feat(ollama): support `response_format` (#34612)
Fixes #34610
---
This PR resolves an issue where `ChatOllama` would raise an `unexpected
keyword argument 'response_format'` error when used with `create_agent`
or when passed an OpenAI-style `response_format`.
When using `create_agent` (especially with models like `gpt-oss`),
LangChain creates a `response_format` argument (e.g., `{"type":
"json_schema", ...}`). `ChatOllama` previously passed this argument
directly to the underlying Ollama client, which does not support
`response_format` and instead expects a `format` parameter.
## The Fix
I updated `_chat_params` in
`libs/partners/ollama/langchain_ollama/chat_models.py` to:
1. Intercept the `response_format` argument.
2. Map it to the native Ollama `format` parameter:
* `{"type": "json_schema", "json_schema": {"schema": ...}}` ->
`format=schema`
* `{"type": "json_object"}` -> `format="json"`
3. Remove `response_format` from the kwargs passed to the client.
## Validation
* **Reproduction Script**: Verified the fix with a script covering
`json_schema`, `json_object`, and explicit `format` priority scenarios.
* **New Tests**: Added 3 new unit tests to
`libs/partners/ollama/tests/unit_tests/test_chat_models.py` covering
these scenarios.
* **Regression**: Ran the full test suite (`make -C libs/partners/ollama
test`), passing 29 tests (previously 26).
* **Lint/Format**: Verified with `make lint_package` and `make format`.
---------
Co-authored-by: Mohan Kumar Sagadevan <mohankumarsagadevan@Mohans-MacBook-Air.local>
Co-authored-by: Mason Daugherty <mason@langchain.dev>
Co-authored-by: Mason Daugherty <github@mdrxy.com>
