diff --git a/.changes/unreleased/Added-20260512-101000.yaml b/.changes/unreleased/Added-20260512-101000.yaml
new file mode 100644
index 00000000..07bd53d2
--- /dev/null
+++ b/.changes/unreleased/Added-20260512-101000.yaml
@@ -0,0 +1,3 @@
+kind: Added
+body: 'ci: Add ''gs ci merge-guard'' to block out-of-order stacked PR merges'
+time: 2026-05-12T10:10:00.762024-04:00
diff --git a/branch_restack.go b/branch_restack.go
index 32b8dc59..f84b803b 100644
--- a/branch_restack.go
+++ b/branch_restack.go
@@ -32,5 +32,5 @@ func (cmd *branchRestackCmd) AfterApply(ctx context.Context, wt *git.Worktree) e
 }
 
 func (cmd *branchRestackCmd) Run(ctx context.Context, handler RestackHandler) error {
-	return handler.RestackBranch(ctx, cmd.Branch)
+	return handler.RestackBranch(ctx, cmd.Branch, nil)
 }
diff --git a/ci.go b/ci.go
new file mode 100644
index 00000000..4e2a0246
--- /dev/null
+++ b/ci.go
@@ -0,0 +1,6 @@
+package main
+
+// ciCmd groups subcommands for CI/CD integration.
+type ciCmd struct {
+	MergeGuard ciMergeGuardCmd `cmd:"merge-guard" help:"Block merging a PR whose base is not trunk"`
+}
diff --git a/ci_merge_guard.go b/ci_merge_guard.go
new file mode 100644
index 00000000..d8c7b92d
--- /dev/null
+++ b/ci_merge_guard.go
@@ -0,0 +1,165 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"go.abhg.dev/gs/internal/forge"
+	"go.abhg.dev/gs/internal/handler/submit"
+	"go.abhg.dev/gs/internal/silog"
+	"go.abhg.dev/gs/internal/text"
+)
+
+type ciMergeGuardCmd struct {
+	Number int    `arg:"" help:"Change request number to check"`
+	Trunk  string `help:"Override trunk branch name"`
+	All    bool   `help:"Block all non-trunk-based PRs, not just git-spice managed ones"`
+}
+
+func (*ciMergeGuardCmd) Help() string {
+	return text.Dedent(`
+		Checks whether a change request is safe to merge
+		by verifying its base branch is trunk.
+
+		Use this in forge CI/CD pipelines to prevent
+		out-of-order merges in a stacked PR workflow.
+
+		By default, only git-spice managed PRs are checked.
+		Unmanaged PRs are allowed through.
+		Use --all to block any PR whose base is not trunk.
+
+		The trunk branch is detected from the git-spice
+		navigation comment on the PR.
+		Use --trunk to override this detection.
+
+		Exit codes:
+		  0  PR is safe to merge (base is trunk, or unmanaged)
+		  1  PR should not be merged yet
+	`)
+}
+
+func (cmd *ciMergeGuardCmd) Run(
+	ctx context.Context,
+	log *silog.Logger,
+	repo forge.Repository,
+) error {
+	changeID, err := cmd.resolveChangeID(repo)
+	if err != nil {
+		return err
+	}
+
+	change, err := repo.FindChangeByID(ctx, changeID)
+	if err != nil {
+		return fmt.Errorf("find change #%d: %w", cmd.Number, err)
+	}
+
+	trunk, managed, err := cmd.detectTrunk(ctx, log, repo, changeID)
+	if err != nil {
+		return err
+	}
+
+	return cmd.evaluate(log, change, trunk, managed)
+}
+
+func (cmd *ciMergeGuardCmd) resolveChangeID(
+	repo forge.Repository,
+) (forge.ChangeID, error) {
+	raw := fmt.Appendf(nil, "%d", cmd.Number)
+	id, err := repo.Forge().UnmarshalChangeID(raw)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"construct change ID for #%d: %w", cmd.Number, err,
+		)
+	}
+	return id, nil
+}
+
+// detectTrunk determines the trunk branch name and whether
+// the PR is managed by git-spice.
+// Returns (trunk, managed, error).
+func (cmd *ciMergeGuardCmd) detectTrunk(
+	ctx context.Context,
+	log *silog.Logger,
+	repo forge.Repository,
+	changeID forge.ChangeID,
+) (trunk string, managed bool, _ error) {
+	if cmd.Trunk != "" {
+		return cmd.Trunk, true, nil
+	}
+
+	trunk, managed = cmd.trunkFromNavComment(ctx, log, repo, changeID)
+	if trunk != "" {
+		return trunk, managed, nil
+	}
+
+	if !managed {
+		return "", false, nil
+	}
+
+	return "", true, fmt.Errorf(
+		"could not determine trunk for #%d: "+
+			"use --trunk to specify it explicitly",
+		cmd.Number,
+	)
+}
+
+// trunkFromNavComment searches for a git-spice navigation comment
+// on the given change and extracts the trunk branch name.
+// Returns ("", false) if no navigation comment is found.
+func (cmd *ciMergeGuardCmd) trunkFromNavComment(
+	ctx context.Context,
+	log *silog.Logger,
+	repo forge.Repository,
+	changeID forge.ChangeID,
+) (trunk string, managed bool) {
+	opts := &forge.ListChangeCommentsOptions{
+		BodyMatchesAll: submit.NavCommentRegexes,
+	}
+
+	for comment, err := range repo.ListChangeComments(
+		ctx, changeID, opts,
+	) {
+		if err != nil {
+			log.Warn("Error listing comments", "error", err)
+			return "", false
+		}
+
+		trunk := submit.ExtractTrunkFromComment(comment.Body)
+		return trunk, true
+	}
+
+	return "", false
+}
+
+// evaluate decides whether the PR is safe to merge
+// based on the change's base branch and the detected trunk.
+func (cmd *ciMergeGuardCmd) evaluate(
+	log *silog.Logger,
+	change *forge.FindChangeItem,
+	trunk string,
+	managed bool,
+) error {
+	// Unmanaged PR: allow unless --all is set.
+	if !managed {
+		if cmd.All {
+			return fmt.Errorf(
+				"#%d: base %q is not trunk (unmanaged PR blocked by --all)",
+				cmd.Number, change.BaseName,
+			)
+		}
+		log.Infof("#%d: not managed by git-spice, allowing", cmd.Number)
+		return nil
+	}
+
+	if change.BaseName == trunk {
+		log.Infof("#%d: base is %q (trunk), safe to merge",
+			cmd.Number, trunk)
+		return nil
+	}
+
+	return fmt.Errorf(
+		"#%d: base is %q, expected trunk %q. "+
+			"Merge the downstack PR first or retarget to trunk",
+		cmd.Number, change.BaseName, trunk,
+	)
+}
diff --git a/doc/includes/cli-reference.md b/doc/includes/cli-reference.md
index 507c9d1c..4d598acd 100644
--- a/doc/includes/cli-reference.md
+++ b/doc/includes/cli-reference.md
@@ -1415,6 +1415,43 @@ going back to the state before the rebase.
 The command can be used in place of 'git rebase --abort'
 even if a git-spice operation is not currently in progress.
 
