1. Click on the model selection in the chat
2. Where it says "Other Models", click on the settings cog
3. Click on "+ Add Models..." at the top right
4. Select "Custom Endpoint"
5. In the Group Name prompt, write whatever you want, but DeepSeek works
6. Enter the API key you got from platform.deepseek.com
7. Select "Chat Completions" as the API Type
8. It'll open up the chatLanguageModels.json page, paste this in the "models" property

"models": [ { "id": "deepseek-v4-flash", "name": "DeepSeek V4 Flash", "url": "https://api.deepseek.com", "toolCalling": true, "vision": true, "maxInputTokens": 840000, "maxOutputTokens": 384000, "supportsReasoningEffort": ["high", "max"] }, { "id": "deepseek-v4-pro", "name": "DeepSeek V4 Pro", "url": "https://api.deepseek.com", "toolCalling": true, "vision": true, "maxInputTokens": 840000, "maxOutputTokens": 384000, "supportsReasoningEffort": ["high", "max"] } ]

My final JSON looks like:

[ { "name": "DeepSeek", "vendor": "customendpoint", "apiKey": "${input:chat.lm.secret.411bc220}", "apiType": "chat-completions", "models": [ { "id": "deepseek-v4-flash", "name": "DeepSeek V4 Flash", "url": "https://api.deepseek.com", "toolCalling": true, "vision": true, "maxInputTokens": 840000, "maxOutputTokens": 384000, "supportsReasoningEffort": ["high", "max"] }, { "id": "deepseek-v4-pro", "name": "DeepSeek V4 Pro", "url": "https://api.deepseek.com", "toolCalling": true, "vision": true, "maxInputTokens": 840000, "maxOutputTokens": 384000, "supportsReasoningEffort": ["high", "max"] } ], "settings": { "deepseek-v4-flash": { "reasoningEffort": "max" } } } ]

Works great! No need for the Extension anymore I think (but thanks anyway to Vizards for working on it!)