diff --git a/.cirrus.yml b/.cirrus.yml
index 9f0c438d..d7d3d172 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -78,7 +78,7 @@ build_task:
   container: {image: "python:3.11-bullseye"}
   clone_script: *clone
   <<: *task-template
-  install_script: pip install tox
+  install_script: pip install tox tox-uv
   build_script:
     - tox -e clean,lint,typecheck,build
     - tar czf dist.tar.gz dist
@@ -102,7 +102,7 @@ linux_task:
       container: {image: "python:3.13-rc-bookworm"}
       allow_failures: true  # RC
   install_script:
-    - python -m pip install --upgrade pip tox pipx
+    - python -m pip install --upgrade pip tox tox-uv pipx
   <<: *test-template
   alias: base-test
 
@@ -127,7 +127,7 @@ macos_task:
 
 freebsd_task:
   name: test (freebsd - 3.11)
-  freebsd_instance: {image_family: freebsd-14-0}
+  freebsd_instance: {image_family: freebsd-14-2}
   install_script:
     - pkg remove -y python lang/python
     - pkg install -y git python311 py311-pip py311-gdbm py311-sqlite3 py311-tox py311-tomli py311-pipx
@@ -149,7 +149,7 @@ windows_task:
     - choco install -y --no-progress python3 --version=3.12.5 --params "/NoLockdown"
     - choco install -y --no-progress curl
     - pip install --upgrade certifi
-    - python -m pip install -U pip tox pipx
+    - python -m pip install -U pip tox tox-uv pipx
   <<: *test-template
   depends_on: [build, base-test]
 
@@ -168,7 +168,7 @@ linkcheck_task:
   depends_on: [finalize]
   allow_failures: true
   <<: *task-template
-  install_script: pip install tox
+  install_script: pip install tox tox-uv
   download_artifact_script: *download-artifact
   linkcheck_script: tox --installpkg dist/*.whl -e linkcheck -- -q
 
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9e06eb5e..695032e0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -34,10 +34,15 @@ jobs:
         with: {fetch-depth: 0}  # deep clone for setuptools-scm
       - uses: actions/setup-python@v5
         with: {python-version: "3.10"}
+      - uses: astral-sh/setup-uv@v5
       - name: Run static analysis and format checkers
-        run: pipx run --python python3.10 tox -e lint,typecheck
+        run: >-
+          uvx --with tox-uv
+          tox -e lint,typecheck
       - name: Build package distribution files
-        run: pipx run --python python3.10 tox -e clean,build
+        run: >-
+          uvx --with tox-uv
+          tox -e clean,build
       - name: Record the path of wheel distribution
         id: wheel-distribution
         run: echo "path=$(ls dist/*.whl)" >> $GITHUB_OUTPUT
@@ -65,8 +70,8 @@ jobs:
     strategy:
       matrix:
         python:
-        - "3.8"  # oldest Python supported by PSF
-        - "3.12"  # newest Python that is stable
+        - "3.8"   # oldest Python supported by validate-pyproject
+        - "3.x"   # newest Python that is stable
         platform:
         - ubuntu-latest
         - macos-13
@@ -77,6 +82,7 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
+      - uses: astral-sh/setup-uv@v5
       - name: Retrieve pre-built distribution files
         uses: actions/download-artifact@v4
         with: {name: python-distribution-files, path: dist/}
@@ -87,7 +93,8 @@ jobs:
           path: ${{ env.VALIDATE_PYPROJECT_CACHE_REMOTE }}
       - name: Run tests
         run: >-
-          pipx run tox
+          uvx --with tox-uv
+          tox
           --installpkg '${{ needs.prepare.outputs.wheel-distribution }}'
           -- -n 5 -rFEx --durations 10 --color yes
       - name: Generate coverage report
@@ -118,6 +125,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with: {python-version: "3.10"}
+      - uses: astral-sh/setup-uv@v5
       - name: Retrieve pre-built distribution files
         uses: actions/download-artifact@v4
         with: {name: python-distribution-files, path: dist/}
@@ -127,4 +135,6 @@ jobs:
           TWINE_REPOSITORY: pypi
           TWINE_USERNAME: __token__
           TWINE_PASSWORD: ${{ secrets.PYPI_TOKEN }}
-        run: pipx run tox -e publish
+        run: >-
+          uvx --with tox-uv
+          tox -e publish
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index c78f7cd3..03c1d90e 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -20,13 +20,13 @@ repos:
     args: ['--fix=auto']  # replace 'auto' with 'lf' to enforce Linux/Mac line endings or 'crlf' for Windows
 
 - repo: https://github.com/codespell-project/codespell
-  rev: v2.3.0
+  rev: v2.4.1
   hooks:
   - id: codespell
     args: [-w, -L, "THIRDPARTY"]
 
 - repo: https://github.com/astral-sh/ruff-pre-commit
-  rev: v0.7.2  # Ruff version
+  rev: v0.9.10  # Ruff version
   hooks:
   - id: ruff
     args: [--fix, --show-fixes]
@@ -63,7 +63,7 @@ repos:
       - validate-pyproject[all]>=0.13
 
 - repo: https://github.com/python-jsonschema/check-jsonschema
-  rev: 0.29.4
+  rev: 0.31.3
   hooks:
     - id: check-metaschema
       files: \.schema\.json$
@@ -71,7 +71,7 @@ repos:
     - id: check-github-workflows
 
