diff --git a/doc/doc-support/package.nix b/doc/doc-support/package.nix
index 728bbe334501..7a0993cfcea5 100644
--- a/doc/doc-support/package.nix
+++ b/doc/doc-support/package.nix
@@ -118,7 +118,7 @@ stdenvNoCC.mkDerivation (
         --script ./anchor.min.js \
         --script ./anchor-use.js \
         --sidebar-depth 3 \
-        --nav ./nav.json \
+        --experimental-config ./nav.json \
         --header ${./header.html}\
         --no-navheader \
         manual.md \
diff --git a/doc/nav.json b/doc/nav.json
index 15d4926421cf..8c564c94294d 100644
--- a/doc/nav.json
+++ b/doc/nav.json
@@ -1,3 +1,4 @@
 {
-  "open": []
+  "open": [],
+  "items": []
 }
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
index 3f61608d60fa..551465ad4881 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual.py
@@ -3,9 +3,9 @@ import hashlib
 import html
 import json
 import re
-import xml.sax.saxutils as xml
 from abc import abstractmethod
 from collections.abc import Mapping, Sequence
+from dataclasses import dataclass
 from pathlib import Path
 from typing import Any, Callable, ClassVar, Generic, NamedTuple, cast, get_args
 
@@ -44,6 +44,7 @@ class BaseConverter(Converter[md.TR], Generic[md.TR]):
         self._current_type = ['book']
         try:
             tokens = self._parse(infile.read_text())
+            self._prepend_config(infile, tokens)
             self._postprocess(infile, outfile, tokens)
             converted = self._renderer.render(tokens)
             outfile.write_text(converted)
@@ -53,6 +54,9 @@ class BaseConverter(Converter[md.TR], Generic[md.TR]):
     def _postprocess(self, infile: Path, outfile: Path, tokens: Sequence[Token]) -> None:
         pass
 
+    def _prepend_config(self, infile: Path, tokens: list[Token]) -> None:
+        pass
+
     def _handle_headings(self, tokens: list[Token], *, src: str, on_heading: Callable[[Token,str],None]) -> None:
         # Headings in a globally numbered order
         # h1 to h6
@@ -232,6 +236,7 @@ class RendererMixin(Renderer):
             'included_preface': lambda *args: self._included_thing("preface", *args),
             'included_parts': lambda *args: self._included_thing("part", *args),
             'included_appendix': lambda *args: self._included_thing("appendix", *args),
+            'included_content': lambda *args: self._included_thing("content", *args),
             'included_options': self.included_options,
         }
 
@@ -264,7 +269,6 @@ class HTMLParameters(NamedTuple):
     # structural depth of the navigation sidebar tree
     sidebar_depth: int
     media_dir: Path
-    sidebar_open: frozenset[str] = frozenset()
     header: Path | None = None
     no_navheader: bool = False
 
@@ -273,15 +277,18 @@ class ManualHTMLRenderer(RendererMixin, HTMLRenderer):
     _in_dir: Path
     _html_params: HTMLParameters
     _redirects: Redirects | None
+    _sidebar_open: frozenset[str]
 
     def __init__(self, toplevel_tag: str, revision: str, html_params: HTMLParameters,
                  manpage_urls: Mapping[str, str], xref_targets: dict[str, XrefTarget],
-                 redirects: Redirects | None, in_dir: Path, base_path: Path):
+                 redirects: Redirects | None, in_dir: Path, base_path: Path,
+                 sidebar_open: frozenset[str]):
         super().__init__(toplevel_tag, revision, manpage_urls, xref_targets)
         self._in_dir = in_dir
         self._base_path = base_path.absolute()
         self._html_params = html_params
         self._redirects = redirects
+        self._sidebar_open = sidebar_open
 
     def _pull_image(self, src: str) -> str:
         src_path = Path(src)
@@ -542,7 +549,10 @@ document.addEventListener("DOMContentLoaded", createObserver);
                 link = f'<a href="{e.target.href()}">{e.target.toc_html}</a>'
                 cls = html.escape(e.kind, True)
                 if children:
-                    open_attr = " open" if e.target.id in self._html_params.sidebar_open else ""
+                    # a group without an 'id' in the config gets open_id "".
+                    # the empty key then fails the truth test, so the group stays closed.
+                    key = e.target.id if e.open_id is None else e.open_id
+                    open_attr = " open" if key and key in self._sidebar_open else ""
                     items.append(
                         f'<li class="{cls}">'
                         f'<details{open_attr}><summary>{link}</summary>{children}</details>'
@@ -607,6 +617,25 @@ document.addEventListener("DOMContentLoaded", createObserver);
 def _to_base26(n: int) -> str:
     return (_to_base26(n // 26) if n > 26 else "") + chr(ord("A") + n % 26)
 
