diff --git a/src/specify_cli/bundler/commands_impl/catalog_config.py b/src/specify_cli/bundler/commands_impl/catalog_config.py
index 37e45bf990..42485fd1a8 100644
--- a/src/specify_cli/bundler/commands_impl/catalog_config.py
+++ b/src/specify_cli/bundler/commands_impl/catalog_config.py
@@ -14,14 +14,13 @@
 from ..lib.yamlio import dump_yaml, ensure_within, load_yaml
 from ..models.catalog import (
     CONFIG_FILENAME,
+    CONFIG_SCHEMA_VERSION,
     BUILTIN_DEFAULT_STACK,
     CatalogSource,
     InstallPolicy,
     Scope,
 )
 
-CONFIG_SCHEMA_VERSION = "1.0"
-
 _BUILTIN_IDS = {raw["id"] for raw in BUILTIN_DEFAULT_STACK}
 
 # Windows absolute paths like ``C:\catalog.json`` parse with a single-letter
diff --git a/src/specify_cli/bundler/models/catalog.py b/src/specify_cli/bundler/models/catalog.py
index 9621b5a334..88a08818f9 100644
--- a/src/specify_cli/bundler/models/catalog.py
+++ b/src/specify_cli/bundler/models/catalog.py
@@ -15,6 +15,11 @@
 from ..lib.yamlio import ensure_within, load_yaml
 
 CONFIG_FILENAME = "bundle-catalogs.yml"
+# Supported bundle-catalogs.yml schema (major version). Both readers of the
+# file — this module's _merge_config and commands_impl/catalog_config._read —
+# reject an unsupported major version so a file written by a newer/incompatible
+# Spec Kit fails fast instead of being parsed under the wrong assumptions.
+CONFIG_SCHEMA_VERSION = "1.0"
 
 
 class InstallPolicy(str, Enum):
@@ -267,6 +272,23 @@ def _merge_config(by_id: dict[str, CatalogSource], config_path: Path, scope: Sco
             f"Malformed catalog config at {config_path}: expected a mapping at "
             f"the top level, got {type(data).__name__}."
         )
+    # Reject an unsupported major schema version, matching the sibling reader
+    # commands_impl/catalog_config._read. Without this, a file written by a
+    # newer/incompatible Spec Kit was silently parsed under v1 assumptions on
+    # the resolution path (bundle search/install), while the other reader
+    # rejected it — the two readers disagreed. An absent schema_version stays
+    # valid (backward compatible with configs that omit it).
+    schema_version = data.get("schema_version")
+    if schema_version is not None and (
+        str(schema_version).strip().split(".")[0]
+        != CONFIG_SCHEMA_VERSION.split(".")[0]
+    ):
+        raise BundlerError(
+            f"Unsupported catalog config schema version "
+            f"'{str(schema_version).strip()}' at {config_path}; this Spec Kit "
+            f"understands version {CONFIG_SCHEMA_VERSION}. The file may have been "
+            "written by a newer version or is corrupt."
+        )
     catalogs = data.get("catalogs")
     if catalogs is None:
         return
diff --git a/tests/contract/test_catalog_schema.py b/tests/contract/test_catalog_schema.py
index 97600d21ef..5ec3d88973 100644
--- a/tests/contract/test_catalog_schema.py
+++ b/tests/contract/test_catalog_schema.py
@@ -113,6 +113,44 @@ def test_absent_or_empty_catalogs_is_noop(tmp_path: Path, body: str):
     assert len(sources) > 0
 
 
+def test_load_source_stack_rejects_unknown_schema_version(tmp_path: Path):
+    """A bundle-catalogs.yml with an unsupported MAJOR schema_version must raise
+    on the resolution path (load_source_stack -> _merge_config), matching the
+    sibling reader commands_impl/catalog_config._read. Without this a file
+    written by a newer/incompatible Spec Kit was silently parsed under v1
+    assumptions on the install/search path, while the other reader rejected it."""
+    make_project(tmp_path)
+    config = {
+        "schema_version": "2.0",
+        "catalogs": [{"id": "corp", "url": "https://corp/catalog.json",
+                      "priority": 1, "install_policy": "install-allowed"}],
+    }
+    (tmp_path / ".specify" / "bundle-catalogs.yml").write_text(
+        yaml.safe_dump(config), encoding="utf-8"
+    )
+    with pytest.raises(BundlerError, match="Unsupported catalog config schema version"):
+        load_source_stack(tmp_path)
+
+
+def test_load_source_stack_accepts_matching_or_absent_schema_version(tmp_path: Path):
+    """A matching major version (1.x) and an absent schema_version both stay
+    valid — the guard rejects only a different major, so existing configs that
+    omit the key are unaffected."""
+    make_project(tmp_path)
+    cfg = tmp_path / ".specify" / "bundle-catalogs.yml"
+    cfg.write_text(yaml.safe_dump({
+        "schema_version": "1.5",  # same major as CONFIG_SCHEMA_VERSION (1.0)
+        "catalogs": [{"id": "corp", "url": "https://corp/catalog.json",
+                      "priority": 1, "install_policy": "install-allowed"}],
+    }), encoding="utf-8")
+    assert "corp" in {s.id for s in load_source_stack(tmp_path)}
+    cfg.write_text(yaml.safe_dump({  # no schema_version key
+        "catalogs": [{"id": "corp2", "url": "https://corp2/catalog.json",
+                      "priority": 1, "install_policy": "install-allowed"}],
+    }), encoding="utf-8")
+    assert "corp2" in {s.id for s in load_source_stack(tmp_path)}
+
+
 def test_project_config_overrides_same_id(tmp_path: Path):
     make_project(tmp_path)
     config = {