diff --git a/src/specify_cli/workflows/expressions.py b/src/specify_cli/workflows/expressions.py
index 6259b59de0..ca10b24d1b 100644
--- a/src/specify_cli/workflows/expressions.py
+++ b/src/specify_cli/workflows/expressions.py
@@ -12,6 +12,19 @@
 from typing import Any
 
 
+# The filters the expression evaluator recognizes. Used to tell a
+# *registered* filter used in an unsupported form (e.g. `| join` with no
+# argument) apart from a genuinely unknown filter name, so each raises an
+# error that names the real problem.
+_REGISTERED_FILTERS: tuple[str, ...] = (
+    "default",
+    "join",
+    "map",
+    "contains",
+    "from_json",
+)
+
+
 # -- Custom filters -------------------------------------------------------
 
 def _filter_default(value: Any, default_value: Any = "") -> Any:
@@ -192,7 +205,27 @@ def _evaluate_simple_expression(expr: str, namespace: dict[str, Any]) -> Any:
         filter_name = filter_expr.strip()
         if filter_name == "default":
             return _filter_default(value)
-        return value
+        # No recognized filter matched. Fail loudly rather than silently
+        # returning the unfiltered value: a passthrough turns a mis-typed or
+        # unsupported filter into a wrong result with no signal. Mirrors the
+        # strict `from_json` handling above. Distinguish a *registered* filter
+        # used in an unsupported form (e.g. `| join` or `| map` with no
+        # argument) from a genuinely unknown filter name, so the message names
+        # the real problem instead of calling a known filter "unknown".
+        leading_name = re.match(r"\w+", filter_expr)
+        name = leading_name.group(0) if leading_name else filter_expr
+        expected = (
+            "expected one of default or default('x'), join('sep'), "
+            "map('attr'), contains('s'), or from_json"
+        )
+        if name in _REGISTERED_FILTERS:
+            raise ValueError(
+                f"filter '{name}' used in an unsupported form (got "
+                f"'| {filter_expr}'): {expected}"
+            )
+        raise ValueError(
+            f"unknown filter '{name}': {expected} (got '| {filter_expr}')"
+        )
 
     # Boolean operators — parse 'or' first (lower precedence) so that
     # 'a or b and c' is evaluated as 'a or (b and c)'.
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index a87c09cf05..bfb3c1bbe1 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -342,6 +342,73 @@ def test_filter_from_json_rejects_malformed_forms(self):
                     "{{ steps.emit.output.stdout | " + bad + " }}", ctx
                 )
 
+    def test_filter_unknown_name_raises(self):
+        # An unregistered filter name must fail loudly rather than silently
+        # returning the unfiltered value (which hides a typo / unsupported
+        # filter as a wrong result).
+        import pytest
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(inputs={"items": [1, 2, 3]})
+        with pytest.raises(ValueError, match="unknown filter 'length'"):
+            evaluate_expression("{{ inputs.items | length }}", ctx)
+
+    def test_filter_unknown_name_with_args_raises(self):
+        # The unknown-filter path must also catch the `name(arg)` form, which
+        # otherwise falls through the recognized-args branch silently.
+        import pytest
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(inputs={"text": "hello"})
+        with pytest.raises(ValueError, match="unknown filter 'upper'"):
+            evaluate_expression("{{ inputs.text | upper('x') }}", ctx)
+
+    def test_registered_filters_unaffected(self):
+        # Regression: all five registered filters keep working unchanged.
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(
+            inputs={
+                "tags": ["a", "b", "c"],
+                "text": "hello world",
+                "missing": "",
+                "rows": [{"id": "a"}, {"id": "b"}],
+            },
+            steps={"emit": {"output": {"stdout": '{"n": 1}'}}},
+        )
+        assert (
+            evaluate_expression("{{ inputs.missing | default('fb') }}", ctx) == "fb"
+        )
+        assert evaluate_expression("{{ inputs.tags | join(', ') }}", ctx) == "a, b, c"
+        assert evaluate_expression("{{ inputs.rows | map('id') }}", ctx) == ["a", "b"]
+        assert (
+            evaluate_expression("{{ inputs.text | contains('world') }}", ctx) is True
+        )
+        assert evaluate_expression(
+            "{{ steps.emit.output.stdout | from_json }}", ctx
+        ) == {"n": 1}
+
+    def test_registered_filter_unsupported_form_raises(self):
+        # A *registered* filter used in an unsupported form (e.g. `| join` with
+        # no argument) must fail loudly with a message that names it as a known
+        # filter misused, not as an "unknown filter".
+        import pytest
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(inputs={"tags": ["a", "b", "c"]})
+        with pytest.raises(
+            ValueError, match="filter 'join' used in an unsupported form"
+        ):
+            evaluate_expression("{{ inputs.tags | join }}", ctx)
+        with pytest.raises(
+            ValueError, match="filter 'map' used in an unsupported form"
+        ):
+            evaluate_expression("{{ inputs.tags | map }}", ctx)
+
     def test_condition_evaluation(self):
         from specify_cli.workflows.expressions import evaluate_condition
         from specify_cli.workflows.base import StepContext