fix: don't crash prompt validation on literal braces (valid Jinja text)

packages/data-designer-engine/src/data_designer/engine/validation.py

@@ -3,6 +3,7 @@
 
 from __future__ import annotations
 
+import re
 from enum import Enum
 from fnmatch import fnmatch
 from string import Formatter
@@ -450,9 +451,20 @@ def validate_skip_references(
     return violations
 
 
+_FORMAT_FIELD_REFERENCE_PATTERN = re.compile(r"(?<!\{)\{\s*(\w+)\s*(?:![sra])?(?::[^{}]*(?:\{[^{}]*\}[^{}]*)*)?\}(?!\})")
+
+
 def _get_string_formatter_references(template: str, allowed_references: list[str]) -> list[str]:
-    return [
-        k[1].strip()
-        for k in Formatter().parse(template)
-        if len(k) > 1 and k[1] is not None and k[1].strip() in allowed_references
-    ]
+    try:
+        return [
+            k[1].strip()
+            for k in Formatter().parse(template)
+            if len(k) > 1 and k[1] is not None and k[1].strip() in allowed_references
+        ]
+    except ValueError:
+        # Unmatched literal braces (e.g. JSON examples like 'output format: }') are
+        # invalid f-string syntax but valid Jinja text, so ``Formatter().parse`` can
+        # raise ``ValueError``. Fall back to a tolerant regex scan that skips Jinja
+        # ``{{ ... }}`` expressions so the advisory check still detects ``{column}``
+        # references without crashing validation.
+        return [m for m in _FORMAT_FIELD_REFERENCE_PATTERN.findall(template) if m in allowed_references]

packages/data-designer-engine/tests/engine/test_validation.py

@@ -236,6 +236,91 @@ def test_validate_detect_f_string_syntax():
     assert violations[0].level == ViolationLevel.WARNING
 
 
+def test_validate_prompt_templates_with_literal_braces() -> None:
+    """Literal braces that are valid Jinja text must not crash the f-string advisory check."""
+    columns = [
+        SamplerColumnConfig(
+            name="random_number",
+            sampler_type="uniform",
+            params={"low": 0, "high": 10},
+        ),
+        LLMTextColumnConfig(
+            name="literal_close_brace",
+            prompt="Why is {{ random_number }} your favorite number? End with a literal } brace.",
+            model_alias=STUB_MODEL_ALIAS,
+        ),
+        LLMTextColumnConfig(
+            name="literal_open_brace",
+            system_prompt="Prefix every answer with an opening brace { like this.",
+            prompt="Describe {{ random_number }}.",
+            model_alias=STUB_MODEL_ALIAS,
+        ),
+    ]
+    violations = validate_prompt_templates(columns, [c.name for c in columns])
+    assert len(violations) == 0
+
+
+def test_validate_detect_f_string_syntax_with_literal_braces() -> None:
+    """f-string references are still detected when the prompt also contains literal braces."""
+    columns = [
+        SamplerColumnConfig(
+            name="random_number",
+            sampler_type="uniform",
+            params={"low": 0, "high": 10},
+        ),
+        LLMTextColumnConfig(
+            name="f_string_ref_after_literal_brace",
+            prompt="End with a literal } brace. Why is {random_number} and {{ random_number }} your favorite number?",
+            model_alias=STUB_MODEL_ALIAS,
+        ),
+    ]
+    violations = validate_prompt_templates(columns, [c.name for c in columns])
+    assert len(violations) == 1
+    assert violations[0].type == ViolationType.F_STRING_SYNTAX
+    assert violations[0].column == "f_string_ref_after_literal_brace"
+    assert violations[0].level == ViolationLevel.WARNING
+
+
+def test_validate_detect_nested_format_spec_ref_with_literal_braces() -> None:
+    """Fallback also detects references whose format spec nests another field."""
+    columns = [
+        SamplerColumnConfig(
+            name="random_number",
+            sampler_type="uniform",
+            params={"low": 0, "high": 10},
+        ),
+        LLMTextColumnConfig(
+            name="nested_spec_ref_after_literal_brace",
+            prompt="Literal } value {random_number:{width}} jinja {{ random_number }}.",
+            model_alias=STUB_MODEL_ALIAS,
+        ),
+    ]
+    violations = validate_prompt_templates(columns, [c.name for c in columns])
+    assert len(violations) == 1
+    assert violations[0].type == ViolationType.F_STRING_SYNTAX
+    assert violations[0].column == "nested_spec_ref_after_literal_brace"
+
+
+def test_validate_detect_formatted_f_string_refs_with_literal_braces() -> None:
+    """Fallback also catches references carrying a conversion or format spec."""
+    columns = [
+        SamplerColumnConfig(
+            name="random_number",
+            sampler_type="uniform",
+            params={"low": 0, "high": 10},
+        ),
+        LLMTextColumnConfig(
+            name="formatted_ref_after_literal_brace",
+            prompt="End with a literal } brace. Padded {random_number:03d}, repr {random_number!r}, jinja {{ random_number }}.",
+            model_alias=STUB_MODEL_ALIAS,
+        ),
+    ]
+    violations = validate_prompt_templates(columns, [c.name for c in columns])
+    assert len(violations) == 1
+    assert violations[0].type == ViolationType.F_STRING_SYNTAX
+    assert violations[0].column == "formatted_ref_after_literal_brace"
+
+
 def test_validate_column_config_with_multi_modal_context():
     column = LLMTextColumnConfig(
         name="image_description",

packages/data-designer/tests/interface/test_data_designer.py

@@ -1555,6 +1555,45 @@ def test_validate_raises_error_when_seed_collides(
         data_designer.validate(config_builder)
 
 
+def test_validate_with_literal_braces_in_prompts(
+    stub_artifact_path: Path,
+    stub_model_providers: list[ModelProvider],
+    stub_check_models_model_configs: list[ModelConfig],
+    stub_managed_assets_path: Path,
+) -> None:
+    """Literal braces that are valid Jinja text must not crash validation.
+
+    Regression test for #904: an unmatched ``}`` in a prompt or an unmatched ``{``
+    in a system prompt is invalid f-string syntax but valid Jinja text, so the
+    f-string advisory check must not leak ``ValueError`` from ``string.Formatter``.
+    """
+    config_builder = DataDesignerConfigBuilder(model_configs=stub_check_models_model_configs)
+    config_builder.add_column(
+        SamplerColumnConfig(
+            name="topic",
+            sampler_type=SamplerType.CATEGORY,
+            params=CategorySamplerParams(values=["science"]),
+        )
+    )
+    config_builder.add_column(
+        LLMTextColumnConfig(
+            name="story",
+            model_alias="stub-model",
+            prompt="Write about {{ topic }}. End with a literal } brace.",
+            system_prompt="Open every answer with a literal { brace.",
+        )
+    )
+
+    data_designer = DataDesigner(
+        artifact_path=stub_artifact_path,
+        model_providers=stub_model_providers,
+        secret_resolver=PlaintextResolver(),
+        managed_assets_path=stub_managed_assets_path,
+    )
+
+    assert data_designer.validate(config_builder) is None
+
+
 def test_init_auto_configures_logging_by_default(
     stub_artifact_path: Path,
     stub_model_providers: list[ModelProvider],