 - repo: https://github.com/scientific-python/cookie
-  rev: 2024.08.19
+  rev: 2025.01.22
   hooks:
     - id: sp-repo-review
       name: Validate Python repository
diff --git a/.readthedocs.yml b/.readthedocs.yml
index 5ed1344e..4b1a0f4f 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -5,23 +5,25 @@
 version: 2
 
 build:
-  os: "ubuntu-22.04"
+  os: ubuntu-lts-latest
   tools:
-    python: "3.10"
+    python: latest
+  jobs:
+    pre_create_environment:
+      - asdf plugin add uv
+      - asdf install uv latest
+      - asdf global uv latest
+    create_environment:
+      - uv venv $READTHEDOCS_VIRTUALENV_PATH
+    install:
+      # Use a cache dir in the same mount to halve the install time
+      # pip and uv pip will gain support for groups soon
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH uv sync --active --cache-dir $READTHEDOCS_VIRTUALENV_PATH/../../uv_cache --group docs --extra all
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
 
-# Build documentation with MkDocs
-#mkdocs:
-#  configuration: mkdocs.yml
-
 # Optionally build your docs in additional formats such as PDF
 formats:
   - pdf
-
-python:
-  install:
-    - requirements: docs/requirements.txt
-    - {path: ., extra_requirements: [all], method: pip}
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e5ff7709..952a2b44 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -6,6 +6,30 @@ Changelog
    Development Version
    ====================
 
+Version 0.24
+============
+* Fix integration with ``SchemaStore`` by loading extra/side schemas, #226, #229.
+* Add support for loading extra schemas, #226.
+* Fixed verify author dict is not empty, #232.
+* Added support for ``validate_pyproject.multi_schema`` plugins with extra schemas, #231.
+* ``validate-pyproject`` no longer communicates test dependencies via the ``tests``
+  extra and documentation dependencies dependencies via the ``docs/requirements.txt`` file.
+  Instead :doc:`pypa:dependency-groups` have been adopted to support CI environments, #227.
+
+  As a result, ``uv``'s high level interface also works for developers. You can use the :pypi:`dependency-groups`
+  package on PyPI if you need to convert to a classic requirements list.
+
+Contributions by @henryiii.
+
+
+Version 0.23
+============
+* Validate SPDX license expressions by @cdce8p in #217
+
+Version 0.22
+============
+* Prevent injecting defaults and modifying input in-place, by @henryiii in #213
+
 Version 0.21
 ============
 * Added support PEP 735, #208
diff --git a/docs/dev-guide.rst b/docs/dev-guide.rst
index 8591987a..10c366f5 100644
--- a/docs/dev-guide.rst
+++ b/docs/dev-guide.rst
@@ -122,6 +122,42 @@ Also notice plugins are activated in a specific order, using Python's built-in
 ``sorted`` function.
 
 
+Providing multiple schemas
+--------------------------
+
+A second system is provided for providing multiple schemas in a single plugin.
+This is useful when a single plugin is responsible for multiple subtables
+under the ``tool`` table, or if you need to provide multiple schemas for a
+a single subtable.
+
+To use this system, the plugin function, which does not take any arguments,
+should return a dictionary with two keys: ``tools``, which is a dictionary of
+tool names to schemas, and optionally ``schemas``, which is a list of schemas
+that are not associated with any specific tool, but are loaded via ref's from
+the other tools.
+
+When using a :pep:`621`-compliant backend, the following can be add to your
+``pyproject.toml`` file:
+
+.. code-block:: toml
+
+    # in pyproject.toml
+    [project.entry-points."validate_pyproject.multi_schema"]
+    arbitrary = "your_package.your_module:your_plugin"
+
+An example of the plugin structure needed for this system is shown below:
+
+.. code-block:: python
+
+    def your_plugin(tool_name: str) -> dict:
+        return {
+            "tools": {"my-tool": my_schema},
+            "schemas": [my_extra_schema],
+        }
+
+Fragments for schemas are also supported with this system; use ``#`` to split
+the tool name and fragment path in the dictionary key.
+
 .. _entry-point: https://setuptools.pypa.io/en/stable/userguide/entry_point.html#entry-points
 .. _JSON Schema: https://json-schema.org/
 .. _Python package: https://packaging.python.org/
diff --git a/docs/requirements.txt b/docs/requirements.txt
deleted file mode 100644
index 96aaa4e4..00000000
--- a/docs/requirements.txt
+++ /dev/null
@@ -1,9 +0,0 @@
-# Requirements file for ReadTheDocs, check .readthedocs.yml.
-# To build the module reference correctly, make sure every external package
-# under `install_requires` in `setup.cfg` is also listed here!
-furo>=2023.08.17
-sphinx>=7.2.2
-sphinx-argparse>=0.3.1
-sphinx-copybutton
-sphinx-jsonschema>=1.16.11
-sphinxemoji
diff --git a/pyproject.toml b/pyproject.toml
index e7c901c3..aab9cc35 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -35,6 +35,32 @@ all = [
     "trove-classifiers>=2021.10.20",
 ]
 store = ["validate-pyproject-schema-store"]
+
+[project.scripts]
+validate-pyproject = "validate_pyproject.cli:main"
+
+[project.entry-points."validate_pyproject.tool_schema"]
+setuptools = "validate_pyproject.api:load_builtin_plugin"
+distutils = "validate_pyproject.api:load_builtin_plugin"
+
+[project.entry-points."repo_review.checks"]
+validate_pyproject = "validate_pyproject.repo_review:repo_review_checks"
+
+[project.entry-points."repo_review.families"]
+validate_pyproject = "validate_pyproject.repo_review:repo_review_families"
+
+[dependency-groups]
+dev = [
+    { include-group = "test" },
+]
+docs = [
+    "furo>=2023.08.17",
+    "sphinx>=7.2.2",
+    "sphinx-argparse>=0.3.1",
+    "sphinx-copybutton",
+    "sphinx-jsonschema>=1.16.11",
+    "sphinxemoji",
+]
 test = [
     "setuptools",
     "pytest>=8.3.3",
@@ -49,18 +75,13 @@ typecheck = [
     "importlib-resources",
 ]
 
