diff --git a/pkg-py/src/commons/_catalog/__init__.py b/pkg-py/src/commons/_catalog/__init__.py
index c7f12af4..950e85d7 100644
--- a/pkg-py/src/commons/_catalog/__init__.py
+++ b/pkg-py/src/commons/_catalog/__init__.py
@@ -13,6 +13,9 @@
 
 from . import _databricks, _snowflake
 from ._core import (
+    CatalogAmbiguousColumnError,
+    CatalogAmbiguousTableError,
+    CatalogClaimedRelationError,
     Manifest,
     MergedDictionary,
     Relation,
@@ -40,7 +43,10 @@
 
 __all__ = [
     "CatalogAccessError",
+    "CatalogAmbiguousColumnError",
+    "CatalogAmbiguousTableError",
     "CatalogAuthorizationError",
+    "CatalogClaimedRelationError",
     "CatalogSessionChangedError",
     "CatalogTransientError",
     "Manifest",
diff --git a/pkg-py/src/commons/_catalog/_core.py b/pkg-py/src/commons/_catalog/_core.py
index 25b7e804..94bb928b 100644
--- a/pkg-py/src/commons/_catalog/_core.py
+++ b/pkg-py/src/commons/_catalog/_core.py
@@ -40,6 +40,18 @@
 _GLOB_SPECIAL = re.compile(r"([][{}()+.^$|\\])")
 
 
+class CatalogAmbiguousTableError(ValueError):
+    """An authored table name matches more than one selected relation."""
+
+
+class CatalogClaimedRelationError(ValueError):
+    """Two authored tables match the same selected relation."""
+
+
+class CatalogAmbiguousColumnError(ValueError):
+    """An authored column name matches more than one discovered column."""
+
+
 @dataclass(frozen=True)
 class Selector:
     """One entry of a `tables` selection, naming a relation or a namespace.
@@ -442,7 +454,7 @@ def _dictionary_matches(
         if label is None:
             continue
         if label in claimed:
-            raise ValueError(
+            raise CatalogClaimedRelationError(
                 f"Authored tables {claimed[label]!r} and {authored_name!r} "
                 f"both match selected relation {label!r}."
             )
@@ -504,7 +516,7 @@ def has_suffix(
 
 def _abort_ambiguous(authored_name: str, candidates: list[str]) -> None:
     listed = ", ".join(repr(item) for item in sorted(candidates))
-    raise ValueError(
+    raise CatalogAmbiguousTableError(
         f"Authored table {authored_name!r} matches more than one selected "
         f"relation: {listed}. Use its fully qualified name in the data "
         f"dictionary."
@@ -562,7 +574,7 @@ def _merge_columns(
             ]
         if len(candidates) > 1:
             listed = ", ".join(repr(name) for name in candidates)
-            raise ValueError(
+            raise CatalogAmbiguousColumnError(
                 f"Authored column {authored_name!r} matches more than one "
                 f"discovered column: {listed}."
             )
diff --git a/pkg-py/tests/test_catalog.py b/pkg-py/tests/test_catalog.py
index 699395a4..72e1253d 100644
--- a/pkg-py/tests/test_catalog.py
+++ b/pkg-py/tests/test_catalog.py
@@ -14,6 +14,9 @@
 import pytest
 
 from commons._catalog import (
+    CatalogAmbiguousColumnError,
+    CatalogAmbiguousTableError,
+    CatalogClaimedRelationError,
     Manifest,
     MergedDictionary,
     Relation,
@@ -508,13 +511,20 @@ def run_merge_case(case: dict) -> MergedDictionary:
     )
 
 
+_MERGE_ERRORS = {
+    "ambiguous": CatalogAmbiguousTableError,
+    "claimed": CatalogClaimedRelationError,
+    "ambiguous_column": CatalogAmbiguousColumnError,
+}
+
+
 def test_the_dictionary_merge_matches_the_shared_contract():
     cases = load_shared_fixture("catalog-merge")["merge"]
     assert cases
     for case in cases:
         expect = case["expect"]
         if "error" in expect:
-            with pytest.raises(ValueError):
+            with pytest.raises(_MERGE_ERRORS[expect["error"]]):
                 run_merge_case(case)
             continue
 
diff --git a/pkg-r/NEWS.md b/pkg-r/NEWS.md
index 079dfcda..e8c91a03 100644
--- a/pkg-r/NEWS.md
+++ b/pkg-r/NEWS.md
@@ -8,6 +8,8 @@
 
 * R code run by `run_r` can no longer send a signal to the application process's ID or to its process group. On Linux, the programs it starts can also no longer leave its process group, so they all stop when the R session ends.
 
+* When a data dictionary cannot be matched to the warehouse catalog, the error now has a condition class you can catch: `commons_catalog_ambiguous_table_error` when an authored table name matches more than one selected relation, `commons_catalog_claimed_relation_error` when two authored tables match the same relation, and `commons_catalog_ambiguous_column_error` when an authored column name matches more than one discovered column.
+
 * The CSS classes on `run_r`'s display are now `commons-run-display`, `commons-run-details`, `commons-run-code`, and `commons-run-plot`, without the `-r`, because the Python package's code tool uses the same classes. App CSS that targets the old `commons-run-r-*` names needs updating.
 
 # commons 0.1.1
diff --git a/pkg-r/R/catalog.R b/pkg-r/R/catalog.R
index 6d09e00b..1ee5948a 100644
--- a/pkg-r/R/catalog.R
+++ b/pkg-r/R/catalog.R
@@ -444,6 +444,7 @@ catalog_dictionary_matches <- function(
       cli::cli_abort(
         "Authored tables {.val {c(other, authored_name)}} both match selected
          relation {.val {label}}.",
+        class = "commons_catalog_claimed_relation_error",
         call = call
       )
     }
@@ -530,6 +531,7 @@ catalog_abort_ambiguous_dictionary_table <- function(
        relation: {.val {matches}}.",
       "i" = "Use its fully qualified name in the data dictionary."
     ),
+    class = "commons_catalog_ambiguous_table_error",
     call = call
   )
 }
@@ -593,6 +595,7 @@ catalog_merge_dictionary_columns <- function(
       cli::cli_abort(
         "Authored column {.val {authored_name}} matches more than one discovered
          column: {.val {names(out)[candidates]}}.",
+        class = "commons_catalog_ambiguous_column_error",
         call = call
       )
     }
diff --git a/pkg-r/tests/testthat/test-catalog-merge.R b/pkg-r/tests/testthat/test-catalog-merge.R
index 53bf898b..710a3bf6 100644
--- a/pkg-r/tests/testthat/test-catalog-merge.R
+++ b/pkg-r/tests/testthat/test-catalog-merge.R
@@ -30,6 +30,12 @@ test_that("exclusion globs match the shared contract", {
   }
 })
 
+merge_error_class <- c(
+  ambiguous = "commons_catalog_ambiguous_table_error",
+  claimed = "commons_catalog_claimed_relation_error",
+  ambiguous_column = "commons_catalog_ambiguous_column_error"
+)
+
 test_that("the dictionary merge matches the shared contract", {
   cases <- shared_fixture("catalog-merge")$merge
   expect_gt(length(cases), 0L)
@@ -70,7 +76,7 @@ test_that("the dictionary merge matches the shared contract", {
       )
     }
     if (!is.null(case$expect$error)) {
-      expect_error(run())
+      expect_error(run(), class = merge_error_class[[case$expect$error]])
       next
     }
     merged <- run()