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All AI inference requests to ModelSwitch require a Bearer token in the Authorization header: 
Authorization: Bearer ms-xxxxxxxx
Keys are created in Dashboard → API Keys. Each key starts with the ms- prefix and is hashed before storage — the full value is only shown once at creation.
Copy your API key immediately after creating it. Once you close the creation dialog, it cannot be retrieved. If you lose a key, deactivate it and create a new one.

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Key types

ModelSwitch uses two types of API keys for different purposes:
TypePrefixUsed for
Proxy keyms-AI inference requests (/v1/*)
Billing keymk-Account management API (/api/*)
For sending AI requests — chat completions, embeddings, model listings — use a proxy key (ms-). For programmatic access to your account (balance, usage, transactions), use a billing key (mk-). See API key management for details on creating, rotating, and deactivating keys.

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Authenticating a request

Pass your ms- key as a Bearer token: 
curl https://modelswitch.io/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer ms-YOUR_KEY" \
  -d '{
    "model": "gpt-4o",
    "messages": [{"role": "user", "content": "Hello!"}]
  }'
Store your API key in an environment variable (e.g., MODELSWITCH_API_KEY) and read it at runtime. Never hardcode keys in source code or commit them to version control.

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Error responses

Authentication and authorization failures return a JSON error body: 
{
  "error": {
    "message": "Missing or invalid Authorization header",
    "type": "authentication_error",
    "code": "invalid_api_key"
  }
}
StatusWhen it occurs
401 UnauthorizedAuthorization header is missing, malformed, or the key is invalid or expired
403 ForbiddenKey is deactivated or its spend limit has been reached
402 Payment RequiredAccount balance is insufficient to process the request
Top up your balance in Dashboard → Billing if you receive a 402 response.

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Validate your key

Check that your key is valid and see your current balance limit with one request: 
curl https://modelswitch.io/v1/auth/key \
  -H "Authorization: Bearer ms-YOUR_KEY"
Response: 
{
  "data": {
    "label": "My Production Key",
    "usage": 0,
    "limit": 1500.50,
    "is_free_tier": false,
    "rate_limit": {
      "requests": 1000,
      "interval": "10s"
    }
  }
}
The limit field reflects your current account balance in USD. This endpoint is useful for verifying a key is active before deploying it to production.