diff --git a/src/specify_cli/extensions.py b/src/specify_cli/extensions.py
index 871503f0ae..de67dfcdd8 100644
--- a/src/specify_cli/extensions.py
+++ b/src/specify_cli/extensions.py
@@ -1702,6 +1702,44 @@ def _open_url(self, url: str, timeout: int = 10):
         from specify_cli.authentication.http import open_url
         return open_url(url, timeout)
 
+    def _validate_catalog_payload(self, catalog_data: Any, url: str) -> None:
+        """Validate a parsed catalog payload's shape.
+
+        Applied to both network-fetched and cache-loaded payloads so a
+        once-poisoned cache (older spec-kit version, manual edit, upstream
+        served a bad payload before the network-side guards were added)
+        cannot re-crash ``_get_merged_extensions`` on subsequent calls.
+
+        Checking only key presence would let a payload like
+        ``{"extensions": []}`` or ``{"extensions": null}`` slip through
+        here and then crash with ``AttributeError: 'list' object has no
+        attribute 'items'`` deep inside ``_get_merged_extensions``. The
+        sibling integration catalog reader already guards both the root
+        object and the nested mapping (see ``integrations/catalog.py``);
+        the extension catalog must stay consistent so a malformed payload
+        surfaces as the user-facing ``Invalid catalog format`` error
+        instead of a raw Python traceback.
+
+        Args:
+            catalog_data: Parsed JSON payload from the catalog source.
+            url: Source URL — used in the error message so the user can
+                tell which catalog in a multi-catalog stack is malformed.
+
+        Raises:
+            ExtensionError: If the payload's shape is invalid.
+        """
+        if not isinstance(catalog_data, dict):
+            raise ExtensionError(
+                f"Invalid catalog format from {url}: expected a JSON object"
+            )
+        if "schema_version" not in catalog_data or "extensions" not in catalog_data:
+            raise ExtensionError(f"Invalid catalog format from {url}")
+        if not isinstance(catalog_data.get("extensions"), dict):
+            raise ExtensionError(
+                f"Invalid catalog format from {url}: "
+                "'extensions' must be a JSON object"
+            )
+
     def get_active_catalogs(self) -> List[CatalogEntry]:
         """Get the ordered list of active catalogs.
 
@@ -1827,11 +1865,19 @@ def _fetch_single_catalog(self, entry: CatalogEntry, force_refresh: bool = False
                     # If metadata is invalid or missing expected fields, treat cache as invalid
                     pass
 
-        # Use cache if valid
+        # Use cache if valid. A previously-cached payload must clear the
+        # same shape checks as a freshly-fetched one — otherwise a once-
+        # poisoned cache (older spec-kit version, manual edit, upstream
+        # served a bad payload before the network-side guards were added)
+        # would re-crash on every invocation despite the cache being
+        # "valid" by age. If validation fails on the cached read, fall
+        # through to the network fetch path so the cache gets refreshed.
         if is_valid:
             try:
-                return json.loads(cache_file.read_text())
-            except json.JSONDecodeError:
+                cached_data = json.loads(cache_file.read_text())
+                self._validate_catalog_payload(cached_data, entry.url)
+                return cached_data
+            except (json.JSONDecodeError, ExtensionError):
                 pass
 
         # Fetch from network
@@ -1839,8 +1885,7 @@ def _fetch_single_catalog(self, entry: CatalogEntry, force_refresh: bool = False
             with self._open_url(entry.url, timeout=10) as response:
                 catalog_data = json.loads(response.read())
 
-            if "schema_version" not in catalog_data or "extensions" not in catalog_data:
-                raise ExtensionError(f"Invalid catalog format from {entry.url}")
+            self._validate_catalog_payload(catalog_data, entry.url)
 
