diff --git a/README.md b/README.md
index 71e2e4e..45bb481 100644
--- a/README.md
+++ b/README.md
@@ -114,6 +114,18 @@ open: some store migrations need one more recovery-mode boot, which is a manual
 step, and the Docker path has never completed a migration against a real
 Stalwart image. Details and field reports: [Status](docs/status.md).
 
+## Support
+
+| Upgrade | Status |
+| --- | --- |
+| 0.15.5 → 0.16.27 | Supported. `--target` defaults to 0.16.27, the last 0.16 release |
+| 0.16 → 1.0 | Not yet. The route is designed in [docs/design-1.0.md](docs/design-1.0.md) but not built, so the tool refuses a 1.0 target |
+| Anything else | Not supported |
+
+The 0.15.5 → 0.16 path gets bug and security fixes. If a Stalwart release
+changes its upgrade path in a way the tool can't follow, the tool refuses that
+target, and this table says so.
+
 ## License
 
 Copyright (C) 2026 Coffey Labs LLC. GPL-3.0-or-later: free software, with no
diff --git a/cmd/stalwart-migrate/preflight.go b/cmd/stalwart-migrate/preflight.go
index 73b5970..e51e8ef 100644
--- a/cmd/stalwart-migrate/preflight.go
+++ b/cmd/stalwart-migrate/preflight.go
@@ -23,7 +23,7 @@ func runPreflight(args []string) error {
 	adminUser := fs.String("admin-user", "", "admin username for the reachability check")
 	adminPassword := fs.String("admin-password", os.Getenv("STALWART_MIGRATE_ADMIN_PASSWORD"),
 		"admin password for the reachability check (or set STALWART_MIGRATE_ADMIN_PASSWORD)")
-	targetVersion := fs.String("target", "latest", `target Stalwart version, or "latest"`)
+	targetVersion := fs.String("target", preflight.DefaultTargetVersion, `target Stalwart version, or "latest" (1.0 and later are refused for now)`)
 	targetBinary := fs.String("target-binary", "", "read the target version from this already-downloaded binary instead of the release API (for a host with no route to the internet)")
 	minFree := fs.Float64("min-free-multiple", 2.0, "required free disk space as a multiple of the data directory size")
 	stateDir := fs.String("state-dir", checkpoint.DefaultBaseDir, "directory to store run checkpoints in")
diff --git a/cmd/stalwart-migrate/rehearse.go b/cmd/stalwart-migrate/rehearse.go
index 1ff00a5..f474663 100644
--- a/cmd/stalwart-migrate/rehearse.go
+++ b/cmd/stalwart-migrate/rehearse.go
@@ -48,7 +48,7 @@ func runRehearse(args []string) (err error) {
 	adminUser := fs.String("admin-user", "", "admin username")
 	adminPassword := fs.String("admin-password", os.Getenv("STALWART_MIGRATE_ADMIN_PASSWORD"),
 		"admin password (or set STALWART_MIGRATE_ADMIN_PASSWORD)")
-	targetVersion := fs.String("target", "latest", `target Stalwart version, or "latest"`)
+	targetVersion := fs.String("target", preflight.DefaultTargetVersion, `target Stalwart version, or "latest" (1.0 and later are refused for now)`)
 	stateDir := fs.String("state-dir", checkpoint.DefaultBaseDir, "directory to store run checkpoints in")
 	workDir := fs.String("work-dir", "/var/lib/stalwart-migrator/work", "scratch directory for the dumps and converted plan (cleaned up afterward - see --keep-artifacts)")
 	pythonPath := fs.String("python", "python3", "path to python3")
diff --git a/cmd/stalwart-migrate/run.go b/cmd/stalwart-migrate/run.go
index eab5de7..7069f5c 100644
--- a/cmd/stalwart-migrate/run.go
+++ b/cmd/stalwart-migrate/run.go
@@ -61,7 +61,7 @@ func runRun(args []string) (err error) {
 	adminUser := fs.String("admin-user", "", "admin username - must be a directory account, not a config fallback-admin (see README)")
 	adminPassword := fs.String("admin-password", os.Getenv("STALWART_MIGRATE_ADMIN_PASSWORD"),
 		"admin password (or set STALWART_MIGRATE_ADMIN_PASSWORD)")
-	targetVersion := fs.String("target", "latest", `target Stalwart version, or "latest"`)
+	targetVersion := fs.String("target", preflight.DefaultTargetVersion, `target Stalwart version, or "latest" (1.0 and later are refused for now)`)
 	targetBinary := fs.String("target-binary", "", "use an already-downloaded target binary instead of fetching one")
 	targetImage := fs.String("target-image", "", "for a container deployment: the target image, named in full "+
 		"(e.g. stalwartlabs/stalwart:v0.16.14). Never guessed from the running container - a derived tag is wrong for a "+
diff --git a/docs/docker.md b/docs/docker.md
index b90824c..930611d 100644
--- a/docs/docker.md
+++ b/docs/docker.md
@@ -20,7 +20,7 @@ goes double here.
 ## Flags
 
