diff --git a/nanobot/agent/tools/anthropic/memory.py b/nanobot/agent/tools/anthropic/memory.py
new file mode 100644
index 0000000..ce59c34
--- /dev/null
+++ b/nanobot/agent/tools/anthropic/memory.py
@@ -0,0 +1,122 @@
+"""MemoryTool20250818 - Anthropic's native memory tool.
+
+Enables Claude to create, read, update, and delete files in a persistent
+/memories directory across conversations.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any, Literal
+
+from nanobot.agent.tools.anthropic.base import BaseAnthropicTool, CLIResult
+
+
+class MemoryTool20250818(BaseAnthropicTool):
+    """Anthropic's native memory_20250818 tool.
+
+    Client-side tool for persistent memory storage across conversations.
+    All operations are restricted to the /memories directory.
+
+    Commands:
+        - view: Show directory contents or file contents with line numbers
+        - create: Create a new file with content
+        - str_replace: Replace unique text occurrence in a file
+        - insert: Insert text at a specific line number
+        - delete: Delete a file or directory
+        - rename: Rename or move a file/directory
+    """
+
+    api_type: Literal["memory_20250818"] = "memory_20250818"
+    name: Literal["memory"] = "memory"
+    beta_flag: str = "context-management-2025-06-27"
+
+    def __init__(self, workspace: Path):
+        """Initialize Memory tool.
+
+        Args:
+            workspace: Root workspace directory
+        """
+        self.workspace = workspace
+        self.memories_dir = workspace / "memories"
+        self.memories_dir.mkdir(parents=True, exist_ok=True)
+
+    def _validate_memory_path(self, path: str) -> Path:
+        """Validate and resolve path to prevent directory traversal.
+
+        Args:
+            path: Path string starting with /memories
+
+        Returns:
+            Validated absolute Path within memories directory
+
+        Raises:
+            ValueError: If path is invalid or escapes /memories directory
+        """
+        # Reject paths not starting with /memories
+        if not path.startswith("/memories"):
+            raise ValueError(f"Path must start with /memories, got: {path}")
+
+        # Resolve to absolute path within workspace
+        # lstrip("/") removes leading slash: "/memories/file.txt" -> "memories/file.txt"
+        relative_path = path.lstrip("/")
+        full_path = (self.workspace / relative_path).resolve()
+
+        # Verify resolved path is within memories directory
+        memories_dir_resolved = self.memories_dir.resolve()
+        try:
+            full_path.relative_to(memories_dir_resolved)
+        except ValueError:
+            raise ValueError(f"Path escapes /memories directory: {path}")
+
+        return full_path
+
+    async def __call__(
+        self,
+        command: Literal["view", "create", "str_replace", "insert", "delete", "rename"],
+        path: str | None = None,
+        old_path: str | None = None,
+        new_path: str | None = None,
+        file_text: str | None = None,
+        old_str: str | None = None,
+        new_str: str | None = None,
+        insert_line: int | None = None,
+        insert_text: str | None = None,
+        view_range: list[int] | None = None,
+        **kwargs: Any,
+    ) -> CLIResult:
+        """Execute memory command.
+
+        Args:
+            command: Command to execute
+            path: File/directory path (for view/create/str_replace/insert/delete)
+            old_path: Source path (for rename)
+            new_path: Destination path (for rename)
+            file_text: File content (for create)
+            old_str: Text to find (for str_replace)
+            new_str: Replacement text (for str_replace)
+            insert_line: Line number to insert at (for insert)
+            insert_text: Text to insert (for insert)
+            view_range: [start_line, end_line] for view
+            **kwargs: Additional arguments (ignored)
+
+        Returns:
+            CLIResult with command output or error
+        """
+        # Placeholder - will implement in later tasks
+        return CLIResult(
+            exit_code=1,
+            output="",
+            error="Not implemented yet"
+        )
+
+    def to_params(self) -> dict[str, Any]:
+        """Convert to Anthropic API tool parameter format.
+
+        Returns:
+            Tool definition for Anthropic API
+        """
+        return {
+            "type": self.api_type,
+            "name": self.name,
+        }
diff --git a/tests/test_memory_security.py b/tests/test_memory_security.py
new file mode 100644
index 0000000..0ff7c51
--- /dev/null
+++ b/tests/test_memory_security.py
@@ -0,0 +1,70 @@
+"""Security tests for MemoryTool20250818."""
+
+import pytest
+from pathlib import Path
+from nanobot.agent.tools.anthropic import MemoryTool20250818
+
+
+@pytest.fixture
+def temp_workspace(tmp_path):
+    """Create temporary workspace."""
+    return tmp_path
+
+
+@pytest.fixture
+def memory_tool(temp_workspace):
+    """Create MemoryTool instance."""
+    return MemoryTool20250818(workspace=temp_workspace)
+
+
+class TestPathSecurity:
+    """Test path validation security."""
+
+    def test_validate_path_valid_root(self, memory_tool):
+        """Test that /memories is valid."""
+        result = memory_tool._validate_memory_path("/memories")
+        assert result == memory_tool.memories_dir
+
+    def test_validate_path_valid_file(self, memory_tool):
+        """Test that /memories/notes.txt is valid."""
+        result = memory_tool._validate_memory_path("/memories/notes.txt")
+        assert result == memory_tool.memories_dir / "notes.txt"
+
+    def test_validate_path_valid_nested(self, memory_tool):
+        """Test that /memories/project/status.xml is valid."""
+        result = memory_tool._validate_memory_path("/memories/project/status.xml")
+        assert result == memory_tool.memories_dir / "project" / "status.xml"
+
+    def test_validate_path_rejects_parent_traversal(self, memory_tool):
+        """Test that ../ is rejected."""
+        with pytest.raises(ValueError, match="escapes /memories directory"):
+            memory_tool._validate_memory_path("/memories/../config.json")
+
+    def test_validate_path_rejects_double_parent_traversal(self, memory_tool):
+        """Test that ../../ is rejected."""
+        with pytest.raises(ValueError, match="escapes /memories directory"):
+            memory_tool._validate_memory_path("/memories/../../etc/passwd")
+
+    def test_validate_path_rejects_absolute_path(self, memory_tool):
+        """Test that absolute paths are rejected."""
+        with pytest.raises(ValueError, match="must start with /memories"):
+            memory_tool._validate_memory_path("/etc/passwd")
+
+    def test_validate_path_rejects_workspace_path(self, memory_tool):
+        """Test that /workspace paths are rejected."""
+        with pytest.raises(ValueError, match="must start with /memories"):
+            memory_tool._validate_memory_path("/workspace/data.txt")
+
+    def test_validate_path_rejects_relative_path(self, memory_tool):
+        """Test that relative paths are rejected."""
+        with pytest.raises(ValueError, match="must start with /memories"):
+            memory_tool._validate_memory_path("notes.txt")
+
+    def test_validate_path_url_encoded_is_safe(self, memory_tool):
+        """Test that URL-encoded paths are safe (not decoded by pathlib)."""
+        # Python's pathlib treats %2e%2e as literal characters, not as ..
+        # So this is actually safe - it creates a subdirectory named "%2e%2e"
+        attack_path = "/memories/%2e%2e/config.json"
+        result = memory_tool._validate_memory_path(attack_path)
+        # This should resolve to memories/%2e%2e/config.json (literal characters)
+        assert result == memory_tool.memories_dir / "%2e%2e" / "config.json"