A bare ``httpx.ReadTimeout`` previously surfaced as ``ReadTimeout: timed
out`` — no provider, no base URL, no model — leaving the user with no
signal to tell whether a model server hung, the URL was wrong, or the
model isn't loaded on the backend.
``ChatSession._format_backend_error`` now rewrites known boundary
exceptions (httpx ``ReadTimeout`` / ``ConnectError`` / etc. and OpenAI /
Anthropic SDK ``APITimeoutError`` / ``APIConnectionError`` /
``NotFoundError`` / ``AuthenticationError`` / ``RateLimitError``) into
operator-actionable text that names the provider, base URL (query
string stripped before ``sanitize_error_text`` redacts credentials),
and model. Matching is by class name so the helper carries no SDK
imports. Unrecognised exceptions fall through to the legacy
``f"{type(exc).__name__}: {exc}"`` shape, preserving existing grep
targets.
Patrick Buckley