fix(core): add messages to bare raise ValueError calls (#38158)

Closes #35727

---

Several internal code paths raised a bare `ValueError` with no message,
so when one of these conditions tripped, users saw a traceback with no
explanation of what actually went wrong.

This adds descriptive messages to each of those `raise ValueError`
calls:

- `FewShotPromptWithTemplates._get_examples` / `_aget_examples` — when
neither `examples` nor `example_selector` is set.
- Prompt `loading` — when a referenced template file uses an unsupported
(non-`.txt`) format.
- The LangSmith document loader — when both `client` and `client_kwargs`
are supplied.
- The Anthropic file-search middleware brace expansion — for unbalanced
or empty brace patterns.

Made by [Open SWE](https://openswe.vercel.app)

---------

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/core/langchain_core/document_loaders/langsmith.py

@@ -94,7 +94,8 @@ class LangSmithLoader(BaseLoader):
             ValueError: If both `client` and `client_kwargs` are provided.
         """  # noqa: E501
         if client and client_kwargs:
-            raise ValueError
+            msg = "Only one of 'client' and 'client_kwargs' should be provided."
+            raise ValueError(msg)
         self._client = client or LangSmithClient(**client_kwargs)
         self.content_key = list(content_key.split(".")) if content_key else []
         self.format_content = format_content or _stringify

libs/core/langchain_core/prompts/few_shot_with_templates.py

@@ -111,14 +111,16 @@ class FewShotPromptWithTemplates(StringPromptTemplate):
             return self.examples
         if self.example_selector is not None:
             return self.example_selector.select_examples(kwargs)
-        raise ValueError
+        msg = "One of 'examples' and 'example_selector' should be provided"
+        raise ValueError(msg)
 
     async def _aget_examples(self, **kwargs: Any) -> list[dict[str, Any]]:
         if self.examples is not None:
             return self.examples
         if self.example_selector is not None:
             return await self.example_selector.aselect_examples(kwargs)
-        raise ValueError
+        msg = "One of 'examples' and 'example_selector' should be provided"
+        raise ValueError(msg)
 
     def format(self, **kwargs: Any) -> str:
         """Format the prompt with the inputs.

libs/core/langchain_core/prompts/loading.py

@@ -104,7 +104,11 @@ def _load_template(
         if resolved_path.suffix == ".txt":
             template = resolved_path.read_text(encoding="utf-8")
         else:
-            raise ValueError
+            msg = (
+                f"Unsupported template file format: '{resolved_path.suffix}'. "
+                "Only '.txt' files are supported."
+            )
+            raise ValueError(msg)
         # Set the template variable to the extracted variable.
         config[var_name] = template
     return config

libs/core/tests/unit_tests/document_loaders/test_langsmith.py

@@ -2,6 +2,7 @@ import datetime
 import uuid
 from unittest.mock import MagicMock, patch
 
+import pytest
 from langsmith.schemas import Example
 
 from langchain_core.document_loaders import LangSmithLoader
@@ -13,6 +14,12 @@ def test_init() -> None:
     LangSmithLoader(api_key="secret")
 
 
+def test_init_client_and_client_kwargs_conflict() -> None:
+    """Passing both `client` and `client_kwargs` should raise."""
+    with pytest.raises(ValueError, match="Only one of 'client' and 'client_kwargs'"):
+        LangSmithLoader(client=MagicMock(), api_key="secret")
+
+
 EXAMPLES = [
     Example(
         inputs={"first": {"second": "foo"}},

libs/core/tests/unit_tests/prompts/test_few_shot_with_templates.py

@@ -81,3 +81,26 @@ def test_prompttemplate_validation() -> None:
         example_prompt=EXAMPLE_PROMPT,
         example_separator="\n",
     ).input_variables == ["content", "new_content"]
+
+
+async def test_get_examples_requires_examples_or_selector() -> None:
+    """Both `_get_examples` and `_aget_examples` raise when neither is set.
+
+    The constructor validator forbids the neither-provided case, so the fields
+    are cleared after construction to reach the guard inside the getters.
+    """
+    suffix = PromptTemplate(input_variables=[], template="end")
+    prompt = FewShotPromptWithTemplates(
+        suffix=suffix,
+        input_variables=[],
+        examples=[{"question": "foo", "answer": "bar"}],
+        example_prompt=EXAMPLE_PROMPT,
+    )
+    prompt.examples = None
+    prompt.example_selector = None
+
+    match = "One of 'examples' and 'example_selector' should be provided"
+    with pytest.raises(ValueError, match=match):
+        prompt._get_examples()
+    with pytest.raises(ValueError, match=match):
+        await prompt._aget_examples()

libs/core/tests/unit_tests/prompts/test_loading.py

@@ -316,7 +316,7 @@ def test_symlink_txt_to_py_is_blocked(tmp_path: Path) -> None:
         os.chdir(tmp_path)
         with (
             suppress_langchain_deprecation_warning(),
-            pytest.raises(ValueError),  # noqa: PT011
+            pytest.raises(ValueError, match="files are supported"),
         ):
             load_prompt_from_config(config)
     finally:
@@ -341,7 +341,7 @@ def test_symlink_jinja2_rce_is_blocked(tmp_path: Path) -> None:
     }
     with (
         suppress_langchain_deprecation_warning(),
-        pytest.raises(ValueError),  # noqa: PT011
+        pytest.raises(ValueError, match="files are supported"),
     ):
         load_prompt_from_config(config, allow_dangerous_paths=True)
 

libs/partners/anthropic/langchain_anthropic/middleware/file_search.py

@@ -35,13 +35,15 @@ def _expand_include_patterns(pattern: str) -> list[str] | None:
 
         end = current.find("}", start)
         if end == -1:
-            raise ValueError
+            msg = f"Unbalanced brace in pattern: '{current}' is missing a '}}'."
+            raise ValueError(msg)
 
         prefix = current[:start]
         suffix = current[end + 1 :]
         inner = current[start + 1 : end]
         if not inner:
-            raise ValueError
+            msg = f"Empty brace expansion in pattern: '{current}'."
+            raise ValueError(msg)
 
         for option in inner.split(","):
             _expand(prefix + option + suffix)