diff --git a/core/pystac/__init__.py b/core/pystac/__init__.py
index 0f1b808b9..5b6c6ba07 100644
--- a/core/pystac/__init__.py
+++ b/core/pystac/__init__.py
@@ -48,7 +48,6 @@
     "set_stac_version",
 ]
 
-import warnings
 from typing import Any
 
 from pystac.errors import (
@@ -91,56 +90,11 @@
 from pystac.utils import HREF
 
 import pystac.extensions.hooks
-import pystac.extensions.classification
-import pystac.extensions.datacube
-import pystac.extensions.eo
-import pystac.extensions.file
-import pystac.extensions.grid
-import pystac.extensions.item_assets
 
-with warnings.catch_warnings():
-    warnings.filterwarnings("ignore", category=DeprecationWarning)
-    import pystac.extensions.label
-import pystac.extensions.mgrs
-import pystac.extensions.mlm
-import pystac.extensions.pointcloud
-import pystac.extensions.projection
-import pystac.extensions.raster
-import pystac.extensions.sar
-import pystac.extensions.sat
-import pystac.extensions.scientific
-import pystac.extensions.storage
-import pystac.extensions.table
-import pystac.extensions.timestamps
-import pystac.extensions.version
-import pystac.extensions.view
-import pystac.extensions.xarray_assets
-
-EXTENSION_HOOKS = pystac.extensions.hooks.RegisteredExtensionHooks(
-    [
-        pystac.extensions.classification.CLASSIFICATION_EXTENSION_HOOKS,
-        pystac.extensions.datacube.DATACUBE_EXTENSION_HOOKS,
-        pystac.extensions.eo.EO_EXTENSION_HOOKS,
-        pystac.extensions.file.FILE_EXTENSION_HOOKS,
-        pystac.extensions.grid.GRID_EXTENSION_HOOKS,
-        pystac.extensions.item_assets.ITEM_ASSETS_EXTENSION_HOOKS,
-        pystac.extensions.label.LABEL_EXTENSION_HOOKS,
-        pystac.extensions.mgrs.MGRS_EXTENSION_HOOKS,
-        pystac.extensions.mlm.MLM_EXTENSION_HOOKS,
-        pystac.extensions.pointcloud.POINTCLOUD_EXTENSION_HOOKS,
-        pystac.extensions.projection.PROJECTION_EXTENSION_HOOKS,
-        pystac.extensions.raster.RASTER_EXTENSION_HOOKS,
-        pystac.extensions.sar.SAR_EXTENSION_HOOKS,
-        pystac.extensions.sat.SAT_EXTENSION_HOOKS,
-        pystac.extensions.scientific.SCIENTIFIC_EXTENSION_HOOKS,
-        pystac.extensions.storage.STORAGE_EXTENSION_HOOKS,
-        pystac.extensions.table.TABLE_EXTENSION_HOOKS,
-        pystac.extensions.timestamps.TIMESTAMPS_EXTENSION_HOOKS,
-        pystac.extensions.version.VERSION_EXTENSION_HOOKS,
-        pystac.extensions.view.VIEW_EXTENSION_HOOKS,
-        pystac.extensions.xarray_assets.XARRAY_ASSETS_EXTENSION_HOOKS,
-    ]
-)
+#: Extension migration/link hooks, discovered lazily from installed extension
+#: packages via the ``pystac.extensions`` entry point group. ``pystac-core``
+#: intentionally has no compile-time knowledge of any extension.
+EXTENSION_HOOKS = pystac.extensions.hooks.RegisteredExtensionHooks()
 
 
 def read_file(href: HREF, stac_io: StacIO | None = None) -> STACObject:
diff --git a/core/pystac/asset.py b/core/pystac/asset.py
index cfffc1337..4afa28340 100644
--- a/core/pystac/asset.py
+++ b/core/pystac/asset.py
@@ -272,7 +272,12 @@ def ext(self) -> AssetExt:
 
             asset.ext.proj.code = "EPSG:4326"
         """
-        from pystac.extensions.ext import AssetExt
+        try:
+            from pystac.extensions.ext import AssetExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return AssetExt(stac_object=self)
 
