Tool Schemas with Unsupported Top-Level Composition (langchain-anthropic 1.5.4)

What it does

Anthropic\u2019s tool-calling API accepts schemas describing the function calls a model can make. When a schema uses top-level composition patterns - a root oneOf, an allOf, or a bare array type without an items key - older langchain-anthropic would silently drop those fields, raise an opaque validation error, or send a malformed schema that fails at runtime. The fix in langchain-anthropic==1.5.4 (PR #39273) intercepts these unsupported patterns before serialization: it either flattens them into a compatible form or raises a clear, actionable error upfront.

Why it matters

When you bind tools for Claude via bind_tools or the @tool decorator, LangChain converts your Python signatures into JSON schemas for Anthropic. Real projects use Pydantic models with discriminated unions (oneOf), optional composition (allOf), or bare list types at the top level - valid in LangChain\u2019s internal schema system but unsupported by Anthropic. Before this fix you only discovered the mismatch when a specific tool was actually called, producing confusing runtime errors. Now the failure surfaces at schema-build time with a clear message, saving hours of production debugging.

Example

$ Bind a Pydantic model with a top-level Union and see the new upfront behavior

Schema validation passed — tools bound successfully
Tool call result: AIMessage(content='...' additional_kwargs={'tool_calls': [...]})

$ Inspect which schema shapes trigger the guard

class SearchTool(BaseModel):
    query: str
    filter: Union[str, int, None] = None   # top-level oneOf
class BatchIDs(BaseModel):
    ids: list                              # bare list, no items=
# Both now fail or flatten at bind time, not at call time

Common flags

#39273

fix(anthropic): handle tool schemas with unsupported top-level composition.

bind_tools

Binds Pydantic/tool schemas to a Claude model for tool calling.

convert_to_anthropic_tool_schema

Converts a LangChain schema to Anthropic\u2019s expected JSON format.