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Currently, tools have a strong input contract through `args_schema`, but no
output contract. This means that anything a tool outputs is converted to
string.
Not only the contract is weak, but the "invisible" conversion to string can
have unexpected effects when the tool returns complex objects like dicts and
arrays.
With this PR, a tool can _optionally_ define an output contract with
`output_schema`. CrewAI validates the raw result and sends the agent JSON.
```python
class ProductResult(BaseModel):
sku: str
name: str
in_stock: bool
class ProductLookupTool(BaseTool):
name: str = "Product Lookup"
description: str = "Look up product availability by SKU."
def _run(self, sku: str) -> ProductResult:
return ProductResult(sku=sku, name="USB-C dock", in_stock=True)
```
If the result does not match the schema, CrewAI warns and falls back to
`str(raw_result)` instead of failing the run:
```python
@tool("Product Lookup", output_schema=ProductResult)
def product_lookup(sku: str) -> dict[str, object]:
return {"sku": sku, "name": "USB-C dock", "in_stock": True}
#=> RuntimeWarning: Failed to validate or serialize output from tool 'Bad Product Lookup' using output_schema 'ProductResult'... Falling back to str(raw_result).
```
This is additive and non-breaking. Existing tools do not need to change. Tools
without `output_schema` keep the old string behavior. Invalid typed outputs
warn and fall back to the old formatting path.