diff --git a/src/fromager/packagesettings/_patch.py b/src/fromager/packagesettings/_patch.py
new file mode 100644
index 00000000..73718fbe
--- /dev/null
+++ b/src/fromager/packagesettings/_patch.py
@@ -0,0 +1,279 @@
+from __future__ import annotations
+
+import logging
+import pathlib
+import re
+import typing
+
+import pydantic
+from packaging.requirements import Requirement
+from packaging.version import Version
+
+from ..pyproject import PyprojectFix
+from ._typedefs import (
+    MODEL_CONFIG,
+    Package,
+    SpecifierSetType,
+)
+
+if typing.TYPE_CHECKING:
+    from .. import build_environment, context
+
+logger = logging.getLogger(__name__)
+
+SDIST_STEP = typing.Literal["sdist"]
+DIST_INFO_METADATA_STEP = typing.Literal["dist-info-metadata"]
+
+
+class PatchBase(pydantic.BaseModel):
+    """Base class for patch setting"""
+
+    model_config = MODEL_CONFIG
+
+    step: typing.ClassVar[SDIST_STEP | DIST_INFO_METADATA_STEP]
+    """In which step of the build process does the plugin run?
+
+    - ``sdist`` plugins run between unpackagin and repacking of source
+      distributions
+    - ``dist-info-metadata`` run when the final wheel file is assembled.
+      They also affect ``get_install_dependencies_of_sdist`` hook.
+    """
+
+    op: str
+    """Operation name (discriminator field)"""
+
+    title: str
+    """Human-readable title for the config setting"""
+
+    when_version: SpecifierSetType | None = None
+    """Only patch when specifer set matches"""
+
+    ignore_missing: bool = False
+    """Don't fail when operation does not modify a file"""
+
+
+class SdistPatchBase(PatchBase):
+    """Base class for patching of sdists"""
+
+    step = "sdist"
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        raise NotImplementedError
+
+
+class PatchReplaceLine(SdistPatchBase):
+    """Replace line in sources"""
+
+    op: typing.Literal["replace-line"]
+    files: typing.Annotated[list[str], pydantic.Field(min_length=1)]
+    search: re.Pattern
+    replace: str
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        # TODO
+        raise NotImplementedError
+
+
+class PatchDeleteLine(SdistPatchBase):
+    """Delete line in sources"""
+
+    op: typing.Literal["delete-line"]
+    files: typing.Annotated[list[str], pydantic.Field(min_length=1)]
+    search: re.Pattern
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        # TODO
+        raise NotImplementedError
+
+
+class PatchPyProjectBuildSystem(SdistPatchBase):
+    """Modify pyproject.toml [build-system]
+
+    Replaces project_override setting
+    """
+
+    op: typing.Literal["pyproject-build-system"]
+
+    update_build_requires: list[str] = pydantic.Field(default_factory=list)
+    """Add / update requirements to pyproject.toml `[build-system] requires`
+    """
+
+    # TODO: use list[Package]
+    remove_build_requires: list[Package] = pydantic.Field(default_factory=list)
+    """Remove requirement from pyproject.toml `[build-system] requires`
+    """
+
+    requires_external: list[str] = pydantic.Field(default_factory=list)
+    """Add / update Requires-External core metadata field
+
+    Each entry contains a string describing some dependency in the system
+    that the distribution is to be used. See
+    https://packaging.python.org/en/latest/specifications/core-metadata/#requires-external-multiple-use
+
+    .. note::
+       Fromager does not modify ``METADATA`` file, yet. Read the information
+       from an ``importlib.metadata`` distribution with
+       ``tomlkit.loads(dist(pkgname).read_text("fromager-build-settings"))``.
+    """
+
+    @pydantic.field_validator("update_build_requires")
+    @classmethod
+    def validate_update_build_requires(cls, v: list[str]) -> list[str]:
+        """update_build_requires fields must be valid requirements"""
+        for reqstr in v:
+            Requirement(reqstr)
+        return v
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        if self.update_build_requires or self.remove_build_requires:
+            pbi = ctx.package_build_info(req)
+            fixer = PyprojectFix(
+                req,
+                build_dir=pbi.build_dir(sdist_root_dir),
+                update_build_requires=self.update_build_requires,
+                remove_build_requires=self.remove_build_requires,
+            )
+            fixer.run()
+
+
+class FixPkgInfoVersion(SdistPatchBase):
+    """Fix PKG-INFO Metadata version of an sdist"""
+
+    op: typing.Literal["fix-pkg-info"]
+    metadata_version: str = "2.4"
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        # TODO
+        raise NotImplementedError
+
+
+# ---------------------------------------------------------------------------
+
+
+class DistInfoMetadataPatchBase(PatchBase):
+    """Base class for patching of dist-info metadata
+
+    The patchers affect wheel metadata and outcome of
+    ``get_install_dependencies_of_sdist``.