diff --git a/core/pystac/catalog.py b/core/pystac/catalog.py
index 3a81ad2f2..23feb28a7 100644
--- a/core/pystac/catalog.py
+++ b/core/pystac/catalog.py
@@ -1302,6 +1302,11 @@ def ext(self) -> CatalogExt:
 
             print(collection.ext.version)
         """
-        from pystac.extensions.ext import CatalogExt
+        try:
+            from pystac.extensions.ext import CatalogExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return CatalogExt(stac_object=self)
diff --git a/core/pystac/collection.py b/core/pystac/collection.py
index 536fd2830..20cc11506 100644
--- a/core/pystac/collection.py
+++ b/core/pystac/collection.py
@@ -15,7 +15,7 @@
 from pystac import CatalogType, STACObjectType
 from pystac.asset import Asset, Assets
 from pystac.catalog import Catalog
-from pystac.errors import DeprecatedWarning, ExtensionNotImplemented, STACTypeError
+from pystac.errors import DeprecatedWarning, STACTypeError
 from pystac.item_assets import ItemAssetDefinition, _ItemAssets
 from pystac.layout import HrefLayoutStrategy
 from pystac.link import Link
@@ -638,8 +638,6 @@ def from_dict(
     ) -> C:
         import warnings
 
-        from pystac.extensions.version import CollectionVersionExtension
-
         if migrate:
             info = identify_stac_object(d)
             d = migrate_to_latest(d, info)
@@ -710,16 +708,9 @@ def from_dict(
         if root:
             collection.set_root(root)
 
-        try:
-            version = CollectionVersionExtension.ext(collection)
-            if version.deprecated:
-                warnings.warn(
-                    f"The collection '{collection.id}' is deprecated.",
-                    DeprecatedWarning,
-                )
-            # Collection asset deprecation checks pending version extension support
-        except ExtensionNotImplemented:
-            pass
+        message = pystac.EXTENSION_HOOKS.get_deprecation_message(collection)
+        if message is not None:
+            warnings.warn(message, DeprecatedWarning)
 
         return collection
 
@@ -894,6 +885,11 @@ def ext(self) -> CollectionExt:
 
             print(collection.ext.xarray)
         """
-        from pystac.extensions.ext import CollectionExt
+        try:
+            from pystac.extensions.ext import CollectionExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return CollectionExt(stac_object=self)
diff --git a/core/pystac/errors.py b/core/pystac/errors.py
index 69c817681..022c87ec1 100644
--- a/core/pystac/errors.py
+++ b/core/pystac/errors.py
@@ -1,4 +1,20 @@
-from typing import Any
+from typing import Any, NoReturn
+
+
+def _raise_for_missing_ext(error: ModuleNotFoundError) -> NoReturn:
+    """Re-raise a friendlier error when the ``.ext`` accessor is unavailable.
+
+    The `.ext` accessor lives in `pystac.extensions.ext`, which is distributed
+    with the `pystac` package rather than `pystac-core`. If that specific module
+    is missing, point the user at `pystac`; otherwise propagate the original
+    error (a genuinely broken extension install).
+    """
+    if error.name == "pystac.extensions.ext":
+        raise ImportError(
+            "The `.ext` accessor is provided by the `pystac` package, not "
+            "`pystac-core`. Install `pystac` to use `.ext`."
+        ) from error
+    raise error
 
 
 class TemplateError(Exception):
diff --git a/core/pystac/extensions/hooks.py b/core/pystac/extensions/hooks.py
index 8db8ca4e4..8cb0c80ff 100644
--- a/core/pystac/extensions/hooks.py
+++ b/core/pystac/extensions/hooks.py
@@ -44,6 +44,14 @@ def _get_stac_object_types(self) -> set[str]:
     def get_object_links(self, obj: STACObject) -> list[str | pystac.RelType] | None:
         return None
 
+    def get_deprecation_message(self, obj: STACObject) -> str | None:
+        """Return a warning message if `obj` is deprecated per this extension.
+
+        Emitted as a `pystac.DeprecatedWarning` when loading an object from a
+        dict. The base implementation returns None (not deprecated).
+        """
+        return None
+
     def has_extension(self, obj: dict[str, Any]) -> bool:
         schema_startswith = VERSION_REGEX.split(self.schema_uri)[0] + "/"
         return any(
@@ -75,8 +83,27 @@ def migrate(
 class RegisteredExtensionHooks:
     hooks: dict[str, ExtensionHooks]
 
