fix(s08): make prepare() idempotent after request reinsertion

Copilot review on #580 flagged two issues: the reinsertion guard only
matched the raw request string, so after the first pass the prefixed
"Current user request" message no longer matched and prepare()
reinserted on every agent-loop iteration; and the extra message kept
the list over snip_compact()'s 50-message limit, causing repeated
re-archiving and transcript writes.

Recognize the prefixed insertion in the existence check, and re-run
snip_compact() right after reinserting so prepare() returns a list
that is already within the snip limit. Add a regression test that
calls prepare() twice and asserts the output is stable, contains a
single request message, and writes no extra transcripts.

Author: wudilyy999

s08_context_compact/code.py

@@ -513,7 +513,8 @@ def prepare(self, messages: list, active_request: str) -> list:
             self.is_archive_marker(message) for message in messages
         ) and not any(
             isinstance(message.get("content"), str)
-            and message["content"] == active_request
+            and (message["content"] == active_request
+                 or message["content"].startswith("Current user request:"))
             for message in messages
             if message.get("role") == "user"
         ):
@@ -523,6 +524,10 @@ def prepare(self, messages: list, active_request: str) -> list:
             # reactive_compact so the model never loses the active task.
             messages.insert(0, {"role": "user", "content":
                                 f"Current user request:\n{active_request}"})
+            # The extra message can push the list back over the snip limit;
+            # archive again now so the next prepare() call in the agent loop
+            # stays stable instead of re-archiving on every iteration.
+            messages = self.snip_compact(messages)
         if self.estimate_chars(messages) > self.CONTEXT_CHAR_LIMIT:
             target = int(self.CONTEXT_CHAR_LIMIT * 0.8)
             messages = self.micro_compact(messages, target)

tests/test_s08_context_compact.py

@@ -174,6 +174,51 @@ def test_prepare_reinserts_active_request_archived_by_snip_compact(
     assert request in rendered
 
 
+def test_prepare_is_idempotent_when_still_over_message_limit(
+        tmp_path, monkeypatch):
+    """agent_loop calls prepare() every iteration; the reinsertion must not
+    stack up duplicate request messages or re-archive on every pass."""
+    lesson = load_lesson(monkeypatch, tmp_path)
+    request = "compare s08 and s09 context handling"
+    messages = [
+        {"role": "user", "content": "earlier question one"},
+        {"role": "assistant", "content": [{"type": "text", "text": "answer one"}]},
+        {"role": "user", "content": "earlier question two"},
+        {"role": "assistant", "content": [{"type": "text", "text": "answer two"}]},
+        {"role": "user", "content": request},
+        {"role": "assistant", "content": [{"type": "text", "text": "starting"}]},
+    ]
+    for index in range(26):
+        tool_id = f"tool-{index}"
+        messages.extend([
+            {"role": "assistant", "content": [
+                {"type": "tool_use", "id": tool_id, "name": "bash", "input": {}}
+            ]},
+            {"role": "user", "content": [
+                {"type": "tool_result", "tool_use_id": tool_id, "content": f"out-{index}"}
+            ]},
+        ])
+
+    compactor = lesson["COMPACTOR"]
+    prepared = compactor.prepare(list(messages), request)
+    assert len(prepared) <= 50  # prepare() leaves the list within the snip limit
+    assert prepared[0]["content"] == f"Current user request:\n{request}"
+    transcripts_before = list(
+        (tmp_path / ".transcripts").glob("transcript_*.jsonl"))
+
+    prepared_again = compactor.prepare(list(prepared), request)
+
+    assert prepared_again == prepared
+    request_messages = [
+        message for message in prepared_again
+        if isinstance(message.get("content"), str)
+        and message["content"].startswith("Current user request:")
+    ]
+    assert len(request_messages) == 1
+    assert len(list((tmp_path / ".transcripts").glob("transcript_*.jsonl"))) \
+        == len(transcripts_before)
+
+
 def test_prepare_does_not_duplicate_active_request_visible_in_tail(
         tmp_path, monkeypatch):
     lesson = load_lesson(monkeypatch, tmp_path)