Baml Core

baml

BAML is a statically-typed, expression-oriented language — TypeScript with snake_case methods, name: type, fields, enums, interfaces, generics, closures, optional chaining, backtick strings with ${...} interpolation, .to_string() on any value, a real stdlib. And a declarative DSL for LLM calls (function … { client: prompt: }, test) that desugars into it, so a model's structured output is just a typed return value.

The CLI is the documentation. Discover via baml describe:

brew install baml                # CLI binary: `baml`

baml init                        # new project (baml.toml + baml_src/)
baml help <command>              # CLI options and examples
baml describe baml.json          # reference for any module/type/method/signature/keyword
baml describe Array --budget 120 #   (Array, String, Map, assert, match, patterns, spawn, python, ...)
                                 #   ends with "N more lines"? re-run with `--budget <N>`
baml check                       # compile-check the project
baml run -e 'expr'               # eval an expression; fast feedback + syntax check
baml test --list && baml test    # run every test/testset block
baml fmt baml_src/main.baml      # canonicalize the project's formatting

baml describe <name> prints the full source body of stdlib functions — the fastest way to verify behavior, and the only path for embedded builtins like assert (no on-disk file). Pure functions need no test/client — check them with baml run -e 'add(2, 3)'.

Don’t describe APIs already demonstrated below unless you run into some errors. You can start based off the examples and use it if you run into more errors or you want actual stdlib details.

Mostly it behaves like JavaScript/TypeScript, with very similar syntax — but BAML is more sound/strict.