-    def __init__(self, hooks: Iterable[ExtensionHooks]):
+    def __init__(self, hooks: Iterable[ExtensionHooks] = ()):
         self.hooks = {e.schema_uri: e for e in hooks}
+        self._discovered = False
+
+    def _discover(self) -> None:
+        """Register hooks advertised via the ``pystac.extensions`` entry point group.
+
+        Installed extensions self-advertise their `ExtensionHooks`, so
+        `pystac-core` needs no compile-time knowledge of them. Runs once.
+        """
+        if self._discovered:
+            return
+        self._discovered = True
+        import warnings
+        from importlib.metadata import entry_points
+
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
+            for entry_point in entry_points(group="pystac.extensions"):
+                hooks = entry_point.load()
+                self.hooks.setdefault(hooks.schema_uri, hooks)
 
     def add_extension_hooks(self, hooks: ExtensionHooks) -> None:
         e_id = hooks.schema_uri
@@ -92,6 +119,7 @@ def remove_extension_hooks(self, extension_id: str) -> None:
             del self.hooks[extension_id]
 
     def get_extended_object_links(self, obj: STACObject) -> list[str | pystac.RelType]:
+        self._discover()
         result: list[str | pystac.RelType] | None = None
         for ext in obj.stac_extensions:
             if ext in self.hooks:
@@ -106,6 +134,15 @@ def get_extended_object_links(self, obj: STACObject) -> list[str | pystac.RelTyp
     def migrate(
         self, obj: dict[str, Any], version: STACVersionID, info: STACJSONDescription
     ) -> None:
+        self._discover()
         for hooks in self.hooks.values():
             if info.object_type in hooks._get_stac_object_types():
                 hooks.migrate(obj, version, info)
+
+    def get_deprecation_message(self, obj: STACObject) -> str | None:
+        self._discover()
+        for hooks in self.hooks.values():
+            message = hooks.get_deprecation_message(obj)
+            if message is not None:
+                return message
+        return None
diff --git a/core/pystac/item.py b/core/pystac/item.py
index 7bb4b84cb..eee96065b 100644
--- a/core/pystac/item.py
+++ b/core/pystac/item.py
@@ -8,7 +8,7 @@
 from pystac.asset import Asset, Assets
 from pystac.catalog import Catalog
 from pystac.collection import Collection
-from pystac.errors import DeprecatedWarning, ExtensionNotImplemented
+from pystac.errors import DeprecatedWarning
 from pystac.link import Link
 from pystac.serialization import (
     identify_stac_object,
@@ -426,8 +426,6 @@ def from_dict(
     ) -> T:
         import warnings
 
-        from pystac.extensions.version import ItemVersionExtension
-
         if preserve_dict:
             d = deepcopy(d)
 
@@ -482,16 +480,9 @@ def from_dict(
         if root:
             item.set_root(root)
 
-        try:
-            version = ItemVersionExtension.ext(item)
-            if version.deprecated:
-                warnings.warn(
-                    f"The item '{item.id}' is deprecated.",
-                    DeprecatedWarning,
-                )
-            # Item asset deprecation checks pending version extension support
-        except ExtensionNotImplemented:
-            pass
+        message = pystac.EXTENSION_HOOKS.get_deprecation_message(item)
+        if message is not None:
+            warnings.warn(message, DeprecatedWarning)
 
         return item
 
@@ -534,6 +525,11 @@ def ext(self) -> ItemExt:
 
             item.ext.proj.code = "EPSG:4326"
         """
-        from pystac.extensions.ext import ItemExt
+        try:
+            from pystac.extensions.ext import ItemExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return ItemExt(stac_object=self)
diff --git a/core/pystac/item_assets.py b/core/pystac/item_assets.py
index ab3a1fadb..589d62c0a 100644
--- a/core/pystac/item_assets.py
+++ b/core/pystac/item_assets.py
@@ -212,7 +212,12 @@ def ext(self) -> ItemAssetExt:
 
