MiniMax models fall through to the Claude-oriented default prompt #41031

Description

SystemPrompt.provider() in packages/opencode/src/session/system.ts picks a system prompt by substring-matching the model id. There is a branch for Kimi (kimi.txt, the open-weight agentic prompt) but none for MiniMax, so every MiniMax model falls through to PROMPT_DEFAULT (default.txt), which is written for Claude.

default.txt assumes a model that already acts without being told to. kimi.txt — which despite the filename has no Kimi/Moonshot-specific text — instead spells out the things open-weight agentic models tend to get wrong: that tools must be used rather than described ("Code that only appears in your text response is NOT saved"), that <system-reminder> tags are authoritative (e.g. plan-mode read-only restrictions), that parallel tool calls are encouraged, and that replies must match the user's language. Running MiniMax on default.txt is a poor fit for those reasons.

Note transform.ts already treats minimax as part of the open-weight family (grouped with kimi, deepseek, qwen, glm around line 778), so prompt selection is out of step with a grouping the codebase already makes elsewhere.

Steps to reproduce

1. Run a MiniMax model (e.g. MiniMax/MiniMax-M3).
2. Add a log/inspect SystemPrompt.provider(model)[0] — it returns default.txt, not kimi.txt.

Concretely, MiniMax on default.txt is more prone to describing an edit in prose instead of calling the edit tool, and to ignoring plan-mode <system-reminder> restrictions.

opencode version

dev (reproduces at current dev tip; provider() has no minimax branch).