Client Registry

If you need to modify the model / parameters for an LLM client at runtime, you can modify the ClientRegistry for any specified function.

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result = await b.ExtractResume("...", baml_options={"client": "GPT4"})

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import os
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from baml_py import ClientRegistry
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async def run():
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    cr = ClientRegistry()
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    # Creates a new client
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    cr.add_llm_client(name='MyAmazingClient', provider='openai', options={
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        "model": "gpt-5-mini",
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        "temperature": 0.7,
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        "api_key": os.environ.get('OPENAI_API_KEY')
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    })
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    # Creates a client using the OpenAI Responses API
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    cr.add_llm_client(name='MyResponsesClient', provider='openai-responses', options={
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        "model": "gpt-4.1",
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        "api_key": os.environ.get('OPENAI_API_KEY')
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    })
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    # Sets MyAmazingClient as the client
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    cr.set_primary('MyAmazingClient')
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    # ExtractResume will now use MyAmazingClient as the calling client
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    res = await b.ExtractResume("...", { "client_registry": cr })

The set_primary Method

The set_primary method can be called with either one of the clients added to the ClientRegistry at runtime using add_llm_client or a client defined in BAML files.

You can, however, define a fallback client and then use ClientRegistry to set that client as the calling client for BAML functions. Here’s a simple example:

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function ExtractResume(input: string) -> Resume {
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  client "openai/gpt-5-mini" // Uses GPT-5 Mini by default
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  prompt #"
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    Extract from this content: {{ resume }}
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    {{ ctx.output_format }}
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  "#
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}
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// This client uses GPT-5 first and if it fails it will use Opus 4 as a fallback.
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client<llm> GptOpusFallback {
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  provider fallback
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  options {
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    strategy ["openai/gpt-5", "anthropic/claude-opus-4-1-20250805"]
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  }
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}

Then, in your code, you can use the ClientRegistry to set the GptOpusFallback at runtime, which will try to use GPT 5 and if it fails Opus 4:

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cr = ClientRegistry()
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cr.set_primary("GptOpusFallback")
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res = await b.ExtractResume("...", { "client_registry": cr })

Now the calling client will be GptOpusFallback, nothing else.

ClientRegistry Interface

Methods use snake_case in Python and camelCase in TypeScript.

add_llm_client / addLlmClient

A function to add an LLM client to the registry.

set_primary / setPrimary

This sets the client for the function to use. (i.e. replaces the client property in a function)