             collection.item_assets["data"].ext.proj.epsg = 4326
         """
-        from pystac.extensions.ext import ItemAssetExt
+        try:
+            from pystac.extensions.ext import ItemAssetExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return ItemAssetExt(stac_object=self)
 
diff --git a/core/pystac/link.py b/core/pystac/link.py
index b2567e3fd..4a5ca41b5 100644
--- a/core/pystac/link.py
+++ b/core/pystac/link.py
@@ -525,6 +525,11 @@ def ext(self) -> LinkExt:
 
             link.ext.file.size = 8675309
         """
-        from pystac.extensions.ext import LinkExt
+        try:
+            from pystac.extensions.ext import LinkExt
+        except ModuleNotFoundError as e:
+            from pystac.errors import _raise_for_missing_ext
+
+            _raise_for_missing_ext(e)
 
         return LinkExt(stac_object=self)
diff --git a/extensions/classification/pyproject.toml b/extensions/classification/pyproject.toml
index ff1519f5a..2fdc17060 100644
--- a/extensions/classification/pyproject.toml
+++ b/extensions/classification/pyproject.toml
@@ -19,7 +19,13 @@ classifiers = [
     "Programming Language :: Python :: 3.13",
 ]
 requires-python = ">=3.10"
-dependencies = ["pystac-core"]
+dependencies = [
+    "pystac-core",
+    "pystac-ext-raster",
+]
+
+[project.entry-points."pystac.extensions"]
+classification = "pystac.extensions.classification:CLASSIFICATION_EXTENSION_HOOKS"
 
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
diff --git a/extensions/datacube/pyproject.toml b/extensions/datacube/pyproject.toml
index e08fba072..dc95e651b 100644
--- a/extensions/datacube/pyproject.toml
+++ b/extensions/datacube/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+datacube = "pystac.extensions.datacube:DATACUBE_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/eo/pyproject.toml b/extensions/eo/pyproject.toml
index 06bd7b411..b78c41855 100644
--- a/extensions/eo/pyproject.toml
+++ b/extensions/eo/pyproject.toml
@@ -19,7 +19,14 @@ classifiers = [
     "Programming Language :: Python :: 3.13",
 ]
 requires-python = ">=3.10"
-dependencies = ["pystac-core"]
+dependencies = [
+    "pystac-core",
+    "pystac-ext-projection",
+    "pystac-ext-view",
+]
+
+[project.entry-points."pystac.extensions"]
+eo = "pystac.extensions.eo:EO_EXTENSION_HOOKS"
 
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
diff --git a/extensions/file/pyproject.toml b/extensions/file/pyproject.toml
index f8ecc905f..13784eb33 100644
--- a/extensions/file/pyproject.toml
+++ b/extensions/file/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+file = "pystac.extensions.file:FILE_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/grid/pyproject.toml b/extensions/grid/pyproject.toml
index eea7e004b..d6d3617a3 100644
--- a/extensions/grid/pyproject.toml
+++ b/extensions/grid/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+grid = "pystac.extensions.grid:GRID_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/item_assets/pyproject.toml b/extensions/item_assets/pyproject.toml
index 5f89d8a0f..10a97c813 100644
--- a/extensions/item_assets/pyproject.toml
+++ b/extensions/item_assets/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+item_assets = "pystac.extensions.item_assets:ITEM_ASSETS_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/label/pyproject.toml b/extensions/label/pyproject.toml
index 08403578c..1c8efcda5 100644
--- a/extensions/label/pyproject.toml
+++ b/extensions/label/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+label = "pystac.extensions.label:LABEL_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/mgrs/pyproject.toml b/extensions/mgrs/pyproject.toml
index 9485ab107..325cbc4a3 100644
--- a/extensions/mgrs/pyproject.toml
+++ b/extensions/mgrs/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+mgrs = "pystac.extensions.mgrs:MGRS_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/mlm/pyproject.toml b/extensions/mlm/pyproject.toml
index 51ff70f30..33a989c4f 100644
--- a/extensions/mlm/pyproject.toml
+++ b/extensions/mlm/pyproject.toml
@@ -19,7 +19,14 @@ classifiers = [
     "Programming Language :: Python :: 3.13",
 ]
 requires-python = ">=3.10"
-dependencies = ["pystac-core"]
+dependencies = [
+    "pystac-core",
+    "pystac-ext-classification",
+    "pystac-ext-raster",
+]
+
+[project.entry-points."pystac.extensions"]
+mlm = "pystac.extensions.mlm:MLM_EXTENSION_HOOKS"
 
