fix interrupt handling: propagate context to subprocess tree

- Pass caller context into Executor.Execute so cancellation reaches
  the subprocess immediately
- Use Setpgid + SIGKILL on process group to kill grandchild processes
- Add context to ToolExecutor signature and thread it through dispatch
- Reset agent state to idle after drainAgent
- Rollback incomplete session turn on interrupt

agent/context.go

@@ -89,6 +89,13 @@ func (cb *ContextBuilder) Messages() []backend.Message {
 	return cb.messages
 }
 
+// Truncate discards all messages after index i.
+func (cb *ContextBuilder) Truncate(i int) {
+	if i < len(cb.messages) {
+		cb.messages = cb.messages[:i]
+	}
+}
+
 // BoundedHistory returns a context-window-safe slice of messages.
 func (cb *ContextBuilder) BoundedHistory() []backend.Message {
 	return cb.buildBounded(false)

agent/loop.go

@@ -13,7 +13,7 @@ import (
 
 const maxRateLimitRetries = 3
 
-type ToolExecutor func(name string, args json.RawMessage) (string, error)
+type ToolExecutor func(ctx context.Context, name string, args json.RawMessage) (string, error)
 type OutputFn func(msg OutputMsg)
 
 type OutputMsg struct {
@@ -99,7 +99,7 @@ func Run(ctx context.Context, cfg Config, state State) error {
 			var result string
 			var isErr bool
 			if cfg.Exec != nil {
-				out, err := cfg.Exec(tc.Name, tc.Arguments)
+				out, err := cfg.Exec(ctx, tc.Name, tc.Arguments)
 				if err != nil {
 					result = fmt.Sprintf("error: %v", err)
 					isErr = true

agent/session.go

@@ -57,7 +57,17 @@ func (s *Session) MarkComplete() error {
 	return nil
 }
 
-// AppendUserMessage adds a follow-up user message and resets the complete
+// Rollback removes any trailing non-user messages from history, discarding
+// an incomplete assistant turn caused by an interruption.
+func (s *Session) Rollback() {
+	s.complete = false
+	msgs := s.ctx.Messages()
+	i := len(msgs)
+	for i > 0 && msgs[i-1].Role != "user" {
+		i--
+	}
+	s.ctx.Truncate(i)
+}
 // flag so the loop will run again on the next call to Loop.Run.
 func (s *Session) AppendUserMessage(content string) {
 	s.complete = false

exec/exec.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"9fans.net/go/plan9/client"
@@ -266,7 +267,7 @@ func loadLayeredConfig(name string) (*sandbox.Config, error) {
 }
 
 // Execute runs code in a sandbox and returns combined stdout+stderr.
-func (e *Executor) Execute(code, language string, timeout int, sandboxName string, trusted bool) (string, error) {
+func (e *Executor) Execute(ctx context.Context, code, language string, timeout int, sandboxName string, trusted bool) (string, error) {
 	start := time.Now()
 
 	if timeout <= 0 {
@@ -314,7 +315,7 @@ func (e *Executor) Execute(code, language string, timeout int, sandboxName strin
 		defer os.RemoveAll(workDir)
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeout)*time.Second)
+	ctx, cancel := context.WithTimeout(ctx, time.Duration(timeout)*time.Second)
 	defer cancel()
 
 	var cmd *exec.Cmd
@@ -323,6 +324,13 @@ func (e *Executor) Execute(code, language string, timeout int, sandboxName strin
 		wrapped := sandbox.WrapCommand(cfg, []string{"bash", "-c", code}, workDir)
 		cmd = exec.CommandContext(ctx, wrapped[0], wrapped[1:]...)
 		cmd.Dir = workDir
+		cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+		cmd.Cancel = func() error {
+			if cmd.Process != nil {
+				syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+			}
+			return nil
+		}
 	default:
 		return "", fmt.Errorf("unsupported language: %s (supported: bash)", language)
 	}

main.go

@@ -264,7 +264,7 @@ func buildAgentEnv(cfg *config.Config, builtinExec *execpkg.Executor) agentEnv {
 		builtinNames[t.Name] = struct{}{}
 	}
 
-	execFn := func(name string, args json.RawMessage) (string, error) {
+	execFn := func(ctx context.Context, name string, args json.RawMessage) (string, error) {
 		if server, ok := serverOf[name]; ok {
 			raw, err := mcpExec.Execute(server, name, args)
 			if err != nil {
@@ -273,7 +273,7 @@ func buildAgentEnv(cfg *config.Config, builtinExec *execpkg.Executor) agentEnv {
 			return extractMCPText(raw), nil
 		}
 		if _, ok := builtinNames[name]; ok {
-			return dispatchBuiltinExec(name, builtinExec, args)
+			return dispatchBuiltinExec(ctx, name, builtinExec, args)
 		}
 		return "", fmt.Errorf("unknown tool: %s", name)
 	}
@@ -537,6 +537,7 @@ func (m *model) apply(am agentMsg) {
 }
 
 // drainAgent cancels the in-flight goroutine and drains remaining events.
+// It also rolls back any incomplete assistant turn from the session history.
 func (m *model) drainAgent() {
 	if m.cancel == nil {
 		return
@@ -555,6 +556,11 @@ func (m *model) drainAgent() {
 		}
 		m.agentCh = nil
 	}
+	if m.session != nil {
+		m.session.Rollback()
+	}
+	m.state = agentIdle
+	m.currentTool = ""
 }
 
 func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
@@ -893,7 +899,7 @@ func extractMCPText(raw json.RawMessage) string {
 	return strings.Join(parts, "\n")
 }
 
-func dispatchBuiltinExec(name string, e *execpkg.Executor, args json.RawMessage) (string, error) {
+func dispatchBuiltinExec(ctx context.Context, name string, e *execpkg.Executor, args json.RawMessage) (string, error) {
 	var a struct {
 		Code     string             `json:"code"`
 		Language string             `json:"language"`
@@ -954,5 +960,5 @@ func dispatchBuiltinExec(name string, e *execpkg.Executor, args json.RawMessage)
 		code = a.Code
 	}
 
-	return e.Execute(code, a.Language, a.Timeout, a.Sandbox, trusted)
+	return e.Execute(ctx, code, a.Language, a.Timeout, a.Sandbox, trusted)
 }