diff --git a/zig/build/cuda.zig b/zig/build/cuda.zig
index c224600140..e3b603eac0 100644
--- a/zig/build/cuda.zig
+++ b/zig/build/cuda.zig
@@ -382,7 +382,7 @@ pub fn hostTests(b: *std.Build, draft_ids: *std.Build.Module, build_options: *st
     @import("qwen_cost_open.zig").tests(b, mods, native_modules, step);
     const windows_port = @import("windows_host.zig").steps(b, cuda, native_modules.api);
     const adapter_tests = b.createModule(.{ .root_source_file = b.path("zig/src/native/slide_adapter_test.zig"), .target = host, .link_libc = true, .imports = &.{.{ .name = "engine_api", .module = native_modules.api }} });
-    b.step("test-slide-adapter-host", "Initial learner progress before its first long unit").dependOn(&b.addRunArtifact(b.addTest(.{ .root_module = adapter_tests })).step);
+    b.step("test-slide-adapter-host", "Slide adapter: initial learner progress, and --slide's startup refusals").dependOn(&b.addRunArtifact(b.addTest(.{ .root_module = adapter_tests })).step);
     @import("drama3_host.zig").native(b, step, drama3, mods.core, native_modules.api, cuda, "drama_cuda_native_host");
     @import("drama3_host.zig").metal(b, mods.core, native_modules.api);
     const safety = b.step("test-kv-safety-host", "Actual native KV startup budget and paged constructors under allocation/completion faults");
diff --git a/zig/src/core/checkpoint_index.zig b/zig/src/core/checkpoint_index.zig
index 7636a31357..1d60cf3114 100644
--- a/zig/src/core/checkpoint_index.zig
+++ b/zig/src/core/checkpoint_index.zig
@@ -15,16 +15,31 @@ pub fn plainName(name: []const u8) bool {
     return true;
 }
 
+/// An index's shard: a plain name, or plain names joined by '/' with none starting with '.', inside the model folder.
+pub fn shardName(name: []const u8) bool {
+    if (std.mem.indexOfScalar(u8, name, '/') == null) return plainName(name);
+    var parts = std.mem.splitScalar(u8, name, '/');
+    while (parts.next()) |part| if (!plainName(part) or part[0] == '.') return false;
+    return true;
+}
+
 pub fn weightMap(root: std.json.Value) error{BadIndex}!std.json.ObjectMap {
     if (root != .object) return error.BadIndex;
     const map = root.object.get("weight_map") orelse return error.BadIndex;
     if (map != .object) return error.BadIndex;
     for (map.object.values()) |value| {
-        if (value != .string or !plainName(value.string)) return error.BadIndex;
+        if (value != .string or !shardName(value.string)) return error.BadIndex;
     }
     return map.object;
 }
 
+/// The weight map with each shard directly in the model folder, as writes need: shard_edit.linked sees only those.
+pub fn plainWeightMap(root: std.json.Value) error{BadIndex}!std.json.ObjectMap {
+    const map = try weightMap(root);
+    for (map.values()) |value| if (!plainName(value.string)) return error.BadIndex;
+    return map;
+}
+
 test "portable shard names exclude path and Windows device aliases" {
     const bad = [_][]const u8{
         "",                 ".",         "..",        ".. ",     "model.",    "model ",       "model\x00suffix", "model\nname",
@@ -35,3 +50,15 @@ test "portable shard names exclude path and Windows device aliases" {
     for ([_][]const u8{ "model.safetensors", "model-00001-of-00002.safetensors", "model.v1", "unicode-λ" }) |name|
         try std.testing.expect(plainName(name));
 }
+
+test "a shard name may name a folder inside the model folder, each part plain and none starting with '.'" {
+    const bad = [_][]const u8{
+        "",                   "/model.safetensors", "sub/",       "sub//model",  "./model",      "sub/./model",    "sub/../model",
+        "../sub",             "sub/..",             "sub\\model", "C:/model",    "CON/model",    "sub/CON",        "sub./model",
+        "sub /model",         "sub/model ",         "sub/m\x00x", "a/b/../../x", ".cache/model", "sub/.hid/model", "sub/.model",
