diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index b6fe6dcb7..4c8da0fe7 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -16,7 +16,7 @@ jobs:
         include:
           - python-version: "3.12"
             is-dev-version: true
-            run_expensive_tests: true
+            run-expensive-tests: true
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -53,7 +53,7 @@ jobs:
       - run: pytest
         env:
           BIOIMAGEIO_CACHE_PATH: bioimageio_cache
-          SKIP_EXPENSIVE_TESTS: ${{ matrix.run_expensive_tests && 'false' || 'true' }}
+          RUN_EXPENSIVE_TESTS: ${{ matrix.run-expensive-tests && 'true' || 'false' }}
       - uses: actions/cache/save@v4
         # explicit restore/save instead of cache action to cache even if coverage fails
         with:
diff --git a/README.md b/README.md
index c8ed40eb5..fdacc9194 100644
--- a/README.md
+++ b/README.md
@@ -124,6 +124,10 @@ To keep the bioimageio.spec Python package version in sync with the (model) desc
 
 ### bioimageio.spec Python package
 
+#### bioimageio.spec 0.5.3.6
+
+* fix URL validation (checking with actual http requests was erroneously skipped)
+
 #### bioimageio.spec 0.5.3.5
 
 * fix loading tifffile in python 3.8 (pin tifffile)
diff --git a/bioimageio/spec/VERSION b/bioimageio/spec/VERSION
index aa7631d22..01f946340 100644
--- a/bioimageio/spec/VERSION
+++ b/bioimageio/spec/VERSION
@@ -1,3 +1,3 @@
 {
-    "version": "0.5.3.5"
+    "version": "0.5.3.6"
 }
diff --git a/bioimageio/spec/_internal/common_nodes.py b/bioimageio/spec/_internal/common_nodes.py
index 4a2588597..09d1a616d 100644
--- a/bioimageio/spec/_internal/common_nodes.py
+++ b/bioimageio/spec/_internal/common_nodes.py
@@ -60,6 +60,7 @@
 from .io_utils import write_content_to_zip
 from .node import Node
 from .packaging_context import PackagingContext
+from .root_url import RootHttpUrl
 from .url import HttpUrl
 from .utils import (
     assert_all_params_set_explicitly,
@@ -343,12 +344,12 @@ def validation_summary(self) -> ValidationSummary:
         assert self._validation_summary is not None, "access only after initialization"
         return self._validation_summary
 
-    _root: Union[HttpUrl, DirectoryPath] = PrivateAttr(
+    _root: Union[RootHttpUrl, DirectoryPath, ZipPath] = PrivateAttr(
         default_factory=lambda: validation_context_var.get().root
     )
 
     @property
-    def root(self) -> Union[HttpUrl, DirectoryPath]:
+    def root(self) -> Union[RootHttpUrl, DirectoryPath, ZipPath]:
         return self._root
 
     @classmethod
diff --git a/bioimageio/spec/_internal/type_guards.py b/bioimageio/spec/_internal/type_guards.py
index 8a0c6a28e..7296a70c7 100644
--- a/bioimageio/spec/_internal/type_guards.py
+++ b/bioimageio/spec/_internal/type_guards.py
@@ -1,6 +1,8 @@
 import collections.abc
-from typing import Any, Dict, Mapping, Sequence, Tuple
+from typing import Any, Dict, List, Mapping, Sequence, Tuple
 
+import numpy as np
+from numpy.typing import NDArray
 from typing_extensions import TypeGuard
 
 
@@ -25,6 +27,15 @@ def is_sequence(v: Any) -> TypeGuard[Sequence[Any]]:
     return isinstance(v, collections.abc.Sequence)
 
