diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index e5adc47..66a2162 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -21,7 +21,7 @@ No Makefile, no code generation, no external linter config. Standard Go toolchai
 
 - `cmd/`: One Cobra command per file. Each exports `<Name>Cmd(cfg *config.Config)` with logic in `run<Name>()`.
 - `internal/git/`: `Ops` interface wrapping git CLI. `MockOps` for tests. Use `ForWorktree(path)` for scoped execution, `CommonDir()` for shared storage, and `GitDir()` for native per-worktree state. Never use production `os.Chdir` or `SetOps` to switch context.
-- `internal/github/`: `ClientOps` interface (18 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`); merges use the async merge API (`/repos/{owner}/{repo}/pulls/{n}/merge-async`) with an explicit `merge_action` (`direct_merge` or `merge_queue`) chosen from the base branch's merge-queue detection. `merge_action` is optional — omitting it (or sending `default`) lets the server auto-route (merge queue if one is configured, else direct merge) — but the CLI sends it explicitly so a wrong detection fails loudly instead of silently merging directly.
+- `internal/github/`: `ClientOps` interface (19 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`); merges use the async merge API (`/repos/{owner}/{repo}/pulls/{n}/merge-async`) with an explicit `merge_action` (`direct_merge` or `merge_queue`) chosen from the base branch's merge-queue detection. `merge_action` is optional — omitting it (or sending `default`) lets the server auto-route (merge queue if one is configured, else direct merge) — but the CLI sends it explicitly so a wrong detection fails loudly instead of silently merging directly.
 - `internal/config/`: `Config` struct passed to all commands. Holds I/O, colors, and test hooks (`SelectFn`, `ConfirmFn`, `InputFn`, `GitHubClientOverride`).
 - `internal/stack/`: Shared catalog (`<common-dir>/gh-stack`, JSON), conservative legacy migration, atomic saves, and short catalog locks.
 - `internal/worktree/`: Origin/owner identities, cleanliness preflight, scoped operations, and touched-ref recovery.
diff --git a/AGENTS.md b/AGENTS.md
index 1edf0dd..12e7c38 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -37,7 +37,7 @@ internal/
     gitops.go                # Ops interface, including scoped worktree execution
     mock_ops.go              # MockOps. Each method has a corresponding *Fn field.
   github/                    # github.ClientOps interface + real Client
-    client_interface.go      # ClientOps interface (18 methods)
+    client_interface.go      # ClientOps interface (19 methods)
     mock_client.go           # MockClient. Uses function-pointer fields for testing.
   stack/                     # common-directory catalog, JSON schema, migration, locking
     schema.json              # JSON Schema for the stack file format
@@ -114,7 +114,7 @@ if errors.As(err, &exitErr) { ... }
 
 - **`git.Ops`** (`internal/git/gitops.go`): wraps git CLI calls. Package-level functions (e.g., `git.CurrentBranch()`) delegate to a swappable package-level `ops` variable. `git.ForWorktree(path)` returns an explicitly scoped executor for HEAD/index/working-file operations. Never switch production context with `os.Chdir` or `git.SetOps`; reserve `SetOps` for tests. `GitDir()` remains per-worktree; `CommonDir()` is repository-wide.
 - **Scoped Git errors:** `ForWorktree(path)` returns `(Ops, error)` and no executor for invalid contexts. Each scoped operation rechecks directory identity. `BranchExists`, `HasStagedChanges`, `IsRebaseInProgress`, and `IsCherryPickInProgress` return `(bool, error)`; callers must handle lookup failures before mutating Git or recovery state, not treat them as absence.
-- **`github.ClientOps`** (`internal/github/client_interface.go`): 18 methods for GitHub API (PRs, stacks, merges). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Async stack merges use `RepoMergeConfig` (GraphQL: allowed merge methods + viewer's default), `BaseBranchUsesMergeQueue` (GraphQL: detects a base-branch merge queue to select the explicit `merge_action`), `MergeStackAsync`, and `GetAsyncMergeResult` (`/repos/{owner}/{repo}/pulls/{n}/merge-async`). Injected via `cfg.GitHubClientOverride` in tests.
+- **`github.ClientOps`** (`internal/github/client_interface.go`): 19 methods for GitHub API (PRs, stacks, merges, repository default branch). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Async stack merges use `RepoMergeConfig` (GraphQL: allowed merge methods + viewer's default), `BaseBranchUsesMergeQueue` (GraphQL: detects a base-branch merge queue to select the explicit `merge_action`), `MergeStackAsync`, and `GetAsyncMergeResult` (`/repos/{owner}/{repo}/pulls/{n}/merge-async`). Injected via `cfg.GitHubClientOverride` in tests.
 - **`config.Config`** (`internal/config/config.go`): Central configuration passed to all commands. Holds I/O streams, color functions, and test hook fields (`SelectFn`, `ConfirmFn`, `InputFn`, `RepoOverride`).
 