             # Save to cache
             self.cache_dir.mkdir(parents=True, exist_ok=True)
@@ -1895,6 +1940,16 @@ def _get_merged_extensions(self, force_refresh: bool = False) -> List[Dict[str,
                 continue
 
             for ext_id, ext_data in catalog_data.get("extensions", {}).items():
+                # Per-entry guard: ``_fetch_single_catalog`` already validates
+                # that ``catalog_data["extensions"]`` is a mapping, but it
+                # does not (and should not) validate every entry shape there
+                # — one malformed entry shouldn't poison an otherwise valid
+                # catalog. Skip non-mapping entries here so a payload like
+                # ``{"extensions": {"foo": [], "bar": {...}}}`` still merges
+                # the valid entries without crashing on ``**ext_data``.
+                # Mirrors ``integrations/catalog.py:245``.
+                if not isinstance(ext_data, dict):
+                    continue
                 if ext_id not in merged:  # Higher-priority catalog wins
                     merged[ext_id] = {
                         **ext_data,
diff --git a/src/specify_cli/presets.py b/src/specify_cli/presets.py
index fd9d4745f1..89fd033187 100644
--- a/src/specify_cli/presets.py
+++ b/src/specify_cli/presets.py
@@ -1860,6 +1860,48 @@ def _open_url(self, url: str, timeout: int = 10):
         from specify_cli.authentication.http import open_url
         return open_url(url, timeout)
 
+    def _validate_catalog_payload(self, catalog_data: Any, url: str) -> None:
+        """Validate a parsed preset-catalog payload's shape.
+
+        Applied to both network-fetched and cache-loaded payloads so a
+        once-poisoned cache (older spec-kit version, manual edit, upstream
+        served a bad payload before the network-side guards were added)
+        cannot re-crash ``_get_merged_packs`` on subsequent calls.
+
+        Checking only key presence would let a payload like
+        ``{"presets": []}`` or ``{"presets": null}`` slip through here and
+        then crash with ``AttributeError: 'list' object has no attribute
+        'items'`` deep inside ``_get_merged_packs``. The sibling
+        integration catalog reader already guards both the root object and
+        the nested mapping (see ``integrations/catalog.py``); the preset
+        catalog must stay consistent so a malformed payload surfaces as
+        the user-facing ``Invalid preset catalog format`` error instead of
+        a raw Python traceback.
+
+        Args:
+            catalog_data: Parsed JSON payload from the catalog source.
+            url: Source URL — used in the error message so the user can
+                tell which catalog in a multi-catalog stack is malformed.
+
+        Raises:
+            PresetError: If the payload's shape is invalid.
+        """
+        if not isinstance(catalog_data, dict):
+            raise PresetError(
+                f"Invalid preset catalog format from {url}: "
+                "expected a JSON object"
+            )
+        if (
+            "schema_version" not in catalog_data
+            or "presets" not in catalog_data
+        ):
+            raise PresetError("Invalid preset catalog format")
+        if not isinstance(catalog_data.get("presets"), dict):
+            raise PresetError(
+                f"Invalid preset catalog format from {url}: "
+                "'presets' must be a JSON object"
+            )
+
     def _load_catalog_config(self, config_path: Path) -> Optional[List[PresetCatalogEntry]]:
         """Load catalog stack configuration from a YAML file.
 
@@ -2035,21 +2077,25 @@ def _fetch_single_catalog(self, entry: PresetCatalogEntry, force_refresh: bool =
         """
         cache_file, metadata_file = self._get_cache_paths(entry.url)
 
+        # Use cache if valid. A previously-cached payload must clear the
+        # same shape checks as a freshly-fetched one — otherwise a once-
+        # poisoned cache would re-crash on every invocation despite the
+        # cache being "valid" by age. If validation fails on the cached
+        # read, fall through to the network fetch path so the cache gets
+        # refreshed.
         if not force_refresh and self._is_url_cache_valid(entry.url):
             try:
-                return json.loads(cache_file.read_text())
-            except json.JSONDecodeError:
+                cached_data = json.loads(cache_file.read_text())
+                self._validate_catalog_payload(cached_data, entry.url)
+                return cached_data
+            except (json.JSONDecodeError, PresetError):
                 pass
 
         try:
             with self._open_url(entry.url, timeout=10) as response:
                 catalog_data = json.loads(response.read())
 