+## CI
+
+### git-spice ci merge-guard {#gs-ci-merge-guard}
+
+```
+gs ci merge-guard <number> [flags]
+```
+
+Block merging a PR whose base is not trunk
+
+Checks whether a change request is safe to merge
+by verifying its base branch is trunk.
+
+Use this in forge CI/CD pipelines to prevent
+out-of-order merges in a stacked PR workflow.
+
+By default, only git-spice managed PRs are checked.
+Unmanaged PRs are allowed through.
+Use --all to block any PR whose base is not trunk.
+
+The trunk branch is detected from the git-spice
+navigation comment on the PR.
+Use --trunk to override this detection.
+
+Exit codes:
+  0  PR is safe to merge (base is trunk, or unmanaged)
+  1  PR should not be merged yet
+
+**Arguments**
+
+* `number`: Change request number to check
+
+**Flags**
+
+* `--trunk=STRING`: Override trunk branch name
+* `--all`: Block all non-trunk-based PRs, not just git-spice managed ones
+
 ## Navigation
 
 ### git-spice up {#gs-up}
diff --git a/downstack_restack.go b/downstack_restack.go
index e3f95a42..87a273f6 100644
--- a/downstack_restack.go
+++ b/downstack_restack.go
@@ -44,5 +44,5 @@ func (cmd *downstackRestackCmd) Run(
 		return errors.New("nothing to restack below trunk")
 	}
 
-	return handler.RestackDownstack(ctx, cmd.Branch)
+	return handler.RestackDownstack(ctx, cmd.Branch, nil)
 }
diff --git a/internal/handler/merge/handler.go b/internal/handler/merge/handler.go
index 8172b233..0f921e88 100644
--- a/internal/handler/merge/handler.go
+++ b/internal/handler/merge/handler.go
@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"go.abhg.dev/gs/internal/handler/restack"
 	"go.abhg.dev/gs/internal/handler/submit"
 	"go.abhg.dev/gs/internal/handler/sync"
 
@@ -36,7 +37,7 @@ type Service interface {
 
 // RestackHandler restacks branches after their bases are merged.
 type RestackHandler interface {
-	RestackBranch(context.Context, string) error
+	RestackBranch(ctx context.Context, branch string, opts *restack.Options) error
 }
 
 // SubmitHandler updates change requests after branch restacks.
@@ -599,7 +600,7 @@ func (e *mergePlanExecutor) prepareForMerge(
 			return fmt.Errorf("verify restacked: %w", err)
 		}
 
-		if err := e.Restack.RestackBranch(ctx, item.branch); err != nil {
+		if err := e.Restack.RestackBranch(ctx, item.branch, nil); err != nil {
 			return fmt.Errorf("restack branch: %w", err)
 		}
 
diff --git a/internal/handler/merge/handler_test.go b/internal/handler/merge/handler_test.go
index 317d2a05..f1d1f165 100644
--- a/internal/handler/merge/handler_test.go
+++ b/internal/handler/merge/handler_test.go
@@ -317,10 +317,10 @@ func TestExecutePlan_retargets(t *testing.T) {
 
 	mockRestack := NewMockRestackHandler(ctrl)
 	mockRestack.EXPECT().
-		RestackBranch(gomock.Any(), "feat2").
+		RestackBranch(gomock.Any(), "feat2", gomock.Nil()).
 		Return(nil)
 	mockRestack.EXPECT().
-		RestackBranch(gomock.Any(), "feat3").
+		RestackBranch(gomock.Any(), "feat3", gomock.Nil()).
 		Return(nil)
 
 	mockSubmit := NewMockSubmitHandler(ctrl)
@@ -396,7 +396,7 @@ func TestExecutePlan_waitsForPreparedChangeHeadBeforeChecks(t *testing.T) {
 
 	mockRestack := NewMockRestackHandler(ctrl)
 	mockRestack.EXPECT().
-		RestackBranch(gomock.Any(), "feat2").
+		RestackBranch(gomock.Any(), "feat2", gomock.Nil()).
 		Return(nil)
 
 	mockSubmit := NewMockSubmitHandler(ctrl)
diff --git a/internal/handler/merge/mocks_test.go b/internal/handler/merge/mocks_test.go
index 9d4523a5..c59410e4 100644
--- a/internal/handler/merge/mocks_test.go
+++ b/internal/handler/merge/mocks_test.go
@@ -13,6 +13,7 @@ import (
 	reflect "reflect"
 
 	git "go.abhg.dev/gs/internal/git"
+	restack "go.abhg.dev/gs/internal/handler/restack"
 	submit "go.abhg.dev/gs/internal/handler/submit"
 	sync "go.abhg.dev/gs/internal/handler/sync"
 	spice "go.abhg.dev/gs/internal/spice"
@@ -207,17 +208,17 @@ func (m *MockRestackHandler) EXPECT() *MockRestackHandlerMockRecorder {
 }
 
 // RestackBranch mocks base method.
-func (m *MockRestackHandler) RestackBranch(arg0 context.Context, arg1 string) error {
+func (m *MockRestackHandler) RestackBranch(ctx context.Context, branch string, opts *restack.Options) error {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "RestackBranch", arg0, arg1)
+	ret := m.ctrl.Call(m, "RestackBranch", ctx, branch, opts)
 	ret0, _ := ret[0].(error)
 	return ret0
 }
 
 // RestackBranch indicates an expected call of RestackBranch.
-func (mr *MockRestackHandlerMockRecorder) RestackBranch(arg0, arg1 any) *MockRestackHandlerRestackBranchCall {
+func (mr *MockRestackHandlerMockRecorder) RestackBranch(ctx, branch, opts any) *MockRestackHandlerRestackBranchCall {
 	mr.mock.ctrl.T.Helper()
-	call := mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RestackBranch", reflect.TypeOf((*MockRestackHandler)(nil).RestackBranch), arg0, arg1)
+	call := mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RestackBranch", reflect.TypeOf((*MockRestackHandler)(nil).RestackBranch), ctx, branch, opts)
 	return &MockRestackHandlerRestackBranchCall{Call: call}
 }
 
@@ -233,13 +234,13 @@ func (c *MockRestackHandlerRestackBranchCall) Return(arg0 error) *MockRestackHan
 }
 
 // Do rewrite *gomock.Call.Do
-func (c *MockRestackHandlerRestackBranchCall) Do(f func(context.Context, string) error) *MockRestackHandlerRestackBranchCall {
+func (c *MockRestackHandlerRestackBranchCall) Do(f func(context.Context, string, *restack.Options) error) *MockRestackHandlerRestackBranchCall {
 	c.Call = c.Call.Do(f)
 	return c
 }
 
 // DoAndReturn rewrite *gomock.Call.DoAndReturn
-func (c *MockRestackHandlerRestackBranchCall) DoAndReturn(f func(context.Context, string) error) *MockRestackHandlerRestackBranchCall {
+func (c *MockRestackHandlerRestackBranchCall) DoAndReturn(f func(context.Context, string, *restack.Options) error) *MockRestackHandlerRestackBranchCall {
 	c.Call = c.Call.DoAndReturn(f)
 	return c
 }
diff --git a/internal/handler/restack/branch.go b/internal/handler/restack/branch.go
index 8d880183..889d9a27 100644
--- a/internal/handler/restack/branch.go
+++ b/internal/handler/restack/branch.go
@@ -3,10 +3,13 @@ package restack
 import "context"
 