 
-def is_tuple(v: Any) -> TypeGuard[Tuple[Any]]:
-    """to avoid Tuple[Unknown]"""
+def is_tuple(v: Any) -> TypeGuard[Tuple[Any, ...]]:
+    """to avoid Tuple[Unknown, ...]"""
     return isinstance(v, tuple)
+
+
+def is_list(v: Any) -> TypeGuard[List[Any]]:
+    """to avoid List[Unknown]"""
+    return isinstance(v, list)
+
+
+def is_ndarray(v: Any) -> TypeGuard[NDArray[Any]]:
+    return isinstance(v, np.ndarray)
diff --git a/bioimageio/spec/_internal/url.py b/bioimageio/spec/_internal/url.py
index 603e3d5f8..de998d96a 100644
--- a/bioimageio/spec/_internal/url.py
+++ b/bioimageio/spec/_internal/url.py
@@ -4,7 +4,7 @@
 import requests
 import requests.exceptions
 from loguru import logger
-from pydantic import RootModel, model_validator
+from pydantic import RootModel
 from typing_extensions import Literal, assert_never
 
 from .field_warning import issue_warning
@@ -12,7 +12,7 @@
 from .validation_context import validation_context_var
 
 
-def _validate_url(url: Union[str, pydantic.HttpUrl]) -> pydantic.AnyUrl:
+def _validate_url(url: Union[str, pydantic.HttpUrl]) -> pydantic.HttpUrl:
     return _validate_url_impl(url, request_mode="head")
 
 
@@ -20,7 +20,7 @@ def _validate_url_impl(
     url: Union[str, pydantic.HttpUrl],
     request_mode: Literal["head", "get_stream", "get"],
     timeout: int = 3,
-) -> pydantic.AnyUrl:
+) -> pydantic.HttpUrl:
 
     url = str(url)
     val_url = url
@@ -76,7 +76,9 @@ def _validate_url_impl(
             msg_context={"error": str(e)},
         )
     else:
-        if response.status_code == 302:  # found
+        if response.status_code == 200:  # ok
+            pass
+        elif response.status_code == 302:  # found
             pass
         elif response.status_code in (301, 303, 308):
             issue_warning(
@@ -88,17 +90,10 @@ def _validate_url_impl(
                     "location": response.headers.get("location"),
                 },
             )
-        elif response.status_code == 403:  # forbidden
-            if request_mode == "head":
-                return _validate_url_impl(
-                    url, request_mode="get_stream", timeout=timeout
-                )
-            elif request_mode == "get_stream":
-                return _validate_url_impl(url, request_mode="get", timeout=timeout)
-            elif request_mode == "get":
-                raise ValueError(f"{response.status_code}: {response.reason} {url}")
-            else:
-                assert_never(request_mode)
+        elif request_mode == "head":
+            return _validate_url_impl(url, request_mode="get_stream", timeout=timeout)
+        elif request_mode == "get_stream":
+            return _validate_url_impl(url, request_mode="get", timeout=timeout)
         elif response.status_code == 405:
             issue_warning(
                 "{status_code}: {reason} {value}",
@@ -108,10 +103,15 @@ def _validate_url_impl(
                     "reason": response.reason,
                 },
             )
-        elif response.status_code != 200:
+        elif request_mode == "get":
             raise ValueError(f"{response.status_code}: {response.reason} {url}")
+        else:
+            assert_never(request_mode)
 
-    return pydantic.AnyUrl(url)
+    return (  # pyright: ignore[reportUnknownVariableType]
+        # TODO: remove pyright ignore for pydantic > 2.9
+        pydantic.HttpUrl(url)  # pyright: ignore[reportCallIssue]
+    )
 
 
 class HttpUrl(RootHttpUrl):
@@ -120,12 +120,14 @@ class HttpUrl(RootHttpUrl):
     root_model: ClassVar[Type[RootModel[Any]]] = RootModel[pydantic.HttpUrl]
     _exists: Optional[bool] = None
 