 ### Stack file
diff --git a/cmd/add_test.go b/cmd/add_test.go
index 2c9b545..e192009 100644
--- a/cmd/add_test.go
+++ b/cmd/add_test.go
@@ -761,7 +761,7 @@ func TestAdd_InitializesStackWithExplicitBranch(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return "unstacked", nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn:    func(name string) (bool, error) { return name == "main" && trunkExists, nil },
 		RevParseFn: func(ref string) (string, error) {
@@ -815,7 +815,7 @@ func TestAdd_InitializesStackWithPromptedBranch(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return "main", nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		CreateBranchFn: func(name, base string) error {
 			createdBranch = name
@@ -856,7 +856,7 @@ func TestAdd_InitializesGeneratedBranchAndCommits(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
 		CurrentBranchFn:   func() (string, error) { return currentBranch, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn:    func(name string) (bool, error) { return name == "main", nil },
 		CreateBranchFn: func(name, base string) error {
diff --git a/cmd/init.go b/cmd/init.go
index fe895ee..a92143b 100644
--- a/cmd/init.go
+++ b/cmd/init.go
@@ -87,7 +87,7 @@ func runInit(cfg *config.Config, opts *initOptions) error {
 	}
 
 	if trunk == "" {
-		trunk, err = git.DefaultBranch()
+		trunk, err = resolveDefaultBranch(cfg, "")
 		if err != nil {
 			cfg.Errorf("unable to determine default branch\nUse -b to specify the trunk branch")
 			return ErrNotInStack
diff --git a/cmd/init_test.go b/cmd/init_test.go
index fe2785f..3ae2c91 100644
--- a/cmd/init_test.go
+++ b/cmd/init_test.go
@@ -102,7 +102,7 @@ func TestInit_CreatesStackWithCorrectTrunk(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 	})
 	defer restore()
@@ -151,7 +151,7 @@ func TestInit_RestoresMissingLocalTrunkWhenTagResolves(t *testing.T) {
 
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		CurrentBranchFn:   func() (string, error) { return "renamed-branch", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		BranchExistsFn: func(name string) (bool, error) {
@@ -205,7 +205,7 @@ func TestInit_AdoptExistingBranches(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -231,7 +231,7 @@ func TestInit_ExplicitBranchNamesUsedVerbatim(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -257,7 +257,7 @@ func TestInit_AdoptFlagShowsDeprecationWarning(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -276,7 +276,7 @@ func TestInit_RerereAlreadyEnabled(t *testing.T) {
 	enableRerereCalled := false
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:          func() (string, error) { return gitDir, nil },
-		DefaultBranchFn:   func() (string, error) { return "main", nil },
+		DefaultBranchFn:   func(string) (string, error) { return "main", nil },
 		CurrentBranchFn:   func() (string, error) { return "main", nil },
 		IsRerereEnabledFn: func() (bool, error) { return true, nil },
 		EnableRerereFn: func() error {
@@ -308,7 +308,7 @@ func TestInit_RefuseIfBranchAlreadyInStack(t *testing.T) {
 