 // RestackBranch restacks the given branch onto its base.
-func (h *Handler) RestackBranch(ctx context.Context, branch string) error {
+func (h *Handler) RestackBranch(
+	ctx context.Context, branch string, opts *Options,
+) error {
 	_, err := h.Restack(ctx, &Request{
 		Branch:          branch,
 		ContinueCommand: []string{"branch", "restack"},
+		AutoResolve:     opts.autoResolvePtr(),
 	})
 	return err
 }
diff --git a/internal/handler/restack/branch_test.go b/internal/handler/restack/branch_test.go
index 95664e68..c7435831 100644
--- a/internal/handler/restack/branch_test.go
+++ b/internal/handler/restack/branch_test.go
@@ -47,7 +47,7 @@ func TestHandler_RestackBranch(t *testing.T) {
 			Store:    statetest.NewMemoryStore(t, "main", "", log),
 			Service:  mockService,
 		}
-		require.NoError(t, handler.RestackBranch(t.Context(), "feature"))
+		require.NoError(t, handler.RestackBranch(t.Context(), "feature", nil))
 		assert.Contains(t, logBuffer.String(), "feature: restacked on main")
 	})
 
@@ -82,7 +82,7 @@ func TestHandler_RestackBranch(t *testing.T) {
 			Service:  mockService,
 		}
 
-		require.NoError(t, handler.RestackBranch(t.Context(), "feature"))
+		require.NoError(t, handler.RestackBranch(t.Context(), "feature", nil))
 	})
 
 	t.Run("UntrackedBranch", func(t *testing.T) {
@@ -113,7 +113,7 @@ func TestHandler_RestackBranch(t *testing.T) {
 			Service:  mockService,
 		}
 
-		err := handler.RestackBranch(t.Context(), "untracked")
+		err := handler.RestackBranch(t.Context(), "untracked", nil)
 		require.Error(t, err)
 		assert.ErrorContains(t, err, "untracked branch")
 		assert.Contains(t, logBuffer.String(), "untracked: branch not tracked: run '"+cli.Name()+" branch track")
@@ -146,7 +146,7 @@ func TestHandler_RestackBranch(t *testing.T) {
 			Store:    statetest.NewMemoryStore(t, "main", "", log),
 			Service:  mockService,
 		}
-		require.NoError(t, handler.RestackBranch(t.Context(), "already-restacked"))
+		require.NoError(t, handler.RestackBranch(t.Context(), "already-restacked", nil))
 		assert.Contains(t, logBuffer.String(), "already-restacked: branch does not need to be restacked.")
 	})
 
@@ -177,7 +177,7 @@ func TestHandler_RestackBranch(t *testing.T) {
 			Store:    statetest.NewMemoryStore(t, "main", "", log),
 			Service:  mockService,
 		}
-		err := handler.RestackBranch(t.Context(), "feature")
+		err := handler.RestackBranch(t.Context(), "feature", nil)
 		require.Error(t, err)
 		assert.ErrorContains(t, err, "restack branch")
 		assert.ErrorIs(t, err, unexpectedErr)
diff --git a/internal/handler/restack/downstack.go b/internal/handler/restack/downstack.go
index e4df6f7a..60c0ab42 100644
--- a/internal/handler/restack/downstack.go
+++ b/internal/handler/restack/downstack.go
@@ -4,11 +4,14 @@ import "context"
 
 // RestackDownstack restacks the downstack of the given branch.
 // This includes the branch itself.
-func (h *Handler) RestackDownstack(ctx context.Context, branch string) error {
+func (h *Handler) RestackDownstack(
+	ctx context.Context, branch string, opts *Options,
+) error {
 	_, err := h.Restack(ctx, &Request{
 		Branch:          branch,
 		Scope:           ScopeDownstack,
 		ContinueCommand: []string{"downstack", "restack"},
+		AutoResolve:     opts.autoResolvePtr(),
 	})
 	return err
 }
diff --git a/internal/handler/restack/downstack_test.go b/internal/handler/restack/downstack_test.go
index fa633e06..a99ac339 100644
--- a/internal/handler/restack/downstack_test.go
+++ b/internal/handler/restack/downstack_test.go
@@ -54,7 +54,7 @@ func TestHandler_RestackDownstack(t *testing.T) {
 			Service:  mockService,
 		}
 
-		require.NoError(t, handler.RestackDownstack(t.Context(), "feature3"))
+		require.NoError(t, handler.RestackDownstack(t.Context(), "feature3", nil))
 		assert.Contains(t, logBuffer.String(), "feature1: restacked on main")
 		assert.Contains(t, logBuffer.String(), "feature2: restacked on feature1")
 		assert.Contains(t, logBuffer.String(), "feature3: restacked on feature2")
@@ -99,6 +99,6 @@ func TestHandler_RestackDownstack(t *testing.T) {
 			Service:  mockService,
 		}
 
-		require.NoError(t, handler.RestackDownstack(t.Context(), "feature2"))
+		require.NoError(t, handler.RestackDownstack(t.Context(), "feature2", nil))
 	})
 }
diff --git a/internal/handler/restack/handler.go b/internal/handler/restack/handler.go
index 7ff1f6d4..93d0dadc 100644
--- a/internal/handler/restack/handler.go
+++ b/internal/handler/restack/handler.go
@@ -93,6 +93,33 @@ type Request struct {
 	//
 	// Defaults to ScopeBranch.
 	Scope Scope
+
+	// AutoResolve, if non-nil, overrides
+	// [Handler.DefaultAutoResolve] for this invocation. A true
+	// value enables the configured resolver; a false value disables
+	// it even when configured.
+	AutoResolve *bool
+}
+
+// Options are optional parameters shared by the high-level restack
+// entry points ([Handler.RestackBranch], [Handler.RestackStack],
+// [Handler.RestackDownstack]).
+type Options struct {
+	// AutoResolve, if non-nil, overrides
+	// [Handler.DefaultAutoResolve] for this invocation. A true
+	// value enables the configured resolver; a false value disables
+	// it even when configured.
+	AutoResolve *bool
+}
+
+// autoResolvePtr returns the AutoResolve pointer if opts is
+// non-nil, or nil. Callers pass the result through to
+// [Request.AutoResolve].
+func (o *Options) autoResolvePtr() *bool {
+	if o == nil {
+		return nil
+	}
+	return o.AutoResolve
 }
 
