diff --git a/README.md b/README.md
index 906351e..3997ecf 100644
--- a/README.md
+++ b/README.md
@@ -77,7 +77,7 @@ Or ask your agent to use a skill:
 Vault organization requires the Obsidian CLI. In Obsidian, enable
 **Settings > General > Command line interface**, **Use [[Wikilinks]]**, and
 **Automatically update internal links**. Put naming conventions in your vault's
-`CLAUDE.md` and configure [managed zones](docs/configuration.md#vault-configuration).
+`CLAUDE.md` and configure [managed zones](docs/configuration.md#shared-preferences-and-host-locations).
 Maintenance tools ship in the CLI: run `obsidian-knowledge garden --help`.
 See [vault maintenance](docs/CLI.md#vault-maintenance) for audit and repair commands.
 
diff --git a/docs/MCP.md b/docs/MCP.md
index 1488621..feccf81 100644
--- a/docs/MCP.md
+++ b/docs/MCP.md
@@ -52,6 +52,12 @@ sending a token across a network. Tunnel provisioning is separate from this serv
   overwriting. Parent directories are created as needed. New Markdown notes
   apply the declarative [configured creation defaults](configuration.md#note-creation)
   before writing; replacements never rerun them. No running app is required.
+  Replacements preserve the existing UID, GID and permission bits on the
+  temporary inode before publication. If ownership cannot be preserved, the
+  write fails and leaves the original file intact. New files use the writer's
+  identity and the destination directory's group inheritance; shared-vault
+  operators must configure those permissions rather than relying on a package
+  default group. This does not copy extended ACLs or other extended attributes.
 - `vault_patch(path, old_str, new_str, expected_sha256=None)` replaces exactly
   one occurrence of `old_str` in a UTF-8 file, including frontmatter. Empty,
   missing, and ambiguous matches (including overlaps) are rejected. Untouched
diff --git a/lib/vault_index/vault_files.py b/lib/vault_index/vault_files.py
index 4da7497..d6349ed 100644
--- a/lib/vault_index/vault_files.py
+++ b/lib/vault_index/vault_files.py
@@ -73,10 +73,14 @@ def write_vault_file(
     target.parent.mkdir(parents=True, exist_ok=True)
     target = resolve_vault_file(vault_root, relative_path)
 
-    mode = stat.S_IMODE(target.stat().st_mode) if target.exists() else 0o644
+    original_stat = target.stat() if target.exists() else None
+    mode = stat.S_IMODE(original_stat.st_mode) if original_stat is not None else 0o644
     fd, temp_name = tempfile.mkstemp(prefix=f".{target.name}.", suffix=".tmp", dir=target.parent)
     temp = Path(temp_name)
     try:
+        # NOTE: docs/MCP.md, vault_write: retain existing access ownership before publication.
+        if original_stat is not None:
+            os.fchown(fd, original_stat.st_uid, original_stat.st_gid)
         os.fchmod(fd, mode)
         with os.fdopen(fd, "wb") as stream:
             fd = -1
diff --git a/tests/conftest.py b/tests/conftest.py
index 2e456d8..e5ee3a3 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -28,8 +28,11 @@
 
 @pytest.fixture(autouse=True)
 def _isolated_preferences(tmp_path, monkeypatch):
-    """Never read or mutate the operator's shared preferences during tests."""
+    """Isolate preferences and clear ambient vault registry overrides."""
     monkeypatch.setenv("OBSIDIAN_KNOWLEDGE_CONFIG", str(tmp_path / "preferences.yaml"))
+    # Tests exercise HOME-based discovery; caller overrides must not supersede it.
+    monkeypatch.delenv("XDG_CONFIG_HOME", raising=False)
+    monkeypatch.delenv("OBSIDIAN_KNOWLEDGE_VAULTS_CONFIG", raising=False)
 
 
 @pytest.fixture(autouse=True)
diff --git a/tests/test_vault_file_ownership.py b/tests/test_vault_file_ownership.py
new file mode 100644
index 0000000..371ca4e
--- /dev/null
+++ b/tests/test_vault_file_ownership.py
@@ -0,0 +1,60 @@
+"""Atomic replacement must preserve the existing note's access ownership."""
+
+from __future__ import annotations
+
+import os
+import stat
+from pathlib import Path
+
+import pytest
+
+from lib.vault_index.vault_files import write_vault_file
+
+
+def test_replacement_applies_original_owner_before_mode_and_publication(tmp_path, monkeypatch):
+    target = tmp_path / "note.txt"
+    target.write_bytes(b"original")
+    target.chmod(0o660)
+    before = target.stat()
+    calls = []
+    chown = os.fchown
+    chmod = os.fchmod
+
+    def preserve_owner(fd, uid, gid):
+        assert target.read_bytes() == b"original"
+        calls.append(("owner", uid, gid))
+        chown(fd, uid, gid)
+
+    def preserve_mode(fd, mode):
+        calls.append(("mode", mode))
+        chmod(fd, mode)
+
+    monkeypatch.setattr(os, "fchown", preserve_owner)
+    monkeypatch.setattr(os, "fchmod", preserve_mode)
+    write_vault_file(tmp_path, Path("note.txt"), b"replacement", replace=True)
+    after = target.stat()
+    assert calls == [("owner", before.st_uid, before.st_gid), ("mode", 0o660)]
+    assert (after.st_uid, after.st_gid, stat.S_IMODE(after.st_mode)) == (before.st_uid, before.st_gid, 0o660)
+    assert target.read_bytes() == b"replacement"
+
+
+def test_ownership_failure_preserves_original_inode_and_removes_temporary(tmp_path, monkeypatch):
+    target = tmp_path / "note.txt"
+    target.write_bytes(b"original")
+    before = target.stat()
+
+    def denied(fd, uid, gid):
+        raise PermissionError("ownership preservation denied")
+
+    monkeypatch.setattr(os, "fchown", denied)
+    with pytest.raises(PermissionError, match="ownership preservation denied"):
+        write_vault_file(tmp_path, Path("note.txt"), b"replacement", replace=True)
+    after = target.stat()
+    assert (after.st_ino, after.st_uid, after.st_gid, after.st_mode) == (
+        before.st_ino,
+        before.st_uid,
+        before.st_gid,
+        before.st_mode,
+    )
+    assert target.read_bytes() == b"original"
+    assert list(tmp_path.iterdir()) == [target]