feat: threading context through `create_agent` flows + middleware (#34978)
Closes https://github.com/langchain-ai/langchain/issues/33956
* Making `ModelRequest` generic on `ContextT` and `ResponseT` so that we
can thread type information through to `wrap_model_call`
* Making builtin middlewares generic on `ContextT` and `ResponseT` so
their context and response types can be inferred from the `create_agent`
signature
See new tests that verify backwards compatibility (for cases where folks
use custom middleware that wasn't parametrized).
This fixes:
1. Lack of access to context and response types in `wrap_model_call`
2. Lack of cohesion between middleware context + response types with
those specified in `create_agent`
See examples below:
### Type-safe context and response access
```python
class MyMiddleware(AgentMiddleware[AgentState[AnalysisResult], UserContext, AnalysisResult]):
def wrap_model_call(
self,
request: ModelRequest[UserContext],
handler: Callable[[ModelRequest[UserContext]], ModelResponse[AnalysisResult]],
) -> ModelResponse[AnalysisResult]:
# ✅ Now type-safe: IDE knows user_id exists and is str
user_id: str = request.runtime.context["user_id"]
# ❌ mypy error: "session_id" doesn't exist on UserContext
request.runtime.context["session_id"]
response = handler(request)
if response.structured_response is not None:
# ✅ Now type-safe: IDE knows sentiment exists and is str
sentiment: str = response.structured_response.sentiment
# ❌ mypy error: "summary" doesn't exist on AnalysisResult
response.structured_response.summary
return response
```
### Mismatched middleware/schema caught at `create_agent`
```python
class SessionMiddleware(AgentMiddleware[AgentState[Any], SessionContext, Any]):
...
# ❌ mypy error: SessionMiddleware expects SessionContext, not UserContext
create_agent(
model=model,
middleware=[SessionMiddleware()],
context_schema=UserContext, # mismatch!
)
class AnalysisMiddleware(AgentMiddleware[AgentState[AnalysisResult], ContextT, AnalysisResult]):
...
# ❌ mypy error: AnalysisMiddleware expects AnalysisResult, not SummaryResult
create_agent(
model=model,
middleware=[AnalysisMiddleware()],
response_format=SummaryResult, # mismatch!
)
```
langchain
dde2012b - feat: threading context through `create_agent` flows + middleware (#34978)
