bridges

baml:bridges

Load this when the task crosses the BAML / host boundary. Host code calls BAML through the generated baml_sdk Python package; BAML reaches host capabilities through baml.sys.shell + a thin generic bridge.

Today's supported host is Python. TypeScript / Go / Ruby clients exist upstream but do not assume them here until the repo adds generators and examples for them.

1. The generator block

Generators live in a .baml file (usually baml_src/generators.baml), not in baml.toml. The block syntax is BAML, not TOML.

// baml_src/generators.baml
//
// NOTE: Generator field names and valid output_type values change across BAML
// releases. Treat this block as illustrative; copy from your checked-in
// generators.baml and run `baml generate` after edits.
generator target {
  output_type "python/pydantic"

  // Relative to baml_src/. Layout must match where `baml_sdk` is imported from.
  output_dir "../"

  default_client_mode "sync"
}

Run:

baml generate

This emits a baml_sdk Python package (typed entrypoints + types) at the configured output_dir. The package is baml_sdk, not the older baml_client name that appears in old docs.

2. The generated Python client

# Import path matches generated layout — this guide assumes the package name `baml_sdk`.
from baml_sdk.sync_client import b

ticket = b.extract_ticket("refund request text")
print(ticket.title, ticket.priority)

• Function names in Python match the BAML names (snake_case stays snake_case).
• Types come through as pydantic models (when output_type "python/pydantic").
• For async: import from baml_sdk.async_client (when default_client_mode "async" or per-call).

Use generated clients for product code. Use baml run for local debugging, scripts, demos, and CI checks.

3. Fan-out in host code

BAML's for / in is sequential. For real concurrency, fan out at the host layer:

import asyncio
from baml_sdk.async_client import b

async def triage_many(emails):
    return await asyncio.gather(*(b.classify_email(e) for e in emails))

When parallelism inside BAML is acceptable, write the loop in BAML and let it run sequentially.

4. Bridge pattern — BAML -> host capability

Use a host bridge only when the task needs a capability BAML doesn't yet provide well: databases, sockets, crypto, advanced byte processing, browser/server runtimes, vector ops, or a missing stdlib primitive.

The bridge should be thin, generic, and protocol-driven.

// ---------------------------------------------------------------------------
// BRIDGE SKETCH — inline comments go stale fast.
// Path, interpreter (python3 vs uv run vs venv), and argv layout drift from
// what this markdown shows. Read the actual bridge entrypoint + JSON schema in
// the repo before editing a real bridge.
// ---------------------------------------------------------------------------
class BridgeRequest {
  op: "query" | "execute" | "sign",
  payload: json,
}

class BridgeResponse {
  ok: bool,
  data: json?,
  error: string?,
}

function call_bridge(req: BridgeRequest) -> BridgeResponse {
  let payload_json = baml.json.stringify(req.to_json());
  let request_path = ".baml_bridge_request.json";

  baml.fs.write(request_path, payload_json);
  // Same caveat: command line below is illustrative; sync with your bridge script.
  let out = baml.sys.shell("python3 bridge.py " + request_path, null);

  if (!out.ok()) {
    throw "bridge exited " + baml.unstable.string(out.exit_code) + ": " + out.stderr.to_string();
  };

  baml.json.from_string<BridgeResponse>(out.stdout.to_string())
}

Note: baml.sys.shell(cmd, options?) returns ShellOutput { stdout: uint8array, stderr: uint8array, exit_code: int } plus .ok(). Decode the byte buffer with .to_string() before handing it to baml.json.from_string<T>. baml.json.stringify(req.to_json()) round-trips a typed request to JSON.

Bridge rules:

• Keep domain policy in BAML; keep external capability in the bridge.
• Don't hardcode one component, route, table, or test case in the bridge.
• Use JSON or a documented line protocol. Avoid fragile string scraping.
• When the host calls BAML, use baml run --output json.
• Surface bridge errors via stderr, exit code, and context.
• Add tests for BAML-side protocol classes (the bridge wire format) and bridge happy/error paths.

Inline comments in bridge sketches go stale. Always cross-check python3 vs uv run, argv shape, env wiring, and the JSON envelope against the real bridge entrypoint and tests in your repo before relying on them.

5. When to bridge (vs. keep it in BAML)

BAML handles well:

• Typed structured-output LLM calls
• Multi-stage LLM pipelines
• Parse-only tests with cached JSON fixtures
• Validation, scoring, formatting
• Line-oriented string processing
• Small CLIs and typed APIs over external systems

Reach for a bridge when:

• Long-lived network serving
• Database connection lifetime
• Filesystem watching
• Cryptography
• Binary protocols
• High-concurrency fanout before BAML exposes the needed runtime primitive
• Stdlib functionality that baml describe confirms is missing

Keep the BAML surface clean even when you bridge. The bridge should be replaceable when the stdlib catches up.

6. Common pitfalls

• TOML [[generator]] in baml.toml — wrong. Generators are a generator <name> { ... } block in a .baml file.
• Package name baml_client — that's the old name. Today it's baml_sdk.
• Stale generated client — baml generate after every schema change. Wire it into your dev loop (file watcher, pre-commit, make generate).
• Assuming TS / Go / Ruby support — Python is the only host currently exercised in this repo. Don't paste host snippets for other languages from old docs.
• Shelling out to baml run from product code when baml_sdk is available — slow and brittle. Use generated clients in product paths.
• Per-call baml.sys.shell inside tight loops — repeated shell calls dominate runtime and make tests flaky. Prefer one structured bridge call over many tiny ones.
• Hardcoding component-specific logic in the bridge — keep the bridge generic with a documented op / payload protocol; keep domain rules in BAML.
• Trying to invoke host functions from BAML via syntax — there's no extern function. BAML reaches host capability via baml.sys.shell + a generic bridge.