Changelog · Cleat

Agents, guards & HTTP servers.

The biggest release since v0.1.0. Four new compiler-enforced primitives — agent, guard, server, and first-class fn — bring safe AI agents, typed HTTP, and policy enforcement into the language core. Structured LLM output ships with compile-time schema extraction.

★ Highlights

Four new primitives — agent for tool-using LLM loops, guard for pre-flight policy checks, server for declarative HTTP, fn as a first-class keyword.
Structured LLM output — annotate a struct, get a typed value. The compiler lifts JSON Schema out of your types and deserializes responses automatically.
Three new stdlib packages — std/server, std/supervisor, std/memory — bringing the total to 15.
826 tests passing across 11 internal packages, up from 626.
VS Code extension with syntax highlighting for .cleat files, published to the marketplace.

+ Added

agent primitive with declarative model, tools, max_turns, and system prompt 1a22e30
guard primitive with Allow / Deny / Transform return types and composition 3f8b91c
server primitive — declarative HTTP routes with typed handlers and middleware 7e42a0d
fn promoted to first-class keyword with explicit effect declarations b09f112
Compile-time JSON Schema extraction from struct types for structured LLM output c44ea80
@description annotation for steering LLM field generation d91c7a2
Correlation IDs propagated automatically across tool, agent, and chain boundaries f1028bb
std/server stdlib package with JSON request/response helpers 52c6f89
std/supervisor for agent retry, timeout, tracing, and execution summaries a704d13
std/memory namespaced KV store with context injection into agents 88e2049
VS Code extension with syntax highlighting, published as cleat-lang.cleat-vscode 60a1bef
Six new example programs covering agents, guards, and HTTP servers ff3e812

~ Changed

std/llm now returns typed structured values when the call site has a known struct type 4b9a7d1
Diagnostic output for effect mismatches now includes the full needs-inheritance trace 2daf906
cleat run now loads .env files by default; disable with --no-env 71e33cc

✓ Fixed

State machine reachability analyzer no longer reports false positives on mutually recursive guards 9c40211
Tool timeout parameter now correctly propagates through supervisor retry loops 5fa8e42
Formatter preserves blank lines between @description-annotated struct fields a0bc339

The first public alpha.

We're cutting the first version of Cleat that the world gets to look at. The compiler, formatter, test runner, and standard library all ship in this release. The language is alpha — usable for real programs, not yet stable for production.

★ Highlights

First-class primitives — tool, stream, state, and chain are now compiler-enforced keywords.
Effect system — every function declares side effects with needs { ... }; the type checker enforces it.
12 stdlib packages shipped in the binary — json, http, fs, strings, math, env, log, os, io, crypto, time, llm.
Built-in tooling — cleat run, cleat build, cleat test, cleat fmt, cleat check.

+ Added

chain primitive with Ed25519 signing and configurable retention policies a3f12c7
using clause for typed mock substitution inside test blocks b7e891d
Pipe operator |> with full type inference across stages 19f0a44
String interpolation: "hello, ${name}" with arbitrary expressions cc4d3b1
Pattern matching with exhaustiveness checks for enums and Result types e2d9f8a
Maps: map[string]int with .keys(), .values(), .contains() 4b1c7e0
Default and named arguments for tool and function declarations 7a8e2f3
--no-warn flag to suppress warnings on build and test for CI f3c1098

~ Changed

Diagnostic renderer rewritten in Rustc style with squiggly underlines and span highlighting 9d4e22b
Result[T, E] error propagation operator narrowed: ? now requires explicit return type 2f6a103
Stream buffer parameter is now required (default behavior was too easy to footgun) 81bd9c4

✓ Fixed

Effect inference no longer drops needs across method calls on struct receivers c5e7811
Formatter preserves trailing commas in multi-line struct literals df21044
Test runner now exits with non-zero code on any failure (was previously 0) 6629a3e
Multi-package import resolver no longer caches stale versions across cleat run calls b8c91f7

! Breaking

Tool decoration syntax moved from comments to first-class keywords. Migration: // timeout: 30s → timeout: 30s outside the tool body block 7e3df01
std/llm now requires an explicit backend selection. Previously defaulted to mock; now must call llm.use_mock() in tests a14f9b2

State machines land.

The state primitive ships with reachability and deadlock detection. Plus the formatter learned to handle long match expressions without going feral.

+ Added

state primitive with declarative transitions, guards, and terminal states 3c9ff21
Compile-time reachability analysis for state machines (unreachable states are errors) 5a8e042
Deadlock detection across when guards and external triggers e1b3c84
std/time with deterministic test clocks and wall-clock access 22f790a

~ Changed

Formatter wraps match arms across multiple lines when total width exceeds 80 cols 7d44b91
Loader now reports cyclic imports with the full cycle path, not just the entry edge 198cda7

✓ Fixed

Type inference no longer panics on empty struct literals f44e10b
Codegen produces correct Go for Result types nested 3+ levels deep cc81023

Streams & backpressure.

First-class stream primitive with typed yields and bounded buffers.

+ Added

stream keyword with yield expressions and explicit buffer sizing 11ee03f
Stream consumers via for x in stream with backpressure semantics 8f12ab9
std/llm with mockable prompt and stream APIs a0c4d51

✓ Fixed

Lexer now correctly handles UTF-8 identifiers in error spans d7fa118
cleat fmt -w no longer rewrites files when no formatting changes are needed 3c2906e

The effect system arrives.

The single largest design change in Cleat's life so far: needs becomes a first-class part of the type system, not an annotation.

+ Added

needs { ... } clause on functions, tools, and methods — required when calling effectful operations 2bb44e0
Effect inference for top-level fn main() with helpful diagnostic when missing 61a039c
Effect aliases via type io = needs { fs, log } for grouping common combinations c4d7e88

! Breaking

All stdlib packages that touch the outside world now require effect declarations. Migration is mechanical but every existing program will need updates bb71f29

Test runner improvements.

Quality of life on the testing path — better filtering, better output, better failures.

+ Added

cleat test --filter "pattern" for selective test execution a91e220
Test output now groups by package with a per-package pass/fail summary 4f7e88c

✓ Fixed

Failing assertions now print expected vs actual values, not just "false" 7c20e93
Test discovery no longer skips files with leading underscores e88af19

¶ Docs

Added testing guide and cookbook of common patterns to docs site
Standard library reference now includes effect requirements for every function

Multi-package modules.

Cleat learns to look across files. Loader, import resolver, and the foundations of the standard library all land in this release.

+ Added

Multi-file package loader with deterministic import resolution 90a3c7f
import "std/json", "std/http", "std/strings" as the first stdlib packages 2c8e144
tool primitive with timeout and retry as comment-form decorations (later promoted to keywords in 0.1.0) 5e62a99
Public/private visibility based on identifier capitalization f1bca37

− Removed

The single-file-only mode is gone. All programs now use the multi-package model, even one-file scripts 3a78d20