 - `--target-image` names the image in full, e.g.
-  `stalwartlabs/stalwart:v0.16.14`. It is never derived from the running
+  `stalwartlabs/stalwart:v0.16.27`. It is never derived from the running
   container by swapping the tag — that is wrong for a digest-pinned image, a
   mirror or a fork, and being wrong means pulling the wrong software into a
   mail server.
diff --git a/docs/rehearsal.md b/docs/rehearsal.md
index b08e077..b217c2a 100644
--- a/docs/rehearsal.md
+++ b/docs/rehearsal.md
@@ -35,7 +35,7 @@ password with `--admin-password` or `STALWART_MIGRATE_ADMIN_PASSWORD`.
 
 ```sh
 stalwart-migrate rehearse --admin-url https://mail.example.com \
-    --admin-user admin --target 0.16.14
+    --admin-user admin --target 0.16.27
 ```
 
 It runs preflight, dumps your settings and principals, converts them with
diff --git a/internal/plan/plan.go b/internal/plan/plan.go
index cb17da9..f10e188 100644
--- a/internal/plan/plan.go
+++ b/internal/plan/plan.go
@@ -103,6 +103,13 @@ func Decide(sourceVersion, targetVersion string) (*Plan, error) {
 	if src.Compare(tgt) >= 0 {
 		return nil, fmt.Errorf("plan: source %s is already at or beyond target %s - nothing to migrate", src, tgt)
 	}
+	// 1.0 can't run on a 0.16 store: it needs the data copied into a new
+	// one, which this tool doesn't do yet (docs/design-1.0.md). Treating
+	// 0.16 -> 1.0 as a patch bump would swap the binary over a store it
+	// can't read.
+	if tgt.Major >= 1 {
+		return nil, fmt.Errorf("plan: target %s is Stalwart 1.0 or later, which this tool can't migrate to yet - target 0.16.27, the last 0.16 release", tgt)
+	}
 
 	crosses := src.Major == 0 && src.Minor < 16 && (tgt.Major > 0 || tgt.Minor >= 16)
 	if crosses {
diff --git a/internal/plan/plan_test.go b/internal/plan/plan_test.go
index a9d3d82..9a8f400 100644
--- a/internal/plan/plan_test.go
+++ b/internal/plan/plan_test.go
@@ -52,15 +52,18 @@ func TestDecideRefusesRegression(t *testing.T) {
 	}
 }
 
-func TestDecideFutureMajorCrossesBoundaryToo(t *testing.T) {
-	// A hypothetical 0.15.x -> 1.0.0 jump should still be treated as
-	// crossing the boundary this tool knows how to automate.
-	p, err := Decide("0.15.9", "1.0.0")
-	if err != nil {
-		t.Fatalf("Decide: %v", err)
+func TestDecideRefusesStalwart1(t *testing.T) {
+	// This used to accept 0.15.x -> 1.0.0 as one more crossing of the
+	// 0.15/0.16 boundary. Stalwart 1.0 shipped needing its data copied into
+	// a new store, which no plan here does, so both routes to it are
+	// refused - 0.16.x -> 1.0.0 would otherwise pass as a patch bump.
+	for _, src := range []string{"0.15.5", "0.16.22", "0.16.27"} {
+		if p, err := Decide(src, "1.0.0"); err == nil {
+			t.Errorf("Decide(%s, 1.0.0) = %+v, want a refusal", src, p)
+		}
 	}
-	if !p.CrossesMajorBoundary {
-		t.Error("CrossesMajorBoundary = false, want true for 0.15.9 -> 1.0.0")
+	if _, err := Decide("0.15.5", "0.16.27"); err != nil {
+		t.Errorf("Decide(0.15.5, 0.16.27): %v", err)
 	}
 }
 
