Keep tool pairs and fold reminders into the last result

Why: Old tool results were filling the window, compaction could split a tool group or commit a stale summary, and reminders arrived as extra user messages.
Evidence: Compaction retries a conflict and opens the circuit after three failures; clearing keeps the last five tool results; a stale file write is rejected; RPC reports started, queued, or handled.
Impact: Requests over the context window replace older tool results with a placeholder. Rewriting an existing file now requires a read in the same run. Subagent text over 32000 characters is capped.
Refs: feature/context-engineering
Co-authored-by: Cursor <cursoragent@cursor.com>

runtime/session/compact.go

@@ -67,15 +67,35 @@ func (s *Store) ReadMeta() map[string]string {
 	return meta
 }
 
+const maxCompactAttempts = 3
+
 // Compact appends a compaction entry. The file keeps every earlier record.
 // Load hides the summarized prefix behind the summary plus the last
 // keepLastN non-system messages. Returns false when there is nothing to fold.
+// A session change during summary is retried from the new transcript.
+// Three consecutive summary failures or conflicts trip the breaker and
+// leave the file unchanged.
 func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (string, error)) (bool, error) {
+	var last error
+	for attempt := 0; attempt < maxCompactAttempts; attempt++ {
+		done, err, retry := s.compactOnce(keepLastN, summarize)
+		if !retry {
+			return done, err
+		}
+		last = err
+	}
+	if last == nil {
+		last = fmt.Errorf("compaction conflict: session changed during summary")
+	}
+	return false, fmt.Errorf("compaction circuit open: %w", last)
+}
+
+func (s *Store) compactOnce(keepLastN int, summarize func(messages []llm.Message) (string, error)) (bool, error, bool) {
 	s.mu.Lock()
 	entries, err := s.readEntriesLocked()
 	if err != nil {
 		s.mu.Unlock()
-		return false, err
+		return false, err, false
 	}
 	leaf := s.leafLocked(entries)
 	generation := len(entries)
@@ -90,7 +110,7 @@ func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (s
 		}
 	}
 	if len(messages) <= keepLastN+1 {
-		return false, nil
+		return false, nil, false
 	}
 	rest := messages
 	restEntries := msgEntries
@@ -100,13 +120,13 @@ func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (s
 	}
 	tailStart := len(rest) - keepLastN
 	if tailStart <= 0 {
-		return false, nil
+		return false, nil, false
 	}
 	tailStart = alignToolTail(rest, tailStart)
 	head := rest[:tailStart]
 	summary, err := summarize(head)
 	if err != nil {
-		return false, err
+		return false, err, true
 	}
 	var tailIDs []string
 	for _, e := range restEntries[tailStart:] {
@@ -116,13 +136,13 @@ func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (s
 	defer s.mu.Unlock()
 	now, err := s.readEntriesLocked()
 	if err != nil {
-		return false, err
+		return false, err, false
 	}
 	if len(now) != generation {
-		return false, fmt.Errorf("compaction conflict: session changed during summary")
+		return false, fmt.Errorf("compaction conflict: session changed during summary"), true
 	}
 	_, err = s.appendEntryLocked(Entry{Type: TypeCompaction, Summary: summary, TailIDs: tailIDs})
-	return err == nil, err
+	return err == nil, err, false
 }
 
 // alignToolTail 让保留尾巴从一条非 tool 消息开始，避免把 tool_calls 和结果拆开。