Fixes workspace member detection to properly handle shared path prefixes

docs/history/hatch.md

@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 - Fixes hatch shell type error for keep_env. 
 - SBOM documentation for including SBOM files in `sdist`
+- Fixes workspace member detection to properly handle shared path prefixes.
 
 ## [1.16.3](https://github.com/pypa/hatch/releases/tag/hatch-v1.16.3) - 2026-01-20 ## {: #hatch-v1.16.3 }
 

src/hatch/env/plugin/interface.py

@@ -1238,7 +1238,7 @@ def members(self) -> list[WorkspaceMember]:
             path_spec = data["path"]
             normalized_path = os.path.normpath(os.path.join(root, path_spec))
             absolute_path = os.path.abspath(normalized_path)
-            shared_prefix = os.path.commonprefix([root, absolute_path])
+            shared_prefix = os.path.commonpath([root, absolute_path])
             relative_path = os.path.relpath(absolute_path, shared_prefix)
 
             # Now we have the necessary information to perform an optimized glob search for members

tests/env/plugin/test_interface.py

@@ -3221,6 +3221,59 @@ def test_correct(self, hatch, temp_dir, isolated_data_dir, platform, global_appl
         assert members[1].project.location == member2_path
         assert members[2].project.location == member3_path
 
+    def test_member_outside_root_with_shared_prefix(self, temp_dir, isolated_data_dir, platform, global_application):
+        """Verify correct workspace member discovery with shared path prefix.
+
+        os.path.commonprefix works character-by-character, so for paths that
+        share a partial directory name (e.g. 'local_app' and 'lib_member' both
+        start with 'l'), it would return an invalid path like '.../l' instead
+        of the true common ancestor directory. os.path.commonpath correctly
+        returns the nearest common directory, which is what we need as the
+        base for the member glob search.
+
+        Example of the mismatch:
+            os.path.commonprefix(['/usr/lib', '/usr/local/lib']) == '/usr/l'
+            os.path.commonpath(['/usr/lib', '/usr/local/lib'])   == '/usr'
+        """
+        project_root = temp_dir / "local_app"
+        project_root.mkdir()
+
+        member_path = temp_dir / "lib_member"
+        member_path.mkdir()
+        (member_path / "pyproject.toml").write_text(
+            """\
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[project]
+name = "lib-member"
+version = "0.1.0"
+"""
+        )
+
+        config = {
+            "project": {"name": "my_app", "version": "0.0.1"},
+            "tool": {"hatch": {"envs": {"default": {"workspace": {"members": [{"path": "../lib_member"}]}}}}},
+        }
+        project = Project(project_root, config=config)
+        environment = MockEnvironment(
+            project_root,
+            project.metadata,
+            "default",
+            project.config.envs["default"],
+            {},
+            isolated_data_dir,
+            isolated_data_dir,
+            platform,
+            0,
+            global_application,
+        )
+
+        members = environment.workspace.members
+        assert len(members) == 1
+        assert members[0].project.location == member_path
+
 
 class TestWorkspaceDependencies:
     def test_basic(self, temp_dir, isolated_data_dir, platform, global_application):