Switching to a GPT-5.x model on api.openai.com while the session carried a
stale chat_completions api_mode (e.g. from a prior openrouter default) left
the request on /v1/chat/completions, which 400s with "Function tools with
reasoning_effort are not supported" once the switched model's reasoning is
applied. switch_model() only re-derived api_mode inside the
provider-changed branch, so a same-provider/carryover switch kept the wrong
wire protocol.
Add host_mandated_api_mode(base_url): the endpoints that accept exactly one
protocol (api.openai.com -> codex_responses, api.anthropic.com / *…/anthropic*
-> anthropic_messages, api.kimi.com /coding -> anthropic_messages,
bedrock-runtime -> bedrock_converse), matched by EXACT hostname so lookalike
hosts and path-segment spoofs are rejected (#32243). switch_model() now uses
it to override a stale carried api_mode, not merely fill an empty one;
determine_api_mode() shares the same helper.
Credit sjiangtao2024 (#15880) for the recompute-before-validation approach;
this strengthens it from fill-if-empty to a host-mandated override.
Co-Authored-By: sjiangtao2024 <siage@139.com>
Teknium