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "feature-1", nil },
 	})
 	defer restore()
@@ -326,7 +326,7 @@ func TestInit_AdoptNonexistentBranch_CreatesIt(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return false, nil },
 		CreateBranchFn: func(name, base string) error {
@@ -350,7 +350,7 @@ func TestInit_MultipleBranches_CreatesAll(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -375,7 +375,7 @@ func TestInit_AdoptWithExistingOpenPR(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -425,7 +425,7 @@ func TestInit_AdoptIgnoresClosedAndMergedPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -466,7 +466,7 @@ func TestInit_ImplicitAdopt_AllExist(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -501,7 +501,7 @@ func TestInit_ImplicitAdopt_AllMissing(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -528,7 +528,7 @@ func TestInit_ImplicitAdopt_Mixed(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return existing[name], nil },
 		CreateBranchFn: func(name, base string) error {
@@ -556,7 +556,7 @@ func TestInit_WhatsNext_Fresh(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn:  func(name, base string) error { return nil },
 	})
@@ -581,7 +581,7 @@ func TestInit_WhatsNext_AdoptedWithPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -614,7 +614,7 @@ func TestInit_WhatsNext_AdoptedNoPRs(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(string) (bool, error) { return true, nil },
 	})
@@ -639,7 +639,7 @@ func TestInit_WhatsNext_MixedWithPR(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return name == "existing", nil },
 		CreateBranchFn:  func(name, base string) error { return nil },
@@ -670,7 +670,7 @@ func TestInit_Interactive_OnTrunk(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 	})
 	defer restore()
@@ -689,7 +689,7 @@ func TestInit_Interactive_OnFeatureBranch_UseCurrent(t *testing.T) {
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "feat/auth", nil },
 		BranchExistsFn:  func(name string) (bool, error) { return name == "feat/auth", nil },
 	})
@@ -732,7 +732,7 @@ func TestInit_TwoPassValidation_NoBranchCreatedOnError(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		CreateBranchFn: func(name, base string) error {
 			created = append(created, name)
@@ -756,7 +756,7 @@ func TestInit_TwoPassValidation_InvalidRefName(t *testing.T) {
 	var created []string
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
-		DefaultBranchFn: func() (string, error) { return "main", nil },
+		DefaultBranchFn: func(string) (string, error) { return "main", nil },
 		CurrentBranchFn: func() (string, error) { return "main", nil },
 		ValidateRefNameFn: func(name string) error {
 			if name == "invalid..name" {
diff --git a/cmd/link.go b/cmd/link.go
index b9997cd..b8ef9e1 100644
--- a/cmd/link.go
+++ b/cmd/link.go
@@ -235,7 +235,7 @@ func runLinkCreateOrUpdate(cfg *config.Config, client github.ClientOps, opts *li
 	// given, default to the repository's default branch (like `gh stack init`).
 	bottomBase := opts.base
 	if bottomBase == "" {
-		base, err := git.DefaultBranch()
+		base, err := resolveDefaultBranch(cfg, opts.remote)
 		if err != nil {
 			cfg.Errorf("unable to determine default branch\nUse --base to specify the base branch")
 			return ErrSilent
diff --git a/cmd/link_test.go b/cmd/link_test.go
index d652846..20c854d 100644
--- a/cmd/link_test.go
+++ b/cmd/link_test.go
@@ -1409,7 +1409,7 @@ func TestLink_DefaultBase_RetargetsBottomPRToDefaultBranch(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:       func() (string, error) { return "", fmt.Errorf("not a git repository") },
 		BranchExistsFn: func(string) (bool, error) { return false, nil },
-		DefaultBranchFn: func() (string, error) {
+		DefaultBranchFn: func(string) (string, error) {
 			defaultBranchCalled = true
 			return "develop", nil
 		},
@@ -1477,7 +1477,7 @@ func TestLink_DefaultBase_CreatesBottomPROnDefaultBranch(t *testing.T) {
 		BranchExistsFn:  func(name string) (bool, error) { return name == "feat-a" || name == "feat-b", nil },
 		PushFn:          func(string, []string, bool, bool) error { return nil },
 		ResolveRemoteFn: func(string) (string, error) { return "origin", nil },
-		DefaultBranchFn: func() (string, error) { return "develop", nil },
+		DefaultBranchFn: func(string) (string, error) { return "develop", nil },
 	})
 	defer restore()
 
@@ -1521,7 +1521,7 @@ func TestLink_DefaultBase_ErrorWhenUnresolvable(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return "", fmt.Errorf("not a git repository") },
 		BranchExistsFn:  func(string) (bool, error) { return false, nil },
-		DefaultBranchFn: func() (string, error) { return "", fmt.Errorf("no default branch") },
+		DefaultBranchFn: func(string) (string, error) { return "", fmt.Errorf("no default branch") },
 	})
 	defer restore()
 