-[project.scripts]
-validate-pyproject = "validate_pyproject.cli:main"
-
-[project.entry-points."validate_pyproject.tool_schema"]
-setuptools = "validate_pyproject.api:load_builtin_plugin"
-distutils = "validate_pyproject.api:load_builtin_plugin"
-
-[project.entry-points."repo_review.checks"]
-validate_pyproject = "validate_pyproject.repo_review:repo_review_checks"
-
-[project.entry-points."repo_review.families"]
-validate_pyproject = "validate_pyproject.repo_review:repo_review_families"
+[tool.uv]
+environments = [
+  "python_version >= '3.9'",
+]
+dev-dependencies = [
+  "validate_pyproject[all]",
+]
 
 [tool.setuptools_scm]
 version_scheme = "no-guess-dev"
@@ -76,8 +97,10 @@ addopts = """
 """
 norecursedirs = ["dist", "build", ".*"]
 testpaths = ["src", "tests"]
+log_cli_level = "info"
 
 [tool.mypy]
+python_version = "3.8"
 enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 show_traceback = true
 warn_unreachable = true
diff --git a/src/validate_pyproject/api.py b/src/validate_pyproject/api.py
index 7fbdb6d5..2122d811 100644
--- a/src/validate_pyproject/api.py
+++ b/src/validate_pyproject/api.py
@@ -4,6 +4,7 @@
 
 import json
 import logging
+import sys
 import typing
 from enum import Enum
 from functools import partial, reduce
@@ -33,15 +34,18 @@
     from .plugins import PluginProtocol
 
 
-try:  # pragma: no cover
+if sys.version_info >= (3, 9):  # pragma: no cover
     from importlib.resources import files
 
     def read_text(package: Union[str, ModuleType], resource: str) -> str:
         """:meta private:"""
         return files(package).joinpath(resource).read_text(encoding="utf-8")
 
-except ImportError:  # pragma: no cover
-    from importlib.resources import read_text
+else:  # pragma: no cover
+    from importlib.resources import read_text as read_text  # noqa: PLC0414
+
+
+__all__ = ["Validator"]
 
 
 T = TypeVar("T", bound=Mapping)
@@ -109,19 +113,25 @@ def __init__(self, plugins: Sequence["PluginProtocol"] = ()):
 
         # Add tools using Plugins
         for plugin in plugins:
-            allow_overwrite: Optional[str] = None
-            if plugin.tool in tool_properties:
-                _logger.warning(f"{plugin.id} overwrites `tool.{plugin.tool}` schema")
-                allow_overwrite = plugin.schema.get("$id")
+            if plugin.tool:
+                allow_overwrite: Optional[str] = None
+                if plugin.tool in tool_properties:
+                    _logger.warning(
+                        f"{plugin.id} overwrites `tool.{plugin.tool}` schema"
+                    )
+                    allow_overwrite = plugin.schema.get("$id")
+                else:
+                    _logger.info(f"{plugin.id} defines `tool.{plugin.tool}` schema")
+                compatible = self._ensure_compatibility(
+                    plugin.tool, plugin.schema, allow_overwrite
+                )
+                sid = compatible["$id"]
+                sref = f"{sid}#{plugin.fragment}" if plugin.fragment else sid
+                tool_properties[plugin.tool] = {"$ref": sref}
+                self._schemas[sid] = (f"tool.{plugin.tool}", plugin.id, plugin.schema)
             else:
-                _logger.info(f"{plugin.id} defines `tool.{plugin.tool}` schema")
-            compatible = self._ensure_compatibility(
-                plugin.tool, plugin.schema, allow_overwrite
-            )
-            sid = compatible["$id"]
-            sref = f"{sid}#{plugin.fragment}" if plugin.fragment else sid
-            tool_properties[plugin.tool] = {"$ref": sref}
-            self._schemas[sid] = (f"tool.{plugin.tool}", plugin.id, plugin.schema)
+                _logger.info(f"Extra schema: {plugin.id}")
+                self._schemas[plugin.id] = (plugin.id, plugin.id, plugin.schema)
 
         self._main_id: str = top_level["$id"]
         main_schema = Schema(top_level)
@@ -139,17 +149,22 @@ def main(self) -> str:
         return self._main_id
 
     def _ensure_compatibility(
-        self, reference: str, schema: Schema, allow_overwrite: Optional[str] = None
+        self,
+        reference: str,
+        schema: Schema,
+        allow_overwrite: Optional[str] = None,
     ) -> Schema:
         if "$id" not in schema or not schema["$id"]:
-            raise errors.SchemaMissingId(reference)
+            raise errors.SchemaMissingId(reference or "<extra>")
         sid = schema["$id"]
         if sid in self._schemas and sid != allow_overwrite:
             raise errors.SchemaWithDuplicatedId(sid)
         version = schema.get("$schema")
         # Support schemas with missing trailing # (incorrect, but required before 0.15)
         if version and version.rstrip("#") != self.spec_version.rstrip("#"):
-            raise errors.InvalidSchemaVersion(reference, version, self.spec_version)
+            raise errors.InvalidSchemaVersion(
+                reference or sid, version, self.spec_version
+            )
         return schema
 
     def __getitem__(self, key: str) -> Schema:
diff --git a/src/validate_pyproject/caching.py b/src/validate_pyproject/caching.py
index f09e633c..458bcd28 100644
--- a/src/validate_pyproject/caching.py
+++ b/src/validate_pyproject/caching.py
@@ -33,7 +33,7 @@ def as_file(
             cache_path.write_text(f.getvalue(), encoding="utf-8")
             _logger.debug(f"Caching {arg} into {cache_path}")
 