+        "tensors/native-QB.",
+    };
+    for (bad) |name| try std.testing.expect(!shardName(name));
+    for ([_][]const u8{ "model.safetensors", "tensors/native-QB.safetensors", "a/b/c.safetensors" }) |name|
+        try std.testing.expect(shardName(name));
+}
diff --git a/zig/src/core/checkpoint_security_test.zig b/zig/src/core/checkpoint_security_test.zig
index 220a3fb2b4..643eb6e60a 100644
--- a/zig/src/core/checkpoint_security_test.zig
+++ b/zig/src/core/checkpoint_security_test.zig
@@ -33,10 +33,10 @@ test "security: checkpoint shard names cannot escape their model folder" {
     const dir = try tmp.dir.realPathFileAlloc(io, ".", a);
     defer a.free(dir);
     const invalid = [_][]const u8{
-        "../outside.safetensors", "/tmp/outside.safetensors",   "sub/../outside.safetensors",
-        "sub/model.safetensors",  "..\\outside.safetensors",    "C:\\outside.safetensors",
-        "C:outside.safetensors",  "model.safetensors\x00extra", "",
-        ".",                      "..",
+        "../outside.safetensors",     "/tmp/outside.safetensors", "sub/../outside.safetensors",
+        "..\\outside.safetensors",    "C:\\outside.safetensors",  "C:outside.safetensors",
+        "model.safetensors\x00extra", "",                         ".",
+        "..",
     };
     for (invalid) |name| {
         const json = try std.json.Stringify.valueAlloc(a, .{ .weight_map = .{ .a = name } }, .{});
@@ -46,6 +46,30 @@ test "security: checkpoint shard names cannot escape their model folder" {
     }
 }
 
+test "security: a checkpoint shard may sit in a folder inside its model folder" {
+    const a = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.createDirPath(io, "sub");
+    const dir = try tmp.dir.realPathFileAlloc(io, ".", a);
+    defer a.free(dir);
+    const shard = try std.fs.path.join(a, &.{ dir, "sub", "model.safetensors" });
+    defer a.free(shard);
+    try edit.create(a, shard, &.{}, &.{.{ .name = "a", .dtype = .u8, .shape = &.{4}, .bytes = "here" }});
+    try tmp.dir.writeFile(io, .{
+        .sub_path = "model.safetensors.index.json",
+        .data = "{\"weight_map\":{\"a\":\"sub/model.safetensors\"}}",
+    });
+    const files = try ck.shardFiles(a, io, dir);
+    defer ck.freeShardFiles(a, files);
+    try std.testing.expectEqual(@as(usize, 1), files.len);
+    try std.testing.expectEqualStrings("sub/model.safetensors", files[0][dir.len + 1 ..]);
+    var model = try ck.Checkpoint.openModel(a, io, dir);
+    defer model.close();
+    try std.testing.expectEqualStrings("here", (try model.get("a")).bytes);
+}
+
 test "security: an unreadable checkpoint index is not silently ignored" {
     const a = std.testing.allocator;
     const io = std.testing.io;
@@ -84,6 +108,61 @@ test "security: a checkpoint index cannot redirect Living Weights into another f
     try std.testing.expectError(error.BadIndex, changed);
 }
 
+test "security: Living Weights refuse an index with a shard in a folder before writing to it" {
+    const a = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.createDirPath(io, "sub");
+    const dir = try tmp.dir.realPathFileAlloc(io, ".", a);
+    defer a.free(dir);
+    const shard = try std.fs.path.join(a, &.{ dir, "sub", "model.safetensors" });
+    defer a.free(shard);
+    try edit.create(a, shard, &.{}, &.{.{ .name = "a.weight", .dtype = .u8, .shape = &.{4}, .bytes = "safe" }});
+    const before = try tmp.dir.readFileAlloc(io, "sub/model.safetensors", a, .limited(4096));
+    defer a.free(before);
+    try tmp.dir.writeFile(io, .{
+        .sub_path = "model.safetensors.index.json",
+        .data = "{\"weight_map\":{\"a.weight\":\"sub/model.safetensors\"}}",
+    });
+    const evil: []const edit.Replacement = &.{.{ .name = "a.weight", .dtype = .u8, .shape = &.{4}, .bytes = "evil" }};
+    try std.testing.expectError(error.BadIndex, edit.overwrite(a, io, dir, evil));
