Ash — Feature Reference

What is Ash?

Ash is a task runner for AI agents — deterministic, repeatable, no scripting required. Point Ash at a directory of markdown files and it walks the tree in sorted order, sending each file to your configured AI agent. One task per file.

tasks/
├── 1-plan/
│   ├── 01-requirements.md
│   └── 02-architecture.md
├── 2-implement/
│   ├── 01-auth.md
│   ├── 02-api.md
│   └── 03-tests.md
└── 3-review/
    └── 01-code-review.md

→ ash tasks/

When you need more than one-shot prompts — chaining, conditionals, parallelism — write an .ash script. Start simple. Grow as needed.

Installation

npm

npm install -g @ash-lang/cli

On npm install, the correct platform binary is downloaded from GitHub Releases automatically.

Prebuilt binaries

Download from GitHub Releases.

Place the binary alongside ash.js in the npm package directory if the automatic download fails.

Requirements

Rust 1.70+ (for building from source)
Node.js (for npm install)

Quick Start

Configure your agent

Create ash.yml in your project root:

default_agent: opencode

Or set it per-run:

ash --agent opencode tasks/

Run your first project

my-project/
├── ash.yml
└── tasks/
    ├── 1-init/
    │   └── 01-setup.md
    └── 2-feature/
        └── 01-add-login.md

ash my-project/tasks/

Ash prints each task and its result as the agent completes it. Tasks that return a non-zero exit code are marked as failures.

Skip failures, keep going

ash --continue-on-error tasks/

Validate without running

ash --check tasks/

See what would run

ash --dry-run tasks/

Writing Tasks

Each .md file is a standalone prompt sent to the agent. Optional YAML frontmatter sets per-task config (agent, model, etc.). The filename sets the order — Ash sorts alphanumerically. Subdirectories group related tasks.

tasks/1-plan/01-requirements.md:

---
agent: opencode
---

Write a requirements document for a login system that supports:

- Email/password authentication
- OAuth with Google and GitHub
- Session management with JWT
- Rate limiting on failed attempts

tasks/2-implement/01-auth.md:

---
agent: claude-code
---

Implement the login API endpoint. Cover:

- POST /auth/login — validates email/password, returns JWT
- POST /auth/register — creates user, sends verification email
- POST /auth/refresh — refreshes expired tokens

Use the requirements from tasks/1-plan/01-requirements.md.

Frontmatter reference

Key	Values	Default	Description
`agent`	agent name	—	Override agent for this task
`model`	model name	—	Override model for this task
`on_fail`	`stop`, `continue`	`stop`	Behavior when the task fails
`compact`	directive	—	Context window strategy for this task

Scripting with .ash Files

When you need more than one-shot prompts — chaining, conditionals, parallelism — write an .ash script:

#!opencode
do "Write a hello world program in Rust"
print stdout

ash hello.ash

Agent shebang

Declare the agent with a shebang:

#!opencode:1.0

do "Review src/" with opencode

Default agent

Set the default agent for all subsequent do calls with use:

use opencode
do "Review src/"                # uses opencode

use claude-code
do "Refactor the implementation" # uses claude-code

The agent can still be overridden per-call with do "..." with <agent>.

Language overview

use opencode                             # set default agent

do "Review src/"                         # call an agent

fn rollback(FILE) {                      # functions
  exec git restore "${FILE}"
  do "Summarize what has been done"
}

for FILE in FILES {                      # loops, conditionals, retry
  try {
    do "Fix bugs in ${FILE}"
  } fail {
    print "failed on ${FILE}"
  } upto 3
}

Directory Mode

Pass a directory to walk its task tree in numeric-prefix order:

ash ./tasks
ash ./tasks --dry-run
ash ./tasks --continue-on-error

Task tree layout

tasks/
├── 01-intro.md
├── 02-setup/
│   ├── 01-db.md
│   ├── 02-config.ash
│   └── 03-seed-data.md
├── 03-build.ash
└── 04-review.md

Files get a numeric prefix (01-, 02-, etc.). Subdirectories group related tasks — the walker recurses into them in order.