@@ -1556,7 +1556,7 @@ func TestLink_ExplicitBase_SkipsDefaultBranchResolution(t *testing.T) {
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:       func() (string, error) { return "", fmt.Errorf("not a git repository") },
 		BranchExistsFn: func(string) (bool, error) { return false, nil },
-		DefaultBranchFn: func() (string, error) {
+		DefaultBranchFn: func(string) (string, error) {
 			defaultBranchCalled = true
 			return "develop", nil
 		},
diff --git a/cmd/utils.go b/cmd/utils.go
index db43aa9..ab27d3b 100644
--- a/cmd/utils.go
+++ b/cmd/utils.go
@@ -1923,6 +1923,38 @@ func ensureRerere(cfg *config.Config) error {
 	return nil
 }
 
+// resolveDefaultBranch finds the repository's default branch without prompting.
+// It asks git about the configured remote (falling back to origin), then the
+// GitHub API, and finally guesses a local main or master branch.
+func resolveDefaultBranch(cfg *config.Config, remoteOverride string) (string, error) {
+	remote := remoteOverride
+	if remote == "" {
+		remote = "origin"
+		branch, _ := git.CurrentBranch()
+		if resolved, err := git.ResolveRemote(branch); err == nil {
+			remote = resolved
+		}
+	}
+	if name, err := git.DefaultBranch(remote); err == nil && name != "" {
+		return name, nil
+	}
+	if client, err := cfg.GitHubClient(); err == nil {
+		if name, err := client.DefaultBranch(); err == nil && name != "" {
+			return name, nil
+		}
+	}
+	for _, name := range []string{"main", "master"} {
+		exists, err := git.BranchExists(name)
+		if err != nil {
+			return "", err
+		}
+		if exists {
+			return name, nil
+		}
+	}
+	return "", git.ErrNoDefaultBranch
+}
+
 // pickRemote determines which remote to use. If remoteOverride is
 // non-empty, it is returned directly. Otherwise it delegates to
 // git.ResolveRemote for config-based resolution and remote listing.
diff --git a/cmd/utils_test.go b/cmd/utils_test.go
index f421188..e67ee70 100644
--- a/cmd/utils_test.go
+++ b/cmd/utils_test.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 
@@ -2057,6 +2058,102 @@ func TestUpdateBaseSHAsPreservesLastValidBase(t *testing.T) {
 // TestFullScreenCanvas verifies that the full-screen TUIs paint the GitHub
 // canvas only on a color-enabled truecolor terminal, and honor the
 // GH_STACK_BACKGROUND opt-out.
+func TestResolveDefaultBranch(t *testing.T) {
+	gitErr := errors.New("unknown")
+	for _, tt := range []struct {
+		name           string
+		override       string
+		resolveRemote  func(string) (string, error)
+		gitDefault     map[string]string
+		apiDefault     string
+		apiErr         error
+		localBranches  []string
+		want           string
+		wantErr        error
+		wantRemoteUsed string
+	}{
+		{
+			name:           "resolved remote",
+			resolveRemote:  func(string) (string, error) { return "upstream", nil },
+			gitDefault:     map[string]string{"upstream": "develop"},
+			want:           "develop",
+			wantRemoteUsed: "upstream",
+		},
+		{
+			name:           "override wins",
+			override:       "fork",
+			gitDefault:     map[string]string{"fork": "trunk"},
+			want:           "trunk",
+			wantRemoteUsed: "fork",
+		},
+		{
+			name:           "origin when remote is ambiguous",
+			resolveRemote:  func(string) (string, error) { return "", &git.ErrMultipleRemotes{Remotes: []string{"a", "b"}} },
+			gitDefault:     map[string]string{"origin": "develop"},
+			want:           "develop",
+			wantRemoteUsed: "origin",
+		},
+		{
+			name:          "GitHub API when git cannot tell",
+			apiDefault:    "release",
+			localBranches: []string{"main"},
+			want:          "release",
+		},
+		{
+			name:          "local main as a last resort",
+			apiErr:        gitErr,
+			localBranches: []string{"master", "main"},
+			want:          "main",
+		},
+		{
+			name:          "local master as a last resort",
+			localBranches: []string{"master"},
+			want:          "master",
+		},
+		{
+			name:    "nothing found",
+			wantErr: git.ErrNoDefaultBranch,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			resolveRemote := tt.resolveRemote
+			if resolveRemote == nil {
+				resolveRemote = func(string) (string, error) { return "origin", nil }
+			}
+			var remoteUsed string
+			restore := git.SetOps(&git.MockOps{
+				ResolveRemoteFn: resolveRemote,
+				DefaultBranchFn: func(remote string) (string, error) {
+					remoteUsed = remote
+					if name, ok := tt.gitDefault[remote]; ok {
+						return name, nil
+					}
+					return "", git.ErrNoDefaultBranch
+				},
+				BranchExistsFn: func(name string) (bool, error) {
+					return slices.Contains(tt.localBranches, name), nil
+				},
+			})
+			defer restore()
+			cfg, _, _ := config.NewTestConfig()
+			cfg.GitHubClientOverride = &github.MockClient{
+				DefaultBranchFn: func() (string, error) { return tt.apiDefault, tt.apiErr },
+			}
+
+			got, err := resolveDefaultBranch(cfg, tt.override)
+			if tt.wantErr != nil {
+				assert.ErrorIs(t, err, tt.wantErr)
+				return
+			}
+			require.NoError(t, err)
+			assert.Equal(t, tt.want, got)
+			if tt.wantRemoteUsed != "" {
+				assert.Equal(t, tt.wantRemoteUsed, remoteUsed)
+			}
+		})
+	}
+}
+
 func TestFullScreenCanvas(t *testing.T) {
 	tests := []struct {
 		name    string
diff --git a/internal/git/git.go b/internal/git/git.go
index 37bd7c4..54e22c9 100644
--- a/internal/git/git.go
+++ b/internal/git/git.go
@@ -297,9 +297,14 @@ func FetchBranches(remote string, branches []string) error {
 	return ops.FetchBranches(remote, branches)
 }
 
