When typing /model deepseek-chat while on a different provider, the
model name now auto-resolves to the correct provider instead of
silently staying on the wrong one and causing API errors.
Detection priority:
1. Direct provider with credentials (e.g. DEEPSEEK_API_KEY set)
2. OpenRouter catalog match with proper slug remapping
3. Direct provider without creds (clear error beats silent failure)
Also adds DeepSeek as a first-class API-key provider — just set
DEEPSEEK_API_KEY and /model deepseek-chat routes directly.
Bare model names get remapped to proper OpenRouter slugs:
/model gpt-5.4 → openai/gpt-5.4
/model claude-opus-4.6 → anthropic/claude-opus-4.6
Salvages the concept from PR #1177 by @virtaava with credential
awareness and OpenRouter slug mapping added.
Co-authored-by: virtaava <virtaava@users.noreply.github.com>
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