 // Restack restacks one or more branches according to the request.
diff --git a/internal/handler/restack/stack.go b/internal/handler/restack/stack.go
index f37ad358..592e99ea 100644
--- a/internal/handler/restack/stack.go
+++ b/internal/handler/restack/stack.go
@@ -5,11 +5,14 @@ import "context"
 // RestackStack restacks the stack of the given branch.
 // This includes all upstack and downtrack branches,
 // as well as the branch itself.
-func (h *Handler) RestackStack(ctx context.Context, branch string) error {
+func (h *Handler) RestackStack(
+	ctx context.Context, branch string, opts *Options,
+) error {
 	_, err := h.Restack(ctx, &Request{
 		Branch:          branch,
 		Scope:           ScopeStack,
 		ContinueCommand: []string{"stack", "restack"},
+		AutoResolve:     opts.autoResolvePtr(),
 	})
 	return err
 }
diff --git a/internal/handler/submit/handler.go b/internal/handler/submit/handler.go
index f1587929..ddcc08f2 100644
--- a/internal/handler/submit/handler.go
+++ b/internal/handler/submit/handler.go
@@ -533,6 +533,7 @@ func (h *Handler) SubmitBatch(ctx context.Context, req *BatchRequest) error {
 		opts.NavCommentSync,
 		opts.NavCommentDownstack,
 		opts.NavCommentMarker,
+		h.Store.Trunk(),
 		branchesToComment,
 		h.upstreamRepository,
 	)
@@ -597,6 +598,7 @@ func (h *Handler) Submit(ctx context.Context, req *Request) error {
 		opts.NavCommentSync,
 		opts.NavCommentDownstack,
 		opts.NavCommentMarker,
+		h.Store.Trunk(),
 		[]string{req.Branch},
 		h.upstreamRepository,
 	)
@@ -739,7 +741,7 @@ func (h *Handler) submitBranch(
 			// If we're importing an existing CR,
 			// also check if there's a stack navigation comment to import.
 			listCommentOpts := forge.ListChangeCommentsOptions{
-				BodyMatchesAll: _navCommentRegexes,
+				BodyMatchesAll: NavCommentRegexes,
 				CanUpdate:      true,
 			}
 
diff --git a/internal/handler/submit/nav_comment.go b/internal/handler/submit/nav_comment.go
index 9501f9c4..9dbf906d 100644
--- a/internal/handler/submit/nav_comment.go
+++ b/internal/handler/submit/nav_comment.go
@@ -91,6 +91,7 @@ func updateNavigationComments(
 	navCommentSync NavCommentSync,
 	navCommentDownstack NavCommentDownstack,
 	navCommentMarker string,
+	trunk string,
 	submittedBranches []string,
 	getRemoteRepo func(context.Context) (forge.Repository, error),
 ) error {
@@ -395,7 +396,7 @@ func updateNavigationComments(
 		}
 
 		info := infos[idx]
-		commentBody := generateStackNavigationComment(nodes, idx, navCommentMarker, remoteRepo.Forge())
+		commentBody := generateStackNavigationComment(nodes, idx, navCommentMarker, trunk, remoteRepo.Forge())
 		if info.Meta.NavigationCommentID() == nil {
 			postc <- &postComment{
 				Branch: info.Branch,
@@ -462,23 +463,54 @@ const (
 // Uses Markdown link definition syntax which is invisible when rendered.
 const _markdownCommentMarker = "[gs]: # (navigation comment)"
 
-// Regular expressions that must ALL match a comment
-// for it to be considered a navigation comment
-// when detecting existing comments.
-var _navCommentRegexes = []*regexp.Regexp{
+// Trunk metadata markers embed the trunk branch name
+// in the navigation comment for CI merge-guard detection.
+const (
+	_trunkMarkerHTML     = "<!-- gs:trunk %s -->"
+	_trunkMarkerMarkdown = "[gs:trunk]: # (%s)"
+)
+
+// NavCommentRegexes are regular expressions
+// that must ALL match a comment body
+// for it to be considered a git-spice navigation comment.
+var NavCommentRegexes = []*regexp.Regexp{
 	regexp.MustCompile(`(?m)^\Q` + _commentHeader + `\E$`),
 	// Match either standard HTML comment or Markdown link definition marker.
 	regexp.MustCompile(`(?m)^(\Q` + _commentMarker + `\E|\Q` + _markdownCommentMarker + `\E)$`),
 }
 
+// _trunkMetadataRegex extracts the trunk branch name
+// from either HTML or Markdown trunk metadata markers.
+var _trunkMetadataRegex = regexp.MustCompile(
+	`(?m)^(?:<!-- gs:trunk (\S+) -->|\[gs:trunk\]: # \((\S+)\))$`,
+)
+
+// ExtractTrunkFromComment extracts the trunk branch name
+// from a git-spice navigation comment body.
+// Returns "" if not found.
+func ExtractTrunkFromComment(body string) string {
+	m := _trunkMetadataRegex.FindStringSubmatch(body)
+	if m == nil {
+		return ""
+	}
+
+	// Group 1 = HTML format, group 2 = Markdown format.
+	if m[1] != "" {
+		return m[1]
+	}
+	return m[2]
+}
+
 func generateStackNavigationComment(
 	nodes []*stackedChange,
 	current int,
 	marker string,
+	trunk string,
 	f forge.Forge,
 ) string {
 	footer := _commentFooter
 	commentMarker := _commentMarker
+	trunkMarker := _trunkMarkerHTML
 	if fc, ok := f.(forge.WithCommentFormat); ok {
 		format := fc.CommentFormat()
 		if format.Footer != "" {
@@ -486,6 +518,7 @@ func generateStackNavigationComment(
 		}
 		if format.Marker != "" {
 			commentMarker = format.Marker
+			trunkMarker = _trunkMarkerMarkdown
 		}
 	}
 
@@ -501,9 +534,12 @@ func generateStackNavigationComment(
 
 	sb.WriteString("\n")
 	sb.WriteString(footer)
-
 	sb.WriteString("\n")
 	sb.WriteString(commentMarker)
+	if trunk != "" {
+		sb.WriteString("\n")
+		fmt.Fprintf(&sb, trunkMarker, trunk)
+	}
 	sb.WriteString("\n")
 	return sb.String()
 }
diff --git a/internal/handler/submit/nav_comment_test.go b/internal/handler/submit/nav_comment_test.go
index 9eb347d2..eba2fed9 100644
--- a/internal/handler/submit/nav_comment_test.go
+++ b/internal/handler/submit/nav_comment_test.go
@@ -4,6 +4,7 @@ import (
 	"cmp"
 	"context"
 	"encoding/json"
+	"fmt"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -450,6 +451,7 @@ func TestUpdateNavigationComments(t *testing.T) {
 				tt.sync,
 				tt.downstack,
 				"",
+				"main",
 				tt.submit,
 				func(context.Context) (forge.Repository, error) {
 					return mockRemoteRepo, nil
@@ -462,9 +464,12 @@ func TestUpdateNavigationComments(t *testing.T) {
 				if assert.Len(t, commentIDs, 1, "change %v doesn't have exactly one comment", changeID) {
 					body, ok := comments[commentIDs[0]]
 					if assert.True(t, ok, "comment %v on change %v has no body", commentIDs[0], changeID) {
-						// Strip header, footer, and marker to get just the navigation content
+						// Strip header, footer, marker, and trunk metadata
+						// to get just the navigation content.
 						stripped := strings.TrimPrefix(body, _commentHeader+"\n\n")
-						stripped = strings.TrimSuffix(stripped, "\n"+_commentFooter+"\n"+_commentMarker+"\n")
+						stripped = strings.TrimSuffix(stripped,
+							"\n"+_commentFooter+"\n"+_commentMarker+
+								"\n"+fmt.Sprintf(_trunkMarkerHTML, "main")+"\n")
 						gotComments[int(changeID)] = stripped
 					}
 				}
@@ -562,6 +567,7 @@ func TestUpdateNavigationComments_deletedExternally(t *testing.T) {
 			NavCommentSyncBranch,
 			NavCommentDownstackAll,
 			"",
+			"main",
 			[]string{"feat1"},
 			func(context.Context) (forge.Repository, error) {
 				return mockRemoteRepo, nil
@@ -677,6 +683,7 @@ func TestUpdateNavigationComments_deletedExternally(t *testing.T) {
 			NavCommentSyncDownstack,
 			NavCommentDownstackAll,
 			"",
+			"main",
 			[]string{"feat3"},
 			func(context.Context) (forge.Repository, error) {
 				return mockRemoteRepo, nil
@@ -784,13 +791,13 @@ func TestGenerateStackNavigationComment(t *testing.T) {
 				tt.want + "\n" +
 				_commentFooter + "\n" +
 				_commentMarker + "\n"
-			got := generateStackNavigationComment(tt.graph, tt.current, "", nil)
+			got := generateStackNavigationComment(tt.graph, tt.current, "", "", nil)
 			assert.Equal(t, want, got)
 
