diff --git a/packages/cultcache-py/src/cultcache_py/backing_store.py b/packages/cultcache-py/src/cultcache_py/backing_store.py
index 5a21e496..7a93bbd0 100644
--- a/packages/cultcache-py/src/cultcache_py/backing_store.py
+++ b/packages/cultcache-py/src/cultcache_py/backing_store.py
@@ -67,10 +67,12 @@ class BackingStore(Protocol):
     def delete(self, type: str, key: str) -> None:
         ...
 
-    def push_all(self, envelopes: list[CultCacheEnvelope]) -> None:
+    def push_all(self, envelopes: list[CultCacheEnvelope], soft: bool = False) -> None:
         existing = {
             (envelope.type, envelope.key): envelope for envelope in self.pull_all()
         }
+        if soft and existing:
+            return
         for envelope in envelopes:
             existing[(envelope.type, envelope.key)] = envelope
         self._replace_all(list(existing.values()))
diff --git a/packages/cultcache-py/src/cultcache_py/stores.py b/packages/cultcache-py/src/cultcache_py/stores.py
index c15ac32e..35b53af3 100644
--- a/packages/cultcache-py/src/cultcache_py/stores.py
+++ b/packages/cultcache-py/src/cultcache_py/stores.py
@@ -106,9 +106,11 @@ class JsonLinesBackingStore:
             existing[(envelope.type, envelope.key)] = envelope
             self._replace_all(list(existing.values()))
 
-    def push_all(self, envelopes: list[CultCacheEnvelope]) -> None:
+    def push_all(self, envelopes: list[CultCacheEnvelope], soft: bool = False) -> None:
         with self._lock:
             existing = {(item.type, item.key): item for item in self.pull_all()}
+            if soft and self.path.exists():
+                return
             for envelope in envelopes:
                 existing[(envelope.type, envelope.key)] = envelope
             self._replace_all(list(existing.values()))
@@ -163,9 +165,14 @@ class SingleFileMessagePackBackingStore:
         with self._write_transaction():
             self._replace_all([envelope], self.pull_all())
 
-    def push_all(self, envelopes: list[CultCacheEnvelope]) -> None:
+    def push_all(self, envelopes: list[CultCacheEnvelope], soft: bool = False) -> None:
+        """A soft push writes only where nothing is at the path: a store the reader reads is left byte for byte, and a file it
+        cannot read is refused by the same read, as in TS and Rust."""
         with self._write_transaction():
-            self._replace_all(list(envelopes), self.pull_all())
+            arrived = self.pull_all()
+            if soft and self.path.exists():
+                return
+            self._replace_all(list(envelopes), arrived)
 
     def delete(self, type: str, key: str) -> None:
         with self._write_transaction():
diff --git a/packages/cultcache-py/tests/test_soft_push_all.py b/packages/cultcache-py/tests/test_soft_push_all.py
new file mode 100644
index 00000000..a1874c7c
--- /dev/null
+++ b/packages/cultcache-py/tests/test_soft_push_all.py
@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+import shutil
+import tempfile
+import unittest
+from pathlib import Path
+
+from cultcache_py import JsonLinesBackingStore, SingleFileMessagePackBackingStore, StoreUnreadableError
+from cultcache_py.backing_store import CultCacheEnvelope
+
+
+def envelope(key: str) -> CultCacheEnvelope:
+    return CultCacheEnvelope(key=key, type="t", payload=b"\x01", stored_at="2026-11-06T00:00:00Z")
+
+
+class SoftPushAllTests(unittest.TestCase):
+    """A soft push writes where nothing is, skips a store it reads, and refuses one it cannot read (TS and Rust parity)."""
+
+    def setUp(self) -> None:
+        self.dir = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.dir, ignore_errors=True)
+        self.path = self.dir / "state.cc"
+
+    def test_over_a_readable_store_it_leaves_the_file_byte_for_byte(self) -> None:
+        for make in (SingleFileMessagePackBackingStore, JsonLinesBackingStore):
+            with self.subTest(make.__name__):
+                path = self.dir / make.__name__
+                store = make(path)
+                store.push_all([envelope("a")])
+                before = path.read_bytes()
+                store.push_all([envelope("b")], soft=True)
+                self.assertEqual(before, path.read_bytes())
+                self.assertEqual(["a"], [item.key for item in make(path).pull_all()])
+                store.push_all([envelope("b")])  # a plain push still lands what the caller holds
+                self.assertEqual(["a", "b"], sorted(item.key for item in make(path).pull_all()))
+
+    def test_over_an_absent_path_it_writes(self) -> None:
+        for make in (SingleFileMessagePackBackingStore, JsonLinesBackingStore):
+            with self.subTest(make.__name__):
+                path = self.dir / make.__name__
+                make(path).push_all([envelope("a")], soft=True)
+                self.assertEqual(["a"], [item.key for item in make(path).pull_all()])
+
+    def test_over_an_unreadable_file_it_refuses_and_leaves_it(self) -> None:
+        junk = b"\xc1\x01\x02\x03"
+        self.path.write_bytes(junk)
+        with self.assertRaises(StoreUnreadableError):
+            SingleFileMessagePackBackingStore(self.path).push_all([envelope("a")], soft=True)
+        self.assertEqual(junk, self.path.read_bytes())
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/packages/cultnet-py/src/cultnet_py/benchmark.py b/packages/cultnet-py/src/cultnet_py/benchmark.py
index 83834635..0793bfc7 100644
--- a/packages/cultnet-py/src/cultnet_py/benchmark.py
+++ b/packages/cultnet-py/src/cultnet_py/benchmark.py
@@ -137,7 +137,9 @@ class InMemoryBackingStore:
     def push(self, envelope: CultCacheEnvelope) -> None:
         self._records[(envelope.type, envelope.key)] = envelope
 
-    def push_all(self, envelopes: list[CultCacheEnvelope]) -> None:
+    def push_all(self, envelopes: list[CultCacheEnvelope], soft: bool = False) -> None:
+        if soft and self._records:
+            return
         for envelope in envelopes:
             self.push(envelope)