Execution order above: 01-intro.md → 02-setup/01-db.md → 02-setup/02-config.ash → 02-setup/03-seed-data.md → 03-build.ash → 04-review.md

File types

Type	Extension	How it's handled
Markdown	`.md`	Content is sent as a prompt to the configured agent. `${VAR}` interpolation is resolved from the evaluator scope.
Ash script	`.ash`	Parsed and executed as an ash script. Has full access to variables, functions, control flow — including `do` statements.

Numeric prefix

Files and directories must start with a numeric prefix (01-, 02-step-, etc.) to be included. Files without a prefix are silently skipped.

Duplicate prefixes at the same level (e.g., 01-foo.md and 01-bar.md in the same directory) are reported as an error. Each prefix must be unique within its directory to maintain a deterministic ordering contract.

Frontmatter (`.md` files)

Markdown tasks can set per-task configuration with YAML frontmatter:

---
agent: claude-code
model: sonnet
compact: truncate 32000
on_fail: continue
---

# Task Title

The prompt content for the agent goes here...

Key	Values	Default	Description
`agent`	agent name	—	Override agent for this task
`model`	model name	—	Override model for this task
`compact`	directive	—	Context window strategy for this task
`on_fail`	`stop`, `continue`	`stop`	Behavior when the task fails

Shebang (`.ash` files)

Ash scripts set their agent via shebang, same as standalone scripts:

#!opencode:1.0:sonnet

do "Fix the migration script"
if $? != 0 {
  do "Rollback changes" with rollback-agent
}

The shebang's engine and model become the defaults for do statements inside the script. Individual do calls can override with with/using clauses.

CLI flags

Flag	Description
`--dry-run`	Print the task list without executing
`--continue-on-error` / `-k`	Keep running after a task fails
`--check` / `-c`	Validate syntax without executing
`--agent <name>:<model>`	Default agent and model for all tasks

Skip behavior

The following files and directories are silently skipped during the walk:

Hidden files and directories (starting with .)
Files without a numeric prefix
Non-task file extensions (not .md or .ash)
Empty markdown files (no content after frontmatter)
Empty ash scripts (no statements)

Directory orchestration inside scripts

The tree walker can also be invoked from within an ash script using do @"path/":

do @"tasks/" with opencode

When the @ path points to a directory, the same tree walker runs — walking the directory, discovering tasks, and executing them in order. See File-based Prompts.

Ash Script Language

Ash scripts (.ash) provide a full scripting language for composing AI agents into automated workflows.

Agent calls

The do statement invokes an agent:

do "Fix the login bug in src/auth/login.ts" with opencode

Clause	Example	Description
`with <agent>`	`with opencode`	Which agent to invoke
`using <model>`	`using sonnet`	Override the AI model
`in <path>`	`in "/workspace"`	Working directory for the agent
`compact <directive>`	`compact "truncate 32000"`	Context management

CLI flags

ash script.ash                     # run a script
ash tasks/                         # run directory mode
ash --check script.ash             # validate syntax only
ash -c script.ash                  # shorthand for --check
ash --agent opencode:sonnet        # default agent and model
ash tasks/ --dry-run               # preview without executing
ash --config path/to/ash.yml      # custom config file path
ash --continue-on-error            # keep going after task failure
ash -k                             # shorthand for --continue-on-error

Variables

Assign with = and reference by name:

NAME = "world"
COUNT = 42
MSG = "hello ${NAME}"

Built-in variables

Variable	Set by	Description
`$?`	`do`, `exec`, fn calls	Exit code (0 = success)
`stdout`	`do`, `exec`	Stdout from the last call
`stderr`	`do`, `exec`	Stderr from the last call
`error`	eval try body failure	Error message when a body statement fails
`report`	eval try evaluator block	Captured `print` output from the `evaluate with` block
`$_attempt`	`evaluate` body	Current attempt number, 1-indexed
`$_max_attempts`	`evaluate` body	Total allowed attempts
`$_feedback`	`evaluate` body	Findings from the previous evaluation iteration
`$_evaluator_output`	`evaluate` evaluator	Full stdout from the evaluator
`$score`	`evaluate` post-loop	The final score (accepted or last attempted)
`$accepted`	`evaluate` post-loop	Whether the threshold was met

The $ prefix is supported for backward compatibility ($FILES, $DIFF). Bare names without $ are preferred.