+    """
+
+    step = "dist-info-metadata"
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        dist_info_dir: pathlib.Path,
+        build_env: build_environment.BuildEnvironment,
+    ) -> None:
+        raise NotImplementedError
+
+
+class PinRequiresDistToConstraint(DistInfoMetadataPatchBase):
+    """Pin install requirements to constraint
+
+    Update an installation requirement version and pin it to the same
+    version as configured in constraints.
+    """
+
+    op: typing.Literal["pin-requires-dist-to-constraint"]
+    requirements: typing.Annotated[list[Package], pydantic.Field(min_length=1)]
+
+    def __call__(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        dist_info_dir: pathlib.Path,
+        build_env: build_environment.BuildEnvironment,
+    ) -> None:
+        # TODO
+        raise NotImplementedError
+
+
+PatchUnion = typing.Annotated[
+    PatchReplaceLine
+    | PatchDeleteLine
+    | PatchPyProjectBuildSystem
+    | FixPkgInfoVersion
+    | PinRequiresDistToConstraint,
+    pydantic.Field(..., discriminator="op"),
+]
+
+
+class Patches(pydantic.RootModel[list[PatchUnion]]):
+    def run_sdist_patcher(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        sdist_root_dir: pathlib.Path,
+    ) -> None:
+        for patcher in self.root:
+            if patcher == SDIST_STEP:
+                assert isinstance(patcher, SdistPatchBase)
+                patcher(
+                    ctx=ctx,
+                    req=req,
+                    version=version,
+                    sdist_root_dir=sdist_root_dir,
+                )
+
+    def run_dist_info_metadata_patcher(
+        self,
+        *,
+        ctx: context.WorkContext,
+        req: Requirement,
+        version: Version,
+        dist_info_dir: pathlib.Path,
+        build_env: build_environment.BuildEnvironment,
+    ) -> None:
+        for patcher in self.root:
+            if patcher.step == DIST_INFO_METADATA_STEP:
+                assert isinstance(patcher, DistInfoMetadataPatchBase)
+                patcher(
+                    ctx=ctx,
+                    req=req,
+                    version=version,
+                    dist_info_dir=dist_info_dir,
+                    build_env=build_env,
+                )
diff --git a/src/fromager/packagesettings/_typedefs.py b/src/fromager/packagesettings/_typedefs.py
index aeae5909..802f5ecc 100644
--- a/src/fromager/packagesettings/_typedefs.py
+++ b/src/fromager/packagesettings/_typedefs.py
@@ -7,6 +7,7 @@
 from collections.abc import Mapping
 
 import pydantic
+from packaging.specifiers import SpecifierSet
 from packaging.utils import NormalizedName, canonicalize_name
 from packaging.version import Version
 from pydantic_core import CoreSchema, core_schema
@@ -58,6 +59,21 @@ def __get_pydantic_core_schema__(
         )
 
 
+class SpecifierSetType(SpecifierSet):
+    """Pydantic-aware specifier set"""
+
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls, source_type: typing.Any, handler: pydantic.GetCoreSchemaHandler
+    ) -> CoreSchema:
+        return core_schema.with_info_plain_validator_function(
+            lambda v, _: SpecifierSet(v),
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                str, when_used="json"
+            ),
+        )
+
+
 # environment variables map
 def _validate_envkey(v: typing.Any) -> str:
     """Validate env key, converts int, float, bool"""
diff --git a/tests/test_patchsettings.py b/tests/test_patchsettings.py
new file mode 100644
index 00000000..1fc30ff3
--- /dev/null
+++ b/tests/test_patchsettings.py
@@ -0,0 +1,51 @@
+import pydantic
+import yaml
+
+from fromager.packagesettings import MODEL_CONFIG
+from fromager.packagesettings._patch import Patches
+
+# example from new patcher proposal
+EXAMPLE = """
+patch:
+  - title: Comment out 'foo' requirement for version >= 1.2
+    op: replace-line
+    files:
+      - 'requirements.txt'
+    search: '^(foo.*)$'
+    replace: '# \\1'
+    when_version: '>=1.2'
+    ignore_missing: true
+
+  - title: Remove 'bar' from constraints.txt
+    op: delete-line
+    files:
+      - 'constraints.txt'
+    search: 'bar.*'
+
+  - title: Fix PKG-INFO metadata version
+    op: fix-pkg-info
+    metadata_version: '2.4'
+    when_version: '<1.0'
+
+  - title: Add missing setuptools to pyproject.toml
+    op: pyproject-build-system
+    update_build_requires:
+      - setuptools
+
+  - title: Update Torch install requirement to version in build env
+    op: pin-requires-dist-to-constraint
+    requirements:
+     - torch
+"""
+
+
+def test_patch_settings_basics() -> None:
+    # temporary test case until patch settings are hooked up to PBI
+
+    class Settings(pydantic.BaseModel):
+        model_config = MODEL_CONFIG
+        patch: Patches
+
+    settings = Settings(**yaml.safe_load(EXAMPLE))
+    patchers = settings.patch.root
+    assert len(patchers) == 5