+@dataclass
+class ConfigLeaf:
+    file: str
+    label: str | None = None
+
+@dataclass
+class ConfigGroup:
+    label: str
+    children: list["ConfigNode"]
+    id: str | None = None
+
+ConfigNode = ConfigLeaf | ConfigGroup
+
+@dataclass
+class ConfigManifest:
+    items: list[ConfigNode]
+    open: frozenset[str]
+
+
 class HTMLConverter(BaseConverter[ManualHTMLRenderer]):
     INCLUDE_ARGS_NS = "html"
     INCLUDE_FRAGMENT_ALLOWED_ARGS = { 'into-file' }
@@ -618,24 +647,127 @@ class HTMLConverter(BaseConverter[ManualHTMLRenderer]):
     _xref_targets: dict[str, XrefTarget]
     _redirection_targets: set[str]
     _appendix_count: int = 0
+    _config_path: Path | None
+    _config: ConfigManifest | None
 
     def _next_appendix_id(self) -> str:
         self._appendix_count += 1
         return _to_base26(self._appendix_count - 1)
 
-    def __init__(self, revision: str, html_params: HTMLParameters, manpage_urls: Mapping[str, str], redirects: Redirects | None = None):
+    def __init__(
+            self,
+            revision: str,
+            html_params: HTMLParameters,
+            manpage_urls: Mapping[str, str],
+            redirects: Redirects | None = None,
+            config_path: Path | None = None
+        ):
         super().__init__()
         self._revision, self._html_params, self._manpage_urls, self._redirects = revision, html_params, manpage_urls, redirects
+        self._config_path = config_path
+        self._config = None
         self._xref_targets = {}
         self._redirection_targets = set()
         # renderer not set on purpose since it has a dependency on the output path!
 
     def convert(self, infile: Path, outfile: Path) -> None:
+        try:
+            self._config = self._load_config()
+        except Exception as e:
+            raise RuntimeError(f"failed to load config '{self._config_path}'") from e
+        sidebar_open = self._config.open if self._config is not None else frozenset()
         self._renderer = ManualHTMLRenderer(
             'book', self._revision, self._html_params, self._manpage_urls, self._xref_targets,
-            self._redirects, infile.parent, outfile.parent)
+            self._redirects, infile.parent, outfile.parent, sidebar_open)
         super().convert(infile, outfile)
 
