fix(compression): debug-log checkpoint capability probe failures

Keep init fail-open on a broken probe, but emit DEBUG so flaky probes are diagnosable.

Co-authored-by: Cursor <cursoragent@cursor.com>

agent/conversation_compression.py

@@ -1292,6 +1292,8 @@ def _warn_checkpoint_required_without_capable_provider(agent: Any) -> None:
 
     Capability-probe exceptions are suppressed so a broken provider cannot crash
     agent construction — same fail-open style as the micro-compact warning.
+    Probe failures log at DEBUG so a flaky probe can be diagnosed without
+    refusing init or emitting the incapable-provider WARNING.
     """
     if getattr(agent, "compression_checkpoint_required", False) is not True:
         return
@@ -1302,7 +1304,12 @@ def _warn_checkpoint_required_without_capable_provider(agent: Any) -> None:
             try:
                 if bool(supports_checkpoint(PRE_COMPRESS_CHECKPOINT_API_VERSION)):
                     return
-            except Exception:
+            except Exception as exc:
+                logger.debug(
+                    "checkpoint-required capability probe failed; init continues: %s",
+                    exc,
+                    exc_info=True,
+                )
                 return
     logger.warning(
         "compression.checkpoint_required is enabled but the active memory "

tests/agent/test_pre_compress_checkpoint_contract.py

@@ -696,7 +696,15 @@ def test_startup_probe_exception_fails_open_without_crash_or_warning(caplog):
         _memory_manager=_ExplodingManager(),
     )
 
-    with caplog.at_level(logging.WARNING, logger="agent.conversation_compression"):
+    with caplog.at_level(logging.DEBUG, logger="agent.conversation_compression"):
         _warn_checkpoint_required_without_capable_provider(agent)
 
-    assert "compression.checkpoint_required" not in _warn_text(caplog)
+    warnings = "\n".join(
+        r.getMessage() for r in caplog.records if r.levelno >= logging.WARNING
+    )
+    debugs = "\n".join(
+        r.getMessage() for r in caplog.records if r.levelno == logging.DEBUG
+    )
+    assert "compression.checkpoint_required" not in warnings
+    assert "probe exploded" in debugs
+    assert "capability probe failed" in debugs