diff --git a/docs_src/changes.md b/docs_src/changes.md
index 6f9b25c0..e1b73963 100644
--- a/docs_src/changes.md
+++ b/docs_src/changes.md
@@ -6,6 +6,9 @@ WeeWX change history
 Fix problem where `[StdReport]` does not include `HTML_ROOT`. Fixes 
 [Issue #1082](https://github.com/weewx/weewx/issues/1082).
 
+Install extensions atomically, never overwrite in place. 
+[PR #1104](https://github.com/weewx/weewx/pull/1104). Thanks to user John K!
+
 
 ### 5.4.0 06/16/2026
 
diff --git a/src/weecfg/extension.py b/src/weecfg/extension.py
index 53e1e23d..d0ce89aa 100644
--- a/src/weecfg/extension.py
+++ b/src/weecfg/extension.py
@@ -282,7 +282,25 @@ class ExtensionEngine:
                     os.makedirs(os.path.dirname(destination_path))
                 except OSError:
                     pass
-                shutil.copy(source_path, destination_path)
+                # Copy to a temporary file, then rename it into place, so
+                # that the destination is replaced atomically. Overwriting
+                # it in place would truncate it under a running weewxd,
+                # which may have the old copy open or memory-mapped (a
+                # mapped file, such as a .bsp ephemeris, kills the process
+                # with SIGBUS when truncated underneath it).
+                tmp_fd, tmp_path = tempfile.mkstemp(
+                    dir=os.path.dirname(destination_path),
+                    prefix=os.path.basename(destination_path) + '.tmp')
+                os.close(tmp_fd)
+                try:
+                    shutil.copy(source_path, tmp_path)
+                    os.replace(tmp_path, destination_path)
+                except BaseException:
+                    try:
+                        os.unlink(tmp_path)
+                    except OSError:
+                        pass
+                    raise
             N += 1
 
         if self.dry_run:
diff --git a/src/weecfg/tests/test_config.py b/src/weecfg/tests/test_config.py
index 8582d0c0..6d68312a 100644
--- a/src/weecfg/tests/test_config.py
+++ b/src/weecfg/tests/test_config.py
@@ -450,6 +450,35 @@ class TestExtensionInstall:
         # It should be the same as our original:
         assert test_dict == self.config_dict
 
+    @pytest.mark.skipif(sys.platform == 'win32',
+                        reason="st_ino and rename-over-open-file semantics are POSIX")
+    def test_reinstall_replaces_files_atomically(self):
+        """Installing over an existing extension must replace each file by
+        renaming a new file into place, never by overwriting it in place: a
+        running weewxd may hold the old file open or memory-mapped (a mapped
+        file, such as a .bsp ephemeris, kills the process with SIGBUS when
+        truncated underneath it)."""
+        self.engine.install_extension('./pmon.tgz', no_confirm=True)
+        installed_path = os.path.join(self.engine.root_dict['USER_DIR'], 'pmon.py')
+        with open(installed_path, 'rb') as held:
+            original = held.read()
+            ino_before = os.fstat(held.fileno()).st_ino
+            # Install again while the file is held open, as a running
+            # weewxd would hold it:
+            self.engine.install_extension('./pmon.tgz', no_confirm=True)
+            # The held file must be untouched -- same inode, full content...
+            assert os.fstat(held.fileno()).st_ino == ino_before
+            held.seek(0)
+            assert held.read() == original
+        # ... while the destination path is a new file, swapped in whole:
+        assert os.stat(installed_path).st_ino != ino_before
+        with open(installed_path, 'rb') as f:
+            assert f.read() == original
+        # No temporary files were left behind:
+        leftovers = [f for f in os.listdir(os.path.dirname(installed_path))
+                     if '.tmp' in f]
+        assert leftovers == []
+
 
 # ############# Utilities #################