Best practices and info

• LLM function = typed return. The RETURN TYPE is the schema the model must produce (class, enum, literal union, string[], T?). Structured output is just a typed value — hand it to ordinary code.
• Prompts are backtick strings with ${...} interpolation. Write prompt: … ${arg} …, and always inject ${ctx.output_format} for a structured return. Escape with \`` / ${`; nest with extra backticks.
• Clients are values, not config blocks. client Fast = openai.ResponsesClient.new(model = "…", api_key = env.OPENAI_API_KEY); — the old client<llm> Name { provider: …, options: {…} } block is removed. Anything implementing ai.Client works (openai.ResponsesClient, openai.ChatClient, anthropic.AnthropicClient, …; constructor parameters differ by provider). api_key and base_url accept ai.Credential: a literal string, null for the provider default, or a late-bound env.NAME reference resolved at request time. Compose reliability by wrapping: ai.clients.Retry.new(inner = c, max_attempts = 3) and ai.clients.RoundRobin.new(members = […]) have .new, but ai.clients.Fallback { members: […] } does not — construct it as a class literal. Then use client: Fast in the function, or the shorthand client: "openai/gpt-4o-mini". baml describe openai / baml describe ai.Credential / baml describe ai.clients.
• Shape the schema with field attributes. @description("…") adds a /// hint the model sees in ${ctx.output_format}; @alias("name") renames the emitted JSON key. Chain: tags: string[] @alias("labels") @description("…").
• Test the pure code, not the model. Unit-test orchestration/post-processing on literal data with assert.*. Calling an LLM function in a test makes a real request — not an offline test. (f$parse/f$render_prompt/f$build_request exist for debugging.)
• Build strings with interpolation, not coercion. score=${n} stringifies any value (implicit .to_string()); call .to_string() for the string alone. + needs both sides already strings ("n=" + 5 won't compile).
• catch for some, catch_all for all. expr catch (e) { baml.errors.ParseError => fallback } handles a specific error; expr catch_all (e) { _ => fallback } is exhaustive — for a workflow top / entrypoint. Errors propagate implicitly; callers needn't re-declare. Raise with throw baml.errors.InvalidArgument { message: "…" } (error types are the builtin baml.errors.* classes — InvalidArgument/ParseError/Io/Timeout/…; baml describe baml.errors); annotate a fallible signature with -> T throws ErrType. Prefer a typed result union (type R = Ok | Err) over throwing for ordinary control flow.
• Interfaces = shared behavior + dynamic dispatch. interface I { function m(self) -> T } (methods may have default bodies); a class opts in via implements I { … }; a value typed I (or I[]) dispatches to the implementor at runtime. Interfaces can also declare associated types and generic bounds. baml describe interfaces.
• Pattern matching. match (v) { … } over values/types; arms are pattern => expr — literals, let x: T (bind + narrow), class destructure T { f: let y }, or-patterns A | B, guards … if cond, _; must be exhaustive. Also v is T → bool (narrows) and if let x: T = v { … } else { … }. baml describe patterns.
• Concurrency = green threads. spawn { … } returns a Future; await collects it. Combine many with baml.future.all / all_complete / race / any (JS Promise.*). Configure a spawn with a with clause: spawn with baml.spawn.options(group = g, cancel = tok, detach = true) { … } — baml.spawn.TaskGroup.new(n) caps concurrency (excess spawns queue FIFO), a baml.spawn.CancelToken cancels cooperatively. baml describe spawn / baml describe baml.future.
• Resource safety — defer, cleanup, catch (e, ctx). defer { … } runs a block at scope exit, LIFO, on every path (return / throw / fall-through) — like Go. A class method named function cleanup(self) -> void is a finalizer: it runs at most once per instance whether you call it, defer it, or the GC reclaims it. catch (e, ctx) binds an ErrorContext alongside the error — an error thrown while handling another chains onto it, so ctx.root_cause() / ctx.cause walk back to the original failure and ctx.to_string() renders the whole chain (Python __context__-style). while let PATTERN = expr { … } loops until the pattern fails (e.g. draining a T?-returning .pop()).
• Call BAML from Python / TS. Declare a [generator.<name>] in baml.toml, run baml generate, then import the typed baml_sdk. Install + usage: baml describe python / baml describe typescript / baml describe baml_sdk.
• Safe access over indexing. Subscript panics on a missing index/key; use .at(i)/.get(k) (→ T?), reach through with ?., default with ?? (parenthesize: (m.get(k) ?? 0) + 1).
• Stdlib methods are snake_case, called on a value. Some return new, some mutate in place, a few do both (sort_by_key sorts the receiver and returns it) — to read the docs, baml describe <word/type/identifier/keyword/etc>.
• Class fields name: type,; construct Type { field: val }. Methods take a bare self; factories are free functions. Fields are mutable (like TS): obj.field = v and obj.field += n work, and a self method can mutate in place — a side-effect method returns void. Classes are reference types: find/at(i)/subscript return a live alias, not a copy, so mutating the result mutates that element inside the array (xs.find(p)?.n += 1 updates xs), and a class passed to a function can be mutated by the callee. Struct-update spread is supported: User { ...u, tier: Tier.Free }. Empty classes are legal (class Marker {}) — handy as union variants. Enums are plain variants — no methods, no associated data (E.A.foo() won't compile); put behavior in free functions that match. enum E { A, B }, access E.A.
• Blocks are expressions — last expression is the value (no ;); return x; for early exit. A side-effect-only function returns void; its block's unit value is null. for (let x in xs) iterates VALUES; while (cond) { … } loops. Closures (x) -> { ... } infer param/return from context (annotate (x: T) -> R only when ambiguous; the -> is required). .map/.filter return arrays directly (no .collect()). Empty map needs a type: let m: map<string, int> = {};.
• No ternary — if/else is the expression. There's no cond ? a : b; if (cond) { a } else { b } is an expression that returns a value, so assign it directly: let label = if (x > 3) { "big" } else { "small" };. Each branch is a block whose last expression is its value (no return). Chain with else if, and pair with if let PATTERN = expr { … } else { … } for bind-and-narrow.
• Conditions use truthiness. if, while, match guards, &&, ||, and ! accept any value. Falsy values are false, null, numeric zero, empty strings, empty arrays/maps, and empty bytes; everything else is truthy. && and || still return bool, not an operand value. A truthy optional narrows in the taken branch.
• Arrays have a JS-like method set — map/filter/filter_map/reduce/find/some/every/flat_map/slice/concat/join/includes/length(), plus in-place push/pop/shift/unshift/sort_by/sort_by_key. Most take closures that can throws. baml describe Array gives more info.
• Local let bindings are reassignable (x = x + 1) — no mut keyword (it's TS let, not Rust); there's no const either.
• Args: defaults with =, keyword calls with = (never :). Declare a default in the signature: function f(a: int, b: int = 10); call f(1) or f(1, b = 2). A defaulted param must be passed by name — f(1, 2) is an error (defaulted parameter 'b' must be passed by name). Any param (even required) may be passed by name (f(a = 1, b = 2)), and you can skip a middle default to set a later one (f(1, c = 9)). Keyword syntax is name = value; name: value won't parse (: is for types/fields). T? does NOT make an argument optional — unlike TS b?: T, a b: T? param is still required (you must pass null, else expected N argument(s), got …); add = null to make it omittable. Built-ins follow this: baml.http.fetch(url, timeout = baml.time.Duration.from_seconds(10)).
• Where it diverges from TS (the silent traps): arithmetic is type-driven, not TS-style. int / int is truncating integer division (285 / 100 == 2, NOT 2.85) and % is the remainder (285 % 100 == 85); this compiles fine and just gives a quietly-wrong number, so it's the highest-value gotcha. Mix in a float to get float division (285 / 100.0 == 2.85, 285.0 / 100 == 2.85); any mixed int/float op promotes to float (5 + 2.0 == 7.0). There is no .to_float() — convert an int with n * 1.0 (or divide by a float). An int result does not auto-coerce to float on assignment (let x: float = 285 / 100 is a compile error). + is numeric-only: string concat needs both sides already string ("n=" + 5 won't compile — use ${...} interpolation). Comparisons (==, <, …, structural ==) and &&/||/! are TS-like.
• Tests: lone test "name" { ... } (no wrapper); testset only GROUPS. Asserts (only 6): assert.equal/approx_equal/is_true/is_type/not_null/contains. assert.equal compares structurally (deep, across classes/arrays/maps) — and so does plain ==, which is the bool form. assert.equal is exact on floats; use assert.approx_equal(actual, expected, eps) for computed ones. Last assert: no trailing ;. Canonical IDs are root::TestName for a top-level test and root::Testset::TestName inside a testset. Run one with baml test -i "root::TestName" (-x to exclude); baml test --list prints valid selectors.
• Namespaces = ns_* directories, no imports. A folder ns_<name>/ under baml_src/ puts its files in namespace <name>; files in baml_src/ itself are the root namespace (nesting stacks — ns_a/ns_b/ → root.a.b; non-ns_ folders don't namespace). Same namespace = same scope: files share definitions with no import. To reach another namespace, use the absolute path root.<ns>.<name> (for example, root.llm.Response). Run a target by its namespace-relative path (baml run agent.main), but baml run -e evaluates in the root scope, so reach in with the absolute form: baml run -e 'root.agent.main()'. Keep namespaces as flat as practical, like Go packages.
• Run baml fmt when you're done with a feature.
• BAML functions, methods, and types are accessible from other languages. Run baml describe baml_sdk for setup instructions. The selected toolchain must match the installed language bridge; use baml toolchain use <channel-or-version> to select it. Keep AI-related things and workflow logic in BAML as much as possible.
• BAML has log.info(..), log.debug(..), log.warn(..), and log.error(..).
• baml pack can create a binary.
• Use backticks instead of the removed #" "# string syntax.

