu.RUnlock()
 	if workDir == "" {
 		workDir, _ = os.Getwd()
 	}

pkg/tools/tools.go

@@ -119,6 +119,12 @@ type PlanBackendSetter interface {
 	SetPlanBackend(PlanBackend)
 }
 
+// WorkDirSetter is implemented by tool servers that accept a dynamic working
+// directory. SetWorkDir updates the directory used for subsequent tool calls.
+type WorkDirSetter interface {
+	SetWorkDir(string)
+}
+
 // NewServer wraps an mcp.Client as a Server.
 func NewServer(client *mcp.Client) Server {
 	return &mcpServer{client: client}