 			// Sanity check: All generated comments must match
 			// these regular expressions.
 			t.Run("Regexp", func(t *testing.T) {
-				for _, re := range _navCommentRegexes {
+				for _, re := range NavCommentRegexes {
 					assert.True(t, re.MatchString(got), "regexp %q failed", re)
 				}
 			})
@@ -804,7 +811,7 @@ func TestGenerateStackNavigationComment(t *testing.T) {
 		}
 		graph[0].Aboves = []int{1}
 
-		got := generateStackNavigationComment(graph, 1, "<-- you are here", nil)
+		got := generateStackNavigationComment(graph, 1, "<-- you are here", "", nil)
 		want := _commentHeader + "\n\n" +
 			joinLines(
 				"- #123",
@@ -827,7 +834,7 @@ func TestGenerateStackNavigationComment(t *testing.T) {
 		}
 		graph[0].Aboves = []int{1}
 
-		got := generateStackNavigationComment(graph, 1, "", nil)
+		got := generateStackNavigationComment(graph, 1, "", "", nil)
 		want := _commentHeader + "\n\n" +
 			joinLines(
 				"- #123+",
@@ -962,6 +969,33 @@ func TestNavCommentDownstack_UnmarshalText(t *testing.T) {
 	})
 }
 
+func TestExtractTrunkFromComment(t *testing.T) {
+	t.Run("HTML", func(t *testing.T) {
+		body := "some text\n<!-- gs:trunk main -->\n"
+		assert.Equal(t, "main", ExtractTrunkFromComment(body))
+	})
+
+	t.Run("Markdown", func(t *testing.T) {
+		body := "some text\n[gs:trunk]: # (develop)\n"
+		assert.Equal(t, "develop", ExtractTrunkFromComment(body))
+	})
+
+	t.Run("Missing", func(t *testing.T) {
+		body := "no trunk metadata here\n"
+		assert.Equal(t, "", ExtractTrunkFromComment(body))
+	})
+
+	t.Run("FullComment", func(t *testing.T) {
+		body := generateStackNavigationComment(
+			[]*stackedChange{
+				{Change: _changeID("1"), Base: -1},
+			},
+			0, "", "main", nil,
+		)
+		assert.Equal(t, "main", ExtractTrunkFromComment(body))
+	})
+}
+
 type _changeID string
 
 func (s _changeID) String() string {
diff --git a/internal/handler/sync/handler.go b/internal/handler/sync/handler.go
index 4aa7da79..af116412 100644
--- a/internal/handler/sync/handler.go
+++ b/internal/handler/sync/handler.go
@@ -78,7 +78,7 @@ type DeleteHandler interface {
 // RestackHandler allows restacking branches after sync
 // has removed their downstack branches.
 type RestackHandler interface {
-	RestackBranch(ctx context.Context, branch string) error
+	RestackBranch(ctx context.Context, branch string, opts *restack.Options) error
 	RestackUpstack(ctx context.Context, branch string, opts *restack.UpstackOptions) error
 }
 
@@ -983,7 +983,7 @@ func (h *Handler) restackDeletedBranchUpstack(
 			return fmt.Errorf("restack upstack %q: %w", target, err)
 		}
 	case mode.Includes(spice.RestackAboves):
-		if err := h.Restack.RestackBranch(ctx, target); err != nil {
+		if err := h.Restack.RestackBranch(ctx, target, nil); err != nil {
 			return fmt.Errorf("restack branch %q: %w", target, err)
 		}
 	case mode.Includes(spice.RestackNone):
diff --git a/internal/handler/sync/handler_test.go b/internal/handler/sync/handler_test.go
index b6450f28..415897d2 100644
--- a/internal/handler/sync/handler_test.go
+++ b/internal/handler/sync/handler_test.go
@@ -368,7 +368,7 @@ func TestHandler_SyncTrunk_restackDeletedUpstacks(t *testing.T) {
 
 		mockRestack := NewMockRestackHandler(ctrl)
 		mockRestack.EXPECT().
-			RestackBranch(gomock.Any(), "child").
+			RestackBranch(gomock.Any(), "child", gomock.Nil()).
 			Return(nil)
 
 		mockAutostash := NewMockAutostashHandler(ctrl)
diff --git a/internal/handler/sync/mocks_test.go b/internal/handler/sync/mocks_test.go
index 51c7adcf..de0af290 100644
--- a/internal/handler/sync/mocks_test.go
+++ b/internal/handler/sync/mocks_test.go
@@ -782,17 +782,17 @@ func (m *MockRestackHandler) EXPECT() *MockRestackHandlerMockRecorder {
 }
 
 // RestackBranch mocks base method.
-func (m *MockRestackHandler) RestackBranch(ctx context.Context, branch string) error {
+func (m *MockRestackHandler) RestackBranch(ctx context.Context, branch string, opts *restack.Options) error {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "RestackBranch", ctx, branch)
+	ret := m.ctrl.Call(m, "RestackBranch", ctx, branch, opts)
 	ret0, _ := ret[0].(error)
 	return ret0
 }
 
 // RestackBranch indicates an expected call of RestackBranch.
-func (mr *MockRestackHandlerMockRecorder) RestackBranch(ctx, branch any) *MockRestackHandlerRestackBranchCall {
+func (mr *MockRestackHandlerMockRecorder) RestackBranch(ctx, branch, opts any) *MockRestackHandlerRestackBranchCall {
 	mr.mock.ctrl.T.Helper()
-	call := mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RestackBranch", reflect.TypeOf((*MockRestackHandler)(nil).RestackBranch), ctx, branch)
+	call := mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RestackBranch", reflect.TypeOf((*MockRestackHandler)(nil).RestackBranch), ctx, branch, opts)
 	return &MockRestackHandlerRestackBranchCall{Call: call}
 }
 
@@ -808,13 +808,13 @@ func (c *MockRestackHandlerRestackBranchCall) Return(arg0 error) *MockRestackHan
 }
 
