Allow users to define custom API parameters per model (e.g. max_output_tokens)

[blinkagent]

Problem

Mux currently uses hardcoded or provider-derived values for API request parameters like max_output_tokens. Users have no way to override these on a per-model basis. For example, in streamManager.ts:

// Use the runtime model's max_output_tokens if available and caller didn't
// specify. This must be the runtime model (not the mapped metadata model)
// because max_output_tokens is a request parameter sent to the provider —
// a custom model's provider may not support the mapped model's output cap.

This is limiting for users who want to:

• Increase max_output_tokens beyond the default for long-form generation
• Set custom temperature, top_p, or other sampling parameters per model
• Pass provider-specific parameters (e.g. OpenRouter's transforms, Anthropic's top_k)
• Tune parameters differently for different use cases (e.g. coding vs. creative writing)

Proposal

Allow users to define custom API parameters on a per-model or per-provider basis in ~/.mux/providers.jsonc (or a dedicated config section). These would be merged into the API request payload when calling that model/provider.

Example config

{
  "anthropic": {
    "apiKey": "sk-ant-...",
    "modelParameters": {
      "claude-sonnet-4-20250514": {
        "max_output_tokens": 16384
      },
      "claude-opus-4-20250414": {
        "max_output_tokens": 32768,
        "temperature": 0.7
      }
    }
  },
  "openai": {
    "apiKey": "sk-...",
    "modelParameters": {
      "*": {
        "max_output_tokens": 8192
      }
    }
  }
}

Precedence

1. Explicit per-request parameters (e.g. from thinking level, tool use)
2. Per-model custom parameters from config
3. Per-provider wildcard (*) parameters from config
4. Runtime model metadata defaults (current behavior)

Related

• Depends on Allow Chat with Mux to configure models, providers, and other config #2572 (JSON Schema for config files) for safe agent-driven configuration

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Created on behalf of @ibetitsmike