+    try std.testing.expectError(error.BadIndex, edit.grow(a, io, dir, evil));
+    try std.testing.expectError(error.BadIndex, edit.bake(a, io, dir, evil));
+    const after = try tmp.dir.readFileAlloc(io, "sub/model.safetensors", a, .limited(4096));
+    defer a.free(after);
+    try std.testing.expectEqualSlices(u8, before, after);
+}
+
+test "security: Living Weights keep to the model folder's own shards, so a linked folder cannot redirect them" {
+    if (@import("builtin").os.tag == .windows) return error.SkipZigTest;
+    const a = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.createDirPath(io, "model");
+    try tmp.dir.createDirPath(io, "outside");
+    const root = try tmp.dir.realPathFileAlloc(io, ".", a);
+    defer a.free(root);
+    const dir = try std.fs.path.join(a, &.{ root, "model" });
+    defer a.free(dir);
+    const victim = try std.fs.path.join(a, &.{ root, "outside", "victim.safetensors" });
+    defer a.free(victim);
+    try edit.create(a, victim, &.{}, &.{.{ .name = "a.weight", .dtype = .u8, .shape = &.{4}, .bytes = "safe" }});
+    const before = try tmp.dir.readFileAlloc(io, "outside/victim.safetensors", a, .limited(4096));
+    defer a.free(before);
+    try tmp.dir.symLink(io, "../outside", "model/sub", .{ .is_directory = true });
+    try tmp.dir.writeFile(io, .{
+        .sub_path = "model/model.safetensors.index.json",
+        .data = "{\"weight_map\":{\"a.weight\":\"sub/victim.safetensors\"}}",
+    });
+    const changed = edit.overwrite(a, io, dir, &.{.{ .name = "a.weight", .dtype = .u8, .shape = &.{4}, .bytes = "evil" }});
+    const after = try tmp.dir.readFileAlloc(io, "outside/victim.safetensors", a, .limited(4096));
+    defer a.free(after);
+    try std.testing.expectEqualSlices(u8, before, after);
+    try std.testing.expectError(error.BadIndex, changed);
+}
+
 test "security: shard and snapshot atomic writes cannot follow preexisting temporary symlinks" {
     if (@import("builtin").os.tag == .windows) return error.SkipZigTest;
     const a = std.testing.allocator;
diff --git a/zig/src/core/shard_edit.zig b/zig/src/core/shard_edit.zig
index 9bd4caecde..499bb91540 100644
--- a/zig/src/core/shard_edit.zig
+++ b/zig/src/core/shard_edit.zig
@@ -54,7 +54,7 @@ pub fn bake(gpa: Allocator, io: Io, dir: []const u8, edits: []const Replacement)
     for (edits) |r| if (r.bytes.len != numel(r.shape) * r.dtype.size() or !std.mem.endsWith(u8, r.name, ".weight")) return error.BadReplacement;
     const index_path = try std.fs.path.join(arena, &.{ dir, index_name });
     const index: ?Value = readJson(arena, io, index_path) catch |e| if (e == error.FileNotFound) null else return e;
-    if (index) |ix| _ = try index_format.weightMap(ix);
+    if (index) |ix| _ = try index_format.plainWeightMap(ix);
     var shards: std.StringArrayHashMapUnmanaged(std.ArrayList(Replacement)) = .empty;
     for (edits) |r| {
         const shard = if (index) |ix| str((try field(try field(ix, "weight_map"), r.name))) orelse return error.BadIndex else single_name;
@@ -95,7 +95,7 @@ pub fn grow(gpa: Allocator, io: Io, dir: []const u8, edits: []const Replacement)
     for (edits) |r| if (r.bytes.len != numel(r.shape) * r.dtype.size()) return error.BadReplacement;
     const index_path = try std.fs.path.join(arena, &.{ dir, index_name });
     const index: ?Value = readJson(arena, io, index_path) catch |e| if (e == error.FileNotFound) null else return e;
-    if (index) |ix| _ = try index_format.weightMap(ix);
+    if (index) |ix| _ = try index_format.plainWeightMap(ix);
     var shards: std.StringArrayHashMapUnmanaged(std.ArrayList(Replacement)) = .empty;
     for (edits) |r| {
         const shard = if (index) |ix| str((try field(try field(ix, "weight_map"), r.name))) orelse return error.BadIndex else single_name;
@@ -167,7 +167,7 @@ pub fn overwrite(gpa: Allocator, io: Io, dir: []const u8, edits: []const Replace
