diff --git a/src/specify_cli/workflows/expressions.py b/src/specify_cli/workflows/expressions.py
index 38a29890ae..e30346f019 100644
--- a/src/specify_cli/workflows/expressions.py
+++ b/src/specify_cli/workflows/expressions.py
@@ -400,6 +400,22 @@ def _apply_filter(value: Any, filter_expr: str, namespace: dict[str, Any]) -> An
     # branch above. The greedy ``.+`` still handles literal ``)`` and ``|``
     # inside quoted args.
     filter_match = re.fullmatch(r"(\w+)\((.+)\)", filter_expr)
+    # A multi-argument call is not a supported form: every filter here takes
+    # exactly one argument, and the whole captured argument text was handed to
+    # ``_evaluate_simple_expression`` as ONE expression. "1, 2" is not a valid
+    # expression, so it evaluated to None -- making ``default(1, 2)`` return
+    # None (silently wrong) and ``join(",", "extra")`` raise a message blaming
+    # the separator rather than the extra argument. Fall through to the
+    # unsupported-form error below instead, which names the filter and lists
+    # the accepted forms.
+    #
+    # Use ``_find_top_level``, the same scanner the operator splitting uses: it
+    # skips commas inside quotes AND inside nested brackets, so a single
+    # argument that happens to contain a comma still works -- ``join(", ")``,
+    # ``default("a, b")``, and the list/dict literals the evaluator supports
+    # (``default([1, 2])``, ``default({"a": 1, "b": 2})``).
+    if filter_match and _find_top_level(filter_match.group(2), ",") != -1:
+        filter_match = None
     if filter_match:
         fname = filter_match.group(1)
         farg = _evaluate_simple_expression(filter_match.group(2).strip(), namespace)
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index afd70adecf..ef4aa265e2 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -711,6 +711,71 @@ def test_filter_call_with_trailing_tokens_fails_loudly(self):
                 StepContext(inputs={"tags": ["a", "b"]}),
             )
 
+    def test_multi_argument_filter_call_fails_loudly(self):
+        """A second argument must be reported, not silently mis-evaluated.
+
+        The whole captured argument text was handed to
+        `_evaluate_simple_expression` as ONE expression. `"1, 2"` is not a valid
+        expression, so it evaluated to None — making `default(1, 2)` return None
+        (silently wrong) and `join(",", "extra")` raise a message blaming the
+        separator rather than the extra argument.
+        """
+        import pytest
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        with pytest.raises(ValueError, match="unsupported form"):
+            evaluate_expression(
+                "{{ inputs.missing | default(1, 2) }}", StepContext(inputs={})
+            )
+        with pytest.raises(ValueError, match="unsupported form"):
+            evaluate_expression(
+                '{{ inputs.tags | join(",", "extra") }}',
+                StepContext(inputs={"tags": ["a", "b"]}),
+            )
+
+    def test_single_argument_containing_a_comma_still_works(self):
+        """The multi-argument check must skip quotes AND nested brackets.
+
+        A single argument may legitimately contain a comma in two ways:
+
+        * inside quotes — `join(", ")`, `default("a, b")`
+        * inside a bracketed literal — `default([1, 2])`, which the expression
+          evaluator supports and which resolves to a real list
+
+        so the check uses `_find_top_level` (the same scanner the operator
+        splitting uses) rather than a quote-only scan.
+        """
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(inputs={"tags": ["a", "b"]})
+        assert evaluate_expression('{{ inputs.tags | join(", ") }}', ctx) == "a, b"
+        assert evaluate_expression('{{ inputs.tags | join(",") }}', ctx) == "a,b"
+        assert (
+            evaluate_expression('{{ inputs.missing | default("a, b") }}', ctx)
+            == "a, b"
+        )
+        # List literals: a comma inside brackets is not an argument separator.
+        assert evaluate_expression(
+            "{{ inputs.missing | default([1, 2]) }}", ctx
+        ) == [1, 2]
+        assert evaluate_expression(
+            "{{ inputs.missing | default([1,2]) }}", ctx
+        ) == [1, 2]
+        assert evaluate_expression("{{ inputs.missing | default([]) }}", ctx) == []
+
+    def test_multi_argument_after_a_literal_is_still_rejected(self):
+        """A real second argument is rejected even when the first is a literal."""
+        import pytest
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        with pytest.raises(ValueError, match="unsupported form"):
+            evaluate_expression(
+                "{{ inputs.missing | default([1,2], 3) }}", StepContext(inputs={})
+            )
+
     def test_chained_filters_apply_left_to_right(self):
         # Filters chain: each filter's result feeds the next. `map` yields a
         # list and `join` is the only filter that renders a list to a string,