-            if (
-                "schema_version" not in catalog_data
-                or "presets" not in catalog_data
-            ):
-                raise PresetError("Invalid preset catalog format")
+            self._validate_catalog_payload(catalog_data, entry.url)
 
             self.cache_dir.mkdir(parents=True, exist_ok=True)
             cache_file.write_text(json.dumps(catalog_data, indent=2))
@@ -2083,6 +2129,17 @@ def _get_merged_packs(self, force_refresh: bool = False) -> Dict[str, Dict[str,
             try:
                 data = self._fetch_single_catalog(entry, force_refresh)
                 for pack_id, pack_data in data.get("presets", {}).items():
+                    # Per-entry guard: ``_fetch_single_catalog`` already
+                    # validates that ``data["presets"]`` is a mapping, but it
+                    # does not (and should not) validate every entry shape
+                    # there — one malformed entry shouldn't poison an
+                    # otherwise valid catalog. Skip non-mapping entries here
+                    # so a payload like ``{"presets": {"foo": [], "bar":
+                    # {...}}}`` still merges the valid entries without
+                    # crashing on ``**pack_data``. Mirrors
+                    # ``integrations/catalog.py:245``.
+                    if not isinstance(pack_data, dict):
+                        continue
                     pack_data_with_catalog = {**pack_data, "_catalog_name": entry.name, "_install_allowed": entry.install_allowed}
                     merged[pack_id] = pack_data_with_catalog
             except PresetError:
diff --git a/tests/test_extensions.py b/tests/test_extensions.py
index 153388a541..4e89663a3d 100644
--- a/tests/test_extensions.py
+++ b/tests/test_extensions.py
@@ -2577,6 +2577,163 @@ def fake_open(req, timeout=None):
 
         assert captured["req"].get_header("Authorization") == "Bearer ghp_testtoken"
 