-    @model_validator(mode="after")
-    def _validate_url(self):
-        url = self._validated
+    def _after_validator(self):
+        self = super()._after_validator()
         context = validation_context_var.get()
-        if context.perform_io_checks and str(url) not in context.known_files:
-            self._validated = _validate_url(url)
+        if (
+            context.perform_io_checks
+            and str(self._validated) not in context.known_files
+        ):
+            self._validated = _validate_url(self._validated)
             self._exists = True
 
         return self
diff --git a/bioimageio/spec/_internal/validated_string.py b/bioimageio/spec/_internal/validated_string.py
index 94a05264d..b195f60ad 100644
--- a/bioimageio/spec/_internal/validated_string.py
+++ b/bioimageio/spec/_internal/validated_string.py
@@ -6,6 +6,7 @@
     CoreSchema,
     no_info_after_validator_function,
 )
+from typing_extensions import Self
 
 
 class ValidatedString(str):
@@ -18,6 +19,10 @@ class ValidatedString(str):
     def __new__(cls, object: object):
         self = super().__new__(cls, object)
         self._validated = cls.root_model.model_validate(str(self)).root
+        return self._after_validator()
+
+    def _after_validator(self) -> Self:
+        """add validation after the `root_model`"""
         return self
 
     @classmethod
diff --git a/bioimageio/spec/_internal/validation_context.py b/bioimageio/spec/_internal/validation_context.py
index 7195c0b7a..43caa2069 100644
--- a/bioimageio/spec/_internal/validation_context.py
+++ b/bioimageio/spec/_internal/validation_context.py
@@ -10,7 +10,7 @@
 from pydantic import DirectoryPath
 
 from ._settings import settings
-from .io_basics import AbsoluteDirectory, FileName, Sha256
+from .io_basics import FileName, Sha256
 from .root_url import RootHttpUrl
 from .warning_levels import WarningLevel
 
@@ -21,7 +21,7 @@ class ValidationContext:
         init=False, default_factory=list
     )
 
-    root: Union[RootHttpUrl, AbsoluteDirectory, ZipFile] = Path()
+    root: Union[RootHttpUrl, DirectoryPath, ZipFile] = Path()
     """url/directory serving as base to resolve any relative file paths"""
 
     warning_level: WarningLevel = 50
diff --git a/bioimageio/spec/_io.py b/bioimageio/spec/_io.py
index 4473b7d40..d5fe11287 100644
--- a/bioimageio/spec/_io.py
+++ b/bioimageio/spec/_io.py
@@ -32,6 +32,7 @@ def load_description(
     format_version: Union[Literal["discover"], Literal["latest"], str] = DISCOVER,
     perform_io_checks: bool = settings.perform_io_checks,
     known_files: Optional[Dict[str, Sha256]] = None,
+    sha256: Optional[Sha256] = None,
 ) -> Union[ResourceDescr, InvalidDescr]:
     """load a bioimage.io resource description
 
@@ -45,6 +46,7 @@ def load_description(
                            absolute file paths is still being checked.
         known_files: Allows to bypass download and hashing of referenced files
                      (even if perform_io_checks is True).
+        sha256: Optional SHA-256 value of **source**
 
     Returns:
         An object holding all metadata of the bioimage.io resource
@@ -55,7 +57,7 @@ def load_description(
         logger.warning("returning already loaded description '{}' as is", name)
         return source  # pyright: ignore[reportReturnType]
 
-    opened = open_bioimageio_yaml(source)
+    opened = open_bioimageio_yaml(source, sha256=sha256)
 
     context = validation_context_var.get().replace(
         root=opened.original_root,
@@ -78,6 +80,7 @@ def load_model_description(
     format_version: Union[Literal["discover"], Literal["latest"], str] = DISCOVER,
     perform_io_checks: bool = settings.perform_io_checks,
     known_files: Optional[Dict[str, Sha256]] = None,
+    sha256: Optional[Sha256] = None,
 ) -> AnyModelDescr:
     """same as `load_description`, but addtionally ensures that the loaded
     description is valid and of type 'model'.
@@ -90,6 +93,7 @@ def load_model_description(
         format_version=format_version,
         perform_io_checks=perform_io_checks,
         known_files=known_files,
+        sha256=sha256,
     )
     return ensure_description_is_model(rd)
 