     const arena = arena_state.allocator();
     const index_path = try std.fs.path.join(arena, &.{ dir, index_name });
     const index: ?Value = readJson(arena, io, index_path) catch |e| if (e == error.FileNotFound) null else return e;
-    if (index) |ix| _ = try index_format.weightMap(ix);
+    if (index) |ix| _ = try index_format.plainWeightMap(ix);
     for (edits) |r| {
         const shard = if (index) |ix| str((try field(try field(ix, "weight_map"), r.name))) orelse return error.BadIndex else single_name;
         const path = try std.fs.path.join(arena, &.{ dir, shard });
diff --git a/zig/src/families/radio/weights.zig b/zig/src/families/radio/weights.zig
index f52db83bea..2ba0c1d2b4 100644
--- a/zig/src/families/radio/weights.zig
+++ b/zig/src/families/radio/weights.zig
@@ -191,7 +191,7 @@ fn open(gpa: Allocator, io: std.Io, dir: []const u8) !std.ArrayList(st.File) {
     var named = try indexed(gpa, io, dir);
     defer if (named) |*n| freeNames(gpa, n);
     for (paths) |path| {
-        if (named) |n| if (!n.contains(std.fs.path.basename(path))) continue;
+        if (named) |n| if (!n.contains(path[dir.len + 1 ..])) continue;
         var f = try st.File.open(gpa, io, path);
         const has = for (f.names.keys()) |k| {
             if (ours(k)) break true;
@@ -242,6 +242,29 @@ test "a layer's tensors are known; a layer scale is not" {
     try std.testing.expect(!Planner.known(tower ++ "blocks.3.ls1.gamma"));
 }
 
+test "the image tensors' shard is found in the folder the index names" {
+    const gpa = std.testing.allocator;
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.createDirPath(io, "tensors");
+    const dir = try tmp.dir.realPathFileAlloc(io, ".", gpa);
+    defer gpa.free(dir);
+    const path = try std.fs.path.join(gpa, &.{ dir, "tensors", "vision.safetensors" });
+    defer gpa.free(path);
+    try core.shard_edit.create(gpa, path, &.{}, &.{.{ .name = "vision_model.x", .dtype = .u8, .shape = &.{4}, .bytes = "seen" }});
+    try tmp.dir.writeFile(io, .{
+        .sub_path = "model.safetensors.index.json",
+        .data = "{\"weight_map\":{\"vision_model.x\":\"tensors/vision.safetensors\"}}",
+    });
+    var files = try open(gpa, io, dir);
+    defer {
+        for (files.items) |*f| f.close(io);
+        files.deinit(gpa);
+    }
+    try std.testing.expectEqual(@as(usize, 1), files.items.len);
+}
+
 const Failed = struct {
     gpa: Allocator,
     w: Weights,
diff --git a/zig/src/native/slide_adapter.zig b/zig/src/native/slide_adapter.zig
index 24f63f76aa..f8cef10659 100644
--- a/zig/src/native/slide_adapter.zig
+++ b/zig/src/native/slide_adapter.zig
@@ -10,6 +10,21 @@ const max_neurons = 64;
 pub fn refusal(comptime Checks: type, a: Allocator, io: std.Io, dir: []const u8, copy: []const u8) !?[]const u8 {
     if (@hasDecl(Checks, "plainShared")) if (try Checks.plainShared(a, io, dir)) |name| return try std.fmt.allocPrint(a, "--slide adds learned neurons beside every shared expert, and {s} in {s} is unquantized, as TensorFold 1.0.3 to 1.0.6 saved their lessons: serve this folder without --slide (what it learned stays), or teach a fresh copy", .{ name, dir });
     if (try Checks.linked(a, io, dir)) |name| return try std.fmt.allocPrint(a, "--slide rewrites the model's own files, and {s} in {s} is a link to data another file shares (a Hugging Face cache's blob, or a hard link), so learning would change that file too: serve a copy made with {s}", .{ name, dir, copy });
+    if (try folderShard(a, io, dir)) |name| return try std.fmt.allocPrint(a, "--slide rewrites the model's own shards, and {s} in {s} sits in a folder, where the checks for links do not look: serve this folder without --slide", .{ name, dir });
+    return null;
+}
+
+/// The first shard the index places in a folder (shard_edit writes only the shards beside it), or null.