-    return open(cache_path, "rb")  # noqa: SIM115 -- not relevant
+    return open(cache_path, "rb")
 
 
 def path_for(arbitrary_id: str, cache: Optional[PathLike] = None) -> Optional[Path]:
diff --git a/src/validate_pyproject/cli.py b/src/validate_pyproject/cli.py
index 37a59713..a356640c 100644
--- a/src/validate_pyproject/cli.py
+++ b/src/validate_pyproject/cli.py
@@ -30,7 +30,7 @@
 from . import _tomllib as tomllib
 from .api import Validator
 from .errors import ValidationError
-from .plugins import PluginWrapper
+from .plugins import PluginProtocol, PluginWrapper
 from .plugins import list_from_entry_points as list_plugins_from_entry_points
 from .remote import RemotePlugin, load_store
 
@@ -124,7 +124,7 @@ class CliParams(NamedTuple):
     dump_json: bool = False
 
 
-def __meta__(plugins: Sequence[PluginWrapper]) -> Dict[str, dict]:
+def __meta__(plugins: Sequence[PluginProtocol]) -> Dict[str, dict]:
     """'Hyper parameters' to instruct :mod:`argparse` how to create the CLI"""
     meta = {k: v.copy() for k, v in META.items()}
     meta["enable"]["choices"] = {p.tool for p in plugins}
@@ -135,9 +135,9 @@ def __meta__(plugins: Sequence[PluginWrapper]) -> Dict[str, dict]:
 @critical_logging()
 def parse_args(
     args: Sequence[str],
-    plugins: Sequence[PluginWrapper],
+    plugins: Sequence[PluginProtocol],
     description: str = "Validate a given TOML file",
-    get_parser_spec: Callable[[Sequence[PluginWrapper]], Dict[str, dict]] = __meta__,
+    get_parser_spec: Callable[[Sequence[PluginProtocol]], Dict[str, dict]] = __meta__,
     params_class: Type[T] = CliParams,  # type: ignore[assignment]
 ) -> T:
     """Parse command line parameters
@@ -167,11 +167,14 @@ def parse_args(
     return params_class(**params)  # type: ignore[call-overload, no-any-return]
 
 
+Plugins = TypeVar("Plugins", bound=PluginProtocol)
+
+
 def select_plugins(
-    plugins: Sequence[PluginWrapper],
+    plugins: Sequence[Plugins],
     enabled: Sequence[str] = (),
     disabled: Sequence[str] = (),
-) -> List[PluginWrapper]:
+) -> List[Plugins]:
     available = list(plugins)
     if enabled:
         available = [p for p in available if p.tool in enabled]
@@ -219,7 +222,7 @@ def run(args: Sequence[str] = ()) -> int:
           (for example  ``["--verbose", "setup.cfg"]``).
     """
     args = args or sys.argv[1:]
-    plugins: List[PluginWrapper] = list_plugins_from_entry_points()
+    plugins = list_plugins_from_entry_points()
     params: CliParams = parse_args(args, plugins)
     setup_logging(params.loglevel)
     tool_plugins = [RemotePlugin.from_str(t) for t in params.tool]
@@ -263,7 +266,7 @@ def _split_lines(self, text: str, width: int) -> List[str]:
         return list(chain.from_iterable(wrap(x, width) for x in text.splitlines()))
 
 
-def plugins_help(plugins: Sequence[PluginWrapper]) -> str:
+def plugins_help(plugins: Sequence[PluginProtocol]) -> str:
     return "\n".join(_format_plugin_help(p) for p in plugins)
 
 
@@ -273,7 +276,7 @@ def _flatten_str(text: str) -> str:
     return (text[0].lower() + text[1:]).strip()
 
 
-def _format_plugin_help(plugin: PluginWrapper) -> str:
+def _format_plugin_help(plugin: PluginProtocol) -> str:
     help_text = plugin.help_text
     help_text = f": {_flatten_str(help_text)}" if help_text else ""
     return f"* {plugin.tool!r}{help_text}"
diff --git a/src/validate_pyproject/extra_validations.py b/src/validate_pyproject/extra_validations.py
index 789411d0..b99d9c91 100644
--- a/src/validate_pyproject/extra_validations.py
+++ b/src/validate_pyproject/extra_validations.py
@@ -19,8 +19,7 @@ class RedefiningStaticFieldAsDynamic(ValidationError):
     """
     __doc__ = _DESC
     _URL = (
-        "https://packaging.python.org/en/latest/specifications/"
-        "pyproject-toml/#dynamic"
+        "https://packaging.python.org/en/latest/specifications/pyproject-toml/#dynamic"
     )
 
 
diff --git a/src/validate_pyproject/plugins/__init__.py b/src/validate_pyproject/plugins/__init__.py
index 19ca2c14..cb1dd8dd 100644
--- a/src/validate_pyproject/plugins/__init__.py
+++ b/src/validate_pyproject/plugins/__init__.py
@@ -7,15 +7,24 @@
 
 import typing
 from importlib.metadata import EntryPoint, entry_points
+from itertools import chain
 from string import Template
 from textwrap import dedent
-from typing import Any, Callable, Iterable, List, Optional, Protocol
+from typing import (
+    Any,
+    Callable,
+    Generator,
+    Iterable,
+    List,
+    NamedTuple,
+    Optional,
+    Protocol,
+    Union,
+)
 
 from .. import __version__
 from ..types import Plugin, Schema
 
-ENTRYPOINT_GROUP = "validate_pyproject.tool_schema"
-
 
 class PluginProtocol(Protocol):
     @property
@@ -66,34 +75,63 @@ def __repr__(self) -> str:
         return f"{self.__class__.__name__}({self.tool!r}, {self.id})"
 
