The opencode-go relay defaults max_tokens to 262144 when none is sent,
but Xiami mimo-v2.5-pro only supports 131072 completion tokens — every
request 400s with "max_tokens is too large: 262144" before the agent
can do anything.
Add a get_max_tokens(model) hook on ProviderProfile (default returns
default_max_tokens) so profiles fronting multiple upstreams can vary
the cap per-model. Wire chat_completions transport through the hook.
Override on OpenCodeGoProfile with mimo-v2.5-pro=131072.
Only mimo-v2.5-pro is capped — other opencode-go models (kimi, glm,
qwen, minimax, other mimo variants) unchanged.
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