 // Do rewrite *gomock.Call.Do
-func (c *MockRestackHandlerRestackBranchCall) Do(f func(context.Context, string) error) *MockRestackHandlerRestackBranchCall {
+func (c *MockRestackHandlerRestackBranchCall) Do(f func(context.Context, string, *restack.Options) error) *MockRestackHandlerRestackBranchCall {
 	c.Call = c.Call.Do(f)
 	return c
 }
 
 // DoAndReturn rewrite *gomock.Call.DoAndReturn
-func (c *MockRestackHandlerRestackBranchCall) DoAndReturn(f func(context.Context, string) error) *MockRestackHandlerRestackBranchCall {
+func (c *MockRestackHandlerRestackBranchCall) DoAndReturn(f func(context.Context, string, *restack.Options) error) *MockRestackHandlerRestackBranchCall {
 	c.Call = c.Call.DoAndReturn(f)
 	return c
 }
diff --git a/main.go b/main.go
index d020ea4c..50eb0451 100644
--- a/main.go
+++ b/main.go
@@ -314,6 +314,8 @@ type mainCmd struct {
 
 	Rebase rebaseCmd `cmd:"" aliases:"rb" group:"Rebase"`
 
+	CI ciCmd `cmd:"" group:"CI" help:"CI/CD integration commands"`
+
 	// Navigation
 	Up     upCmd     `cmd:"" aliases:"u" group:"Navigation" help:"Move up one branch"`
 	Down   downCmd   `cmd:"" aliases:"d" group:"Navigation" help:"Move down one branch"`
diff --git a/stack_restack.go b/stack_restack.go
index 88ef562b..7d7b4d68 100644
--- a/stack_restack.go
+++ b/stack_restack.go
@@ -88,5 +88,5 @@ func (cmd *stackRestackCmd) Run(
 		return err
 	}
 
-	return handler.RestackStack(ctx, cmd.Branch)
+	return handler.RestackStack(ctx, cmd.Branch, nil)
 }
diff --git a/testdata/help/ci_merge-guard.txt b/testdata/help/ci_merge-guard.txt
new file mode 100644
index 00000000..823186af
--- /dev/null
+++ b/testdata/help/ci_merge-guard.txt
@@ -0,0 +1,35 @@
+Usage: gs ci merge-guard <number> [flags]
+
+Block merging a PR whose base is not trunk
+
+Checks whether a change request is safe to merge by verifying its base branch is
+trunk.
+
+Use this in forge CI/CD pipelines to prevent out-of-order merges in a stacked PR
+workflow.
+
+By default, only git-spice managed PRs are checked. Unmanaged PRs are allowed
+through. Use --all to block any PR whose base is not trunk.
+
+The trunk branch is detected from the git-spice navigation comment on the PR.
+Use --trunk to override this detection.
+
+Exit codes:
+
+    0  PR is safe to merge (base is trunk, or unmanaged)
+    1  PR should not be merged yet
+
+Arguments:
+  <number>    Change request number to check
+
+Flags:
+  --trunk=STRING    Override trunk branch name
+  --all             Block all non-trunk-based PRs, not just git-spice managed
+                    ones
+
+Global Flags:
+  -h, --help           Show help for the command
+      --version        Print version information and quit
+  -v, --verbose        Enable verbose output ($GIT_SPICE_VERBOSE)
+  -C, --dir=DIR        Change to DIR before doing anything
+      --[no-]prompt    Whether to prompt for missing information
diff --git a/testdata/help/gs.txt b/testdata/help/gs.txt
index 43406fde..51e82ea4 100644
--- a/testdata/help/gs.txt
+++ b/testdata/help/gs.txt
@@ -73,6 +73,9 @@ Rebase
   rebase (rb) continue (c)    Continue an interrupted operation
   rebase (rb) abort (a)       Abort an operation
 