 
+class StoredPlugin:
+    def __init__(self, tool: str, schema: Schema):
+        self._tool, _, self._fragment = tool.partition("#")
+        self._schema = schema
+
+    @property
+    def id(self) -> str:
+        return self.schema.get("id", "MISSING ID")
+
+    @property
+    def tool(self) -> str:
+        return self._tool
+
+    @property
+    def schema(self) -> Schema:
+        return self._schema
+
+    @property
+    def fragment(self) -> str:
+        return self._fragment
+
+    @property
+    def help_text(self) -> str:
+        return self.schema.get("description", "")
+
+    def __repr__(self) -> str:
+        args = [repr(self.tool), self.id]
+        if self.fragment:
+            args.append(f"fragment={self.fragment!r}")
+        return f"{self.__class__.__name__}({', '.join(args)}, <schema: {self.id}>)"
+
+
 if typing.TYPE_CHECKING:
     _: PluginProtocol = typing.cast(PluginWrapper, None)
 
 
-def iterate_entry_points(group: str = ENTRYPOINT_GROUP) -> Iterable[EntryPoint]:
-    """Produces a generator yielding an EntryPoint object for each plugin registered
+def iterate_entry_points(group: str) -> Iterable[EntryPoint]:
+    """Produces an iterable yielding an EntryPoint object for each plugin registered
     via ``setuptools`` `entry point`_ mechanism.
 
     This method can be used in conjunction with :obj:`load_from_entry_point` to filter
-    the plugins before actually loading them.
+    the plugins before actually loading them. The entry points are not
+    deduplicated.
     """
     entries = entry_points()
     if hasattr(entries, "select"):  # pragma: no cover
         # The select method was introduced in importlib_metadata 3.9 (and Python 3.10)
         # and the previous dict interface was declared deprecated
         select = typing.cast(
-            Any,
+            Callable[..., Iterable[EntryPoint]],
             getattr(entries, "select"),  # noqa: B009
         )  # typecheck gymnastics
-        entries_: Iterable[EntryPoint] = select(group=group)
-    else:  # pragma: no cover
-        # TODO: Once Python 3.10 becomes the oldest version supported, this fallback and
-        #       conditional statement can be removed.
-        entries_ = (plugin for plugin in entries.get(group, []))
-    deduplicated = {
-        e.name: e for e in sorted(entries_, key=lambda e: (e.name, e.value))
-    }
-    return list(deduplicated.values())
+        return select(group=group)
+    # pragma: no cover
+    # TODO: Once Python 3.10 becomes the oldest version supported, this fallback and
+    #       conditional statement can be removed.
+    return (plugin for plugin in entries.get(group, []))
 
 
 def load_from_entry_point(entry_point: EntryPoint) -> PluginWrapper:
@@ -105,23 +143,64 @@ def load_from_entry_point(entry_point: EntryPoint) -> PluginWrapper:
         raise ErrorLoadingPlugin(entry_point=entry_point) from ex
 
 
+def load_from_multi_entry_point(
+    entry_point: EntryPoint,
+) -> Generator[StoredPlugin, None, None]:
+    """Carefully load the plugin, raising a meaningful message in case of errors"""
+    try:
+        fn = entry_point.load()
+        output = fn()
+    except Exception as ex:
+        raise ErrorLoadingPlugin(entry_point=entry_point) from ex
+
+    for tool, schema in output["tools"].items():
+        yield StoredPlugin(tool, schema)
+    for schema in output.get("schemas", []):
+        yield StoredPlugin("", schema)
+
+
+class _SortablePlugin(NamedTuple):
+    priority: int
+    name: str
+    plugin: Union[PluginWrapper, StoredPlugin]
+
+    def __lt__(self, other: Any) -> bool:
+        return (self.plugin.tool or self.plugin.id, self.name, self.priority) < (
+            other.plugin.tool or other.plugin.id,
+            other.name,
+            other.priority,
+        )
+
+
 def list_from_entry_points(
-    group: str = ENTRYPOINT_GROUP,
     filtering: Callable[[EntryPoint], bool] = lambda _: True,
-) -> List[PluginWrapper]:
+) -> List[Union[PluginWrapper, StoredPlugin]]:
     """Produces a list of plugin objects for each plugin registered
     via ``setuptools`` `entry point`_ mechanism.
 
     Args:
-        group: name of the setuptools' entry point group where plugins is being
-            registered
         filtering: function returning a boolean deciding if the entry point should be
             loaded and included (or not) in the final list. A ``True`` return means the
             plugin should be included.
     """
-    return [
-        load_from_entry_point(e) for e in iterate_entry_points(group) if filtering(e)
-    ]
+    tool_eps = (
+        _SortablePlugin(0, e.name, load_from_entry_point(e))
+        for e in iterate_entry_points("validate_pyproject.tool_schema")
+        if filtering(e)
+    )
+    multi_eps = (
+        _SortablePlugin(1, e.name, p)
+        for e in sorted(
+            iterate_entry_points("validate_pyproject.multi_schema"),
+            key=lambda e: e.name,
+            reverse=True,
+        )
+        for p in load_from_multi_entry_point(e)
+        if filtering(e)
+    )
+    eps = chain(tool_eps, multi_eps)
+    dedup = {e.plugin.tool or e.plugin.id: e.plugin for e in sorted(eps, reverse=True)}
+    return list(dedup.values())[::-1]
 
 
 class ErrorLoadingPlugin(RuntimeError):
diff --git a/src/validate_pyproject/pre_compile/cli.py b/src/validate_pyproject/pre_compile/cli.py
index 985ba741..46e538e4 100644
--- a/src/validate_pyproject/pre_compile/cli.py
+++ b/src/validate_pyproject/pre_compile/cli.py
@@ -10,7 +10,7 @@
 from typing import Any, Dict, List, Mapping, NamedTuple, Sequence
 
 from .. import cli
-from ..plugins import PluginWrapper
+from ..plugins import PluginProtocol, PluginWrapper
 from ..plugins import list_from_entry_points as list_plugins_from_entry_points
 from ..remote import RemotePlugin, load_store
 from . import pre_compile
@@ -85,7 +85,9 @@ class CliParams(NamedTuple):
     store: str = ""
 