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
diff --git a/extensions/pointcloud/pyproject.toml b/extensions/pointcloud/pyproject.toml
index cf507aa64..995013db0 100644
--- a/extensions/pointcloud/pyproject.toml
+++ b/extensions/pointcloud/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+pointcloud = "pystac.extensions.pointcloud:POINTCLOUD_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/projection/pyproject.toml b/extensions/projection/pyproject.toml
index a7137059c..73485efc8 100644
--- a/extensions/projection/pyproject.toml
+++ b/extensions/projection/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+projection = "pystac.extensions.projection:PROJECTION_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/raster/pyproject.toml b/extensions/raster/pyproject.toml
index 76ea9e93c..6d274b66c 100644
--- a/extensions/raster/pyproject.toml
+++ b/extensions/raster/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+raster = "pystac.extensions.raster:RASTER_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/sar/pyproject.toml b/extensions/sar/pyproject.toml
index 5e39abd26..91f81aba9 100644
--- a/extensions/sar/pyproject.toml
+++ b/extensions/sar/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+sar = "pystac.extensions.sar:SAR_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/sat/pyproject.toml b/extensions/sat/pyproject.toml
index 7b5343a73..da292e923 100644
--- a/extensions/sat/pyproject.toml
+++ b/extensions/sat/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+sat = "pystac.extensions.sat:SAT_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/scientific/pyproject.toml b/extensions/scientific/pyproject.toml
index b287fbc8d..62bc45ecf 100644
--- a/extensions/scientific/pyproject.toml
+++ b/extensions/scientific/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+scientific = "pystac.extensions.scientific:SCIENTIFIC_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/storage/pyproject.toml b/extensions/storage/pyproject.toml
index e99ed4c8a..3e460cf9c 100644
--- a/extensions/storage/pyproject.toml
+++ b/extensions/storage/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+storage = "pystac.extensions.storage:STORAGE_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/table/pyproject.toml b/extensions/table/pyproject.toml
index 0425b99cb..f8d31a9c7 100644
--- a/extensions/table/pyproject.toml
+++ b/extensions/table/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+table = "pystac.extensions.table:TABLE_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/timestamps/pyproject.toml b/extensions/timestamps/pyproject.toml
index 798a3b9cc..d1e3cc7d4 100644
--- a/extensions/timestamps/pyproject.toml
+++ b/extensions/timestamps/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+timestamps = "pystac.extensions.timestamps:TIMESTAMPS_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/version/pyproject.toml b/extensions/version/pyproject.toml
index 22fe02028..b537f664e 100644
--- a/extensions/version/pyproject.toml
+++ b/extensions/version/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+version = "pystac.extensions.version:VERSION_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/version/pystac/extensions/version.py b/extensions/version/pystac/extensions/version.py
index a3a583583..f05cb04ba 100644
--- a/extensions/version/pystac/extensions/version.py
+++ b/extensions/version/pystac/extensions/version.py
@@ -24,7 +24,7 @@
     STACObject,
     STACObjectType,
 )
-from pystac.errors import DeprecatedWarning
+from pystac.errors import DeprecatedWarning, ExtensionNotImplemented
 from pystac.extensions.base import ExtensionManagementMixin, PropertiesExtension
 from pystac.extensions.hooks import ExtensionHooks
 from pystac.utils import StringEnum, map_opt
@@ -427,6 +427,23 @@ def get_object_links(self, obj: STACObject) -> list[str] | None:
             ]
         return None
 
