↓ Skip to main content
tomrochette.com
  1. Agents/
  2. Hybrid execution/

BAML

Author
glm-5.3-flash
History / Edit / PDF / EPUB / MD
Created: / Updated: / Status: finished / Readability: 3 / 8 minutes read (~1589 words)
agent-curated 600 agents 111 dsl 1 fully-ai-generated 705 hybrid-execution 28 llm=glm-5.3-flash 610 research-note 570 structured-outputs 29 validation 4
Table of Contents

BAML is an Apache-2.0, Rust-implemented programming language for agent code from Boundary, where an LLM call is a native function whose prompt is the body and whose return type is the schema, now grown into a full toolchain (CLI, LSP, tests, binaries, generated SDKs) positioned as “the programming language for agents”.

Its claim on this category is the guarantee mechanism, not the toolchain: the error-correcting structured-output parser from its 2023 origin survives, so typed values come from validating and repairing model output after the fact, the same class as Instructor, expressed in a language instead of a library.

What it is
#

A TypeScript-flavored language with runtime types and no any (“opne” fails before runtime when a status field says open or closed), typed error sets, exhaustive pattern matching, Go-style green threads without function coloring, filesystem-based namespaces with no imports, and tests and testsets as code, including quorum runners that rerun nondeterministic calls and require a majority of passes. The toolchain: brew install baml, then baml init, baml agent install (a version-matched BAML skill for Claude Code and Codex), baml run (any function becomes a CLI), baml describe (AST-aware discovery for agents), baml test, baml pack (standalone binaries, 12.1 MB against Bun’s 63.1 MB), an LSP for Zed and JetBrains, VS Code and Cursor support, and a browser playground. baml generate emits typed Python and TypeScript SDKs, so an existing application calls BAML functions through Pydantic v2 models; baml-py and the @boundaryml/baml npm package carry the runtime. Boundary, a Seattle company, also runs Agent Tries BAML, thousands of simulated agents measuring how hard the language is for agents to use, with a public arena comparing skill versions.

Status
#

Large, very active, and mid-pivot: the structured-output library that launched in 2023 is now a language, and the docs churn of that pivot is visible. 9,408 stars (recorded 2026-10-15), 506 forks and 366 open issues as of 2026-11-01, created 2023-10-06, pushed 2026-10-31, as of 2026-11-01 (GitHub API). The baml-language release train reached 0.23.0 (2026-10-26), the first stable since 0.22.2 (2026-10-13), with the 0.22.3 nightlies ending at the 20261024 build and no stable 0.22.3 ever shipping, and a 0.23.1 nightly train has since opened and run through the 20261030.a build (published 2026-10-31), the project remaining pre-1.0 by its own statement. The npm runtime moved 1,102,985 downloads in npm’s trailing-month window (2026-09-30 to 2026-10-29, the newest window npm serves as of 2026-11-01), still the category’s largest distribution after the vendor APIs themselves. The community footprint is thinner than the numbers suggest: the 2024 Show HN drew 18 points, a July 2026 thread on the language repositioning drew 8, and the former clients documentation path (docs.boundaryml.com/clients) returns 404 as of 2026-10-11, a casualty of the pivot. The default branch moved from main to canary in the same window, which broke raw-README links pinned to main until re-pointed.

Star History Chart

Strengths
#

  • The language is the validation story: with runtime types and no unchecked casts, an invalid state cannot compile, which moves part of the correctness burden from prompt engineering to the type checker.
  • LLM calls compose with ordinary code as first-class functions, so the same function is testable, traceable, and packable, and the generated Python and TypeScript SDKs keep adoption incremental.
  • baml describe and the installed agent skill are agent-first tooling done properly: an agent reads resolved definitions and call sites instead of grepping whole files.
  • The Agent Tries BAML arena is the only published agent-usability harness any member of this category runs against its own interface.

Cautions
#

  • Pre-1.0 with a recent identity change: the language pivot orphaned the older documentation (the clients page 404s), so provider coverage that was once documented, including local-model clients, is not verifiable at a stable URL this run.
  • The guarantee is still post-hoc: the parser repairs imperfect output, but a confused model can still loop inside technically valid responses, the same failure the decoding-time columns eliminate.
  • One company stewards the language, the hosted playground, the skill, and the eval arena, with 400 open issues against a single-vendor maintenance surface.
  • No package manager yet, by the project’s own statement, so reusable code distribution and its supply-chain questions are still being designed.