 
-def parser_spec(plugins: Sequence[PluginWrapper]) -> Dict[str, dict]:
+def parser_spec(
+    plugins: Sequence[PluginProtocol],
+) -> Dict[str, dict]:
     common = ("version", "enable", "disable", "verbose", "very_verbose")
     cli_spec = cli.__meta__(plugins)
     meta = {k: v.copy() for k, v in META.items()}
@@ -101,7 +103,7 @@ def run(args: Sequence[str] = ()) -> int:
     prms = cli.parse_args(args, plugins, desc, parser_spec, CliParams)
     cli.setup_logging(prms.loglevel)
 
-    tool_plugins = [RemotePlugin.from_str(t) for t in prms.tool]
+    tool_plugins: List[PluginProtocol] = [RemotePlugin.from_str(t) for t in prms.tool]
     if prms.store:
         tool_plugins.extend(load_store(prms.store))
 
diff --git a/src/validate_pyproject/project_metadata.schema.json b/src/validate_pyproject/project_metadata.schema.json
index 00c3d03e..c04e6d3a 100644
--- a/src/validate_pyproject/project_metadata.schema.json
+++ b/src/validate_pyproject/project_metadata.schema.json
@@ -326,7 +326,11 @@
           "format": "idn-email",
           "description": "MUST be a valid email address"
         }
-      }
+      },
+      "anyOf": [
+        { "required": ["name"] },
+        { "required": ["email"] }
+      ]
     },
     "entry-point-group": {
       "$id": "#/definitions/entry-point-group",
diff --git a/src/validate_pyproject/remote.py b/src/validate_pyproject/remote.py
index 2194c174..adf9c82e 100644
--- a/src/validate_pyproject/remote.py
+++ b/src/validate_pyproject/remote.py
@@ -67,16 +67,23 @@ def load_store(pyproject_url: str) -> Generator[RemotePlugin, None, None]:
 
     fragment, contents = load_from_uri(pyproject_url)
     if fragment:
-        _logger.error(f"Must not be called with a fragment, got {fragment!r}")
+        _logger.error(
+            f"Must not be called with a fragment, got {fragment!r}"
+        )  # pragma: no cover
     table = contents["properties"]["tool"]["properties"]
     for tool, info in table.items():
         if tool in {"setuptools", "distutils"}:
             pass  # built-in
         elif "$ref" in info:
             _logger.info(f"Loading {tool} from store: {pyproject_url}")
-            yield RemotePlugin.from_url(tool, info["$ref"])
+            rp = RemotePlugin.from_url(tool, info["$ref"])
+            yield rp
+            for values in rp.schema["properties"].values():
+                url = values.get("$ref", "")
+                if url.startswith(("https://", "https://")):
+                    yield RemotePlugin.from_url("", url)
         else:
-            _logger.warning(f"{tool!r} does not contain $ref")
+            _logger.warning(f"{tool!r} does not contain $ref")  # pragma: no cover
 
 
 if typing.TYPE_CHECKING:
diff --git a/src/validate_pyproject/repo_review.py b/src/validate_pyproject/repo_review.py
index 09fc779d..51c93a6c 100644
--- a/src/validate_pyproject/repo_review.py
+++ b/src/validate_pyproject/repo_review.py
@@ -28,9 +28,9 @@ def repo_review_checks() -> Dict[str, VPP001]:
 
 def repo_review_families(pyproject: Dict[str, Any]) -> Dict[str, Dict[str, str]]:
     has_distutils = "distutils" in pyproject.get("tool", {})
-    plugin_names = (ep.name for ep in plugins.iterate_entry_points())
-    plugin_list = (
-        f"`[tool.{n}]`" for n in plugin_names if n != "distutils" or has_distutils
+    plugin_list = plugins.list_from_entry_points(
+        lambda e: e.name != "distutils" or has_distutils
     )
-    descr = f"Checks `[build-system]`, `[project]`, {', '.join(plugin_list)}"
+    plugin_names = (f"`[tool.{n.tool}]`" for n in plugin_list if n.tool)
+    descr = f"Checks `[build-system]`, `[project]`, {', '.join(plugin_names)}"
     return {"validate-pyproject": {"name": "Validate-PyProject", "description": descr}}
diff --git a/tests/examples/simple/empty-author.toml b/tests/examples/simple/empty-author.toml
new file mode 100644
index 00000000..51e5abdb
--- /dev/null
+++ b/tests/examples/simple/empty-author.toml
@@ -0,0 +1,4 @@
+[project]
+name = 'foo'
+version = '1.0'
+authors = []
diff --git a/tests/invalid-examples/pep621/missing-fields/empty-author.errors.txt b/tests/invalid-examples/pep621/missing-fields/empty-author.errors.txt
new file mode 100644
index 00000000..6d2d01cf
--- /dev/null
+++ b/tests/invalid-examples/pep621/missing-fields/empty-author.errors.txt
@@ -0,0 +1 @@
+`project.authors[0]` cannot be validated by any definition
diff --git a/tests/invalid-examples/pep621/missing-fields/empty-author.toml b/tests/invalid-examples/pep621/missing-fields/empty-author.toml
new file mode 100644
index 00000000..770e5d40
--- /dev/null
+++ b/tests/invalid-examples/pep621/missing-fields/empty-author.toml
@@ -0,0 +1,4 @@
+[project]
+name = 'foo'
+version = '1.0'
+authors = [{}]
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 7fd30f34..2b15e45d 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -198,6 +198,12 @@ def test_bad_url(tmp_path, capsys):
         )
 
