fix: accept case-aliased worktree repository paths (#3837)

apps/backend/internal/agent/runtime/lifecycle/env_preparer_local.go

@@ -218,7 +218,16 @@ func validateLocalRepositoryWorkspace(ctx context.Context, workspacePath, reposi
 	if err != nil {
 		return err
 	}
-	if workspaceCommonDir != repositoryCommonDir {
+	workspaceCommonInfo, err := os.Stat(workspaceCommonDir)
+	if err != nil {
+		return worktree.ErrReuseWorktreeUnavailable
+	}
+	repositoryCommonInfo, err := os.Stat(repositoryCommonDir)
+	if err != nil {
+		return worktree.ErrReuseWorktreeUnavailable
+	}
+	if !workspaceCommonInfo.IsDir() || !repositoryCommonInfo.IsDir() ||
+		!os.SameFile(workspaceCommonInfo, repositoryCommonInfo) {
 		return worktree.ErrReuseWorktreeUnavailable
 	}
 	return nil
@@ -243,9 +252,9 @@ func localGitTopLevel(ctx context.Context, path string) (string, error) {
 }
 
 // localGitCommonDir returns the canonical Git directory shared by a checkout
-// and all of its linked worktrees. Comparing this value, instead of the
-// worktree top-level paths, proves that a linked worktree belongs to the
-// selected repository while still rejecting an unrelated Git checkout.
+// and all of its linked worktrees. Its filesystem identity, rather than its
+// path spelling, proves that a linked worktree belongs to the selected
+// repository while still rejecting an unrelated Git checkout.
 func localGitCommonDir(ctx context.Context, path string) (string, error) {
 	info, err := os.Stat(path)
 	if err != nil || !info.IsDir() {

apps/backend/internal/agent/runtime/lifecycle/env_preparer_local_test.go

@@ -78,6 +78,27 @@ func TestLocalPreparer_ReuseRequiredValidatesRepositoryIdentity(t *testing.T) {
 	}
 }
 
+// @covers AC-TASKS-ADDITIONAL-SESSION-WORKSPACE-REUSE-002.1
+func TestValidateLocalRepositoryWorkspaceAcceptsCaseAliasedCommonDirectory(t *testing.T) {
+	parent := t.TempDir()
+	repositoryPath := filepath.Join(parent, "CaseIdentity")
+	if err := os.Mkdir(repositoryPath, 0o755); err != nil {
+		t.Fatalf("create mixed-case repository directory: %v", err)
+	}
+	initGitRepoAt(t, repositoryPath)
+	caseAlias := filepath.Join(parent, "caseidentity")
+	if _, err := os.Stat(caseAlias); err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			t.Skip("filesystem is case-sensitive")
+		}
+		t.Fatalf("inspect case-aliased repository path: %v", err)
+	}
+
+	if err := validateLocalRepositoryWorkspace(context.Background(), repositoryPath, caseAlias); err != nil {
+		t.Fatalf("validateLocalRepositoryWorkspace() rejected the same repository through a case alias: %v", err)
+	}
+}
+
 func TestLocalPreparer_RejectsRepoBackedNonGitWorkspace(t *testing.T) {
 	workspacePath := t.TempDir()
 	preparer := NewLocalPreparer(newTestLocalLogger())
@@ -116,6 +137,11 @@ func TestLocalPreparer_AcceptsMatchingRepoBackedGitWorkspace(t *testing.T) {
 func initGitRepo(t *testing.T) string {
 	t.Helper()
 	dir := t.TempDir()
+	return initGitRepoAt(t, dir)
+}
+
+func initGitRepoAt(t *testing.T, dir string) string {
+	t.Helper()
 	// Start from a clean env: filter all GIT_* vars that may leak from parent
 	// processes (e.g. pre-commit hooks set GIT_DIR, GIT_WORK_TREE, GIT_INDEX_FILE).
 	gitEnv := filterLocalTestGitEnv(os.Environ())