diff --git a/internal/preflight/checks.go b/internal/preflight/checks.go
index c892b25..951612d 100644
--- a/internal/preflight/checks.go
+++ b/internal/preflight/checks.go
@@ -179,6 +179,12 @@ func (c *Checker) Run(ctx context.Context, store *checkpoint.Store, rs *checkpoi
 				Detail: fmt.Sprintf("current version %s is already at or beyond target %s - nothing to migrate", curV, tgtV),
 			}, ""
 		}
+		if tgtV.Major >= 1 {
+			return CheckResult{
+				Status: StatusFail,
+				Detail: fmt.Sprintf("target %s is Stalwart 1.0 or later, which needs its data copied into a new store - this tool can't do that yet (docs/design-1.0.md); pass --target %s, the last 0.16 release", tgtV, DefaultTargetVersion),
+			}, ""
+		}
 		if curV.Major == 0 && curV.Minor < 16 && (tgtV.Major > 0 || tgtV.Minor >= 16) {
 			return CheckResult{
 				Status: StatusOK,
diff --git a/internal/preflight/checks_test.go b/internal/preflight/checks_test.go
index 67e560c..371b03b 100644
--- a/internal/preflight/checks_test.go
+++ b/internal/preflight/checks_test.go
@@ -607,3 +607,41 @@ func TestRehearseWarnsRatherThanBlocksOnAnUnrecreatableContainer(t *testing.T) {
 		t.Fatalf("advisory mode should not block, got:\n%s", report.String())
 	}
 }
+
+func TestLatestResolvingToStalwart1IsRefused(t *testing.T) {
+	// Since 2026-10-26 Stalwart's latest release is 1.0.0. A 0.16 server
+	// asked to go to "latest" would otherwise plan a patch bump: the new
+	// binary swapped in over a store it can't read.
+	fakeTool(t, "stalwart-cli", "stalwart-cli 1.0.12")
+	fakeTool(t, "python3", "Python 3.13.5")
+	binaryPath := writeFakeBinary(t, "0.16.22", filepath.Join(t.TempDir(), "invocations"))
+	configPath := filepath.Join(t.TempDir(), "config.json")
+	if err := os.WriteFile(configPath, []byte(`{}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	withFakeGithub(t, func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(Release{TagName: "v1.0.0"})
+	})
+
+	store := checkpoint.NewStore(t.TempDir())
+	rs, err := store.Create("", "latest")
+	if err != nil {
+		t.Fatalf("store.Create: %v", err)
+	}
+	report, err := New(Options{
+		BinaryPath:    binaryPath,
+		ConfigPath:    configPath,
+		DataDir:       t.TempDir(),
+		TargetVersion: "latest",
+	}).Run(context.Background(), store, rs)
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	res := resultFor(t, report, "upgrade-direction")
+	if res.Status != StatusFail || !strings.Contains(res.Detail, DefaultTargetVersion) {
+		t.Errorf("upgrade-direction = %s %q, want a failure naming %s", res.Status, res.Detail, DefaultTargetVersion)
+	}
+	if !report.Blocking() {
+		t.Error("report should block a run that targets 1.0")
+	}
+}
diff --git a/internal/preflight/release.go b/internal/preflight/release.go
index 59658f4..1844aca 100644
--- a/internal/preflight/release.go
+++ b/internal/preflight/release.go
@@ -14,6 +14,11 @@ import (
 
 // githubAPIBase is a var, not a const, so tests can point it at an
 // httptest server instead of hitting the real GitHub API.
+// DefaultTargetVersion is what --target defaults to: the last 0.16 release.
+// It was "latest" until Stalwart's latest became 1.0, which this tool can't
+// migrate to yet.
+const DefaultTargetVersion = "0.16.27"
+
 var githubAPIBase = "https://api.github.com/repos/stalwartlabs/stalwart/releases"
 
 type ReleaseAsset struct {