+    @pytest.mark.parametrize(
+        "payload",
+        [
+            # Root is not a JSON object.
+            [],
+            "oops",
+            42,
+            None,
+            # Root is fine but ``extensions`` is the wrong type.
+            {"schema_version": "1.0", "extensions": []},
+            {"schema_version": "1.0", "extensions": "oops"},
+            {"schema_version": "1.0", "extensions": None},
+            {"schema_version": "1.0", "extensions": 42},
+        ],
+    )
+    def test_fetch_single_catalog_rejects_malformed_payload(self, temp_dir, payload):
+        """Malformed catalog payloads raise ExtensionError, not AttributeError.
+
+        Without this guard, a payload like ``{"extensions": []}`` would pass the
+        key-presence check and then crash with ``AttributeError: 'list' object
+        has no attribute 'items'`` deep inside ``_get_merged_extensions``. The
+        sibling integration catalog reader already validates both the root
+        object and the nested mapping (see ``integrations/catalog.py``); the
+        extension catalog must stay consistent.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = self._make_catalog(temp_dir)
+
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(payload).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = CatalogEntry(
+            url="https://example.com/catalog.json",
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response):
+            with pytest.raises(ExtensionError, match="Invalid catalog format"):
+                catalog._fetch_single_catalog(entry, force_refresh=True)
+
+    @pytest.mark.parametrize(
+        "cached_payload",
+        [
+            [],
+            "oops",
+            42,
+            None,
+            {"schema_version": "1.0", "extensions": []},
+            {"schema_version": "1.0", "extensions": "oops"},
+            {"schema_version": "1.0", "extensions": None},
+        ],
+    )
+    def test_fetch_single_catalog_rejects_malformed_cached_payload(
+        self, temp_dir, cached_payload
+    ):
+        """A poisoned cache silently falls back to the network instead of
+        crashing — cached payloads pass through the same shape validation
+        as freshly-fetched ones.
+
+        Without this, a cache poisoned by an older spec-kit version (or a
+        manual edit, or an upstream that briefly served a bad payload
+        before the network guards landed) would re-crash every invocation
+        of ``_get_merged_extensions`` despite the cache being "valid" by
+        age. The recovery contract is: if the cached payload fails
+        validation, drop it and refetch — never propagate
+        ``AttributeError`` to the caller.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = self._make_catalog(temp_dir)
+
+        # Poison the default-URL cache. ``DEFAULT_CATALOG_URL`` is the
+        # branch that goes through ``is_cache_valid()`` (the non-default
+        # branch uses per-URL hashed cache files but the same code path
+        # below).
+        catalog.cache_dir.mkdir(parents=True, exist_ok=True)
+        catalog.cache_file.write_text(json.dumps(cached_payload))
+        catalog.cache_metadata_file.write_text(
+            json.dumps(
+                {
+                    "cached_at": datetime.now(timezone.utc).isoformat(),
+                    "catalog_url": ExtensionCatalog.DEFAULT_CATALOG_URL,
+                }
+            )
+        )
+
+        # Network refetch returns a valid payload so the recovery path
+        # can complete.
+        valid = {
+            "schema_version": "1.0",
+            "extensions": {"foo": {"name": "Foo", "version": "1.0.0"}},
+        }
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(valid).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = CatalogEntry(
+            url=ExtensionCatalog.DEFAULT_CATALOG_URL,
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response):
+            result = catalog._fetch_single_catalog(entry, force_refresh=False)
+
+        # The poisoned cache was discarded and the network payload returned.
+        assert result == valid
+
+    def test_get_merged_extensions_skips_non_mapping_entries(self, temp_dir):
+        """Per-entry guard: one malformed entry shouldn't poison the merge.
+
+        ``_fetch_single_catalog`` validates that ``extensions`` is a mapping,
+        but it doesn't (and shouldn't) validate every entry inside it — a
+        single bad entry in an otherwise-valid catalog should be skipped, not
+        crash the whole resolve path. Mirrors the per-entry skip in
+        ``integrations/catalog.py``: a malformed entry returns no error,
+        valid entries continue to merge normally.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = self._make_catalog(temp_dir)
+        # Mix of valid entry, list-shaped entry, and string-shaped entry.
+        payload = {
+            "schema_version": "1.0",
+            "extensions": {
+                "good": {"name": "Good", "version": "1.0.0"},
+                "bad-list": [],
+                "bad-str": "oops",
+            },
+        }
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(payload).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = CatalogEntry(
+            url="https://example.com/catalog.json",
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response), \
+             patch.object(catalog, "get_active_catalogs", return_value=[entry]):
+            merged = catalog._get_merged_extensions(force_refresh=True)
+
+        # Only the well-formed entry survives; the two malformed entries are
+        # silently dropped rather than raising or crashing.
+        assert [ext["id"] for ext in merged] == ["good"]
+
     def test_download_extension_sends_auth_header(self, temp_dir, monkeypatch):
         """download_extension passes Authorization header when a provider is configured."""
         from unittest.mock import patch, MagicMock
diff --git a/tests/test_presets.py b/tests/test_presets.py
index 8fa700fa77..5322d1454d 100644
--- a/tests/test_presets.py
+++ b/tests/test_presets.py
@@ -1514,6 +1514,163 @@ def fake_open(req, timeout=None):
 
         assert captured["req"].get_header("Authorization") == "Bearer ghp_testtoken"
 