+CI
+  ci merge-guard    Block merging a PR whose base is not trunk
+
 Navigation
   up (u)        Move up one branch
   down (d)      Move down one branch
diff --git a/testdata/script/branch_create_below_with_downstack_history.txt b/testdata/script/branch_create_below_with_downstack_history.txt
index 4ac2630c..196a392b 100644
--- a/testdata/script/branch_create_below_with_downstack_history.txt
+++ b/testdata/script/branch_create_below_with_downstack_history.txt
@@ -81,6 +81,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -91,6 +92,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/final-comments.txt --
 - change: 2
   body: |
@@ -103,6 +105,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -114,6 +117,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 4
   body: |
     This change is part of the following stack:
@@ -125,3 +129,4 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_onto_two_stacks_with_downstack_history.txt b/testdata/script/branch_onto_two_stacks_with_downstack_history.txt
index 0cea91ca..dc1a17d0 100644
--- a/testdata/script/branch_onto_two_stacks_with_downstack_history.txt
+++ b/testdata/script/branch_onto_two_stacks_with_downstack_history.txt
@@ -152,6 +152,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -162,6 +163,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -172,6 +174,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/submit-second-stack-comments.txt --
 - change: 4
   body: |
@@ -182,6 +185,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 5
   body: |
     This change is part of the following stack:
@@ -191,6 +195,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/bottom-prs-merged-comments.txt --
 - change: 2
   body: |
@@ -202,6 +207,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -212,6 +218,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 5
   body: |
     This change is part of the following stack:
@@ -221,6 +228,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/branch-onto-comments.txt --
 - change: 2
   body: |
@@ -233,6 +241,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -243,6 +252,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 5
   body: |
     This change is part of the following stack:
@@ -253,3 +263,4 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_by_name.txt b/testdata/script/branch_submit_by_name.txt
index 73b25bbc..dcfe1787 100644
--- a/testdata/script/branch_submit_by_name.txt
+++ b/testdata/script/branch_submit_by_name.txt
@@ -71,3 +71,4 @@ Contents of feature1
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_detect_existing.txt b/testdata/script/branch_submit_detect_existing.txt
index b905a7c7..246f144b 100644
--- a/testdata/script/branch_submit_detect_existing.txt
+++ b/testdata/script/branch_submit_detect_existing.txt
@@ -120,6 +120,7 @@ feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/pulls-updated.json --
 [
   {
@@ -181,6 +182,7 @@ feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -190,3 +192,4 @@ feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_navigation_comment_deleted.txt b/testdata/script/branch_submit_navigation_comment_deleted.txt
index ef6196a4..1e55da55 100644
--- a/testdata/script/branch_submit_navigation_comment_deleted.txt
+++ b/testdata/script/branch_submit_navigation_comment_deleted.txt
@@ -53,6 +53,7 @@ feature content
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/no-comments.txt --
 []
 -- golden/recreated-comments.txt --
@@ -64,4 +65,5 @@ feature content
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/end --
diff --git a/testdata/script/branch_submit_navigation_comment_downstack_open.txt b/testdata/script/branch_submit_navigation_comment_downstack_open.txt
index 6c46a9d2..0d5a1e34 100644
--- a/testdata/script/branch_submit_navigation_comment_downstack_open.txt
+++ b/testdata/script/branch_submit_navigation_comment_downstack_open.txt
@@ -59,6 +59,7 @@ feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/comments-after.txt --
 - change: 2
   body: |
@@ -68,3 +69,4 @@ feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_navigation_comment_opt_out_multiple.txt b/testdata/script/branch_submit_navigation_comment_opt_out_multiple.txt
index 02c23a45..9eaa426c 100644
--- a/testdata/script/branch_submit_navigation_comment_opt_out_multiple.txt
+++ b/testdata/script/branch_submit_navigation_comment_opt_out_multiple.txt
@@ -64,6 +64,7 @@ foo
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -73,3 +74,4 @@ foo
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_navigation_comment_sync_branch.txt b/testdata/script/branch_submit_navigation_comment_sync_branch.txt
index 58698db8..f7aad447 100644
--- a/testdata/script/branch_submit_navigation_comment_sync_branch.txt
+++ b/testdata/script/branch_submit_navigation_comment_sync_branch.txt
@@ -56,6 +56,7 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -65,6 +66,7 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -75,3 +77,4 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_navigation_comment_sync_downstack.txt b/testdata/script/branch_submit_navigation_comment_sync_downstack.txt
index 28e3cbf4..a7d40f85 100644
--- a/testdata/script/branch_submit_navigation_comment_sync_downstack.txt
+++ b/testdata/script/branch_submit_navigation_comment_sync_downstack.txt
@@ -75,6 +75,7 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -85,6 +86,7 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -95,3 +97,4 @@ feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/branch_submit_rename.txt b/testdata/script/branch_submit_rename.txt
index 5665032f..4af78bfa 100644
--- a/testdata/script/branch_submit_rename.txt
+++ b/testdata/script/branch_submit_rename.txt
@@ -88,6 +88,7 @@ New contents of feature1
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/update.json --
 [
   {
@@ -124,3 +125,4 @@ New contents of feature1
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/ci_merge_guard_already_merged.txt b/testdata/script/ci_merge_guard_already_merged.txt
new file mode 100644
index 00000000..ba08b8ec
--- /dev/null
+++ b/testdata/script/ci_merge_guard_already_merged.txt
@@ -0,0 +1,34 @@
+# 'ci merge-guard' handles an already-merged PR gracefully.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a branch on trunk and submit
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+gs bs --fill
+stderr 'Created #1'
+
+# merge externally
+shamhub merge alice/example 1
+
+# merged PR was based on main (trunk) — still safe
+gs ci merge-guard 1
+stderr 'safe to merge'
+
+-- repo/feature1.txt --
+This is feature 1
diff --git a/testdata/script/ci_merge_guard_blocked.txt b/testdata/script/ci_merge_guard_blocked.txt
new file mode 100644
index 00000000..061efae7
--- /dev/null
+++ b/testdata/script/ci_merge_guard_blocked.txt
@@ -0,0 +1,43 @@
+# 'ci merge-guard' blocks a PR whose base is not trunk.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a stack: main -> feature1 -> feature2
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+git add feature2.txt
+gs bc feature2 -m 'Add feature 2'
+
+# submit the full stack
+gs downstack submit --fill
+stderr 'Created #1'
+stderr 'Created #2'
+
+# PR #2 is based on feature1, not trunk — should be blocked
+! gs ci merge-guard 2
+stderr 'base is "feature1"'
+stderr 'expected trunk "main"'
+
+# PR #1 is based on main — should be safe
+gs ci merge-guard 1
+stderr 'safe to merge'
+
+-- repo/feature1.txt --
+This is feature 1
+-- repo/feature2.txt --
+This is feature 2
diff --git a/testdata/script/ci_merge_guard_closed.txt b/testdata/script/ci_merge_guard_closed.txt
new file mode 100644
index 00000000..6dbfe16e
--- /dev/null
+++ b/testdata/script/ci_merge_guard_closed.txt
@@ -0,0 +1,34 @@
+# 'ci merge-guard' handles a closed (rejected) PR.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a branch on trunk and submit
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+gs bs --fill
+stderr 'Created #1'
+
+# close (reject) the PR
+shamhub reject alice/example 1
+
+# closed PR was based on main (trunk) — still safe
+gs ci merge-guard 1
+stderr 'safe to merge'
+
+-- repo/feature1.txt --
+This is feature 1
diff --git a/testdata/script/ci_merge_guard_deep_stack.txt b/testdata/script/ci_merge_guard_deep_stack.txt
new file mode 100644
index 00000000..9ab64efe
--- /dev/null
+++ b/testdata/script/ci_merge_guard_deep_stack.txt
@@ -0,0 +1,53 @@
+# 'ci merge-guard' blocks at every non-trunk level of a deep stack.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a deep stack: main -> feat1 -> feat2 -> feat3
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+git add feature2.txt
+gs bc feature2 -m 'Add feature 2'
+git add feature3.txt
+gs bc feature3 -m 'Add feature 3'
+
+# submit the full stack
+gs downstack submit --fill
+stderr 'Created #1'
+stderr 'Created #2'
+stderr 'Created #3'
+
+# PR #1 based on main — safe
+gs ci merge-guard 1
+stderr 'safe to merge'
+
+# PR #2 based on feature1, not trunk — blocked
+! gs ci merge-guard 2
+stderr 'base is "feature1"'
+stderr 'expected trunk "main"'
+
+# PR #3 based on feature2, not trunk — blocked
+! gs ci merge-guard 3
+stderr 'base is "feature2"'
