fix: prevent stale lock-skip signal leaking between compress_context calls

Advisor review found a critical stale-signal leak: if auto-compress
sets _compression_skipped_due_to_lock during a lock-skip, a subsequent
successful manual /compress will see the stale signal, falsely report
'Compression already in progress', and discard the compression results.

Fix:
- compress_context clears _compression_skipped_due_to_lock = None at
  entry so each call's outcome alone determines the signal.
- Unified gateway 'holder: unknown' drift to match CLI/TUI pattern
  (omit holder clause when not a descriptive string).
- Added MagicMock opt-outs in 3 sibling test files broken by the new
  signal check (test_compress_here, test_compress_focus,
  test_compress_plugin_engine).
- Added stale-signal-leak invariant test proving the fix.

agent/conversation_compression.py

@@ -1283,6 +1283,12 @@ def compress_context(
     _try_acquire_lock = None
     _lock_lookup_error: Optional[Exception] = None
     _legacy_session_db_without_lock_api = False
+    # Clear any stale lock-skip signal from a prior call so this call's
+    # outcome alone determines what callers see.  Without this an
+    # auto-compress lock-skip followed by a successful manual /compress
+    # would falsely report "Compression already in progress" and discard
+    # the compression results.
+    agent._compression_skipped_due_to_lock = None
     if _lock_db is not None:
         try:
             _legacy_session_db_without_lock_api = _lock_api_is_absent_on_session_db(

gateway/slash_commands.py

@@ -3570,10 +3570,11 @@ class GatewaySlashCommandsMixin:
                 # "No changes from compression" no-op text.
                 _lock_skipped = getattr(tmp_agent, "_compression_skipped_due_to_lock", None)
                 if _lock_skipped:
-                    holder = _lock_skipped if isinstance(_lock_skipped, str) else "unknown"
+                    holder = _lock_skipped if isinstance(_lock_skipped, str) else None
+                    holder_clause = f" (holder: {holder})" if holder else ""
                     return (
-                        f"⏳ Compression already in progress for this session "
-                        f"(holder: {holder}). Please wait for it to finish."
+                        f"⏳ Compression already in progress for this session"
+                        f"{holder_clause}. Please wait for it to finish."
                     )
 
                 if partial and tail:

tests/agent/test_compress_signal_leak.py

@@ -0,0 +1,60 @@
+"""Invariant: stale signal leak between consecutive compress_context calls."""
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+def test_signal_cleared_on_entry_between_calls(monkeypatch):
+    """Call 1: lock held → signal set. Call 2: lock available → signal must
+    be None after call 2 because the entry code cleared it. This prevents
+    a prior auto-compress lock-skip from causing a subsequent successful
+    manual /compress to falsely report 'Compression already in progress'."""
+    from agent.conversation_compression import compress_context
+
+    agent = MagicMock()
+    agent._cached_system_prompt = ""
+    agent.tools = None
+    agent._memory_manager = None
+    agent._build_system_prompt = MagicMock(return_value="sys prompt")
+    agent._emit_warning = MagicMock()
+
+    msgs = [
+        {"role": "user", "content": "a"}, {"role": "assistant", "content": "b"},
+        {"role": "user", "content": "c"}, {"role": "assistant", "content": "d"},
+    ]
+
+    monkeypatch.setattr(
+        "agent.conversation_compression._compression_lock_holder",
+        lambda a: "pid=test:holder",
+    )
+
+    # --- Call 1: lock held ---
+    db1 = MagicMock()
+    db1.try_acquire_compression_lock.return_value = False
+    db1.get_compression_lock_holder.return_value = "pid=holder1"
+    agent._session_db = db1
+
+    compress_context(agent, msgs, "", approx_tokens=100, force=True)
+
+    # Call 1: signal set (lock was held).
+    assert agent._compression_skipped_due_to_lock is not None
+
+    # --- Call 2: lock available, compressor succeeds ---
+    db2 = MagicMock()
+    db2.try_acquire_compression_lock.return_value = True
+    agent._session_db = db2
+    agent.context_compressor = MagicMock()
+    compressed = [
+        {"role": "user", "content": "[summary]"},
+        {"role": "assistant", "content": "ok"},
+    ]
+    agent.context_compressor.compress = MagicMock(return_value=compressed)
+    agent.context_compressor.compression_count = 0
+    agent.context_compressor.last_compression_rough_tokens = 0
+
+    compress_context(agent, msgs, "", approx_tokens=100, force=True)
+
+    # Call 2: signal must be None — entry code cleared stale Call 1 signal.
+    assert agent._compression_skipped_due_to_lock is None, (
+        "stale signal from lock-skip call 1 leaked into successful call 2"
+    )

tests/cli/test_compress_here.py

@@ -26,6 +26,7 @@ def _wire_agent(shell, compressed_head):
     shell.agent.session_id = None
     shell.agent.tools = None
     shell.agent._compress_context.return_value = (compressed_head, "")
+    shell.agent._compression_skipped_due_to_lock = False
 
 
 def test_compress_here_compresses_head_only(capsys):

tests/gateway/test_compress_focus.py

@@ -68,6 +68,7 @@ async def test_compress_focus_topic_passed_to_agent():
     agent_instance.context_compressor.has_content_to_compress.return_value = True
     agent_instance.session_id = "sess-1"
     agent_instance._compress_context.return_value = (compressed, "")
+    agent_instance._compression_skipped_due_to_lock = False
 
     def _estimate(messages):
         return 100
@@ -98,6 +99,7 @@ async def test_compress_no_focus_passes_none():
     agent_instance.context_compressor.has_content_to_compress.return_value = True
     agent_instance.session_id = "sess-1"
     agent_instance._compress_context.return_value = (list(history), "")
+    agent_instance._compression_skipped_due_to_lock = False
 
     with (
         patch("gateway.run._resolve_runtime_agent_kwargs", return_value={"api_key": "***"}),

tests/gateway/test_compress_plugin_engine.py

@@ -127,6 +127,7 @@ async def test_compress_works_with_plugin_context_engine():
     agent_instance.context_compressor = plugin_engine
     agent_instance.session_id = "sess-1"
     agent_instance._compress_context.return_value = (compressed, "")
+    agent_instance._compression_skipped_due_to_lock = False
 
     with (
         patch("gateway.run._resolve_runtime_agent_kwargs", return_value={"api_key": "***"}),