Control Flow

All bodies use { }:

If / else

if $? == 0 {
  print "build passed"
} else if SCORE > 0.8 {
  print "good enough"
} else {
  print "fail"
  exit 1
}

For loops

for FILE in FILES {
  exec eslint FILE
}

While loops

while RETRIES < 3 {
  do "Fix remaining issues"
  RETRIES = RETRIES + 1
}

Try blocks

Binary try — retries on failure, runs an optional fail block:

try {
  do "Deploy to staging"
} fail {
  print "deployment failed, rolling back"
} upto 3

Evaluate blocks

evaluate is a top-level statement that runs a body, evaluates the result using an external evaluator (agent, function, or command), and retries until a numeric score threshold is met or attempts are exhausted.

evaluate {
  do "Write a blog post about Rust"
} by @"reviewer.md" with opencode
   accept by 85
   upto 5

The by clause selects the evaluator — one of three forms:

Evaluator	Syntax	Description
Agent	`@"prompt.md" [with <agent>] [using <model>]`	An agent reviews the output and produces a score
Function	`fn_name(args...)`	A user-defined function returns the score
Command	`exec "<command>"`	A shell command outputs the score

The agent evaluator receives language-injected scoring instructions so prompts don't need to specify the output format. The agent must output:

SCORE: <0-100 integer>
FINDINGS:
<actionable improvement feedback>

Per-iteration variables

Within each iteration's body, these variables are automatically set:

Variable	Type	Description
`$_attempt`	Int	Current attempt number, 1-indexed
`$_max_attempts`	Int	Total allowed attempts
`$_feedback`	String	Findings from the previous iteration (empty on attempt 1)

Post-loop variables

After the evaluate block completes, these variables are set in the parent scope:

Variable	Type	On acceptance	On exhaustion
`$score`	Int	The accepted score (>= threshold)	The last attempted score
`$accepted`	Bool	`true`	`false`
`$_evaluator_output`	String	Full evaluator stdout from the accepting run	Full evaluator stdout from the last run

Outcomes

Acceptance — score >= threshold on any attempt. The loop terminates immediately. Side effects from the accepting iteration are preserved.
Exhaustion — all attempts complete without reaching the threshold. The last iteration's side effects are preserved for inspection.
Error — body crash, evaluator failure, or score extraction failure propagates immediately. No silent retry.

Example: branching on acceptance

evaluate {
  do "Write documentation"
} by score_fn()
   accept by 80
   upto 5

if $accepted {
  print "Approved with score $score"
  exec deploy docs/
} else {
  print "Rejected (score $score), check output"
  exit 1
}

Functions

fn review(FILE, MODEL) {
  do "Review ${FILE}" with opencode using MODEL
}

fn build() {
  exec npm run build
}

Call with parentheses — required for both built-in and user functions:

FILES = get_src_files()
review("src/auth.ts", "sonnet")
build()

Scoping

Variables created inside { } are local — not visible outside. Outer variables are readable but not writable inside a function.

Functions compose naturally with try:

fn review_files(PATTERN, MODEL) {
  for FILE in exec find . -name PATTERN {
    try {
      do "Review ${FILE}" with opencode using MODEL
    } fail {
      do "Fix errors: ${stderr}" with opencode compact "truncate 16000"
    } upto 2
  }
}

Built-in Tools

Statement	Description
`exec cmd`	Run a shell command
`evaluate { } by ... accept by ... upto N`	Retry body until a score threshold is met, evaluated by an agent/function/command
`use <agent>`	Set the default agent for subsequent `do` calls
`print msg`	Print output
`include "file.ash"`	Load another ash script
`env KEY`	Read an environment variable
`exit code`	Exit the script
`return [val]`	Return from a function
`break`	Exit a for/while loop
`continue`	Skip to next loop iteration
`compact "directive"`	Set context window strategy
`within <dir> { }`	Run block in a different directory

Session Blocks

Keep multiple do calls in a shared agent session:

session {
  do "Implement the token types"
  do "Implement the value system"
  do "Refactor the AST"
}

A toggle form (begin / end) spans non-contiguous code without nesting:

session begin
do "Research approach"
do "Draft prototype"
session end

# ... intervening code ...

session begin
do "Finalize implementation"
session end

Mix toggle and block forms as long as they are not nested. session begin errs if a session is already active. session end errs without a matching begin.

