fix(memory): warn when MEMORY.md / USER.md exceed their char limit on load

The cap only fires on add/replace, so an externally written over-budget
file rode silently in the system prompt while every later add was refused
with no visible cause. Warn at load; entries stay loaded (never truncate
a user's memories).

Salvage of #10886 (original hunk targeted memory_tool.py before the
store split); authored by @easyvibecoding.

Refs #10877

tests/tools/test_memory_tool.py

@@ -270,6 +270,26 @@ class TestMemoryStorePersistence:
         assert len(store.memory_entries) == 2
 
 
+class TestMemoryStoreCharLimitOnLoad:
+    @pytest.mark.parametrize("filename, target", [("MEMORY.md", "memory"), ("USER.md", "user")])
+    def test_over_limit_file_loads_but_warns(self, tmp_path, monkeypatch, caplog, filename, target):
+        """An externally written over-budget file is kept (no silent data loss) and named in a
+        warning; an in-budget file loads quietly (#10877)."""
+        import logging
+        monkeypatch.setattr("tools.memory_tool.get_memory_dir", lambda: tmp_path)
+        (tmp_path / filename).write_text("x" * 600, encoding="utf-8")
+        with caplog.at_level(logging.WARNING):
+            store = MemoryStore(memory_char_limit=500, user_char_limit=300)
+            store.load_from_disk()
+        assert filename in caplog.text and "exceeds" in caplog.text
+        assert len(store._entries_for(target)) == 1
+        caplog.clear()
+        (tmp_path / filename).write_text("short", encoding="utf-8")
+        with caplog.at_level(logging.WARNING):
+            MemoryStore(memory_char_limit=500, user_char_limit=300).load_from_disk()
+        assert "exceeds" not in caplog.text
+
+
 class TestMemoryStoreSnapshot:
     def test_snapshot_frozen_at_load(self, store):
         assert store.format_for_system_prompt("memory") is None  # empty store

tools/memory_tool_store.py

@@ -130,6 +130,13 @@ class MemoryStore:
             # Deduplicate (order-preserving, first occurrence wins).
             entries = list(dict.fromkeys(self._read_file(path)))
             self._set_entries(target, entries)
+            # External writers (MCP bridges, hand edits) can exceed the cap; the limit only fires on
+            # add/replace, so the oversized block would silently ride in the prompt while every later
+            # add is refused with no visible cause (#10877). Warn; never truncate a user's memories.
+            if (count := self._char_count(target)) > (limit := self._char_limit(target)):
+                logger.warning("%s exceeds its char limit on load: %d/%d chars. Entries stay loaded; "
+                               "further additions are blocked until it is back under the limit.",
+                               path.name, count, limit)
             self._system_prompt_snapshot[target] = self._render_block(target, [_sanitize(e, path.name) for e in entries])
 
     @staticmethod