For anything not shown (signatures, niche stdlib, advanced features), run baml describe <name> — the CLI is the docs; never guess the stdlib.

Example 1 — LLM DSL + glue (schema, attributes, client, backtick prompt, post-processing)

// The return type IS the schema; @description/@alias shape what the model sees.
enum Priority { High, Low }

class LineItem {
    name: string,
    amount: float,
    priority: Priority,
}

class Invoice {
    vendor: string @alias("seller"),
    status: "draft" | "final" @description("invoice state"),
    items: LineItem[],
    note: string?,
}

// Clients are ordinary values implementing `ai.Client` (no `client<llm> { }` block).
// `env.OPENAI_API_KEY` is a late-bound reference resolved when a request runs.
client Fast = openai.ResponsesClient.new(model = "gpt-4o-mini", api_key = env.OPENAI_API_KEY);
// Compose reliability by wrapping a client. `Retry`/`RoundRobin` have `.new(...)`;
// `Fallback` has no `.new`, so construct it as a class literal.
client Reliable = ai.clients.Retry.new(inner = Fast, max_attempts = 3);
client Safe = ai.clients.Fallback { members: [Reliable, anthropic.AnthropicClient.new(model = "claude-sonnet-5")] };

function Extract(raw: string) -> Invoice {
    client: Reliable                              // or shorthand: "openai/gpt-4o-mini"
    prompt: `Extract the invoice. ${ctx.output_format}\n${raw}`
}