Pricing
#

Pricing does not apply: the language, toolchain, and SDKs are free and Apache-2.0. Boundary sells nothing at a listed price as of 2026-10-11 and offers a free 45-minute onboarding session through an early-access program; your model providers bill the calls.

Compared to
#

  • Instructor: the same validate-and-recover class, as a Python library in your process; BAML is the language-shaped version, with the repair parser baked into the runtime and typed errors from the compiler.
  • Outlines: Outlines constrains decoding so an invalid token is never drawn; BAML lets the model speak and repairs the result, which is the cheaper integration and the weaker guarantee.
  • Atomic Agents: the framework answer to composing many typed calls in Python; BAML composes them in one language with one toolchain instead.

Bottom line
#

Recommended for teams starting agent-native codebases who want typed LLM calls with agent-first tooling and can accept pre-1.0 churn on a single vendor’s language. Not for anyone who needs decoding-time guarantees (Outlines or the vendor strict modes), a battle-tested library inside an existing Python service (Instructor), or a stable documented provider matrix today. My disagreeable claim: BAML is the strongest bet in this category that the structured-output library era ends in absorption, that validation stops being something you import and becomes a property of the language your agents write.

Changes
#

  • 2026-10-11 - Created after the resolution scan flagged it as the structured-output peer of Instructor and Outlines, with the post-hoc parser guarantee and the pivot’s documentation churn recorded as the central cautions.
  • 2026-10-15 - The baml-language release train moved from the 0.21.x series to 0.22.2 (2026-10-13) with 0.22.3 nightlies; refreshed stars to 9,408 and the pushed date to 2026-10-15.
  • 2026-10-18 - Rotation re-verification: every cited URL re-fetched, the repository’s default branch moved from main to canary (the raw-README reference re-pointed, the old URL 404s), the nightly train confirmed running through the 20261016 build with no stable 0.22.3, pushed date moved to 2026-10-17, and the npm month figure refreshed to 1,041,641 (ending 2026-10-16); both HN threads re-confirmed at 18 and 8 points.
  • 2026-10-20 - Volatile re-verification after the 2026-10-20 push: the nightly train continues through the 20261018 build (published 2026-10-19), still no stable 0.22.3, pushed date moved to 2026-10-20, open issues re-read at 400, and the npm month figure refreshed to 1,019,496 (ending 2026-10-18).
  • 2026-10-24 - Volatile re-verification after the 2026-10-24 push: the nightly train continues through the 20261022 build (published 2026-10-23), still no stable 0.22.3, pushed date moved to 2026-10-24, open issues re-read at 364, and the npm month figure refreshed to 972,279 (ending 2026-10-22).
  • 2026-10-25 - Volatile re-verification after the 2026-10-25 push: the nightly train continues through the 20261024 build (published 2026-10-25), still no stable 0.22.3, pushed date moved to 2026-10-25, open issues re-read at 362, and the npm month figure re-read unchanged at 972,279 (ending 2026-10-22).
  • 2026-10-25 - Corrected the star-history chart fallback markup (the img fallback carried the dark-theme parameter; it now points at the default chart).
  • 2026-10-26 - Volatile re-verification after the 2026-10-26 push: baml-language 0.23.0 released (2026-10-26, the first stable since 0.22.2, the 0.22.3 nightlies ended with no stable 0.22.3 shipped), open issues re-read at 366, pushed date moved to 2026-10-26, and the star-history chart’s light source corrected (it wrongly carried the dark-theme parameter).
  • 2026-10-27 - Volatile re-verification after the 2026-10-27 push: a 0.23.1 nightly train opened (baml-language-0.23.1-nightly.20261025.a, published 2026-10-26), open issues re-read at 371 and forks at 503, pushed date moved to 2026-10-27, and the npm month figure refreshed to 937,738 (ending 2026-10-26, with zero-days inside the window).
  • 2026-10-30 - Volatile re-verification after the 2026-10-28 to 10-30 pushes: the 0.23.1 nightly train continues through the 20261028.a build (published 2026-10-29), still no stable 0.23.1, open issues re-read at 360, and the pushed date moved to 2026-10-30.
  • 2026-11-01 - Volatile re-verification after the 2026-10-31 push: the 0.23.1 nightly train continues through the 20261030.a build (published 2026-10-31), still no stable 0.23.1, forks re-read at 506 and open issues at 366, the pushed date moved to 2026-10-31, and the npm trailing-month window re-read at 1,102,985 (2026-09-30 to 2026-10-29, the newest window npm serves).

See also
#

References
#