@@ -101,6 +105,7 @@ def load_dataset_description(
     format_version: Union[Literal["discover"], Literal["latest"], str] = DISCOVER,
     perform_io_checks: bool = settings.perform_io_checks,
     known_files: Optional[Dict[str, Sha256]] = None,
+    sha256: Optional[Sha256] = None,
 ) -> AnyDatasetDescr:
     """same as `load_description`, but addtionally ensures that the loaded
     description is valid and of type 'dataset'.
@@ -110,6 +115,7 @@ def load_dataset_description(
         format_version=format_version,
         perform_io_checks=perform_io_checks,
         known_files=known_files,
+        sha256=sha256,
     )
     return ensure_description_is_dataset(rd)
 
@@ -140,19 +146,9 @@ def load_description_and_validate_format_only(
     format_version: Union[Literal["discover"], Literal["latest"], str] = DISCOVER,
     perform_io_checks: bool = settings.perform_io_checks,
     known_files: Optional[Dict[str, Sha256]] = None,
+    sha256: Optional[Sha256] = None,
 ) -> ValidationSummary:
-    """load a bioimage.io resource description
-
-    Args:
-        source: Path or URL to an rdf.yaml or a bioimage.io package
-                (zip-file with rdf.yaml in it).
-        format_version: (optional) Use this argument to load the resource and
-                        convert its metadata to a higher format_version.
-        perform_io_checks: Wether or not to perform validation that requires file io,
-                           e.g. downloading a remote files. The existence of local
-                           absolute file paths is still being checked.
-        known_files: Allows to bypass download and hashing of referenced files
-                     (even if perform_io_checks is True).
+    """same as `load_description`, but only return the validation summary.
 
     Returns:
         Validation summary of the bioimage.io resource found at `source`.
@@ -163,6 +159,7 @@ def load_description_and_validate_format_only(
         format_version=format_version,
         perform_io_checks=perform_io_checks,
         known_files=known_files,
+        sha256=sha256,
     )
     assert rd.validation_summary is not None
     return rd.validation_summary
diff --git a/bioimageio/spec/application/v0_3.py b/bioimageio/spec/application/v0_3.py
index 5901cd157..f0f6724ac 100644
--- a/bioimageio/spec/application/v0_3.py
+++ b/bioimageio/spec/application/v0_3.py
@@ -14,7 +14,7 @@
 from ..generic.v0_3 import CiteEntry as CiteEntry
 from ..generic.v0_3 import DeprecatedLicenseId as DeprecatedLicenseId
 from ..generic.v0_3 import Doi as Doi
-from ..generic.v0_3 import GenericDescrBase, LinkedResourceNode, ResourceId
+from ..generic.v0_3 import GenericDescrBase, LinkedResourceBase, ResourceId
 from ..generic.v0_3 import LicenseId as LicenseId
 from ..generic.v0_3 import LinkedResource as LinkedResource
 from ..generic.v0_3 import Maintainer as Maintainer
@@ -47,7 +47,7 @@ class ApplicationDescr(GenericDescrBase):
     """The primary source of the application"""
 
 
-class LinkedApplication(LinkedResourceNode):
+class LinkedApplication(LinkedResourceBase):
     """Reference to a bioimage.io application."""
 