+fn folderShard(a: Allocator, io: std.Io, dir: []const u8) !?[]const u8 {
+    const path = try std.fs.path.join(a, &.{ dir, "model.safetensors.index.json" });
+    const text = std.Io.Dir.cwd().readFileAlloc(io, path, a, .limited(1 << 26)) catch |e| switch (e) {
+        error.FileNotFound => return null,
+        else => return e,
+    };
+    const index = try std.json.parseFromSliceLeaky(std.json.Value, a, text, .{});
+    const map = if (index == .object) index.object.get("weight_map") orelse return null else return null;
+    if (map != .object) return null;
+    for (map.object.values()) |v| if (v == .string and std.mem.indexOfScalar(u8, v.string, '/') != null) return v.string;
     return null;
 }
 
diff --git a/zig/src/native/slide_adapter_test.zig b/zig/src/native/slide_adapter_test.zig
index 081e6de5db..de7912158a 100644
--- a/zig/src/native/slide_adapter_test.zig
+++ b/zig/src/native/slide_adapter_test.zig
@@ -1,7 +1,8 @@
-//! Initial reader progress must be visible before its first potentially long step begins.
+//! The slide adapter: initial reader progress before its first potentially long step, and --slide's startup refusals.
 const std = @import("std");
 const api = @import("engine_api");
 const Adapter = @import("slide_adapter.zig").Adapter;
+const refusal = @import("slide_adapter.zig").refusal;
 const Fixture = struct {
     pub const Example = api.Example;
     pub const Learner = struct {
@@ -44,3 +45,26 @@ test "lesson adapter: publish reading zero before the first long unit" {
     try hook.begin(hook.ctx, &.{}, .{ .ctx = &seen, .event = Seen.on });
     try std.testing.expectEqual(@as(usize, 1), seen.reads);
 }
+const NoLinks = struct {
+    pub fn linked(_: std.mem.Allocator, _: std.Io, _: []const u8) !?[]const u8 {
+        return null;
+    }
+};
+test "slide refusal: a shard the index places in a folder is refused at startup, before any save" {
+    var arena: std.heap.ArenaAllocator = .init(std.testing.allocator);
+    defer arena.deinit();
+    const a = arena.allocator();
+    const io = std.testing.io;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    const dir = try tmp.dir.realPathFileAlloc(io, ".", a);
+    try std.testing.expect((try refusal(NoLinks, a, io, dir, "a copy")) == null);
+    try tmp.dir.writeFile(io, .{ .sub_path = "model.safetensors.index.json", .data = "{\"weight_map\":{\"a\":\"model.safetensors\"}}" });
+    try std.testing.expect((try refusal(NoLinks, a, io, dir, "a copy")) == null);
+    try tmp.dir.writeFile(io, .{
+        .sub_path = "model.safetensors.index.json",
+        .data = "{\"weight_map\":{\"a\":\"model.safetensors\",\"b\":\"sub/b.safetensors\"}}",
+    });
+    const why = (try refusal(NoLinks, a, io, dir, "a copy")) orelse return error.TestUnexpectedResult;
+    try std.testing.expect(std.mem.indexOf(u8, why, "sub/b.safetensors") != null);
+}