 
+def test_bad_extra_url(tmp_path, capsys):
+    example = write_example(tmp_path, name="valid-pyproject.toml")
+    with pytest.raises(ValueError, match="URL must start with 'http:' or 'https:'"):
+        cli.run(["--tool", "=file://json.schemastore.org/poetry.toml", str(example)])
+
+
 @pytest.mark.skipif(sys.version_info[:2] < (3, 11), reason="requires 3.11+")
 def test_parser_is_tomllib():
     """Make sure Python >= 3.11 uses tomllib instead of tomli"""
diff --git a/tests/test_formats.py b/tests/test_formats.py
index cdff3ee2..5bcea215 100644
--- a/tests/test_formats.py
+++ b/tests/test_formats.py
@@ -139,9 +139,10 @@ def test_entrypoint_references_with_extras():
     assert formats.python_entrypoint_reference(example) is False
 
 
-@pytest.mark.parametrize("example", ["module" "invalid-module"])
+@pytest.mark.parametrize("example", ["module", "invalid-module"])
 def test_invalid_entrypoint_references(example):
-    assert formats.python_entrypoint_reference(example) is False
+    result = example == "module"
+    assert formats.python_entrypoint_reference(example) is result
 
 
 @pytest.mark.parametrize("example", ["λ", "a", "_"])
diff --git a/tests/test_plugins.py b/tests/test_plugins.py
index 1f07dec5..89f3c8bd 100644
--- a/tests/test_plugins.py
+++ b/tests/test_plugins.py
@@ -1,12 +1,16 @@
 # The code in this module is mostly borrowed/adapted from PyScaffold and was originally
 # published under the MIT license
 # The original PyScaffold license can be found in 'NOTICE.txt'
-from importlib.metadata import EntryPoint  # pragma: no cover
+import functools
+import importlib.metadata
+import sys
+from types import ModuleType
+from typing import List
 
 import pytest
 
 from validate_pyproject import plugins
-from validate_pyproject.plugins import ENTRYPOINT_GROUP, ErrorLoadingPlugin
+from validate_pyproject.plugins import ErrorLoadingPlugin, PluginWrapper, StoredPlugin
 
 EXISTING = (
     "setuptools",
@@ -18,7 +22,9 @@ def test_load_from_entry_point__error():
     # This module does not exist, so Python will have some trouble loading it
     # EntryPoint(name, value, group)
     entry = "mypkg.SOOOOO___fake___:activate"
-    fake = EntryPoint("fake", entry, ENTRYPOINT_GROUP)
+    fake = importlib.metadata.EntryPoint(
+        "fake", entry, "validate_pyproject.tool_schema"
+    )
     with pytest.raises(ErrorLoadingPlugin):
         plugins.load_from_entry_point(fake)
 
@@ -28,7 +34,7 @@ def is_entry_point(ep):
 
 
 def test_iterate_entry_points():
-    plugin_iter = plugins.iterate_entry_points()
+    plugin_iter = plugins.iterate_entry_points("validate_pyproject.tool_schema")
     assert hasattr(plugin_iter, "__iter__")
     pluging_list = list(plugin_iter)
     assert all(is_entry_point(e) for e in pluging_list)
@@ -68,3 +74,155 @@ def _fn2(_):
 
