> For the complete documentation index, see [llms.txt](https://docs.fastrouter.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fastrouter.ai/api-reference/auto-router.md).

# Auto Router

## Get Model Selection

> Returns the optimal model recommendation for a given prompt without making the actual completion request. Uses FastRouter's intelligent routing engine.\
> \
> \*\*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\` for automatic selection.

```json
{"openapi":"3.1.0","info":{"title":"FastRouter API Reference","version":"1.0.0"},"tags":[{"name":"Model Selection","description":"Get the optimal model recommendation for a given prompt without making the actual completion request. Uses FastRouter's intelligent routing engine."}],"servers":[{"url":"https://api.fastrouter.ai","description":"Production API"}],"security":[{"bearerAuth":[]}],"components":{"securitySchemes":{"bearerAuth":{"type":"http","scheme":"bearer","bearerFormat":"API Key","description":"FastRouter API Key. Get yours at https://fastrouter.ai\n\nFormat: `Authorization: Bearer YOUR_API_KEY`"}},"responses":{"BadRequestError":{"description":"Bad Request - The request is malformed. This could be due to missing parameters, invalid formats, or routing errors.\n\n**Note:** the 400 body shape is not uniform. Validation errors return the structured object below, but routing/model errors may instead return a plain string: `{ \"error\": \"<message>\" }`. A few endpoints (e.g. some file operations) may return a plain-text body rather than JSON.","content":{"application/json":{"schema":{"oneOf":[{"type":"object","description":"Structured validation error.","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"}}}}},{"type":"object","description":"Plain-string routing/model error.","properties":{"error":{"type":"string"}}}]}}}},"UnauthorizedError":{"description":"Invalid Credentials - Your API key is invalid, missing, or disabled. Check your credentials.\n\nNote: the 401 error body uses `code`, `message`, `param`, and `type` (there is no `status` field), and `type` is `invalid_request_error`.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"param":{"type":"string","nullable":true},"code":{"type":"string"}}}}}}}},"InternalServerError":{"description":"Internal Error - Something went wrong on our side. Retry the request, and contact support if the issue persists.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"},"status":{"type":"integer"}}}}}}}}}},"paths":{"/api/v1/model/selection":{"post":{"operationId":"getModelSelection","tags":["Model Selection"],"summary":"Get Model Selection","description":"Returns the optimal model recommendation for a given prompt without making the actual completion request. Uses FastRouter's intelligent routing engine.\n\n**Available at both:**\n- `POST https://api.fastrouter.ai/api/v1/model/selection`\n- `POST https://api.fastrouter.ai/v1/model/selection`\n\nSet model to `fastrouter/auto` for automatic selection.","requestBody":{"required":true,"content":{"application/json":{"schema":{"type":"object","required":["model","messages"],"properties":{"model":{"type":"string","description":"Set to `fastrouter/auto` for auto selection, or provide a specific model ID."},"messages":{"type":"array","description":"Conversation messages. The routing engine analyzes these to determine the optimal model.","minItems":1,"items":{"type":"object","required":["role","content"],"properties":{"role":{"type":"string","enum":["system","user","assistant"]},"content":{"type":"string"}}}}}}}}},"responses":{"200":{"description":"Model selection result","content":{"application/json":{"schema":{"type":"object","properties":{"selected_model":{"type":"string","description":"The recommended model ID"},"assigned_tags":{"type":"array","items":{"type":"string"},"description":"Tags assigned to the prompt"},"top_candidates":{"type":"array","items":{"type":"string"},"description":"Top candidate models"},"selection_strategy":{"type":"string","enum":["auto","explicit"]},"error":{"type":"string","description":"Error message if selection encountered issues"}}}}}},"400":{"$ref":"#/components/responses/BadRequestError"},"401":{"$ref":"#/components/responses/UnauthorizedError"},"500":{"$ref":"#/components/responses/InternalServerError"}}}}}}
```


---

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