-// DefaultBranch returns the HEAD branch from origin.
-func DefaultBranch() (string, error) {
-	return ops.DefaultBranch()
+// ErrNoDefaultBranch reports that a remote's default branch is unknown.
+var ErrNoDefaultBranch = errors.New("could not determine default branch")
+
+// DefaultBranch returns the remote's default branch from the local
+// refs/remotes/<remote>/HEAD pointer, or by asking the remote when that
+// pointer is missing.
+func DefaultBranch(remote string) (string, error) {
+	return ops.DefaultBranch(remote)
 }
 
 // CreateBranch creates a new branch from the given base.
diff --git a/internal/git/gitops.go b/internal/git/gitops.go
index 0dd9a96..b09204f 100644
--- a/internal/git/gitops.go
+++ b/internal/git/gitops.go
@@ -45,7 +45,7 @@ type Ops interface {
 	Fetch(remote string) error
 	FetchBranch(remote, branch string) error
 	FetchBranches(remote string, branches []string) error
-	DefaultBranch() (string, error)
+	DefaultBranch(remote string) (string, error)
 	CreateBranch(name, base string) error
 	Push(remote string, branches []string, force, atomic bool) error
 	ResolveRemote(branch string) (string, error)
@@ -230,25 +230,47 @@ func isMissingRemoteRefError(err error) bool {
 	return err != nil && strings.Contains(err.Error(), "couldn't find remote ref")
 }
 
