Auto Router

Get Model Selection

post

https://api.fastrouter.ai/api/v1https://api.fastrouter.ai/v1

/model/selection

Returns the optimal model recommendation for a given prompt and parameters without making the actual completion request. Uses FastRouter's intelligent routing engine to analyze the prompt and suggest the best model.

Available at both:

• POST https://api.fastrouter.ai/api/v1/model/selection

• POST https://api.fastrouter.ai/v1/model/selection

Set model to fastrouter/auto to use automatic model selection. If a specific model is provided, it is returned as-is with selection_strategy: "explicit".

Authorizations

bearerAuth

AuthorizationstringRequired

FastRouter API Key. Get yours at https://fastrouter.ai

Format: Authorization: Bearer YOUR_API_KEY

Body

application/json

modelstringRequired

Set to fastrouter/auto for automatic model selection, or provide a specific model ID.

Example: fastrouter/auto

Responses

200

Model selection result

application/json

selected_modelstringOptional

The recommended model ID in provider/model format

Example: openai/gpt-4o

assigned_tagsstring[]Optional

Tags assigned to the prompt based on analysis (e.g., complexity, domain, task type)

Example: ["complex","coding","reasoning"]

top_candidatesstring[]Optional

Top 5 candidate models considered for this prompt

Example: ["openai/gpt-4o","anthropic/claude-sonnet-4-20250514","google/gemini-2.5-pro"]

selection_strategystring · enumOptional

auto when fastrouter/auto routing was used, explicit when a specific model was provided

Example: autoPossible values:

errorstring · nullableOptional

Error message if the selection process encountered issues

400

Bad Request - The request is malformed. This could be due to missing parameters, invalid formats, or CORS issues.

application/json

401

Invalid Credentials - Your API key is invalid, disabled, or your OAuth session has expired. Check your credentials.

application/json

500Error

post

/model/selection

HTTPcURLJavaScriptPython

POST /api/v1/model/selection HTTP/1.1
Host: api.fastrouter.ai
Authorization: Bearer YOUR_SECRET_TOKEN
Content-Type: application/json
Accept: */*
Content-Length: 160

{
  "model": "fastrouter/auto",
  "messages": [
    {
      "role": "user",
      "content": "Write a complex recursive algorithm to solve the Tower of Hanoi problem with visualization."
    }
  ]
}

Auto Model SelectionExplicit Model (passthrough)

{
  "selected_model": "openai/gpt-4o",
  "assigned_tags": [
    "complex",
    "coding",
    "reasoning"
  ],
  "top_candidates": [
    "openai/gpt-4o",
    "anthropic/claude-sonnet-4-20250514",
    "google/gemini-2.5-pro"
  ],
  "selection_strategy": "auto"
}