     id: ApplicationId
diff --git a/bioimageio/spec/common.py b/bioimageio/spec/common.py
index 470d2edaf..c4100f88f 100644
--- a/bioimageio/spec/common.py
+++ b/bioimageio/spec/common.py
@@ -7,7 +7,13 @@
     FileDescr,
     YamlValue,
 )
-from ._internal.io_basics import AbsoluteDirectory, AbsoluteFilePath, FileName, Sha256
+from ._internal.io_basics import (
+    AbsoluteDirectory,
+    AbsoluteFilePath,
+    FileName,
+    Sha256,
+    ZipPath,
+)
 from ._internal.root_url import RootHttpUrl
 from ._internal.types import (
     FilePath,
@@ -34,4 +40,5 @@
     "Sha256",
     "ValidationError",
     "YamlValue",
+    "ZipPath",
 ]
diff --git a/bioimageio/spec/dataset/v0_3.py b/bioimageio/spec/dataset/v0_3.py
index f1ae9298d..abdc0cf3f 100644
--- a/bioimageio/spec/dataset/v0_3.py
+++ b/bioimageio/spec/dataset/v0_3.py
@@ -15,7 +15,7 @@
 from ..generic.v0_3 import (
     DocumentationSource,
     GenericDescrBase,
-    LinkedResourceNode,
+    LinkedResourceBase,
     _author_conv,  # pyright: ignore[reportPrivateUsage]
     _maintainer_conv,  # pyright: ignore[reportPrivateUsage]
 )
@@ -105,7 +105,7 @@ def _convert(cls, data: Dict[str, Any], /) -> Dict[str, Any]:
         return data
 
 
-class LinkedDataset(LinkedResourceNode):
+class LinkedDataset(LinkedResourceBase):
     """Reference to a bioimage.io dataset."""
 
     id: DatasetId
diff --git a/bioimageio/spec/generic/v0_3.py b/bioimageio/spec/generic/v0_3.py
index 81b9787f6..bafe37c95 100644
--- a/bioimageio/spec/generic/v0_3.py
+++ b/bioimageio/spec/generic/v0_3.py
@@ -25,45 +25,55 @@
     Node,
     ResourceDescrBase,
 )
-from .._internal.constants import (
-    TAG_CATEGORIES,
-)
+from .._internal.constants import TAG_CATEGORIES
 from .._internal.field_validation import validate_gh_user
 from .._internal.field_warning import as_warning, warn
 from .._internal.io import (
     BioimageioYamlContent,
+    FileDescr,
     V_suffix,
     YamlValue,
     include_in_package_serializer,
     validate_suffix,
 )
-from .._internal.io import FileDescr as FileDescr
-from .._internal.io_basics import AbsoluteFilePath
-from .._internal.io_basics import Sha256 as Sha256
-from .._internal.license_id import DeprecatedLicenseId as DeprecatedLicenseId
-from .._internal.license_id import LicenseId as LicenseId
-from .._internal.types import (
-    ImportantFileSource,
-    NotEmpty,
-)
-from .._internal.types import RelativeFilePath as RelativeFilePath
-from .._internal.url import HttpUrl as HttpUrl
+from .._internal.io_basics import AbsoluteFilePath, Sha256
+from .._internal.license_id import DeprecatedLicenseId, LicenseId
+from .._internal.types import ImportantFileSource, NotEmpty, RelativeFilePath
+from .._internal.url import HttpUrl
 from .._internal.validated_string import ValidatedString
 from .._internal.validator_annotations import (
     AfterValidator,
     Predicate,
     RestrictCharacters,
 )
-from .._internal.version_type import Version as Version
+from .._internal.version_type import Version
 from .._internal.warning_levels import ALERT, INFO
 from ._v0_3_converter import convert_from_older_format
 from .v0_2 import Author as _Author_v0_2
-from .v0_2 import BadgeDescr as BadgeDescr
-from .v0_2 import CoverImageSource
-from .v0_2 import Doi as Doi
+from .v0_2 import BadgeDescr, CoverImageSource, Doi, OrcidId, Uploader
 from .v0_2 import Maintainer as _Maintainer_v0_2
-from .v0_2 import OrcidId as OrcidId
-from .v0_2 import Uploader as Uploader
+
+__all__ = [
+    "Author",
+    "BadgeDescr",
+    "CiteEntry",
+    "DeprecatedLicenseId",
+    "Doi",
+    "FileDescr",
+    "GenericDescr",
+    "HttpUrl",
+    "KNOWN_SPECIFIC_RESOURCE_TYPES",
+    "LicenseId",
+    "LinkedResource",
+    "Maintainer",
+    "OrcidId",
+    "RelativeFilePath",
+    "ResourceId",
+    "Sha256",
+    "Uploader",
+    "VALID_COVER_IMAGE_EXTENSIONS",
+    "Version",
+]
 