-func (d *defaultOps) DefaultBranch() (string, error) {
-	ref, err := d.run("symbolic-ref", "--quiet", "refs/remotes/origin/HEAD")
+func (d *defaultOps) DefaultBranch(remote string) (string, error) {
+	prefix := "refs/remotes/" + remote + "/"
+	name := prefix+"HEAD"
+	ref, err := d.run("symbolic-ref", "--quiet", name)
+
+	if err == nil {
+		return strings.TrimPrefix(ref, prefix), nil
+	}
+
+	var gitErr *cligit.GitError
+	if !errors.As(err, &gitErr) || gitErr.ExitCode != 1 || gitErr.Stderr != "" {
+		return "", err
+	}
+
+	// The local pointer is missing, e.g. when the remote was added to an
+	// existing repository instead of cloned, so ask the remote directly.
+	cmd, err := d.command("ls-remote", "--symref", remote, "HEAD")
 	if err != nil {
-		var gitErr *cligit.GitError
-		if !errors.As(err, &gitErr) || gitErr.ExitCode != 1 || gitErr.Stderr != "" {
-			return "", err
+		return "", err
+	}
+
+	cmd.Env = append(cmd.Environ(), "GIT_TERMINAL_PROMPT=0")
+	if out, err := cmd.Output(); err == nil {
+		if branch := parseSymrefHead(string(out)); branch != "" {
+			return branch, nil
 		}
-		for _, name := range []string{"main", "master"} {
-			exists, lookupErr := d.BranchExists(name)
-			if lookupErr != nil {
-				return "", lookupErr
-			}
-			if exists {
-				return name, nil
-			}
+	}
+
+	return "", fmt.Errorf("%w for remote %q", ErrNoDefaultBranch, remote)
+}
+
+// parseSymrefHead extracts the branch from `git ls-remote --symref` output,
+// whose first line is "ref: refs/heads/<branch>\tHEAD".
+func parseSymrefHead(out string) string {
+	for _, line := range strings.Split(out, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) == 3 && fields[0] == "ref:" && fields[2] == "HEAD" {
+			return strings.TrimPrefix(fields[1], "refs/heads/")
 		}
-		return "", err
 	}
-	return strings.TrimPrefix(ref, "refs/remotes/origin/"), nil
+	return ""
 }
 
 func (d *defaultOps) CreateBranch(name, base string) error {
diff --git a/internal/git/gitops_test.go b/internal/git/gitops_test.go
index cb1822c..4b0b9fc 100644
--- a/internal/git/gitops_test.go
+++ b/internal/git/gitops_test.go
@@ -678,6 +678,32 @@ func TestIntegration_ResolveRemote_GitPushConfigTakesPrecedence(t *testing.T) {
 	assert.Equal(t, "origin", remote)
 }
 
+func TestIntegration_DefaultBranch(t *testing.T) {
+	bareDir, cloneDir := setupBareAndClone(t)
+	gitExec(t, cloneDir, "push", "origin", "main:develop")
+	gitExec(t, bareDir, "-c", "safe.bareRepository=all", "symbolic-ref", "HEAD", "refs/heads/develop")
+
+	dir := setupLocalRepo(t)
+	gitExec(t, dir, "remote", "add", "upstream", bareDir)
+	restoreDir := withGitDir(t, dir)
+	defer restoreDir()
+
+	branch, err := DefaultBranch("upstream")
+	require.NoError(t, err)
+	assert.Equal(t, "develop", branch, "asks the remote when no local pointer exists")
+	_, err = gitExecMayFail(t, dir, "symbolic-ref", "refs/remotes/upstream/HEAD")
+	assert.Error(t, err, "querying the remote must not write a local pointer")
+
+	gitExec(t, dir, "update-ref", "refs/remotes/upstream/cached", "HEAD")
+	gitExec(t, dir, "symbolic-ref", "refs/remotes/upstream/HEAD", "refs/remotes/upstream/cached")
+	branch, err = DefaultBranch("upstream")
+	require.NoError(t, err)
+	assert.Equal(t, "cached", branch, "the local pointer avoids a network call")
+
+	_, err = DefaultBranch("missing")
+	assert.ErrorIs(t, err, ErrNoDefaultBranch)
+}
+
 func TestIntegration_SaveAndGetRemote(t *testing.T) {
 	_, cloneDir := setupBareAndClone(t)
 	restoreDir := withGitDir(t, cloneDir)
@@ -1755,14 +1781,13 @@ func TestIntegration_WorktreeLocalMutations(t *testing.T) {
 	linked, path := addTestWorktree(t, root, dir, "feature")
 	restore := forbidGlobalWorktreeQueries(t)
 	defer restore()
-	defaultBranch, err := linked.DefaultBranch()
-	require.NoError(t, err)
-	assert.Equal(t, "main", defaultBranch)
+	_, err := linked.DefaultBranch("origin")
+	require.ErrorIs(t, err, ErrNoDefaultBranch, "local branch names are not guessed at the git layer")
 	gitExec(t, dir, "update-ref", "refs/remotes/origin/remote-trunk", "main")
 	gitExec(t, dir, "symbolic-ref", "refs/remotes/origin/HEAD", "refs/remotes/origin/remote-trunk")
-	defaultBranch, err = linked.DefaultBranch()
+	defaultBranch, err := linked.DefaultBranch("origin")
 	require.NoError(t, err)
-	assert.Equal(t, "remote-trunk", defaultBranch, "a remote HEAD takes precedence over local branch-name fallbacks")
+	assert.Equal(t, "remote-trunk", defaultBranch)
 	initial := gitExec(t, dir, "rev-parse", "HEAD")
 	writeFile(t, dir, "new.txt", "new main commit\n")
 	gitExec(t, dir, "add", ".")
diff --git a/internal/git/mock_ops.go b/internal/git/mock_ops.go
index a3f894c..716a213 100644
--- a/internal/git/mock_ops.go
+++ b/internal/git/mock_ops.go
@@ -22,7 +22,7 @@ type MockOps struct {
 	FetchFn                  func(string) error
 	FetchBranchFn            func(string, string) error
 	FetchBranchesFn          func(string, []string) error
-	DefaultBranchFn          func() (string, error)
+	DefaultBranchFn          func(string) (string, error)
 	CreateBranchFn           func(string, string) error
 	PushFn                   func(string, []string, bool, bool) error
 	ResolveRemoteFn          func(string) (string, error)
@@ -160,9 +160,9 @@ func (m *MockOps) FetchBranches(remote string, branches []string) error {
 	return nil
 }
 
