fix(agent): preserve recent turns during compression

agent/context_compressor.py

@@ -156,6 +156,11 @@ _FALLBACK_TURN_MAX_CHARS = 700
 _AUTO_FOCUS_MAX_TURNS = 3
 _AUTO_FOCUS_TURN_MAX_CHARS = 260
 _AUTO_FOCUS_MAX_CHARS = 700
+# Keep a short run of recent messages verbatim even when the token budget is
+# already exhausted.  The public ``protect_last_n`` default is intentionally
+# high for small/light tails, but using all 20 as a hard floor here would bring
+# back the old large-tool-output case where nothing can be compacted.
+_MAX_TAIL_MESSAGE_FLOOR = 8
 
 
 _PATH_MENTION_RE = re.compile(r"(?:/|~/?|[A-Za-z]:\\)[^\s`'\")\]}<>]+")
@@ -1990,11 +1995,12 @@ This compaction should PRIORITISE preserving all information related to the focu
         derived from ``summary_target_ratio * context_length``, so it
         scales automatically with the model's context window.
 
-        Token budget is the primary criterion.  A hard minimum of 3 messages
-        is always protected, but the budget is allowed to exceed by up to
-        1.5x to avoid cutting inside an oversized message (tool output, file
-        read, etc.).  If even the minimum 3 messages exceed 1.5x the budget
-        the cut is placed right after the head so compression still runs.
+        Token budget is the primary criterion.  A bounded message-count floor
+        keeps a short run of recent turns verbatim even when the budget is
+        exhausted, but the budget is allowed to exceed by up to 1.5x to avoid
+        cutting inside an oversized message (tool output, file read, etc.). If
+        even that floor exceeds 1.5x the budget, the cut is placed right after
+        the head so compression still runs.
 
         Never cuts inside a tool_call/result group.  Always ensures the most
         recent user message is in the tail (see ``_ensure_last_user_message_in_tail``).
@@ -2002,8 +2008,19 @@ This compaction should PRIORITISE preserving all information related to the focu
         if token_budget is None:
             token_budget = self.tail_token_budget
         n = len(messages)
-        # Hard minimum: always keep at least 3 messages in the tail
-        min_tail = min(3, n - head_end - 1) if n - head_end > 1 else 0
+        # Hard minimum: always keep a bounded recent-message floor in the tail.
+        # ``protect_last_n`` remains a minimum up to the cap; the cap avoids
+        # preserving a whole run of bulky tool outputs on every compaction.
+        available_tail = max(0, n - head_end - 1)
+        min_tail_floor = max(3, min(self.protect_last_n, _MAX_TAIL_MESSAGE_FLOOR))
+        # Leave at least two non-head messages available to summarize on short
+        # transcripts; otherwise compression can replace a tiny middle with a
+        # summary and save no messages at all.
+        compressible_tail_cap = max(3, available_tail - 2)
+        min_tail = (
+            min(min_tail_floor, compressible_tail_cap, available_tail)
+            if available_tail > 1 else 0
+        )
         soft_ceiling = int(token_budget * 1.5)
         accumulated = 0
         cut_idx = n  # start from beyond the end