diff --git a/src/specify_cli/workflows/expressions.py b/src/specify_cli/workflows/expressions.py
index 6259b59de0..d54d43c9c4 100644
--- a/src/specify_cli/workflows/expressions.py
+++ b/src/specify_cli/workflows/expressions.py
@@ -192,7 +192,18 @@ 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 — an unknown filter name, or a known
+        # filter used in an unsupported form. 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.
+        leading_name = re.match(r"\w+", filter_expr)
+        name = leading_name.group(0) if leading_name else filter_expr
+        raise ValueError(
+            f"unknown filter '{name}': expected one of default('x'), "
+            "join('sep'), map('attr'), contains('s'), or from_json "
+            f"(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..2dd560eb2e 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -342,6 +342,49 @@ 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: the 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": ""},
+            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.text | contains('world') }}", ctx) is True
+        )
+        assert evaluate_expression(
+            "{{ steps.emit.output.stdout | from_json }}", ctx
+        ) == {"n": 1}
+
     def test_condition_evaluation(self):
         from specify_cli.workflows.expressions import evaluate_condition
         from specify_cli.workflows.base import StepContext