// Structured output is just a typed value - hand it to ordinary code.
// Closure params/return infer from context; only the -> is required.
// `min_amount` has a default - omit it, or pass it by name (`min_amount = 10.0`).
function high_total(inv: Invoice, min_amount: float = 0.0) -> float {
    inv.items
        .filter((i) -> { i.priority == Priority.High && i.amount >= min_amount })
        .reduce((a, i) -> { a + i.amount }, 0.0)
}

test "post-process a literal Invoice - no model call" {
    let inv = Invoice {
        vendor: "Acme", status: "final", note: null,
        items: [LineItem { name: "srv", amount: 900.0, priority: Priority.High },
                LineItem { name: "mug", amount: 12.0, priority: Priority.Low }],
    };
    assert.equal(high_total(inv), 900.0);                        // default min_amount = 0.0
    assert.equal(high_total(inv, min_amount = 1000.0), 0.0);     // keyword arg
    let parsed = Extract$parse(`{"seller":"Acme","status":"draft","items":[],"note":null}`);
    assert.equal(parsed.vendor, "Acme")
}

Example 2 — the language (methods, interpolation, closures, maps, json, errors). Mutations work like Typescript

// BAML is a real language - no LLM here.
enum Tier { Free, Pro }

class User {
    name: string,
    tier: Tier,
    score: int,
    // method (bare self) + ${} interpolation (implicit .to_string() on the int)
    function label(self) -> string { `${self.name.to_upper_case()}:${self.score}` }
    // fields are MUTABLE like TS: assign / += on self in place; a side-effect method returns void
    function celebrate(self) -> void { self.score += 100 }
}

function make_user(name: string, score: int) -> User { User { name: name, tier: Tier.Pro, score: score } }

// inferred closures; sort_by_key; optional chaining + ?? over a possibly-null .at
function top_label(us: User[]) -> string {
    us.sort_by_key((u) -> { 0 - u.score }).at(0)?.label() ?? "none"
}

// map<string,int> via for-let-in; .get ?? default; explicit .to_string()
function tier_counts(us: User[]) -> map<string, int> {
    let counts: map<string, int> = {};
    for (let u in us) { let _ = counts.set(u.tier.to_string(), (counts.get(u.tier.to_string()) ?? 0) + 1); }
    counts
}

function roundtrip(u: User) -> User { baml.json.from_string<User>(baml.json.to_string(u)) }

// `catch` with a typed arm handles ONE specific error
function safe_parse(s: string) -> int { baml.Int.parse(s) catch (e) { baml.errors.ParseError => -1 } }

function truthy_label(s: string?) -> string { if (s) { "set" } else { "empty" } }

test "lang" {
    let us = [make_user("ada", 90), make_user("bo", 30)];
    log.info(us);
    assert.equal(top_label(us), "ADA:90");
    assert.equal((tier_counts(us).get("Pro") ?? 0), 2);
    let kit = make_user("kit", 5);
    kit.celebrate();            // mutate in place
    kit.tier = Tier.Free;       // direct field assignment
    assert.equal(kit.score, 105);
    let upgraded = User { ...kit, tier: Tier.Pro };
    assert.is_type<User>(upgraded);
    assert.equal(upgraded.tier, Tier.Pro);
    assert.equal(285 / 100, 2);
    assert.equal(roundtrip(make_user("zoe", 7)).name, "zoe");
    assert.equal(safe_parse("42"), 42);
    assert.equal(safe_parse("x"), -1);
    assert.equal(truthy_label("value"), "set");
    assert.equal(truthy_label(""), "empty");
    assert.equal(truthy_label(null), "empty")
}

Example 3 — interfaces (shared behavior, default method, dynamic dispatch)

interface Animal {
    function sound(self) -> string
    function describe(self) -> string { `${self.sound()}!` } // default method
}

class Dog {
    name: string,
    implements Animal { function sound(self) -> string { "woof" } }
}

class Cat {
    indoor: bool,
    implements Animal {
        function sound(self) -> string { "meow" }
        function describe(self) -> string { `quiet ${self.sound()}` } // override
    }
}

// an Animal[] holds any implementor; calls dispatch dynamically
function chorus(animals: Animal[]) -> string {
    animals.map((a) -> { a.describe() }).join(" ")
}

test "interfaces" {
    let animals: Animal[] = [Dog { name: "Rex" }, Cat { indoor: true }];
    assert.equal(chorus(animals), "woof! quiet meow")
}

Example 4 — pattern matching (match over values + types, is, if let)

class Circle { r: int }
class Rect { w: int, h: int }
type Shape = Circle | Rect

function area(s: Shape) -> int {
    match (s) {
        Circle { r: 0 } => 0,                           // literal field, no binding
        let c: Circle => 3 * c.r * c.r,                 // typed binding (matches + narrows)
        Rect { w: let w, h: let h } if w == h => w * w, // destructure + guard
        _ => 0,                                         // wildcard
    }
}

function classify(n: int) -> string {
    match (n) {
        0 => "zero",
        1 | 2 | 3 => "small",     // or-pattern
        let x if x < 0 => "neg",  // binding + guard
        _ => "big",
    }
}

