diff --git a/python/arcticdb/flattener.py b/python/arcticdb/flattener.py
index cfe6efaa8ce..bf83fd91197 100644
--- a/python/arcticdb/flattener.py
+++ b/python/arcticdb/flattener.py
@@ -14,11 +14,11 @@
 from arcticdb.exceptions import DataTooNestedException, UnsupportedKeyInDictionary
 
 try:
-    from msgpack.fallback import DEFAULT_RECURSE_LIMIT
+    from msgpack.fallback import DEFAULT_RECURSE_LIMIT as _DEFAULT_RECURSE_LIMIT
 except ImportError:
     # The default as of msgpack 1.1.0 - handle the import error in case the msgpack wheel stops exporting this constant.
     # Want to keep compatibility with a wide range of msgpack versions.
-    DEFAULT_RECURSE_LIMIT = 511
+    _DEFAULT_RECURSE_LIMIT = 511
 
 from arcticdb import _msgpack_compat
 from arcticdb.log import version as log
@@ -27,6 +27,19 @@
 from arcticdb.preconditions import check
 from arcticdb_ext import get_config_int
 
+# Two separate limits govern maximum nesting depth:
+#   Write (msgpack packer): uses ~2 Python stack frames per level; capped at DEFAULT_RECURSE_LIMIT // 2.
+#   Read (create_original_obj_from_metastruct): uses ~3 Python stack frames per level, so reads
+#   fail above sys.getrecursionlimit() // 3 ≈ 333 with Python's default limit of 1000.
+#
+# The read path is tighter, so we pin msgpack's DEFAULT_RECURSE_LIMIT at 511, giving a write
+# cap of 511 // 2 == 255 — safely below the ~333 read ceiling. msgpack 1.2.0 raised its own
+# constant DEFAULT_RECURSE_LIMIT to 1024, which silently allowed writes that the reader could not
+# reconstruct. We keep 511 regardless of what msgpack exports.
+# TODO: once the metastruct reader is non-recursive, raise to (sys.getrecursionlimit() - 10) // 2.
+# https://man312219.monday.com/boards/7852509418/pulses/12254825163
+DEFAULT_RECURSE_LIMIT = min(511, _DEFAULT_RECURSE_LIMIT)
+
 
 class Flattener:
     # Probably a bad idea given the dict key could have this, fine for now as write does not allow this symbol anyways.
diff --git a/python/tests/unit/arcticdb/version_store/test_recursive_normalizers.py b/python/tests/unit/arcticdb/version_store/test_recursive_normalizers.py
index ffe55218090..e89514e89df 100644
--- a/python/tests/unit/arcticdb/version_store/test_recursive_normalizers.py
+++ b/python/tests/unit/arcticdb/version_store/test_recursive_normalizers.py
@@ -8,7 +8,7 @@
 import numpy as np
 import arcticdb
 from arcticdb import QueryBuilder, LibraryOptions
-from arcticdb.flattener import Flattener
+from arcticdb.flattener import Flattener, DEFAULT_RECURSE_LIMIT
 from arcticdb.version_store._custom_normalizers import (
     CustomNormalizer,
     register_normalizer,
@@ -392,12 +392,18 @@ def test_deep_nesting_metastruct_size_over_limit(lmdb_version_store_v1, all_recu
     key = "reasonable_length_key"
     data = {key: pd.DataFrame({"col": [0]})}
 
-    nesting_levels = 256
+    # Mirror the guard in flattener._create_meta_structure (raises above DEFAULT_RECURSE_LIMIT // 2).
+    # flattener clamps DEFAULT_RECURSE_LIMIT to 511, so this is 255 - see the note there for why.
+    nesting_limit = DEFAULT_RECURSE_LIMIT // 2
+    nesting_levels = nesting_limit + 1
     for i in range(nesting_levels - 1):
         data[key] = {key: data[key]}
 
     # When & Then
-    with pytest.raises(DataTooNestedException, match=r"^Symbol sym cannot be recursively normalized.*255 levels.*"):
+    with pytest.raises(
+        DataTooNestedException,
+        match=rf"^Symbol sym cannot be recursively normalized.*{nesting_limit} levels.*",
+    ):
         lib.write(sym, data, recursive_normalizers=True)
 
 
@@ -408,7 +414,7 @@ def test_deep_nesting_metastruct_size_under_limit(lmdb_version_store_v1, all_rec
     key = "reasonable_length_key"
     data = {key: pd.DataFrame({"col": [0]})}
 
-    nesting_levels = 255
+    nesting_levels = (DEFAULT_RECURSE_LIMIT - 1) // 2
     for i in range(nesting_levels - 1):
         data[key] = {key: data[key]}