+    def _load_config(self) -> ConfigManifest | None:
+        if self._config_path is None:
+            return None
+        src = self._config_path.read_text()
+        config = json.loads(src)
+        if not isinstance(config, dict) or not isinstance(config.get('items'), list):
+            raise SrcError(
+                src=src,
+                description=f"config {self._config_path}: expected a top-level object with an 'items' array")
+        open_ids = config.get('open', [])
+        if not isinstance(open_ids, list) or not all(isinstance(i, str) for i in open_ids):
+            raise SrcError(
+                src=src,
+                description=f"config {self._config_path}: 'open' must be an array of strings")
+        items = self._parse_config_nodes(config['items'], "config items", src)
+        return ConfigManifest(items=items, open=frozenset(open_ids))
+
+    def _parse_config_nodes(self, items: Any, where: str, src: str) -> list[ConfigNode]:
+        return [self._parse_config_node(item, f"{where}[{idx}]", src)
+                for idx, item in enumerate(items)]
+
+    def _parse_config_node(self, item: Any, where: str, src: str) -> ConfigNode:
+        if not isinstance(item, dict):
+            raise SrcError(src=src, description=f"{where}: expected an object, got {type(item).__name__}")
+        label = item.get('label')
+        if label is not None and (not isinstance(label, str) or not label):
+            raise SrcError(src=src, description=f"{where}: 'label' must be a non-empty string")
+        has_file, has_children = 'file' in item, 'children' in item
+        if has_file == has_children:
+            raise SrcError(
+                src=src,
+                description=f"{where}: must have exactly one of 'file' or 'children'")
+        if has_file:
+            if not isinstance(item['file'], str):
+                raise SrcError(src=src, description=f"{where}: 'file' must be a string")
+            return ConfigLeaf(file=item['file'], label=label)
+        children = item['children']
+        if not isinstance(children, list) or not children:
+            raise SrcError(src=src, description=f"{where}: 'children' must be a non-empty array")
+        if not isinstance(label, str) or not label:
+            raise SrcError(src=src, description=f"{where}: a group requires a non-empty 'label'")
+        gid = item.get('id')
+        if gid is not None and (not isinstance(gid, str) or not gid):
+            raise SrcError(src=src, description=f"{where}: 'id' must be a non-empty string")
+        return ConfigGroup(label=label, children=self._parse_config_nodes(children, where, src), id=gid)
+
+    def _prepend_config(self, infile: Path, tokens: list[Token]) -> None:
+        if self._config is None:
+            return
+        assert self._config_path is not None
+        include = self._build_config_include(self._config.items, self._config_path.resolve())
+        # tokens 0 to 5 hold the title h1 triple and the subtitle h2 triple.
+        # _render_book renders tokens[6:] as the body.
+        # a change to the preamble must change this index too.
+        tokens[6:6] = [include]
+
+    def _build_config_include(self, nodes: list[ConfigNode], config_file: Path) -> Token:
+        included = [self._build_config_item(node, config_file) for node in nodes]
+        token = Token('included_content', '', 0, map=[0, 1])
+        token.meta['included'] = included
+        token.meta['include-args'] = {}
+        return token
+
+    def _build_config_item(self, node: ConfigNode, config_file: Path) -> tuple[list[Token], Path]:
+        if isinstance(node, ConfigLeaf):
+            path = (config_file.parent / node.file).resolve()
+            leaf_src = path.read_text()
+            self._base_paths.append(path)
+            self._current_type.append('content')
+            try:
+                fragment = self._parse(leaf_src)
+            finally:
+                self._current_type.pop()
+                self._base_paths.pop()
+            if node.label is not None:
+                fragment[1].meta['toc-label'] = node.label
+            return fragment, path
+
+        # TocEntry._collect_entries reads nav-label and nav-id.
+        # it builds the sidebar group from them.
+        group = self._build_config_include(node.children, config_file)
+        group.meta['nav-label'] = node.label
+        if node.id is not None:
+            group.meta['nav-id'] = node.id
+
+        return [group], config_file
+
     def _parse(self, src: str, *, auto_id_prefix: None | str = None) -> list[Token]:
         tokens = super()._parse(src,auto_id_prefix=auto_id_prefix)
         for token in tokens:
@@ -747,10 +879,12 @@ class HTMLConverter(BaseConverter[ManualHTMLRenderer]):
             title_html = self._renderer.renderInline(inlines.children[0:1])
         else:
             toc_html, title = title_html, title_html
+            if (toc_label := inlines.meta.get('toc-label')) is not None:
+                toc_html = html.escape(cast(str, toc_label))
             title_html = (
                 f"<em>{title_html}</em>"
                 if typ == 'chapter'
-                else title_html if typ in [ 'book', 'part' ]
+                else title_html if typ in [ 'book', 'part', 'content' ]
                 else f'the section called “{title_html}”'
             )
         return XrefTarget(id, title_html, toc_html, re.sub('<.*?>', '', title), path, drop_fragment)
@@ -820,9 +954,14 @@ def _build_cli_html(p: argparse.ArgumentParser) -> None:
     p.add_argument('--media-dir', default="media", type=Path)
     p.add_argument('--redirects', type=Path)
     p.add_argument('--sidebar-depth', default=2, type=int)
-    # nav metadata (JSON): {"open": ["anchor-id", ...]} selects which sidebar
-    # entries render expanded; omitted or absent means everything is collapsed.
-    p.add_argument('--nav', type=Path)
+    p.add_argument('--experimental-config', type=Path, help="""
+JSON file of the following form
+{ items: [ { file, label? } | { label, children: [...], id? } ], open: [ id, ... ] }
+
+Files added through this flag prepend '--infile'
+
+This flag will replace --infile and `{=include=}` directives in the future
+""")
     # Deprecated flags,
     p.add_argument('--toc-depth', nargs='?', action=_DeprecatedDepthFlag, default=None)
     p.add_argument('--chunk-toc-depth', nargs='?', action=_DeprecatedDepthFlag, default=None)
@@ -835,10 +974,6 @@ def _build_cli_html(p: argparse.ArgumentParser) -> None:
     p.add_argument('outfile', type=Path)
 