+stderr 'expected trunk "main"'
+
+-- repo/feature1.txt --
+This is feature 1
+-- repo/feature2.txt --
+This is feature 2
+-- repo/feature3.txt --
+This is feature 3
diff --git a/testdata/script/ci_merge_guard_not_found.txt b/testdata/script/ci_merge_guard_not_found.txt
new file mode 100644
index 00000000..27194b36
--- /dev/null
+++ b/testdata/script/ci_merge_guard_not_found.txt
@@ -0,0 +1,24 @@
+# 'ci merge-guard' fails cleanly when the PR does not exist.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# no PRs exist — guard should fail with a clear error
+! gs ci merge-guard 999
+stderr 'find change #999'
+
+-- repo/.gitkeep --
diff --git a/testdata/script/ci_merge_guard_safe.txt b/testdata/script/ci_merge_guard_safe.txt
new file mode 100644
index 00000000..d1ad9295
--- /dev/null
+++ b/testdata/script/ci_merge_guard_safe.txt
@@ -0,0 +1,31 @@
+# 'ci merge-guard' allows a trunk-based PR to merge.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a branch on trunk and submit
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+gs bs --fill
+stderr 'Created #1'
+
+# PR #1 is based on main (trunk) — should be safe
+gs ci merge-guard 1
+stderr 'safe to merge'
+
+-- repo/feature1.txt --
+This is feature 1
diff --git a/testdata/script/ci_merge_guard_trunk_override.txt b/testdata/script/ci_merge_guard_trunk_override.txt
new file mode 100644
index 00000000..3637052d
--- /dev/null
+++ b/testdata/script/ci_merge_guard_trunk_override.txt
@@ -0,0 +1,43 @@
+# 'ci merge-guard --trunk' overrides trunk detection.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a stack: main -> feature1 -> feature2
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+git add feature2.txt
+gs bc feature2 -m 'Add feature 2'
+
+gs downstack submit --fill
+stderr 'Created #1'
+stderr 'Created #2'
+
+# PR #2 is based on feature1.
+# With --trunk=feature1, it should pass.
+gs ci merge-guard --trunk feature1 2
+stderr 'safe to merge'
+
+# With --trunk=main (correct trunk), it should block.
+! gs ci merge-guard --trunk main 2
+stderr 'base is "feature1"'
+stderr 'expected trunk "main"'
+
+-- repo/feature1.txt --
+This is feature 1
+-- repo/feature2.txt --
+This is feature 2
diff --git a/testdata/script/ci_merge_guard_unmanaged.txt b/testdata/script/ci_merge_guard_unmanaged.txt
new file mode 100644
index 00000000..0b7e4a07
--- /dev/null
+++ b/testdata/script/ci_merge_guard_unmanaged.txt
@@ -0,0 +1,39 @@
+# 'ci merge-guard' allows unmanaged PRs by default,
+# blocks them with --all.
+
+as 'Test <test@example.com>'
+at '2024-04-05T16:40:32Z'
+
+# setup
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+
+# set up a fake GitHub remote
+shamhub init
+shamhub new origin alice/example.git
+shamhub register alice
+git push origin main
+
+env SHAMHUB_USERNAME=alice
+gs auth login
+
+# create a branch on trunk and submit
+git add feature1.txt
+gs bc feature1 -m 'Add feature 1'
+gs bs --fill
+stderr 'Created #1'
+
+# delete the navigation comment to simulate an unmanaged PR
+shamhub delete-comment 1
+
+# by default, unmanaged PRs are allowed through
+gs ci merge-guard 1
+stderr 'not managed by git-spice, allowing'
+
+# with --all, unmanaged PRs are blocked
+! gs ci merge-guard --all 1
+stderr 'unmanaged PR blocked by --all'
+
+-- repo/feature1.txt --
+This is feature 1
diff --git a/testdata/script/downstack_submit.txt b/testdata/script/downstack_submit.txt
index 6fa9a506..a11088a8 100644
--- a/testdata/script/downstack_submit.txt
+++ b/testdata/script/downstack_submit.txt
@@ -128,6 +128,7 @@ This is feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -138,6 +139,7 @@ This is feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -148,3 +150,4 @@ This is feature 2
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/repo_sync_unsubmitted_upstack_history.txt b/testdata/script/repo_sync_unsubmitted_upstack_history.txt
index f24b54dd..a9b24501 100644
--- a/testdata/script/repo_sync_unsubmitted_upstack_history.txt
+++ b/testdata/script/repo_sync_unsubmitted_upstack_history.txt
@@ -60,6 +60,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -69,3 +70,4 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/stack_edit_inserted_at_bottom_with_downstack_history.txt b/testdata/script/stack_edit_inserted_at_bottom_with_downstack_history.txt
index c79faea3..17f7ea99 100644
--- a/testdata/script/stack_edit_inserted_at_bottom_with_downstack_history.txt
+++ b/testdata/script/stack_edit_inserted_at_bottom_with_downstack_history.txt
@@ -114,6 +114,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -125,6 +126,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -136,6 +138,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 4
   body: |
     This change is part of the following stack:
@@ -147,6 +150,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/ls.txt --
     ┏━□ feature3 (#3)
   ┏━┻□ feature2 (#2)
@@ -165,6 +169,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -176,6 +181,7 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 4
   body: |
     This change is part of the following stack:
@@ -187,3 +193,4 @@ main
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/stack_submit.txt b/testdata/script/stack_submit.txt
index d836f990..82ec2412 100644
--- a/testdata/script/stack_submit.txt
+++ b/testdata/script/stack_submit.txt
@@ -148,6 +148,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -158,6 +159,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -168,6 +170,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/pr-1-merged.json --
 [
   {
@@ -253,6 +256,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -263,6 +267,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -273,6 +278,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/ls.txt --
     ┏━□ feature3 (#3)
   ┏━┻□ feature2 (#2)
diff --git a/testdata/script/stack_submit_multiple_merged_history.txt b/testdata/script/stack_submit_multiple_merged_history.txt
index ba40d9c6..6be62d59 100644
--- a/testdata/script/stack_submit_multiple_merged_history.txt
+++ b/testdata/script/stack_submit_multiple_merged_history.txt
@@ -67,6 +67,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -77,6 +78,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -87,6 +89,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/post-merge-comments.txt --
 - change: 1
   body: |
@@ -98,6 +101,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -108,6 +112,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -118,6 +123,7 @@ This is feature 3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/ls.txt --
     ┏━□ feature3 (#3)
   ┏━┻□ feature2 (#2)
diff --git a/testdata/script/stack_submit_update_leave_draft.txt b/testdata/script/stack_submit_update_leave_draft.txt
index ca804fbf..13c92791 100644
--- a/testdata/script/stack_submit_update_leave_draft.txt
+++ b/testdata/script/stack_submit_update_leave_draft.txt
@@ -240,6 +240,7 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -249,6 +250,7 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -259,6 +261,7 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 -- golden/comments/pr-1-merged.txt --
 - change: 1
   body: |
@@ -268,6 +271,7 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -278,6 +282,7 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -288,3 +293,4 @@ INF CR #3 is up-to-date: $SHAMHUB_URL/alice/example/change/3
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/testdata/script/upstack_submit_main.txt b/testdata/script/upstack_submit_main.txt
index 3c16bd34..78ec7d50 100644
--- a/testdata/script/upstack_submit_main.txt
+++ b/testdata/script/upstack_submit_main.txt
@@ -192,6 +192,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 2
   body: |
     This change is part of the following stack:
@@ -201,6 +202,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 3
   body: |
     This change is part of the following stack:
@@ -211,6 +213,7 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
 - change: 4
   body: |
     This change is part of the following stack:
@@ -221,3 +224,4 @@ main ◀
 
     <sub>Change managed by [git-spice](https://abhinav.github.io/git-spice/).</sub>
     <!-- gs:navigation comment -->
+    <!-- gs:trunk main -->
diff --git a/upstack_restack.go b/upstack_restack.go
index f6e6e052..4def1ada 100644
--- a/upstack_restack.go
+++ b/upstack_restack.go
@@ -34,10 +34,10 @@ func (*upstackRestackCmd) Help() string {
 // RestackHandler implements high level restack operations.
 type RestackHandler interface {
 	RestackUpstack(ctx context.Context, branch string, opts *restack.UpstackOptions) error
-	RestackDownstack(ctx context.Context, branch string) error
+	RestackDownstack(ctx context.Context, branch string, opts *restack.Options) error
 	Restack(context.Context, *restack.Request) (int, error)
-	RestackStack(ctx context.Context, branch string) error
-	RestackBranch(ctx context.Context, branch string) error
+	RestackStack(ctx context.Context, branch string, opts *restack.Options) error
+	RestackBranch(ctx context.Context, branch string, opts *restack.Options) error
 }
 
 func (cmd *upstackRestackCmd) AfterApply(ctx context.Context, wt *git.Worktree) error {