Nola vs BAML

What BAML is

BAML is a domain-specific language for LLM functions: you declare classes and functions with typed inputs and outputs in .baml files, write the prompt next to the signature, and baml-cli generate emits a client (TypeScript, Python, Ruby and others) whose calls return parsed, validated values. The return type renders the output format into the prompt (ctx.output_format), and a VS Code extension with a playground and test runner comes with it.

The same task, side by side

Classify a support message into a category plus order ids, then write a reply that depends on the category.

BAML — two files: the .baml source and the TypeScript that calls the generated client.

baml_src/triage.baml

class Triage {
  category "billing" | "refund" | "fraud" | "other"
  orderIds string[]
}

function ClassifyMessage(message: string) -> Triage {
  client "openai/gpt-5-mini"
  prompt #"
    {{ _.role("system") }} You are a support classifier.
    {{ ctx.output_format }}
    {{ _.role("user") }} {{ message }}
  "#
}

function WriteReply(category: string, message: string) -> string {
  client "openai/gpt-5-mini"
  prompt #"
    {{ _.role("system") }} You are a support agent.
    {{ _.role("user") }} Reply to this {{ category }} case: {{ message }}
  "#
}

// not-checked — triage.ts, after `npx baml-cli generate`
import { b } from "./baml_client";

const triage = await b.ClassifyMessage(message);
const reply = await b.WriteReply(triage.category, message);

Nola — one file, no generated client:

classify.tsi

export type Triage = {
  category: "billing" | "refund" | "fraud" | "other";
  orderIds: string[];
};

export infer function classifyMessage(.message: string) {
  const triage = ask ..`triage the customer message`<Triage>;
  const reply = ask ..`a reply for a ${triage.category} case`<string>;
  return { ...triage, reply };
}

Where BAML is stronger

• Multi-language clients. One .baml definition serves TypeScript, Python, Ruby and more; Nola is TypeScript only.
• Tooling around the prompt. The playground, the built-in test blocks and the mature structured-output parser are a complete workflow for iterating on prompts.
• Maturity. BAML is established and widely used; Nola is 0.1.x.

Where Nola is stronger

• No second language, no codegen step. Triage is the TypeScript type you already have — usable from plain TS, refactorable with the editor, no generate between an edit and its effect. In BAML, Triage is a BAML class that TypeScript sees only after baml-cli generate.
• Context belongs to the function. Both asks in classifyMessage see .message; the reply is chained with a ${}. In BAML each function is one prompt-to-parse round trip, so the follow-up is a second function you hand the category and the message to again.
• Plain TypeScript orchestration. Loops, branches, await, call intents over your own functions — all in the same file, type-checked together by nola check.

When to pick which

• You need clients in several languages, or a prompt playground with test cases, today → BAML.
• Your stack is TypeScript and you want the typed call layer to be part of the language, with the editor seeing every ask → Nola.
• Both are honest about one thing: the output type is the contract, and the prompt lives next to it.

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