state: extend interface with recentHistory/taskState; loop uses selective context

pkg/agent/core.go

@@ -969,12 +969,6 @@ func (s *agent) executeTurn(ctx context.Context, input string, handler EventHand
 	s.cfg.IncrToolCallCount = func() int64 {
 		return s.toolCallCount.Add(1)
 	}
-	s.cfg.TaskState = func() *TaskState {
-		if s.session == nil {
-			return nil
-		}
-		return s.session.TaskState
-	}
 	s.cfg.SaveSession = func() { s.saveSession() }
 	s.cfg.AutoCompact = func(ctx context.Context) {
 		if ctx.Err() != nil || s.session == nil {

pkg/agent/loop.go

@@ -62,8 +62,6 @@ type agentConfig struct {
 	PlanFile string
 	// IncrToolCallCount increments and returns the session-wide tool call counter.
 	IncrToolCallCount func() int64
-	// TaskState returns the current task state, or nil if not set.
-	TaskState func() *TaskState
 }
 
 // readPlanFile returns the contents of the plan file, or empty string if
@@ -88,15 +86,20 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 	var resultCache sync.Map
 
 	for {
-		history := state.history()
+		var history []backend.Message
+		if ts := state.taskState(); ts != nil {
+			// Structured mode: task state + recent tail only.
+			history = state.recentHistory(recentTailSize)
+		} else {
+			history = state.history()
+		}
 		if cfg.preamble != "" {
 			history = append([]backend.Message{{Role: "system", Content: cfg.preamble}}, history...)
 		}
-		if cfg.TaskState != nil {
-			if ts := cfg.TaskState(); ts != nil {
-				if msg := ts.render(); msg != "" {
-					history = append([]backend.Message{history[0], {Role: "user", Content: msg}}, history[1:]...)
-				}
+		if ts := state.taskState(); ts != nil {
+			if msg := ts.render(); msg != "" {
+				// Insert after system prompt, before conversation.
+				history = append([]backend.Message{history[0], {Role: "user", Content: msg}}, history[1:]...)
 			}
 		}
 

pkg/agent/loop_maxsteps_helpers_test.go

@@ -48,6 +48,17 @@ func (s *simpleState) history() []backend.Message {
 	return s.msgs
 }
 
+func (s *simpleState) recentHistory(n int) []backend.Message {
+	if n >= len(s.msgs) {
+		return s.msgs
+	}
+	return s.msgs[len(s.msgs)-n:]
+}
+
+func (s *simpleState) taskState() *TaskState { return nil }
+
+func (s *simpleState) updateTaskState(TaskState) {}
+
 func (s *simpleState) update(msg backend.Message, results []toolResult) {
 	s.msgs = append(s.msgs, msg)
 	for _, r := range results {

pkg/agent/session.go

@@ -137,6 +137,21 @@ func (s *Session) history() []backend.Message {
 	return s.messages
 }
 
+func (s *Session) recentHistory(n int) []backend.Message {
+	if n >= len(s.messages) {
+		return s.messages
+	}
+	return s.messages[len(s.messages)-n:]
+}
+
+func (s *Session) taskState() *TaskState {
+	return s.TaskState
+}
+
+func (s *Session) updateTaskState(ts TaskState) {
+	s.TaskState = &ts
+}
+
 func (s *Session) addUsage(u backend.Usage, estimated bool) {
 	s.TotalInputTokens += u.InputTokens
 	s.TotalCachedInputTokens += u.CachedInputTokens

pkg/agent/state.go

@@ -17,6 +17,15 @@ type state interface {
 	// The first entry is always the initial user message containing the goal.
 	history() []backend.Message
 
+	// recentHistory returns the last n messages from the history.
+	recentHistory(n int) []backend.Message
+
+	// taskState returns the current structured task state, or nil if not set.
+	taskState() *TaskState
+
+	// updateTaskState replaces the current task state.
+	updateTaskState(ts TaskState)
+
 	// Update appends the assistant's reply and any tool results to the history.
 	// Called once per loop iteration after Act completes.
 	update(assistant backend.Message, results []toolResult)