Latest Results
fix(core): fail fast when tool schemas can't resolve forward refs during serialization (#39570)
fixes #39099
We currently allow forward refs in pydantic v2 schemas upon creation:
```python
class Container(BaseModel):
rows: list["Row"] = [] # "Row" is declared below, after the tool is decorated
@tool
def my_tool(container: Container):
"""A tool whose schema depends on a forward reference that is not resolvable yet."""
return "ok"
class Row(BaseModel):
name: str
```
When it comes time to introspect the tool schema (notably in
`count_tokens_approximately` and `convert_to_openai_tool`), we rely on
[signature
introspection](https://github.com/langchain-ai/langchain/blob/943dd700ef7c33e3f1f21d3e280c9c249b88259c/libs/core/langchain_core/tools/base.py#L1654-L1661)
to extract the tool's input schema. If that contains invalid forward
references, there's no schema fields to extract which results in an
empty dict:
<details>
<summary>Invalid forward reference MRE</summary>
```python
from __future__ import annotations
import inspect
from pydantic import BaseModel, Field
from pydantic.errors import PydanticUndefinedAnnotation
from langchain_core.tools.base import get_all_basemodel_annotations
from langchain_core.utils.pydantic import _create_subset_model, model_json_schema
class Container(BaseModel):
"""A model with a nested forward reference that can never resolve."""
rows: list["UndefinedRow"] = Field(default_factory=list)
def main() -> None:
"""Print the field-selection inputs and their zero-field subset result."""
selected_annotations = get_all_basemodel_annotations(Container)
subset_schema = _create_subset_model(
"ContainerSubset",
Container,
list(selected_annotations),
fn_description=Container.__doc__,
)
print(f"Pydantic complete: {Container.__pydantic_complete__}")
print(f"Pydantic fields: {list(Container.model_fields)}")
print(f"inspect.signature: {inspect.signature(Container)}")
print(f"Fields selected by get_all_basemodel_annotations: {selected_annotations}")
print(f"Subset properties: {model_json_schema(subset_schema)['properties']}")
if __name__ == "__main__":
main()
```
```output
Pydantic complete: False
Pydantic fields: ['rows']
inspect.signature: (**data: 'Any') -> 'None'
Fields selected by get_all_basemodel_annotations: {}
Subset properties: {}
```
</details>
<details>
<summary>Valid forward reference MRE</summary>
```python
from __future__ import annotations
import inspect
from pydantic import BaseModel, Field
from pydantic.errors import PydanticUndefinedAnnotation
from langchain_core.tools.base import get_all_basemodel_annotations
from langchain_core.utils.pydantic import _create_subset_model, model_json_schema
class Container(BaseModel):
"""A model with a nested forward reference that can never resolve."""
rows: list["UndefinedRow"] = Field(default_factory=list)
class UndefinedRow(BaseModel):
name: str = Field()
def main() -> None:
"""Print the field-selection inputs and their zero-field subset result."""
Container.model_rebuild()
selected_annotations = get_all_basemodel_annotations(Container)
subset_schema = _create_subset_model(
"ContainerSubset",
Container,
list(selected_annotations),
fn_description=Container.__doc__,
)
print(f"Pydantic complete: {Container.__pydantic_complete__}")
print(f"Pydantic fields: {list(Container.model_fields)}")
print(f"inspect.signature: {inspect.signature(Container)}")
print(f"Fields selected by get_all_basemodel_annotations: {selected_annotations}")
print(f"Subset properties: {model_json_schema(subset_schema)['properties']}")
if __name__ == "__main__":
main()
```
```output
Pydantic complete: True
Pydantic fields: ['rows']
inspect.signature: (*, rows: list[__main__.UndefinedRow] = <factory>) -> None
Fields selected by get_all_basemodel_annotations: {'rows': list[__main__.UndefinedRow]}
Subset properties: {'rows': {'items': {'$ref': '#/$defs/UndefinedRow'}, 'title': 'Rows', 'type': 'array'}}
```
</details>
---
The fix is to
* at introspection time, resolve forward references using
`.model_rebuild()` that raises a pydantic exception if forward
references cant be resolved
* i'm also widening a pydantic utility to use a type guard instead of
having to use bool + cast
I'm intentionally not rebuilding pydantic v1 schemas in the same way
since
* forward references are specified by explicitly passing names into
`update_forward_refs`
* pydantic v1 is old news Latest Branches
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