 def _run_cli_html(args: argparse.Namespace) -> None:
-    sidebar_open: frozenset[str] = frozenset()
-    if args.nav:
-        with open(args.nav) as nav_file:
-            sidebar_open = frozenset(json.load(nav_file).get("open", []))
     with open(args.manpage_urls) as manpage_urls, open(Path(__file__).parent / "redirects.js") as redirects_script:
         redirects = None
         if args.redirects:
@@ -848,8 +983,8 @@ def _run_cli_html(args: argparse.Namespace) -> None:
         md = HTMLConverter(
             args.revision,
             HTMLParameters(args.generator, args.stylesheet, args.script,
-                           args.sidebar_depth, args.media_dir, sidebar_open, args.header, args.no_navheader),
-            json.load(manpage_urls), redirects)
+                           args.sidebar_depth, args.media_dir, args.header, args.no_navheader),
+            json.load(manpage_urls), redirects, args.experimental_config)
         md.convert(args.infile, args.outfile)
 
 def build_cli(p: argparse.ArgumentParser) -> None:
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual_structure.py b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual_structure.py
index 859ff5197d17..204b5aa9b933 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual_structure.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/nixos_render_docs/manual_structure.py
@@ -13,8 +13,10 @@ from .utils import Freezeable
 # FragmentType is used to restrict structural include blocks.
 FragmentType = Literal['preface', 'part', 'chapter', 'section', 'appendix']
 
-# in the TOC all fragments are allowed, plus the all-encompassing book.
-TocEntryType = Literal['book', 'preface', 'part', 'chapter', 'section', 'appendix', 'example', 'figure']
+# the TOC allows every fragment type. it also allows the enclosing book.
+# it allows 'example' and 'figure'.
+# --experimental-config adds a generic 'content' kind.
+TocEntryType = Literal['book', 'preface', 'part', 'chapter', 'section', 'appendix', 'example', 'figure', 'content']
 
 def is_include(token: Token) -> bool:
     return token.type == "fence" and token.info.startswith("{=include=} ")
@@ -145,6 +147,7 @@ class TocEntry(Freezeable):
     next: TocEntry | None = None
     children: list[TocEntry] = dc.field(default_factory=list)
     starts_new_chunk: bool = False
+    open_id: str | None = None
 
     @property
     def root(self) -> TocEntry:
@@ -193,9 +196,21 @@ class TocEntry(Freezeable):
                 fragment_type_str = token.type[9:].removesuffix('s')
                 assert fragment_type_str in get_args(TocEntryType)
                 fragment_type = cast(TocEntryType, fragment_type_str)
-                for fragment, _path in included:
-                    subentries = cls._collect_entries(xrefs, fragment, fragment_type)
-                    entries[-1][1].children.append(subentries)
+                if (nav_label := token.meta.get('nav-label')) is not None:
+                    children = [cls._collect_entries(xrefs, fragment, fragment_type)
+                                for fragment, _path in included]
+                    first = children[0].target
+                    group = TocEntry(
+                        fragment_type,
+                        dc.replace(first, title_html=nav_label, toc_html=nav_label, title=nav_label),
+                        children=children,
+                        open_id=token.meta.get('nav-id', ''))
+                    entries.append(('h1', group))
+                    token.meta['TocEntry'] = group
+                else:
+                    for fragment, _path in included:
+                        subentries = cls._collect_entries(xrefs, fragment, fragment_type)
+                        entries[-1][1].children.append(subentries)
             elif token.type == 'heading_open' and (id := cast(str, token.attrs.get('id', ''))):
                 while len(entries) > 1 and entries[-1][0] >= token.tag:
                     entries[-2][1].children.append(entries.pop()[1])
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_html_header.py b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_html_header.py
index 7111959bea55..b0af6d8c4b99 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_html_header.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_html_header.py
@@ -9,7 +9,7 @@ def render(tmp_path: Path, header: Path | None) -> str:
     infile = tmp_path / "index.md"
     infile.write_text(SAMPLE_BOOK)
     outfile = tmp_path / "index.html"
-    params = HTMLParameters("", [], [], 2, sidebar_open = [], media_dir=tmp_path, header = header)
+    params = HTMLParameters("", [], [], 2, media_dir=tmp_path, header = header)
     HTMLConverter("1.0.0", params, {}).convert(infile, outfile)
     return outfile.read_text()
 
diff --git a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
index 45bce4cb6e78..5da16ab7659b 100644
--- a/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
+++ b/pkgs/by-name/ni/nixos-render-docs/src/tests/test_manual.py
@@ -1,7 +1,9 @@
+import json
 import xml.parsers.expat as expat
 from html.entities import name2codepoint
 from pathlib import Path
 
