diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 7c0b937..8d77ebb 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -108,7 +108,10 @@ and CLI publication tests. Use the offline environment specified in Stage 10.
 
 ## Stage 2: Add optional checkout-root discovery with precise failure handling
 
-**Status:** pending.
+**Status:** complete. `go test ./internal/git` passes offline, including real
+checkout, worktree, alias, standalone, missing-Git, diagnostic, and failure
+cases. The public Go-only preparation composition check is deferred to Stage 3,
+where preparation adopts discovery.
 
 **Files:** `internal/git/source.go` or a focused companion file, Git tests, and
 `internal/process/process.go` only if an existing invocation capability proves
diff --git a/internal/git/discovery_test.go b/internal/git/discovery_test.go
new file mode 100644
index 0000000..ec986c3
--- /dev/null
+++ b/internal/git/discovery_test.go
@@ -0,0 +1,111 @@
+package git
+
+import (
+	"context"
+	"errors"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDiscoverRepositoryRoot(t *testing.T) {
+	repo := newRemoteFixture(t)
+	child := filepath.Join(repo.work, "child")
+	if err := os.Mkdir(child, 0755); err != nil {
+		t.Fatal(err)
+	}
+	alias := filepath.Join(t.TempDir(), "alias")
+	if err := os.Symlink(repo.work, alias); err != nil {
+		t.Fatal(err)
+	}
+	worktree := filepath.Join(t.TempDir(), "worktree")
+	gitRun(t, repo.work, "worktree", "add", "--detach", worktree)
+	for _, tc := range []struct{ dir, want string }{{repo.work, repo.work}, {child, repo.work}, {alias, repo.work}, {worktree, worktree}} {
+		root, found, err := DiscoverRepositoryRoot(context.Background(), tc.dir)
+		want, e := filepath.EvalSymlinks(tc.want)
+		if e != nil {
+			t.Fatal(e)
+		}
+		if err != nil || !found || root != want {
+			t.Fatalf("Discover(%s) = %q, %t, %v; want %q", tc.dir, root, found, err, want)
+		}
+	}
+	standalone := t.TempDir()
+	if root, found, err := DiscoverRepositoryRoot(context.Background(), standalone); err != nil || found || root != "" {
+		t.Fatalf("standalone = %q, %t, %v", root, found, err)
+	}
+	if _, err := RepositoryRoot(context.Background(), standalone); err == nil {
+		t.Fatal("strict lookup accepted standalone")
+	}
+	broken := t.TempDir()
+	if err := os.WriteFile(filepath.Join(broken, ".git"), []byte("gitdir: missing\n"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), broken); err == nil || found {
+		t.Fatalf("broken marker: found %t, error %v", found, err)
+	}
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), filepath.Join(standalone, "missing")); err == nil || found {
+		t.Fatalf("missing directory: found %t, error %v", found, err)
+	}
+}
+
+func TestNoRepositoryDiagnostic(t *testing.T) {
+	for _, tc := range []struct {
+		message string
+		want    bool
+	}{
+		{"fatal: not a git repository (or any of the parent directories): .git\n", true},
+		{"fatal: not a git repository (or any parent up to mount point /some path)\nStopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).\n", true},
+		{"fatal: not a git repository: '/bad'", false},
+		{"fatal: detected dubious ownership in repository", false},
+	} {
+		if got := noRepositoryDiagnostic(tc.message); got != tc.want {
+			t.Errorf("diagnostic %q = %t", tc.message, got)
+		}
+	}
+}
+
+func TestDiscoverRepositoryRootFailures(t *testing.T) {
+	dir := t.TempDir()
+	bin := t.TempDir()
+	script := "#!/bin/sh\nprintf '%s\\n' 'fatal: detected dubious ownership in repository' >&2\nexit 128\n"
+	if err := os.WriteFile(filepath.Join(bin, "git"), []byte(script), 0755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin)
+	_, found, err := DiscoverRepositoryRoot(context.Background(), dir)
+	var exitErr *exec.ExitError
+	if found || !errors.As(err, &exitErr) || exitErr.ExitCode() != 128 || !strings.Contains(err.Error(), "dubious ownership") {
+		t.Fatalf("found %t, error %v", found, err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if _, _, err := DiscoverRepositoryRoot(ctx, dir); !errors.Is(err, context.Canceled) {
+		t.Fatalf("cancellation = %v", err)
+	}
+}
+
+func TestDiscoverRepositoryRootWithoutGit(t *testing.T) {
+	repo := newRemoteFixture(t)
+	standalone := t.TempDir()
+	t.Setenv("PATH", t.TempDir())
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), standalone); found || err != nil {
+		t.Fatalf("standalone: found %t, error %v", found, err)
+	}
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), repo.work); found || !errors.Is(err, exec.ErrNotFound) {
+		t.Fatalf("checkout: found %t, error %v", found, err)
+	}
+	broken := t.TempDir()
+	if err := os.WriteFile(filepath.Join(broken, ".git"), []byte("broken"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), broken); found || !errors.Is(err, exec.ErrNotFound) {
+		t.Fatalf("broken marker: found %t, error %v", found, err)
+	}
+	t.Setenv("GIT_DIR", "invalid")
+	if _, found, err := DiscoverRepositoryRoot(context.Background(), standalone); found || !errors.Is(err, exec.ErrNotFound) {
+		t.Fatalf("explicit context: found %t, error %v", found, err)
+	}
+}
diff --git a/internal/git/source.go b/internal/git/source.go
index 7cbb44f..14d2362 100644
--- a/internal/git/source.go
+++ b/internal/git/source.go
@@ -3,8 +3,11 @@ package git
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	"os"
+	"os/exec"
 	"path"
 	"path/filepath"
 	"strconv"
@@ -41,6 +44,10 @@ func RepositoryRoot(ctx context.Context, directory string) (string, error) {
 	if err != nil {
 		return "", fmt.Errorf("resolve Git repository root: %w", err)
 	}
+	return normalizeRepositoryRoot(root)
+}
+
+func normalizeRepositoryRoot(root string) (string, error) {
 	root = strings.TrimSuffix(root, "\n")
 	resolved, err := filepath.EvalSymlinks(root)
 	if err != nil {
@@ -49,6 +56,84 @@ func RepositoryRoot(ctx context.Context, directory string) (string, error) {
 	return resolved, nil
 }
 