 KNOWN_SPECIFIC_RESOURCE_TYPES = (
     "application",
@@ -181,7 +191,24 @@ def _check_doi_or_url(self):
         return self
 
 
-class LinkedResource(Node):
+class LinkedResourceBase(Node):
+
+    @model_validator(mode="before")
+    def _remove_version_number(  # pyright: ignore[reportUnknownParameterType]
+        cls, value: Union[Any, Dict[Any, Any]]
+    ):
+        if isinstance(value, dict):
+            vn: Any = value.pop("version_number", None)
+            if vn is not None and value.get("version") is None:
+                value["version"] = vn
+
+        return value  # pyright: ignore[reportUnknownVariableType]
+
+    version: Optional[Version] = None
+    """The version of the linked resource following SemVer 2.0."""
+
+
+class LinkedResource(LinkedResourceBase):
     """Reference to a bioimage.io resource"""
 
     id: ResourceId
@@ -441,20 +468,3 @@ def check_specific_types(cls, value: str) -> str:
             )
 
         return value
-
-
-class LinkedResourceNode(Node):
-
-    @model_validator(mode="before")
-    def _remove_version_number(  # pyright: ignore[reportUnknownParameterType]
-        cls, value: Union[Any, Dict[Any, Any]]
-    ):
-        if isinstance(value, dict):
-            vn: Any = value.pop("version_number", None)
-            if vn is not None and value.get("version") is None:
-                value["version"] = vn
-
-        return value  # pyright: ignore[reportUnknownVariableType]
-
-    version: Optional[Version] = None
-    """The version of the linked resource following SemVer 2.0."""
diff --git a/bioimageio/spec/model/v0_5.py b/bioimageio/spec/model/v0_5.py
index 98e811c9f..be31c62be 100644
--- a/bioimageio/spec/model/v0_5.py
+++ b/bioimageio/spec/model/v0_5.py
@@ -93,7 +93,7 @@
 from ..generic.v0_3 import (
     DocumentationSource,
     GenericModelDescrBase,
-    LinkedResourceNode,
+    LinkedResourceBase,
     _author_conv,  # pyright: ignore[reportPrivateUsage]
     _maintainer_conv,  # pyright: ignore[reportPrivateUsage]
 )
@@ -2031,7 +2031,7 @@ class ModelId(ResourceId):
     pass
 
 
-class LinkedModel(LinkedResourceNode):
+class LinkedModel(LinkedResourceBase):
     """Reference to a bioimage.io model."""
 
     id: ModelId
diff --git a/bioimageio/spec/notebook/v0_3.py b/bioimageio/spec/notebook/v0_3.py
index c5df26d81..f95f2f02d 100644
--- a/bioimageio/spec/notebook/v0_3.py
+++ b/bioimageio/spec/notebook/v0_3.py
@@ -10,7 +10,7 @@
 from ..generic.v0_3 import CiteEntry as CiteEntry
 from ..generic.v0_3 import DeprecatedLicenseId as DeprecatedLicenseId
 from ..generic.v0_3 import Doi as Doi
-from ..generic.v0_3 import GenericDescrBase, LinkedResourceNode
+from ..generic.v0_3 import GenericDescrBase, LinkedResourceBase
 from ..generic.v0_3 import LicenseId as LicenseId
 from ..generic.v0_3 import LinkedResource as LinkedResource
 from ..generic.v0_3 import Maintainer as Maintainer
@@ -42,7 +42,7 @@ class NotebookDescr(GenericDescrBase):
     """The Jupyter notebook"""
 
 
-class LinkedNotebook(LinkedResourceNode):
+class LinkedNotebook(LinkedResourceBase):
     """Reference to a bioimage.io notebook."""
 