+import pytest
 from nixos_render_docs.manual import HTMLConverter, HTMLParameters
 
 
@@ -49,7 +51,7 @@ def _build(tmp_path: Path, sidebar_depth: int = 2, sidebar_open: frozenset[str]
     out.mkdir(exist_ok=True)
     conv = HTMLConverter(
         "1.0.0",
-        HTMLParameters("test-gen", [], [], sidebar_depth, Path("media"), sidebar_open),
+        HTMLParameters("test-gen", [], [], sidebar_depth, Path("media")),
         {},
     )
     conv.convert(tmp_path / "index.md", out / "index.html")
@@ -88,10 +90,42 @@ def test_sidebar_is_collapsible_tree(tmp_path: Path) -> None:
 
 
 def test_nav_metadata_opens_selected_entries(tmp_path: Path) -> None:
-    # ids listed in the nav "open" set render as <details open>
-    html = _build(tmp_path, sidebar_depth=3, sidebar_open=frozenset({"chap-fpa"}))
-    assert '<details open><summary><a href="#chap-fpa"' in html
-    assert '<details><summary><a href="#part-builders"' in html
+    # the 'open' array expands a config group and an --infile heading.
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [
+                {"label": "Guides", "id": "guides", "children": [
+                    {"label": "Intro", "file": "intro.md"},
+                ]},
+            ],
+            "open": ["guides", "manual-chap"],
+        },
+        {"intro.md": "# Introduction {#intro}\n\nBody.\n"},
+        manual_chapter="# Manual chapter {#manual-chap}\n\n## Sub {#manual-sub}\n\nText.\n",
+    )
+    # the group opens by its own id. its link points at the child.
+    assert '<details open><summary><a href="#intro">Guides</a>' in html
+    assert '<details open><summary><a href="#manual-chap"' in html
+
+
+def test_config_group_without_id_is_not_openable(tmp_path: Path) -> None:
+    # the config lists 'intro' in 'open'.
+    # the group has no id, so its key is empty. the group stays closed.
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [
+                {"label": "Guides", "children": [
+                    {"file": "intro.md"},
+                ]},
+            ],
+            "open": ["intro"],
+        },
+        {"intro.md": "# Introduction {#intro}\n\nBody.\n"},
+    )
+    assert '<details><summary><a href="#intro">Guides</a>' in html
+    assert "<details open>" not in html
 
