client Option

info

Added in version 0.216.0

The client option provides a simple way to override which LLM client a function uses at runtime. It's a shorthand for creating a ClientRegistry and calling set_primary().

Basic Usage

Python

from baml_client import b

# Use shorthand provider/model syntax
result = await b.ExtractResume("...", baml_options={"client": "openai/gpt-4o-mini"})

# Use OpenRouter for open-source models
result = await b.ExtractResume("...", baml_options={"client": "openrouter/meta-llama/llama-3.1-70b-instruct"})

# Reference a client defined in .baml files
result = await b.ExtractResume("...", baml_options={"client": "MyCustomClient"})

# Use with_options to set for multiple calls
my_b = b.with_options(client="openai/gpt-4o-mini")
result1 = await my_b.ExtractResume("...")
result2 = await my_b.ExtractInvoice("...")

TypeScript

import { b } from "baml_client"

// Use shorthand provider/model syntax
const result = await b.ExtractResume("...", { client: "openai/gpt-4o-mini" })

// Use OpenRouter for open-source models
const result2 = await b.ExtractResume("...", { client: "openrouter/meta-llama/llama-3.1-70b-instruct" })

// Reference a client defined in .baml files
const result3 = await b.ExtractResume("...", { client: "MyCustomClient" })

// Use withOptions to set for multiple calls
const myB = b.withOptions({ client: "openai/gpt-4o-mini" })
const res1 = await myB.ExtractResume("...")
const res2 = await myB.ExtractInvoice("...")

Go

import b "example.com/myproject/baml_client"

ctx := context.Background()

// Use shorthand provider/model syntax
result, err := b.ExtractResume(ctx, "...", b.WithClient("openai/gpt-4o-mini"))

// Use OpenRouter for open-source models
result, err = b.ExtractResume(ctx, "...", b.WithClient("openrouter/meta-llama/llama-3.1-70b-instruct"))

// Reference a client defined in .baml files
result, err = b.ExtractResume(ctx, "...", b.WithClient("MyCustomClient"))

When to Use

Use the client option when you want to:

• Quickly switch between different LLM providers or models
• A/B test different models
• Use different clients for different environments (dev vs prod)
• Override the default client defined in your .baml files

Precedence

If you provide both client and client_registry, the client option takes precedence:

Python

from baml_py import ClientRegistry

cr = ClientRegistry()
cr.set_primary("ModelA")

# "ModelB" will be used, not "ModelA"
result = await b.ExtractResume("...", baml_options={
    "client": "ModelB",
    "client_registry": cr
})

TypeScript

import { ClientRegistry } from "@boundaryml/baml"

const cr = new ClientRegistry()
cr.setPrimary("ModelA")

// "ModelB" will be used, not "ModelA"
const result = await b.ExtractResume("...", {
    client: "ModelB",
    clientRegistry: cr
})

Valid Client Names

The client value can be:

1. Shorthand provider/model syntax - Use provider/model format:

   • "openai/gpt-4o-mini" - OpenAI models
   • "anthropic/claude-sonnet-4-20250514" - Anthropic models
   • "openrouter/meta-llama/llama-3.1-70b-instruct" - OpenRouter models
   • "google-ai/gemini-2.0-flash" - Google AI models

2. Client defined in .baml files - Reference by name (e.g., "MyCustomClient")

If the client name is not found, you'll get a runtime error.

Comparison with ClientRegistry

Feature	client option	ClientRegistry
Set primary client	Yes	Yes (set_primary)
Add custom clients	No	Yes (add_llm_client)
Override client options	No	Yes
Simplicity	Simple string	More complex

Use client for simple overrides. Use ClientRegistry for adding new clients or customizing client options at runtime.

See Also

• with_options - Set default options for a client
• Client Configuration - Configure clients in BAML