     id: NotebookId
diff --git a/dev/env.yaml b/dev/env.yaml
index 57caa0814..ab4c6ad33 100644
--- a/dev/env.yaml
+++ b/dev/env.yaml
@@ -29,6 +29,7 @@ dependencies:
   - python-dateutil
   - python=3.12
   - requests
+  - requests-mock
   - rich
   - ruff
   - ruyaml
diff --git a/setup.py b/setup.py
index 3a1847391..b1b750af6 100644
--- a/setup.py
+++ b/setup.py
@@ -37,7 +37,7 @@
         "packaging>=17.0",
         "pooch>=1.5,<2",
         "pydantic-settings>=2.5,<3",
-        "pydantic>=2.7.0,<2.10",
+        "pydantic>=2.7.0,<2.10",  # TODO: check and update pin after https://github.com/pydantic/pydantic/pull/11008 is released
         "python-dateutil",
         "requests",
         "rich",
@@ -57,6 +57,7 @@
                 "pytest-cov",
                 "pytest-xdist",  # parallel pytest
                 "pytest",
+                "requests-mock",
             ]
         ),
         "dev": test_extras
diff --git a/tests/test_bioimageio_collection.py b/tests/test_bioimageio_collection.py
index 3089c7558..4458f76d0 100644
--- a/tests/test_bioimageio_collection.py
+++ b/tests/test_bioimageio_collection.py
@@ -1,13 +1,10 @@
-import json
-from pathlib import Path
 from typing import Any, Collection, Dict, Iterable, Mapping, Tuple
 
-import pooch  # pyright: ignore [reportMissingTypeStubs]
 import pytest
+import requests
 
-from bioimageio.spec import settings
 from bioimageio.spec.common import HttpUrl, Sha256
-from tests.utils import ParameterSet, check_bioimageio_yaml, skip_expensive
+from tests.utils import ParameterSet, check_bioimageio_yaml, expensive_test
 
 BASE_URL = "https://uk1s3.embassy.ebi.ac.uk/public-datasets/bioimage.io/"
 
@@ -27,12 +24,7 @@
 
 
 def _get_rdf_sources():
-    all_versions_path: Any = pooch.retrieve(
-        BASE_URL + "all_versions.json", None, path=settings.cache_path
-    )
-    with Path(all_versions_path).open(encoding="utf-8") as f:
-        entries = json.load(f)["entries"]
-
+    entries: Any = requests.get(BASE_URL + "all_versions.json").json()["entries"]
     ret: Dict[str, Tuple[HttpUrl, Sha256]] = {}
     for entry in entries:
         for version in entry["versions"]:
@@ -57,10 +49,10 @@ def yield_bioimageio_yaml_urls() -> Iterable[ParameterSet]:
         yield pytest.param(descr_url, sha, key, id=key)
 
 
-@skip_expensive
+@expensive_test
 @pytest.mark.parametrize("descr_url,sha,key", list(yield_bioimageio_yaml_urls()))
 def test_rdf(
-    descr_url: Path,
+    descr_url: HttpUrl,
     sha: Sha256,
     key: str,
     bioimageio_json_schema: Mapping[Any, Any],
@@ -80,7 +72,7 @@ def test_rdf(
     )
 