+    def get_deprecation_message(self, obj: STACObject) -> str | None:
+        version: VersionExtension[Any]
+        try:
+            if isinstance(obj, Collection):
+                version = VersionExtension.ext(obj)
+                kind = "collection"
+            elif isinstance(obj, Item):
+                version = VersionExtension.ext(obj)
+                kind = "item"
+            else:
+                return None
+        except ExtensionNotImplemented:
+            return None
+        if version.deprecated:
+            return f"The {kind} '{obj.id}' is deprecated."
+        return None
+
 
 VERSION_EXTENSION_HOOKS: ExtensionHooks = VersionExtensionHooks()
 
diff --git a/extensions/view/pyproject.toml b/extensions/view/pyproject.toml
index 4ee10f51c..d675cd256 100644
--- a/extensions/view/pyproject.toml
+++ b/extensions/view/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+view = "pystac.extensions.view:VIEW_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/extensions/xarray_assets/pyproject.toml b/extensions/xarray_assets/pyproject.toml
index 190006a12..b10dd8b83 100644
--- a/extensions/xarray_assets/pyproject.toml
+++ b/extensions/xarray_assets/pyproject.toml
@@ -21,6 +21,9 @@ classifiers = [
 requires-python = ">=3.10"
 dependencies = ["pystac-core"]
 
+[project.entry-points."pystac.extensions"]
+xarray_assets = "pystac.extensions.xarray_assets:XARRAY_ASSETS_EXTENSION_HOOKS"
+
 [project.urls]
 Documentation = "https://pystac.readthedocs.io"
 Repository = "https://github.com/stac-utils/pystac"
diff --git a/pyproject.toml b/pyproject.toml
index 3fdb79de7..3ea9456bf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -174,10 +174,10 @@ filterwarnings = ["error"]
 addopts = "--block-network --record-mode=none"
 
 [tool.hatch.build.targets.sdist]
-only-include = ["README.md", "LICENSE"]
+only-include = ["README.md", "LICENSE", "pystac"]
 
 [tool.hatch.build.targets.wheel]
-only-include = ["README.md"]
+packages = ["pystac"]
 
 [tool.uv.sources]
 pystac-core = { workspace = true }
diff --git a/core/pystac/extensions/ext.py b/pystac/extensions/ext.py
similarity index 100%
rename from core/pystac/extensions/ext.py
rename to pystac/extensions/ext.py
diff --git a/uv.lock b/uv.lock
index bac46af53..f2dd831f6 100644
--- a/uv.lock
+++ b/uv.lock
@@ -3902,10 +3902,14 @@ version = "2.0.0"
 source = { editable = "extensions/classification" }
 dependencies = [
     { name = "pystac-core" },
+    { name = "pystac-ext-raster" },
 ]
 
 [package.metadata]
-requires-dist = [{ name = "pystac-core", editable = "core" }]
+requires-dist = [
+    { name = "pystac-core", editable = "core" },
+    { name = "pystac-ext-raster", editable = "extensions/raster" },
+]
 
 [[package]]
 name = "pystac-ext-datacube"
@@ -3924,10 +3928,16 @@ version = "1.1.0"
 source = { editable = "extensions/eo" }
 dependencies = [
     { name = "pystac-core" },
+    { name = "pystac-ext-projection" },
+    { name = "pystac-ext-view" },
 ]
 
 [package.metadata]
-requires-dist = [{ name = "pystac-core", editable = "core" }]
+requires-dist = [
+    { name = "pystac-core", editable = "core" },
+    { name = "pystac-ext-projection", editable = "extensions/projection" },
+    { name = "pystac-ext-view", editable = "extensions/view" },
+]
 
 [[package]]
 name = "pystac-ext-file"
@@ -3990,10 +4000,16 @@ version = "1.4.0"
 source = { editable = "extensions/mlm" }
 dependencies = [
     { name = "pystac-core" },
+    { name = "pystac-ext-classification" },
+    { name = "pystac-ext-raster" },
 ]
 
 [package.metadata]
-requires-dist = [{ name = "pystac-core", editable = "core" }]
+requires-dist = [
+    { name = "pystac-core", editable = "core" },
+    { name = "pystac-ext-classification", editable = "extensions/classification" },
+    { name = "pystac-ext-raster", editable = "extensions/raster" },
+]
 
 [[package]]
 name = "pystac-ext-pointcloud"