diff --git a/src/agent_loop.py b/src/agent_loop.py
index efc6b4e05..f3609b41b 100644
--- a/src/agent_loop.py
+++ b/src/agent_loop.py
@@ -16711,16 +16711,15 @@ def _existing_workspace_files(paths: list[str], workspace: Optional[str]) -> lis
     """
     if not workspace:
         return []
-    root = Path(str(workspace)).expanduser()
+    from src.tool_execution import _resolve_tool_path_in_workspace
+
     existing: list[str] = []
     for path in paths:
-        candidate = Path(path).expanduser()
-        if not candidate.is_absolute():
-            candidate = root / candidate
         try:
-            if candidate.is_file():
+            resolved = _resolve_tool_path_in_workspace(str(workspace), str(path))
+            if os.path.isfile(resolved):
                 existing.append(path)
-        except OSError:
+        except (OSError, ValueError):
             continue
     return existing
 
diff --git a/tests/test_existing_workspace_files_confinement.py b/tests/test_existing_workspace_files_confinement.py
new file mode 100644
index 000000000..5b528a665
--- /dev/null
+++ b/tests/test_existing_workspace_files_confinement.py
@@ -0,0 +1,76 @@
+from src.agent_loop import _existing_workspace_files
+
+
+def test_existing_workspace_files_accepts_relative_file_inside_workspace(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    target = workspace / "inside.py"
+    target.write_text("pass\n")
+
+    assert _existing_workspace_files(
+        ["inside.py"],
+        str(workspace),
+    ) == ["inside.py"]
+
+
+def test_existing_workspace_files_accepts_absolute_file_inside_workspace(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    target = workspace / "inside.py"
+    target.write_text("pass\n")
+
+    assert _existing_workspace_files(
+        [str(target)],
+        str(workspace),
+    ) == [str(target)]
+
+
+def test_existing_workspace_files_rejects_absolute_file_outside_workspace(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    outside = tmp_path / "outside.py"
+    outside.write_text("pass\n")
+
+    assert _existing_workspace_files(
+        [str(outside)],
+        str(workspace),
+    ) == []
+
+
+def test_existing_workspace_files_rejects_parent_traversal(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    outside = tmp_path / "outside.py"
+    outside.write_text("pass\n")
+
+    assert _existing_workspace_files(
+        ["../outside.py"],
+        str(workspace),
+    ) == []
+
+
+def test_existing_workspace_files_rejects_symlink_escape(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    outside = tmp_path / "outside.py"
+    outside.write_text("pass\n")
+
+    escape = workspace / "escape.py"
+    escape.symlink_to(outside)
+
+    assert _existing_workspace_files(
+        ["escape.py"],
+        str(workspace),
+    ) == []
+
+
+def test_existing_workspace_files_supports_workspace_alias(tmp_path):
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    target = workspace / "inside.py"
+    target.write_text("pass\n")
+
+    assert _existing_workspace_files(
+        ["/workspace/inside.py"],
+        str(workspace),
+    ) == ["/workspace/inside.py"]