A Base URL Changes Routing, Not Capabilities

A base URL chooses the request destination

The OpenAI Python SDK is a client library: it helps your Python application send requests and read responses. By default, it connects to OpenAI. Setting a different base URL tells the client to send those requests somewhere else, such as an internal gateway or an OpenAI-compatible relay.

Think of the base URL as the address of the service, rather than the name of a model. The address determines where a request travels. The model field separately identifies the model you want that service to run. Changing one does not automatically change the other.

The important distinction is routing, not capability. A different destination does not make every SDK feature available there. The receiving service must support the endpoint, request fields, and response format your application uses.

The client needs matching routing and credentials

For a newcomer, the simplest approach is to set the destination when creating the SDK client. The client accepts a base_url argument for the API address and an api_key argument for authentication. You can describe these as two separate settings: where to connect and the credential that destination accepts.

Use the exact SDK base address documented by your chosen service. Some providers require an API version segment at the end of that address. A public website address or service root is not necessarily the complete SDK destination. Do not append an individual operation endpoint yourself; the SDK adds the operation path when you call a method.

Keep credentials outside your application source, typically in environment variables or a secrets manager. When connecting to a relay, use the credential that relay issued. An OpenAI credential should not be assumed to work with another service, and you should not send it to an unfamiliar destination.

Compatible request formats still need individual checks

An OpenAI-compatible service can let you keep much of your existing application structure. Compatibility, however, is not a promise that every OpenAI endpoint works unchanged. Check the particular operation you intend to use, especially when your application needs streaming, tool calling, structured output, or image input.

Endpoint families also matter. Support for chat completions does not necessarily imply support for the Responses API. Start with a small text request using a documented endpoint and an exact model identifier from the service’s catalog. A familiar display name is not always the identifier accepted in a request.

If that request fails, separate the likely causes. Authentication errors suggest a credential problem. A missing endpoint can indicate an incorrect base address or unsupported operation. A model error may mean the identifier is unavailable. Avoid changing all three settings at once; checking one assumption at a time makes troubleshooting easier.

Relay selection adds operational and billing considerations

BestRelay, the publisher of this article, is a quality-first API relay that forwards each request to the official model named in it, without substitution. Its coverage spans the Claude, GPT, Gemini, DeepSeek, Kimi, GLM, Qwen, and Grok families, among others. That makes model selection a deliberate application setting, not something inferred from the base URL.

BestRelay works with Claude Code, Codex, Gemini CLI, and OpenAI SDKs. Those integrations share a destination service, but their setup details can differ. For the Python SDK, follow the service’s SDK-specific address guidance rather than copying settings from another tool.

Billing is pay-as-you-go, based on token counts reported by the provider, with cache reads and writes priced separately. When estimating costs, distinguish ordinary input and output usage from cached usage. Members also earn a referral commission when people they invite top up; that program is separate from request routing and model selection.

Create a BestRelay account to try a small request. Before expanding usage, confirm that the returned model information, application behavior, and usage records match your expectations. A successful connection is the beginning of validation, not the end.