         pw = plugins.PluginWrapper("name", _fn2)
         assert pw.help_text == "Help for `name`"
+
+
+class TestStoredPlugin:
+    def test_empty_help_text(self):
+        def _fn1(_):
+            return {}
+
+        pw = plugins.StoredPlugin("name", {})
+        assert pw.help_text == ""
+
+        def _fn2(_):
+            """Help for `${tool}`"""
+            return {}
+
+        pw = plugins.StoredPlugin("name", {"description": "Help for me"})
+        assert pw.help_text == "Help for me"
+
+
+def fake_multi_iterate_entry_points(name: str) -> List[importlib.metadata.EntryPoint]:
+    if name == "validate_pyproject.multi_schema":
+        return [
+            importlib.metadata.EntryPoint(
+                name="_", value="test_module:f", group="validate_pyproject.multi_schema"
+            )
+        ]
+    return []
+
+
+def test_multi_plugins(monkeypatch):
+    s1 = {"id": "example1"}
+    s2 = {"id": "example2"}
+    s3 = {"id": "example3"}
+    sys.modules["test_module"] = ModuleType("test_module")
+    sys.modules["test_module"].f = lambda: {
+        "tools": {"example#frag": s1},
+        "schemas": [s2, s3],
+    }  # type: ignore[attr-defined]
+    monkeypatch.setattr(
+        plugins, "iterate_entry_points", fake_multi_iterate_entry_points
+    )
+
+    lst = plugins.list_from_entry_points()
+    assert len(lst) == 3
+    assert all(e.id.startswith("example") for e in lst)
+
+    (fragmented,) = (e for e in lst if e.tool)
+    assert fragmented.tool == "example"
+    assert fragmented.fragment == "frag"
+    assert fragmented.schema == s1
+
+
+def fake_both_iterate_entry_points(name: str) -> List[importlib.metadata.EntryPoint]:
+    if name == "validate_pyproject.multi_schema":
+        return [
+            importlib.metadata.EntryPoint(
+                name="_", value="test_module:f", group="validate_pyproject.multi_schema"
+            )
+        ]
+    if name == "validate_pyproject.tool_schema":
+        return [
+            importlib.metadata.EntryPoint(
+                name="example1",
+                value="test_module:f1",
+                group="validate_pyproject.tool_schema",
+            ),
+            importlib.metadata.EntryPoint(
+                name="example3",
+                value="test_module:f3",
+                group="validate_pyproject.tool_schema",
+            ),
+        ]
+    return []
+
+
+def test_combined_plugins(monkeypatch):
+    s1 = {"id": "example1"}
+    s2 = {"id": "example2"}
+    sys.modules["test_module"] = ModuleType("test_module")
+    sys.modules["test_module"].f = lambda: {
+        "tools": {"example1": s1, "example2": s2},
+    }  # type: ignore[attr-defined]
+    sys.modules["test_module"].f1 = lambda _: {"id": "tool1"}  # type: ignore[attr-defined]
+    sys.modules["test_module"].f3 = lambda _: {"id": "tool3"}  # type: ignore[attr-defined]
+    monkeypatch.setattr(plugins, "iterate_entry_points", fake_both_iterate_entry_points)
+
+    lst = plugins.list_from_entry_points()
+    assert len(lst) == 3
+
+    assert lst[0].tool == "example1"
+    assert isinstance(lst[0], StoredPlugin)
+
+    assert lst[1].tool == "example2"
+    assert isinstance(lst[1], StoredPlugin)
+
+    assert lst[2].tool == "example3"
+    assert isinstance(lst[2], PluginWrapper)
+
+
+def fake_several_entry_points(
+    name: str, *, reverse: bool
+) -> List[importlib.metadata.EntryPoint]:
+    if name == "validate_pyproject.multi_schema":
+        items = [
+            importlib.metadata.EntryPoint(
+                name="a",
+                value="test_module:f1",
+                group="validate_pyproject.multi_schema",
+            ),
+            importlib.metadata.EntryPoint(
+                name="b",
+                value="test_module:f2",
+                group="validate_pyproject.multi_schema",
+            ),
+        ]
+        return items[::-1] if reverse else items
+    return []
+
+
+@pytest.mark.parametrize("reverse", [True, False])
+def test_several_multi_plugins(monkeypatch, reverse):
+    s1 = {"id": "example1"}
+    s2 = {"id": "example2"}
+    s3 = {"id": "example3"}
+    sys.modules["test_module"] = ModuleType("test_module")
+    sys.modules["test_module"].f1 = lambda: {
+        "tools": {"example": s1},
+    }  # type: ignore[attr-defined]
+    sys.modules["test_module"].f2 = lambda: {
+        "tools": {"example": s2, "other": s3},
+    }  # type: ignore[attr-defined]
+    monkeypatch.setattr(
+        plugins,
+        "iterate_entry_points",
+        functools.partial(fake_several_entry_points, reverse=reverse),
+    )
+
+    (plugin1, plugin2) = plugins.list_from_entry_points()
+    assert plugin1.id == "example1"
+    assert plugin2.id == "example3"
+
+
+def test_broken_multi_plugin(monkeypatch):
+    def broken_ep():
+        raise RuntimeError("Broken")
+
+    sys.modules["test_module"] = ModuleType("test_module")
+    sys.modules["test_module"].f = broken_ep
+    monkeypatch.setattr(
+        plugins, "iterate_entry_points", fake_multi_iterate_entry_points
+    )
+    with pytest.raises(ErrorLoadingPlugin):
+        plugins.list_from_entry_points()
diff --git a/tests/test_repo_review.py b/tests/test_repo_review.py
index aaf39643..25339f11 100644
--- a/tests/test_repo_review.py
+++ b/tests/test_repo_review.py
@@ -45,9 +45,9 @@ def test_valid_example(repo_review_processor, name: str) -> None:
 @pytest.mark.parametrize("name", ["pdm/invalid-version", "pdm/redefining-as-dynamic"])
 def test_invalid_example(repo_review_processor, name: str) -> None:
     processed = repo_review_processor.process(INVALID_EXAMPLES / name)
-    assert any(
-        not r.result and r.result is not None for r in processed.results
-    ), f"{processed.results}"
+    assert any(not r.result and r.result is not None for r in processed.results), (
+        f"{processed.results}"
+    )
 
 
 def test_no_distutils(repo_review_processor) -> None:
diff --git a/tox.ini b/tox.ini
index e8a2d577..45af7d02 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,7 +3,7 @@
 # THIS SCRIPT IS SUPPOSED TO BE AN EXAMPLE. MODIFY IT ACCORDING TO YOUR NEEDS!
 
 [tox]
-minversion = 3.24
+minversion = 4.22
 envlist = default
 isolated_build = True
 
@@ -16,9 +16,8 @@ passenv =
     HOME
     SETUPTOOLS_*
     VALIDATE_PYPROJECT_*
-extras =
-    all
-    test
+dependency_groups = test
+extras = all
 commands =
     pytest {posargs}
 
@@ -36,14 +35,14 @@ commands =
 
 
 [testenv:typecheck]
+base_python = 3.8
 description = Invoke mypy to typecheck the source code
 changedir = {toxinidir}
 passenv =
     TERM
     # ^ ensure colors
-extras =
-    all
-    typecheck
+extras = all
+dependency_groups = typecheck
 commands =
     python -m mypy {posargs:--pretty --show-error-context src}
 
@@ -78,11 +77,8 @@ setenv =
     linkcheck: BUILD = linkcheck
 passenv =
     SETUPTOOLS_*
-extras =
-    all
-deps =
-    -r {toxinidir}/docs/requirements.txt
-    # ^  requirements.txt shared with Read The Docs
+extras = all
+dependency_groups = docs
 commands =
     sphinx-build -v -T -j auto --color -b {env:BUILD} -d "{env:BUILDDIR}/doctrees" "{env:DOCSDIR}" "{env:BUILDDIR}/{env:BUILD}" {posargs}