+// DiscoverRepositoryRoot finds a containing checkout or confirms standalone use.
+func DiscoverRepositoryRoot(ctx context.Context, directory string) (string, bool, error) {
+	if directory == "" {
+		directory = "."
+	}
+	var stdout, stderr strings.Builder
+	err := process.Run(ctx, process.Invocation{
+		Args: []string{"git", "rev-parse", "--show-toplevel"}, Dir: directory,
+		Env: map[string]string{"LC_ALL": "C"}, Stdout: &stdout, Stderr: &stderr,
+	})
+	if err == nil {
+		root, err := normalizeRepositoryRoot(stdout.String())
+		return root, err == nil, err
+	}
+	if errors.Is(err, exec.ErrNotFound) {
+		standalone, inspectErr := standaloneWithoutGit(directory)
+		if inspectErr != nil {
+			return "", false, inspectErr
+		}
+		if standalone {
+			return "", false, nil
+		}
+	}
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) && exitErr.ExitCode() == 128 && noRepositoryDiagnostic(stderr.String()) {
+		return "", false, nil
+	}
+	if message := strings.TrimSpace(stderr.String()); message != "" {
+		return "", false, fmt.Errorf("resolve Git repository root: %s: %w", message, err)
+	}
+	return "", false, fmt.Errorf("resolve Git repository root: %w", err)
+}
+
+func noRepositoryDiagnostic(message string) bool {
+	message = strings.TrimSpace(message)
+	if message == "fatal: not a git repository (or any of the parent directories): .git" {
+		return true
+	}
+	const prefix = "fatal: not a git repository (or any parent up to mount point "
+	const suffix = ")\nStopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set)."
+	return strings.HasPrefix(message, prefix) && strings.HasSuffix(message, suffix) && len(message) > len(prefix)+len(suffix)
+}
+
+func standaloneWithoutGit(directory string) (bool, error) {
+	_, gitDirSet := os.LookupEnv("GIT_DIR")
+	_, workTreeSet := os.LookupEnv("GIT_WORK_TREE")
+	if gitDirSet || workTreeSet {
+		return false, nil
+	}
+	abs, err := filepath.Abs(directory)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	physical, err := filepath.EvalSymlinks(abs)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	info, err := os.Stat(physical)
+	if err != nil {
+		return false, fmt.Errorf("inspect repository directory: %w", err)
+	}
+	if !info.IsDir() {
+		return false, fmt.Errorf("inspect repository directory %q: not a directory", directory)
+	}
+	for dir := physical; ; dir = filepath.Dir(dir) {
+		_, err := os.Lstat(filepath.Join(dir, ".git"))
+		if err == nil {
+			return false, nil
+		}
+		if !errors.Is(err, os.ErrNotExist) {
+			return false, fmt.Errorf("inspect Git marker in %s: %w", dir, err)
+		}
+		if filepath.Dir(dir) == dir {
+			return true, nil
+		}
+	}
+}
+
 // CaptureHead records the checkout commit before identity derived from files
 // in that commit is inspected.
 func CaptureHead(ctx context.Context, repositoryDir string) (string, error) {