diff --git a/devagent/tools/search_tools.py b/devagent/tools/search_tools.py
index aff6858..17a9279 100644
--- a/devagent/tools/search_tools.py
+++ b/devagent/tools/search_tools.py
@@ -42,7 +42,10 @@ def find_files(args: dict) -> str:
 
         try:
             search_root = (root / path).resolve()
-            if not str(search_root).startswith(str(root)):
+            # A string prefix is not a containment test: `/srv/project-secrets`
+            # starts with `/srv/project` without being inside it. Same rule as
+            # `file_tools._within_roots`.
+            if not search_root.is_relative_to(root):
                 return "[error] path escapes project root"
         except Exception as exc:
             return f"[error] {exc}"
@@ -74,7 +77,11 @@ def find_files(args: dict) -> str:
                 "glob": {"type": "string", "description": "File glob filter, e.g. '*.py'"},
                 "case_sensitive": {"type": "boolean", "default": True},
                 "max_results": {"type": "integer", "default": 50},
-                "context_lines": {"type": "integer", "default": 0, "description": "Lines of context around each match"},
+                "context_lines": {
+                    "type": "integer",
+                    "default": 0,
+                    "description": "Lines of context around each match",
+                },
             },
             "required": ["pattern"],
         },
@@ -88,7 +95,11 @@ def find_files(args: dict) -> str:
             "type": "object",
             "properties": {
                 "pattern": {"type": "string", "description": "Glob pattern, e.g. '*.ts'"},
-                "path": {"type": "string", "description": "Sub-directory to search in", "default": "."},
+                "path": {
+                    "type": "string",
+                    "description": "Sub-directory to search in",
+                    "default": ".",
+                },
                 "max_results": {"type": "integer", "default": 100},
             },
         },
@@ -164,10 +175,7 @@ def _python_grep(
                 if context:
                     start = max(0, i - 1 - context)
                     end = min(len(lines), i + context)
-                    block = [
-                        f"{rel}:{j + 1}: {lines[j]}"
-                        for j in range(start, end)
-                    ]
+                    block = [f"{rel}:{j + 1}: {lines[j]}" for j in range(start, end)]
                     results.extend(block)
                     results.append("--")
                 else:
diff --git a/tests/test_search_tool_containment.py b/tests/test_search_tool_containment.py
new file mode 100644
index 0000000..79b3732
--- /dev/null
+++ b/tests/test_search_tool_containment.py
@@ -0,0 +1,45 @@
+"""find_files must not admit a sibling whose name starts with the root's."""
+
+from __future__ import annotations
+
+from devagent.tools.registry import ToolRegistry
+from devagent.tools.search_tools import register_search_tools
+
+
+def _registry(project_root) -> ToolRegistry:
+    registry = ToolRegistry()
+    register_search_tools(registry, str(project_root))
+    return registry
+
+
+def test_find_files_rejects_a_sibling_sharing_the_name_prefix(tmp_path) -> None:
+    project = tmp_path / "proj"
+    project.mkdir()
+    sibling = tmp_path / "proj-private"
+    sibling.mkdir()
+    (sibling / "secret.txt").write_text("nope", encoding="utf-8")
+
+    result = _registry(project).call("find_files", {"path": "../proj-private", "pattern": "*.txt"})
+
+    assert result == "[error] path escapes project root"
+    assert "secret.txt" not in result
+
+
+def test_find_files_still_lists_files_inside_the_project(tmp_path) -> None:
+    project = tmp_path / "proj"
+    project.mkdir()
+    (project / "notes.txt").write_text("hello", encoding="utf-8")
+
+    result = _registry(project).call("find_files", {"pattern": "*.txt"})
+
+    assert result == "notes.txt"
+
+
+def test_find_files_rejects_plain_traversal(tmp_path) -> None:
+    project = tmp_path / "proj"
+    project.mkdir()
+    (tmp_path / "outside.txt").write_text("nope", encoding="utf-8")
+
+    result = _registry(project).call("find_files", {"path": "..", "pattern": "*.txt"})
+
+    assert result == "[error] path escapes project root"