diff --git a/src/anthropic/lib/tools/_beta_builtin_memory_tool.py b/src/anthropic/lib/tools/_beta_builtin_memory_tool.py
index dd37de615..63ce8f366 100644
--- a/src/anthropic/lib/tools/_beta_builtin_memory_tool.py
+++ b/src/anthropic/lib/tools/_beta_builtin_memory_tool.py
@@ -395,7 +395,10 @@ def __init__(self, base_path: str = "./memory"):
 
     def _validate_path(self, path: str) -> Path:
         """Validate and resolve memory paths"""
-        if not path.startswith("/memories"):
+        # The separator matters: without it "/memoriesX" passes the prefix check and
+        # then slices down to "X", so a path that names something outside the store
+        # is silently re-pointed at <root>/X.
+        if path != "/memories" and not path.startswith("/memories/"):
             raise ToolError(f"Path must start with /memories, got: {path}")
 
         relative_path = path[len("/memories") :].lstrip("/")
@@ -681,7 +684,10 @@ async def _ensure_memory_root(self) -> None:
 
     async def _validate_path(self, path: str) -> AsyncPath:
         """Validate and resolve memory paths"""
-        if not path.startswith("/memories"):
+        # The separator matters: without it "/memoriesX" passes the prefix check and
+        # then slices down to "X", so a path that names something outside the store
+        # is silently re-pointed at <root>/X.
+        if path != "/memories" and not path.startswith("/memories/"):
             raise ToolError(f"Path must start with /memories, got: {path}")
 
         relative_path = path[len("/memories") :].lstrip("/")
diff --git a/tests/lib/tools/memory_tools/test_filesystem.py b/tests/lib/tools/memory_tools/test_filesystem.py
index 8462ffd7a..a9d4a070c 100644
--- a/tests/lib/tools/memory_tools/test_filesystem.py
+++ b/tests/lib/tools/memory_tools/test_filesystem.py
@@ -449,6 +449,27 @@ def test_delete_not_allow_deleting_memories_directory(
         with pytest.raises(ToolError, match="Cannot delete the /memories directory itself"):
             sync_local_filesystem_tool.delete(BetaMemoryTool20250818DeleteCommand(command="delete", path="/memories"))
 
+    @pytest.mark.parametrize("path", ["/memoriesX", "/memoriesXY/z.md", "/memoriessub", "/memories."])
+    def test_paths_sharing_the_prefix_without_a_separator_are_rejected(
+        self, sync_local_filesystem_tool: BetaLocalFilesystemMemoryTool, temp_directory: str, path: str
+    ) -> None:
+        """Paths that share the prefix without a separator name nothing inside the
+        store. Unchecked, they slice down to a relative path and land on a real
+        entry: delete "/memoriessub" removes <root>/sub and reports success."""
+        sync_local_filesystem_tool.create(
+            BetaMemoryTool20250818CreateCommand(command="create", file_text="keep me", path="/memories/sub/a.txt")
+        )
+
+        with pytest.raises(ToolError, match="Path must start with /memories"):
+            sync_local_filesystem_tool.delete(BetaMemoryTool20250818DeleteCommand(command="delete", path=path))
+
+        with pytest.raises(ToolError, match="Path must start with /memories"):
+            sync_local_filesystem_tool.create(
+                BetaMemoryTool20250818CreateCommand(command="create", file_text="written", path=path)
+            )
+
+        assert get_directory_snapshot(temp_directory) == {"memories/sub/a.txt": "keep me"}
+
     def test_rename(self, sync_local_filesystem_tool: BetaLocalFilesystemMemoryTool) -> None:
         sync_local_filesystem_tool.create(
             BetaMemoryTool20250818CreateCommand(
@@ -988,6 +1009,27 @@ async def test_delete_not_allow_deleting_memories_directory(
                 BetaMemoryTool20250818DeleteCommand(command="delete", path="/memories")
             )
 
+    @pytest.mark.parametrize("path", ["/memoriesX", "/memoriesXY/z.md", "/memoriessub", "/memories."])
+    async def test_paths_sharing_the_prefix_without_a_separator_are_rejected(
+        self, async_local_filesystem_tool: BetaAsyncLocalFilesystemMemoryTool, temp_directory: str, path: str
+    ) -> None:
+        """Paths that share the prefix without a separator name nothing inside the
+        store. Unchecked, they slice down to a relative path and land on a real
+        entry: delete "/memoriessub" removes <root>/sub and reports success."""
+        await async_local_filesystem_tool.create(
+            BetaMemoryTool20250818CreateCommand(command="create", file_text="keep me", path="/memories/sub/a.txt")
+        )
+
+        with pytest.raises(ToolError, match="Path must start with /memories"):
+            await async_local_filesystem_tool.delete(BetaMemoryTool20250818DeleteCommand(command="delete", path=path))
+
+        with pytest.raises(ToolError, match="Path must start with /memories"):
+            await async_local_filesystem_tool.create(
+                BetaMemoryTool20250818CreateCommand(command="create", file_text="written", path=path)
+            )
+
+        assert get_directory_snapshot(temp_directory) == {"memories/sub/a.txt": "keep me"}
+
     async def test_rename(self, async_local_filesystem_tool: BetaAsyncLocalFilesystemMemoryTool) -> None:
         await async_local_filesystem_tool.create(
             BetaMemoryTool20250818CreateCommand(