-func (m *MockOps) DefaultBranch() (string, error) {
+func (m *MockOps) DefaultBranch(remote string) (string, error) {
 	if m.DefaultBranchFn != nil {
-		return m.DefaultBranchFn()
+		return m.DefaultBranchFn(remote)
 	}
 	return "main", nil
 }
diff --git a/internal/github/client_interface.go b/internal/github/client_interface.go
index c1281ed..7b98bf6 100644
--- a/internal/github/client_interface.go
+++ b/internal/github/client_interface.go
@@ -22,6 +22,7 @@ type ClientOps interface {
 	GetAsyncMergeResult(prNumber int, uuid string) (*AsyncMergeResult, error)
 	PRTitles(numbers []int) (map[int]string, error)
 	BaseBranchUsesMergeQueue(baseRef string) (bool, error)
+	DefaultBranch() (string, error)
 }
 
 // Compile-time check that Client satisfies ClientOps.
diff --git a/internal/github/github.go b/internal/github/github.go
index 88d3f6f..bc01a66 100644
--- a/internal/github/github.go
+++ b/internal/github/github.go
@@ -546,6 +546,17 @@ func (c *Client) FindStackForPR(prNumber int) (*RemoteStack, error) {
 	return &stacks[0], nil
 }
 
+// DefaultBranch returns the repository's default branch.
+func (c *Client) DefaultBranch() (string, error) {
+	var repo struct {
+		DefaultBranch string `json:"default_branch"`
+	}
+	if err := c.rest.Get(fmt.Sprintf("repos/%s/%s", c.owner, c.repo), &repo); err != nil {
+		return "", fmt.Errorf("fetching repository: %w", err)
+	}
+	return repo.DefaultBranch, nil
+}
+
 // GetStack fetches a single stack by its stack number.
 func (c *Client) GetStack(stackNumber int) (*RemoteStack, error) {
 	path := fmt.Sprintf("repos/%s/%s/stacks/%d", c.owner, c.repo, stackNumber)
diff --git a/internal/github/mock_client.go b/internal/github/mock_client.go
index a31dd4e..26575a3 100644
--- a/internal/github/mock_client.go
+++ b/internal/github/mock_client.go
@@ -22,6 +22,7 @@ type MockClient struct {
 	GetAsyncMergeResultFn      func(int, string) (*AsyncMergeResult, error)
 	PRTitlesFn                 func([]int) (map[int]string, error)
 	BaseBranchUsesMergeQueueFn func(string) (bool, error)
+	DefaultBranchFn            func() (string, error)
 }
 
 // Compile-time check that MockClient satisfies ClientOps.
@@ -170,3 +171,10 @@ func (m *MockClient) BaseBranchUsesMergeQueue(baseRef string) (bool, error) {
 	}
 	return false, nil
 }
+
+func (m *MockClient) DefaultBranch() (string, error) {
+	if m.DefaultBranchFn != nil {
+		return m.DefaultBranchFn()
+	}
+	return "", nil
+}