// `is` -> bool (and narrows); `if let PATTERN = expr { } else { }`
function label(s: Shape) -> string {
    if (s is Circle) {
        "circle"
    } else if let r: Rect = s {
        `rect ${r.w}x${r.h}`
    } else {
        "?"
    }
}

test "patterns" {
    assert.equal(area(Circle { r: 2 }), 12);
    assert.equal(area(Rect { w: 3, h: 3 }), 9);
    assert.equal(classify(2), "small");
    assert.equal(classify(-5), "neg");
    assert.equal(label(Circle { r: 1 }), "circle");
    assert.equal(label(Rect { w: 2, h: 4 }), "rect 2x4")
}

Example 5 — resource safety + structured concurrency (defer, cleanup, ErrorContext, spawn options, futures, while-let)

class DbConn {
    log: string[],
    // `cleanup` is a magic method (recognized by name): runs at most once per
    // instance - whether called explicitly, deferred, or reclaimed by the GC.
    function cleanup(self) -> void { self.log.push("closed") }
}

function use_conn() -> string[] {
    let c = DbConn { log: [] };
    {
        defer { c.cleanup() }              // deferred blocks run LIFO at scope exit,
        defer { c.log.push("commit") }     // on every path (return / throw / fall-through)
        c.log.push("query")
    }
    c.log                                  // ["query", "commit", "closed"]
}

function fail_a() -> string { throw baml.errors.Io { message: "disk full" } }
function fail_b() -> string { throw baml.errors.Timeout { message: "retry timed out" } }

// `catch (e, ctx)` binds the error AND its ErrorContext; throwing while handling
// chains the new error onto the one being handled, so root_cause() walks to the origin.
function root_cause_demo() -> string {
    fail_a() catch (e, ctx) {
        _ => fail_b() catch (e2, ctx2) {
            _ => match (ctx2.root_cause().error) {      // ctx.to_string() renders the full chain
                let io: baml.errors.Io => io.message,   // "disk full" - the original cause
                _ => "unknown",
            }
        }
    }
}

// spawn returns a Future; baml.future.all/all_complete/race/any combine many (JS Promise.*).
function concurrent_squares(xs: int[]) -> int {
    let futures = xs.map((x) -> { spawn { x * x } });   // all run concurrently
    let squares = await baml.future.all(futures);
    squares.reduce((a, b) -> { a + b }, 0)
}

// Configure a spawn with `with baml.spawn.options(...)`: a TaskGroup caps concurrency
// (excess spawns queue), a CancelToken cancels cooperatively, detach reparents the task.
function rate_limited() -> int {
    let g = baml.spawn.TaskGroup.new(2);
    let a = spawn with baml.spawn.options(group = g) { 1 };
    let b = spawn with baml.spawn.options(group = g) { 2 };
    (await a) + (await b)
}

// while-let drains an optional-returning source; the loop exits when the pattern fails.
function drain(stack: string[]) -> string {
    let out = "";
    while let item: string = stack.pop() { out = out + item; }
    out
}

test "resources + concurrency" {
    assert.equal(use_conn(), ["query", "commit", "closed"]);
    assert.equal(root_cause_demo(), "disk full");
    assert.equal(concurrent_squares([1, 2, 3]), 14);
    assert.equal(rate_limited(), 3);
    assert.equal(drain(["a", "b", "c"]), "cba")
}

Concurrency — green threads (parallelize LLM / HTTP calls)

spawn { … } launches a background task; await collects it; baml.future.all(list) awaits many in order. Run baml describe spawn for the details.

function fetch_all(urls: string[]) -> string[] {
    // each request runs concurrently; await all results in order
    await baml.future.all(urls.map((u) -> { spawn { baml.http.fetch(u).text() } }))
}

Workflow: sketch → baml run -e / baml check constantly → baml describe anything unfamiliar → baml test.

Also just start writing some code. This is plenty of information already. Pretend you're writing some typescript but with this new syntax etc.

BAML workflow visualizer annotations

Use '//#' to add comments that will show up in the BAML visualizer. Useful for annotating branches, general flow of the program. When you write baml code you should add some of these in general flow of the program. No need to annotate everything. e.g.

function hello() -> string {
    //# Start loading data
    let greeting = "hello";
    //# Return the result
    greeting
}