[Yarmoluk]

Summary

CohereReranker built its AsyncClient once in __init__ and passed it the same kwargs as the sync client. Two bugs resulted:

• "Event loop is closed" — the connection pool was bound to the first event loop that used it; a reranker that outlived that loop failed on every subsequent call.
• "object Response can't be used in 'await' expression" — a caller-supplied httpx_client (sync) was forwarded to the AsyncClient, so arank got a sync Response back and crashed on await.

Fix

• _get_aclient() builds the AsyncClient on first use and rebuilds it when the running loop changes (same pattern CohereTextVectorizer got in fix(vectorize): use Cohere's async client for aembed and aembed_many #785).
• A caller-supplied async_httpx_client is kept and reused across loops.
• A sync httpx_client without a matching async one disables the async path and falls back to the sync client with a warning.

Test plan

• pytest tests/unit/test_cohere_reranker.py — 181-line new suite covering lazy init, loop-change rebuild, sync-client fallback, and warning behaviour.
• No changes to vectorizer, router, or connection code.

🤖 Generated with Claude Code

---

Note

Medium Risk
Changes async HTTP client lifecycle and httpx kwargs handling for reranking; behavior is covered by new unit tests but affects production async rerank paths.

Overview
Fixes CohereReranker.arank failures from eagerly creating AsyncClient in __init__: connection pools tied to a closed event loop caused "Event loop is closed", and forwarding a sync httpx_client to the async client caused await errors on sync responses.

CohereReranker now lazily builds the async client via _get_aclient(), reusing it within one loop and rebuilding when the running loop changes (same approach as CohereTextVectorizer). Callers can pass async_httpx_client for a dedicated async HTTP client; if only a sync httpx_client is supplied, arank logs a warning and falls back to rank. httpx_client=None still enables the async path.

Adds tests/unit/test_cohere_reranker.py with fakes covering lazy init, loop reuse/rebuild, sync fallback, and async_httpx_client wiring.

^{Reviewed by Cursor Bugbot for commit 3e0646b. Bugbot is set up for automated code reviews on this repo. Configure here.}