+    @pytest.mark.parametrize(
+        "payload",
+        [
+            # Root is not a JSON object.
+            [],
+            "oops",
+            42,
+            None,
+            # Root is fine but ``presets`` is the wrong type.
+            {"schema_version": "1.0", "presets": []},
+            {"schema_version": "1.0", "presets": "oops"},
+            {"schema_version": "1.0", "presets": None},
+            {"schema_version": "1.0", "presets": 42},
+        ],
+    )
+    def test_fetch_single_catalog_rejects_malformed_payload(self, project_dir, payload):
+        """Malformed catalog payloads raise PresetError, not AttributeError.
+
+        Without this guard, a payload like ``{"presets": []}`` would pass the
+        key-presence check and then crash with ``AttributeError: 'list' object
+        has no attribute 'items'`` deep inside ``_get_merged_packs``. The
+        sibling integration catalog reader already validates both the root
+        object and the nested mapping (see ``integrations/catalog.py``); the
+        preset catalog must stay consistent.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = PresetCatalog(project_dir)
+
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(payload).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = PresetCatalogEntry(
+            url="https://example.com/catalog.json",
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response):
+            with pytest.raises(PresetError, match="Invalid preset catalog format"):
+                catalog._fetch_single_catalog(entry, force_refresh=True)
+
+    @pytest.mark.parametrize(
+        "cached_payload",
+        [
+            [],
+            "oops",
+            42,
+            None,
+            {"schema_version": "1.0", "presets": []},
+            {"schema_version": "1.0", "presets": "oops"},
+            {"schema_version": "1.0", "presets": None},
+        ],
+    )
+    def test_fetch_single_catalog_rejects_malformed_cached_payload(
+        self, project_dir, cached_payload
+    ):
+        """A poisoned cache silently falls back to the network instead of
+        crashing — cached payloads pass through the same shape validation
+        as freshly-fetched ones.
+
+        Without this, a cache poisoned by an older spec-kit version (or a
+        manual edit, or an upstream that briefly served a bad payload
+        before the network guards landed) would re-crash every invocation
+        of ``_get_merged_packs`` despite the cache being "valid" by age.
+        The recovery contract is: if the cached payload fails validation,
+        drop it and refetch — never propagate ``AttributeError`` to the
+        caller.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = PresetCatalog(project_dir)
+
+        # Poison the default-URL cache. ``DEFAULT_CATALOG_URL`` and
+        # non-default URLs both flow through the same cache-load branch.
+        cache_file, metadata_file = catalog._get_cache_paths(
+            catalog.DEFAULT_CATALOG_URL
+        )
+        cache_file.parent.mkdir(parents=True, exist_ok=True)
+        cache_file.write_text(json.dumps(cached_payload))
+        metadata_file.write_text(
+            json.dumps(
+                {
+                    "cached_at": datetime.now(timezone.utc).isoformat(),
+                    "catalog_url": catalog.DEFAULT_CATALOG_URL,
+                }
+            )
+        )
+
+        # Network refetch returns a valid payload so the recovery path
+        # can complete.
+        valid = {
+            "schema_version": "1.0",
+            "presets": {"foo": {"name": "Foo", "version": "1.0.0"}},
+        }
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(valid).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = PresetCatalogEntry(
+            url=catalog.DEFAULT_CATALOG_URL,
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response):
+            result = catalog._fetch_single_catalog(entry, force_refresh=False)
+
+        # The poisoned cache was discarded and the network payload returned.
+        assert result == valid
+
+    def test_get_merged_packs_skips_non_mapping_entries(self, project_dir):
+        """Per-entry guard: one malformed entry shouldn't poison the merge.
+
+        ``_fetch_single_catalog`` validates that ``presets`` is a mapping,
+        but it doesn't (and shouldn't) validate every entry inside it — a
+        single bad entry in an otherwise-valid catalog should be skipped,
+        not crash the whole resolve path. Mirrors the per-entry skip in
+        ``integrations/catalog.py``: a malformed entry returns no error,
+        valid entries continue to merge normally.
+        """
+        from unittest.mock import patch, MagicMock
+
+        catalog = PresetCatalog(project_dir)
+        payload = {
+            "schema_version": "1.0",
+            "presets": {
+                "good": {"name": "Good", "version": "1.0.0"},
+                "bad-list": [],
+                "bad-str": "oops",
+            },
+        }
+        mock_response = MagicMock()
+        mock_response.read.return_value = json.dumps(payload).encode()
+        mock_response.__enter__ = lambda s: s
+        mock_response.__exit__ = MagicMock(return_value=False)
+
+        entry = PresetCatalogEntry(
+            url="https://example.com/catalog.json",
+            name="default",
+            priority=1,
+            install_allowed=True,
+        )
+
+        with patch.object(catalog, "_open_url", return_value=mock_response), \
+             patch.object(catalog, "get_active_catalogs", return_value=[entry]):
+            merged = catalog._get_merged_packs(force_refresh=True)
+
+        # Only the well-formed entry survives; the two malformed entries are
+        # silently dropped rather than raising or crashing.
+        assert list(merged.keys()) == ["good"]
+
     def test_download_pack_sends_auth_header(self, project_dir, monkeypatch):
         """download_pack passes Authorization header when configured."""
         from unittest.mock import patch, MagicMock