 
 def test_sidebar_depth_caps_the_tree(tmp_path: Path) -> None:
@@ -169,3 +203,179 @@ def test_output_is_well_formed_xhtml(tmp_path: Path) -> None:
     assert {p.name for p in pages} == {"index.html", "chapter.html"}
     for page in pages:
         _parse_xhtml(page.read_text())
+
+
+_DEFAULT_INDEX = (
+    "# Test manual {#book-test}\n\n"
+    "## Version 1\n\n"
+    "```{=include=} chapters\nmanual-chapter.md\n```\n"
+)
+_DEFAULT_MANUAL_CHAPTER = "# Manual chapter {#manual-chap}\n\nManual body.\n"
+
+
+def _render_with_config(
+    tmp_path: Path,
+    config: object,
+    files: dict[str, str],
+    *,
+    index: str = _DEFAULT_INDEX,
+    manual_chapter: str = _DEFAULT_MANUAL_CHAPTER,
+    sidebar_depth: int = 6,
+) -> str:
+    (tmp_path / "config.json").write_text(json.dumps(config))
+    (tmp_path / "manual-chapter.md").write_text(manual_chapter)
+    for name, content in files.items():
+        path = tmp_path / name
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(content)
+
+    (tmp_path / "index.md").write_text(index)
+    out = tmp_path / "out"
+    out.mkdir(exist_ok=True)
+
+    conv = HTMLConverter(
+        "1.0.0",
+        HTMLParameters("test-gen", [], [], sidebar_depth, Path("media")),
+        {},
+        config_path=tmp_path / "config.json",
+    )
+    conv.convert(tmp_path / "index.md", out / "index.html")
+    return (out / "index.html").read_text()
+
+
+def test_config_content_renders_before_manual(tmp_path: Path) -> None:
+    html = _render_with_config(
+        tmp_path,
+        {"items": [{"label": "Intro", "file": "intro.md"}]},
+        {"intro.md": "# Introduction {#intro}\n\nConfig intro body.\n"},
+    )
+    assert "Config intro body." in html
+    assert "Manual body." in html
+    assert html.index("Config intro body.") < html.index("Manual body.")
+
+
+def test_config_grouping_node_is_sidebar_only(tmp_path: Path) -> None:
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [
+                {
+                    "label": "Guides", "children": [
+                        {"label": "Install", "file": "install.md"},
+                    ]
+                }
+            ]
+        },
+        {"install.md": "# Install {#install}\n\nInstall body.\n"},
+    )
+    # the group adds no heading and no anchor to the body.
+    assert "Guides</h2>" not in html
+    assert 'id="guides"' not in html
+    # the child renders as a page.
+    assert '<h2 id="install" class="title" >Install' in html
+    assert "Install body." in html
+    # the sidebar nests the child under the group.
+    assert '<details><summary><a href="#install">Guides</a></summary>' in html
+    assert '<a href="#install">Install</a>' in html
+
+
+def test_config_leaf_derives_id_and_label_from_file(tmp_path: Path) -> None:
+    # the sidebar link reuses the heading id. the label reuses the heading title.
+    html = _render_with_config(
+        tmp_path,
+        {"items": [{"file": "intro.md"}]},
+        {"intro.md": "# Introduction {#intro}\n\nBody.\n"},
+    )
+    assert '<h2 id="intro" class="title" >Introduction' in html
+    assert '<a href="#intro">Introduction</a>' in html
+
+
+def test_config_leaf_label_differs_from_file_title(tmp_path: Path) -> None:
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [
+                {"label": "Sidebar Label", "file": "leaf1.md"}
+            ]
+        },
+        {"leaf1.md": "# File Title {#leaf1}\n\nBody.\n"},
+    )
+    assert '<h2 id="leaf1" class="title" >File Title' in html
+    assert '<a href="#leaf1">Sidebar Label</a>' in html
+
+
+def test_config_tree_nests_arbitrarily_deep(tmp_path: Path) -> None:
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [
+                {"label": "A", "children": [
+                    {"label": "B", "children": [
+                        {"label": "C", "file": "c.md"},
+                    ]},
+                ]}
+            ]
+        },
+        {"c.md": "# C File {#leaf-c}\n\nDeep body.\n"},
+    )
+    # only the leaf renders a heading. the groups A and B add no anchor.
+    assert '<h2 id="leaf-c" class="title"' in html
+    assert 'id="ga"' not in html and 'id="gb"' not in html
+    # the sidebar nests the groups.
+    # every group links to its first descendant page, #leaf-c.
+    assert '<details><summary><a href="#leaf-c">A</a></summary>' in html
+    assert '<details><summary><a href="#leaf-c">B</a></summary>' in html
+    assert '<a href="#leaf-c">C</a></li>' in html
+    assert html.index('>A</a>') < html.index('>B</a>') < html.index('>C</a>')
+
+
+def test_config_id_is_cross_referenceable(tmp_path: Path) -> None:
+    html = _render_with_config(
+        tmp_path,
+        {
+            "items": [{"label": "Intro", "file": "intro.md"}]
+        },
+        {"intro.md": "# Introduction {#intro}\n\nBody.\n"},
+        manual_chapter="# Manual chapter {#manual-chap}\n\nSee [](#intro).\n",
+    )
+    assert '<a class="xref" href="#intro"' in html
+    assert ">Introduction</a>" in html
+
+
+def test_config_duplicate_id_fails(tmp_path: Path) -> None:
+    with pytest.raises(RuntimeError) as excinfo:
+        _render_with_config(
+            tmp_path,
+            {
+                "items": [{"label": "Dup", "file": "dup.md"}]
+            },
+            {"dup.md": "# Dup file {#dup}\n\nBody.\n"},
+            manual_chapter="# Manual {#dup}\n\nBody.\n",
+        )
+    assert "duplicate id" in str(excinfo.value.__cause__)
+
+
+def test_config_malformed_node_fails(tmp_path: Path) -> None:
+    with pytest.raises(RuntimeError) as excinfo:
+        _render_with_config(
+            tmp_path,
+            {"items": [{"label": "Bad", "file": "x.md", "children": []}]},
+            {},
+        )
+    assert "exactly one of" in str(excinfo.value.__cause__)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        _render_with_config(
+            tmp_path,
+            {"items": [{"file": 123}]},
+            {},
+        )
+    assert "'file' must be a string" in str(excinfo.value.__cause__)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        _render_with_config(
+            tmp_path,
+            {"items": [{"children": [{"file": "x.md"}]}]},
+            {"x.md": "# X {#x}\n\nB.\n"},
+        )
+    assert "a group requires a non-empty 'label'" in str(excinfo.value.__cause__)