Background and Parallel Execution

Run a statement in the background with &:

do "Long running analysis" &
print "main continues immediately"
wait

Run multiple statements in parallel with wait { }:

wait {
  do "Train model A"
  do "Train model B"
}
print "both models done"

File-based Prompts (`@file`)

Load a prompt from a file with variable interpolation:

do @"path/to/prompt.md"

The file is read, ${VAR} placeholders are resolved, and the result is sent to the agent.

@<path> loads the prompt from a file relative to the script:

do @skills/refactor.md with opencode
do @skills/review.md with opencode using sonnet
do @'play_step${n}.md' with opencode
do @${DIR}/prompts/task.md with opencode

Directory orchestration

If the @ path points to a directory rather than a file, the task tree walker runs instead — the same mechanism used by ash tasks/:

do @"tasks/" with opencode

This walks the directory, discovers numbered .md and .ash files, and executes them in sorted order using the specified agent. Each .md file becomes a standalone task sent to the agent. Each .ash file is executed as a script with access to the same evaluator scope.

This lets scripts recursively compose task directories:

do @"review/" with opencode
do @"fix/" with opencode using sonnet
if $? == 0 {
  do @"deploy/" with opencode
}

Individual task files can override the agent and model via YAML frontmatter, just like in directory mode.

REPL

Run ash with no arguments to enter interactive mode:

$ ash
ash> NAME = "world"
ash> print "hello ${NAME}"
hello world

Commands: .help, .clear, .vars, .exit. Up/down arrows navigate history. Multi-line blocks (if, for, session) auto-detect continuation.

Supported Agents

Agent	Description
`echo`	Built-in passthrough for testing
`opencode`	OpenCode CLI agent
`claude-code`	Anthropic Claude Code
`aider`	Aider AI pair programming

Auto-discovery

Agents are auto-discovered on your PATH. Run to refresh:

ash discover

Custom agents

Add custom CLI-based agents in ash.yml:

agents:
  my-tool:
    type: local-cli
    cmd: my-tool
    message_flag: "--prompt"
    yes_flag: "--yes"

Building from Source

git clone https://github.com/kenny1125nz/ash-lang.git
cd ash
cargo build --release

Requirements: Rust 1.70+

License

AGPLv3

VS Code Extension

Syntax highlighting, check, and run commands for Ash (.ash) agent shell scripts.

The extension provides language tooling only — the Ash runtime is a separate CLI. Install it via npm or download from GitHub Releases:

npm i -g @ash-lang/cli

Features

Syntax highlighting for .ash files — variables, strings, control flow, agent calls
Run script — executes the current .ash file with the ash runtime
Check script — validates syntax without executing

Commands

Command	Title	Description
`ash.runScript`	Ash: Run Script	Run the active `.ash` file
`ash.checkScript`	Ash: Check Script	Validate syntax of the active `.ash` file
`ash.stopScript`	Ash: Stop Script	Stop a running script

Run from the Command Palette (Ctrl+Shift+P) or right-click an .ash file in the editor.

Ash — Feature Reference

What is Ash?

Installation

npm

Prebuilt binaries

Requirements

Quick Start

Configure your agent

Run your first project

Skip failures, keep going

Validate without running

See what would run

Writing Tasks

Frontmatter reference

Scripting with .ash Files

Agent shebang

Default agent

Language overview

Directory Mode

Task tree layout

File types

Numeric prefix

Frontmatter (.md files)

Shebang (.ash files)

CLI flags

Skip behavior

Directory orchestration inside scripts

Ash Script Language

Agent calls

CLI flags

Variables

Built-in variables

Control Flow

If / else

For loops

While loops

Try blocks

Evaluate blocks

Per-iteration variables

Post-loop variables

Outcomes

Example: branching on acceptance

Functions

Scoping

Built-in Tools

Session Blocks

Background and Parallel Execution

File-based Prompts (@file)

Directory orchestration

REPL

Supported Agents

Auto-discovery

Custom agents

Building from Source

License

VS Code Extension

Features

Commands

Frontmatter (`.md` files)

Shebang (`.ash` files)

File-based Prompts (`@file`)