diff --git a/package-lock.json b/package-lock.json
index 8c1b7a4b..9100d8db 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1184,7 +1184,7 @@
     },
     "packages/cultcache-ts": {
       "name": "@gamecult/cultcache-ts",
-      "version": "0.14.1",
+      "version": "0.14.2",
       "license": "MIT",
       "dependencies": {
         "@msgpack/msgpack": "^3.1.3",
diff --git a/packages/cultcache-ts/CHANGELOG.md b/packages/cultcache-ts/CHANGELOG.md
index f43bf60a..14d32a33 100644
--- a/packages/cultcache-ts/CHANGELOG.md
+++ b/packages/cultcache-ts/CHANGELOG.md
@@ -3,6 +3,26 @@
 All notable changes to `@gamecult/cultcache-ts` are recorded here, following
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## [0.14.2]
+
+### Added
+- `SingleFileMessagePackBackingStore` takes a cross-process write lock on `<file>.lock`,
+  interoperable with the C# and Rust runtimes (`flock` on Unix, `LockFileEx` on Windows).
+  Each write reads the current file, merges and replaces it under the lock, so concurrent
+  writers no longer lose each other's records.
+- The staged file is fsynced before the replace, and on Unix the directory after it. A
+  directory sync failure after the replace is reported as replaced but not durable.
+
+### Changed
+- The Windows replace is a POSIX-semantics rename first (it lands while C# and Rust readers
+  hold the store open), falling back to a write-through `MoveFileExW`, with 25 attempts 20 ms
+  apart on errors 5, 32 and 33. Writers wait up to 30 s for the lock.
+- Opening a legacy store that needs healing rewrites it under the lock, from a read taken
+  under the lock.
+
+### Dependencies
+- `koffi` 3.3.0, loaded only on the first write.
+
 ## [0.14.1]
 
 ### Added
diff --git a/packages/cultcache-ts/README.md b/packages/cultcache-ts/README.md
index 9e208dfe..192d4b5d 100644
--- a/packages/cultcache-ts/README.md
+++ b/packages/cultcache-ts/README.md
@@ -358,7 +358,8 @@ That is the TypeScript version of the original CultCache trick:
 - each record is `[key, schemaId, storedAt, payload]`
 - each catalog entry is `[schemaId, schemaName, schemaVersion, contentHash, canonicalSchemaJson, compatibleSchemaIds, members]`
 - it still reads the older TS/Rust-style envelope array and rewrites repaired legacy payloads as v1 snapshots
-- it queues local writes so one process does not step on its own shoelaces
+- it queues local writes in order, then takes the shared `<file>.lock` (`flock` on Unix, `LockFileEx` on Windows, via koffi, loaded on the first write) for each one, so C#, Rust and TypeScript writers exclude each other; the read a write merges from is taken under that lock
+- it fsyncs the staged file and, on Unix, the directory after the replace; on Windows it replaces with a POSIX-semantics rename and falls back to a write-through `MoveFileExW`, retrying briefly on sharing violations
 - it persists raw MessagePack payload bytes inside schema-addressed records
 
-It is a control-plane store, not a tiny god. If multiple processes hammer the same file, you still need a coordinator or an external lock instead of prayer.
+It is a control-plane store, not a tiny god: every write rewrites the whole file.
diff --git a/packages/cultcache-ts/package.json b/packages/cultcache-ts/package.json
index 501b511d..c48e54e5 100644
--- a/packages/cultcache-ts/package.json
+++ b/packages/cultcache-ts/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@gamecult/cultcache-ts",
-  "version": "0.14.1",
+  "version": "0.14.2",
   "description": "CultCache-style typed persistence for TypeScript with schema-backed polymorphic MessagePack storage.",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
diff --git a/src/GameCult.Caching/Contracts/cultcache-store-composition.md b/src/GameCult.Caching/Contracts/cultcache-store-composition.md
index a19a8839..173662f4 100644
--- a/src/GameCult.Caching/Contracts/cultcache-store-composition.md
+++ b/src/GameCult.Caching/Contracts/cultcache-store-composition.md
@@ -240,8 +240,13 @@ replaces; writers bump a minted timestamp by one tick when it is not later.
   wait bound (30 seconds).
 - A conditional commit on a store with staged single-record writes throws:
   flush first.
-- Single-file stores lock a sidecar `<path>.lock` opened exclusively. Directory
-  stores use their existing commit lease.
+- Single-file stores lock a sidecar `<path>.lock`: C# by an exclusive
+  (share-none) open, Rust and TypeScript by an exclusive `LockFileEx` on a
+  shared open on Windows, and every runtime by `flock(LOCK_EX)` on Unix. Each
+  excludes the others (TypeScript tests: "a TypeScript write waits while a C#
+  writer holds the store lock", "... a Rust writer ...", "a C# writer waits
+  while a TypeScript writer holds the store lock", "a Rust writer waits ...").
+  Directory stores use their existing commit lease.
 - A directory store with no `.commit.lock` (no writer has created it yet, or
   the store was copied without it) gives a pull no lease, and opening it
   creates none. Such a pull re-reads the manifest after loading its pages and
@@ -265,9 +270,11 @@ against another writer; processes sharing a store must all use it.
   in that store, and leaves the store clean.
 - A conditional commit lands the batch onto the file as it is under the lock.
 
-A store is written by one runtime at a time. Whether a Rust `fs2` lock and a C#
-exclusive open of the same lock file exclude each other is not established, and
-another runtime need not obey the `storedAt` rule.
+C#, Rust and TypeScript writers exclude each other on the shared lock file;
+Python takes no cross-process lock yet (`variants:follow_up:py-shared-write-lock`),
+and Rust's open fails rather than waits while a C# writer holds the lock
+(`variants:follow_up:rust-lock-open-waits`). Another runtime need not obey the
+`storedAt` rule.
 
 ## Serialization options