 
-@skip_expensive
+@expensive_test
 @pytest.mark.parametrize(
     "rdf_id",
     [
diff --git a/tests/test_internal/test_url.py b/tests/test_internal/test_url.py
new file mode 100644
index 000000000..73f3027d1
--- /dev/null
+++ b/tests/test_internal/test_url.py
@@ -0,0 +1,111 @@
+from typing import Type
+
+import pytest
+import requests.exceptions
+from requests_mock import Mocker as RequestsMocker
+
+from bioimageio.spec._internal.validation_context import ValidationContext
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "https://example.com",
+        "https://colab.research.google.com/github/bioimage-io/spec-bioimage-io/blob/main/example/load_model_and_create_your_own.ipynb",
+        "https://www.kaggle.com/c/data-science-bowl-2018",
+    ],
+)
+def test_httpurl_valid(url: str):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    with ValidationContext(perform_io_checks=True):
+        assert HttpUrl(url).exists(), url
+
+
+@pytest.mark.parametrize(
+    "text,status_code",
+    [
+        ("OK", 200),
+        ("found", 302),
+        ("redirected", 301),
+        ("redirected", 303),
+        ("redirected", 308),
+        ("let's ignore this I guess??", 405),
+    ],
+)
+def test_httpurl_mock_valid(text: str, status_code: int, requests_mock: RequestsMocker):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    url = "https://example.com"
+    _ = requests_mock.get(url, text=text, status_code=status_code)
+    assert HttpUrl(url).exists()
+
+
+@pytest.mark.parametrize(
+    "text,status_code",
+    [
+        ("forbidden", 403),
+        ("Not found", 404),
+        ("just wrong", 199),
+    ],
+)
+def test_httpurl_mock_invalid(
+    text: str, status_code: int, requests_mock: RequestsMocker
+):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    url = "https://example.com"
+    _ = requests_mock.head(url, text=text, status_code=status_code)
+    _ = requests_mock.get(url, text=text, status_code=status_code)
+    with ValidationContext(perform_io_checks=True):
+        with pytest.raises(ValueError):
+            _ = HttpUrl(url)
+
+    with ValidationContext(perform_io_checks=False):
+        assert not HttpUrl(url).exists()
+
+
+@pytest.mark.parametrize(
+    "exc",
+    [
+        requests.exceptions.InvalidURL,
+    ],
+)
+def test_httpurl_mock_exc(exc: Type[Exception], requests_mock: RequestsMocker):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    url = "https://example.com"
+    _ = requests_mock.head(url, exc=exc)
+    with ValidationContext(perform_io_checks=True):
+        with pytest.raises(ValueError):
+            _ = HttpUrl(url)
+
+    with ValidationContext(perform_io_checks=False):
+        assert not HttpUrl(url).exists()
+
+
+@pytest.mark.parametrize(
+    "url,io_check",
+    [
+        ("https://example.invalid", True),
+        ("https://example.invalid", False),
+    ],
+)
+def test_httpurl_nonexisting(url: str, io_check: bool):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    with ValidationContext(perform_io_checks=io_check):
+        assert HttpUrl(url).exists(), url
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "invalid-url",
+    ],
+)
+def test_httpurl_invalid(url: str):
+    from bioimageio.spec._internal.url import HttpUrl
+
+    with pytest.raises(ValueError), ValidationContext(perform_io_checks=False):
+        _ = HttpUrl(url)
diff --git a/tests/test_specific_reexports_generics.py b/tests/test_specific_reexports_generics.py
index 208c4e0fb..3e6ad19fc 100644
--- a/tests/test_specific_reexports_generics.py
+++ b/tests/test_specific_reexports_generics.py
@@ -92,7 +92,7 @@ def get_members(m: ModuleType):
     "GenericDescrBase",
     "GenericModelDescrBase",
     "KNOWN_SPECIFIC_RESOURCE_TYPES",
-    "LinkedResourceNode",
+    "LinkedResourceBase",
     "ResourceDescrBase",
     "ResourceDescrType",
 }
diff --git a/tests/utils.py b/tests/utils.py
index 14fd71e85..3fd9fc24e 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -46,9 +46,9 @@
 
 unset = object()
 
-skip_expensive = pytest.mark.skipif(
-    (run := os.getenv("SKIP_EXPENSIVE_TESTS")) == "true",
-    reason=f"Skipping expensive test (SKIP_EXPENSIVE_TESTS {run}!=true )",
+expensive_test = pytest.mark.skipif(
+    (run := os.getenv("RUN_EXPENSIVE_TESTS")) != "true",
+    reason="Skipping expensive test (enable by RUN_EXPENSIVE_TESTS='true')",
 )