Halo DSL Dictionary

The formal lexicon for the Halo DSL. Reserved keywords are strictly limited to five categories: Entities, Modifiers, Flow Operators, Directives, and Native Primitives.

1. Core Entities (The Nouns)

Keywords that declare foundational infrastructure blocks. Instantiated globally and called by pipelines.

policy

Field	Value
Definition	Defines strict, non-negotiable boundaries for security, budget, or compliance.
Evaluated	At runtime before and after every model inference
Syntax	policy Name : Rule(args) & Rule(args)
Example	policy Privacy : Redact(PII) & Block(PromptInjection)

tool

Field	Value
Definition	Declares an external capability — database query, REST API, etc.
Enforced	Zero trust: an agent can only use the tools it equips
Syntax	tool Name : Provider(args)
Example	tool CoreDB : PostgreSQL(Env.DB_URI)

store

Field	Value
Definition	Persistent storage: a local vector store (VectorStore) or a conversation history (File)
Equipped	An agent that equips a VectorStore gets the entries closest to each call's input; a File store keeps its conversation
Syntax	store Name : Provider(args)
Example	store KnowledgeBase : VectorStore(namespace: "engineering")

schema

Field	Value
Definition	Declares the shape of a pipeline's output
Field types	String, Number, Integer, Boolean, Array / List (or Array<T>), Object / JSON, Any, another schema
Enforced	A pipeline declared -> Schema fails with SchemaValidationError if its output doesn't match
Example	schema Profile { name: String, tags: Array<String> }

agent

Field	Value
Definition	Instantiates an AI actor. Binds a foundation model to parameters
Modifiers	system (system prompt), equips (binds tools/stores), requires (binds policies)
Syntax	agent Name [model-ref, key: value]
Example	agent DataAnalyst [gemini-3-1-pro, temp: 0.1]

pipeline

Field	Value
Definition	Defines a pipeline: its parameters, return type, and stages
Syntax	pipeline Name(param: Type) -> ReturnType: ...
Example	pipeline ProcessClaim(event: Webhook) -> Result:

2. Relational Modifiers (The Verbs)

Keywords that define relationships and permissions between entities.

equips

Field	Value
Definition	Grants an agent permission to use specific tools or store
Enforcement	Compile-time — unequipped API calls fail before runtime
Syntax	equips Tool1, Tool2

requires

Field	Value
Definition	Binds policy guardrails to an agent
Enforcement	Agent cannot execute without policy wrapper active
Syntax	requires Guard1, Guard2

system

Field	Value
Definition	Sets the agent's persona prompt
Syntax	system "You are a QA engineer..."

3. Flow Operators (The Routing)

Reactive pipeline operators that dictate how the implicit context flows. No if/else needed.

|> — Push Pipe

Pushes the current context forward to the next node or agent.

context |> Agent.prompt("process this")

|? — Gate (Conditional)

Evaluates structured data in the context and only continues if true.

|? (context.data.risk_score > 90) =>

|! — Interrupt Pipe

The Human-in-the-Loop trigger. Asks the runtime's interrupt handler; with no handler, the run pauses until signal() / halo resume continues it.

|! yield(channel: Slack, msg: "Approve deployment?")

|* — Fan-Out Pipe

Triggers parallel execution. Runs multiple nodes concurrently, merging outputs when the {} block closes.

|* { AgentA.prompt("task1"), AgentB.prompt("task2") }

|? { ... } — Branch

Field	Value
Definition	Runs the first arm whose condition holds; else matches anything and must come last
Result	The chosen arm's result (or the unchanged value when nothing matches)
Example	|? { (context.data.score > 0.9) => System.Notify("removed"), else => Review(post) } as outcome

|* each(...) — Each

Field	Value
Definition	Runs one call per list item, in parallel (concurrency: n, default 4), in input order
Scope	The item under its loop name (issue, {{issue.number}}) and index, inside the call only
Example	|* each(issue in context.data.issues, concurrency: 2) => GitHub.add_labels(labels: issue.labels) as labeled

|<> — Loop

Field	Value
Definition	While the condition (a string) holds, jump back to the agent's most recent stage so it can try again
Limit	max: n (default 3); then the run fails with LoopLimitError
Example	|<> loop(SWE, "context.data.Shell.run.exitCode != 0", max: 3) as retries

~> — Listen (Events)

Waits for an external event (from the listen handler, or signal() after a pause) and stores it at context.data.<Source>.<Event>.

~> listen(UI.Button_Click)

4. Compiler Directives

Annotations placed above agents or pipelines that control how they run.

@checkpoint

Field	Value
Definition	On a pipeline: saves a snapshot after every stage (and at a closed gate) to .halo/checkpoints
Benefit	Continue a failed or halted run with halo run --checkpoint <file>

@shadow(Name)

Field	Value
Definition	On an agent: runs it alongside every call to the named agent (or model) and records its output, which is never used
Use case	Comparing a new model or prompt on real traffic

@retry(count, backoff)

Field	Value
Definition	Retries model and tool calls on transient failures (network, 429, 5xx, timeouts); never on policy violations
Example	@retry(3, 2s)

@timeout(duration)

Field	Value
Definition	On a pipeline: aborts the whole run. On an agent: aborts each call
Units	ms, s, m, h, d
Example	@timeout(5m)

@correct(n)

Field	Value
Definition	When an output policy rejects an agent's answer, show the agent the reason and ask again, up to n times
Applies to	An agent, or a pipeline (all its agents)
Example	@correct(2)

5. Native Primitives (The Built-ins)

Reserved functions for state mechanics.

yield

Field	Value
Definition	Asks a human: the request (message, channel, attachments, timeout) goes to the interrupt handler, or the run pauses
Used with	The `
Args	channel, msg, timeout, attachments

context

Field	Value
Definition	The state of a run
Namespaces	context.current (flowing value), context.input (arguments), context.data (results), context.meta (cost, calls, violations), context.trace (hash chain)

as

Field	Value
Definition	Binds a stage's result to a name under context.data
Applies to	|> calls, fan-out items and blocks, |! yield, ~> listen
Example	|> Analyst.score(deal) as score then |? (context.data.score.value > 80)

System

Field	Value
Definition	Built-in callee for runtime utilities and events
Methods	Print, Log (output, pass the value through), Sleep(duration), Fail(msg), Value(x) (make x the flowing value), Pick(value, fields: [...], maxChars: n) (keep only dotted fields of an object or of each list item; labels.name maps over a list), and the events ResolveIncident, Publish, Distribute, TriggerWebhook, Notify (sent to onSystemEvent)
Example	|> System.Pick(context.data.issues.items, fields: ["number", "title", "labels.name"], maxChars: 1500) as items

6. Lexical Tokens

Brackets & Punctuation

Token	Usage
[ ]	Agent model config block
( )	Function arguments, conditions
{ }	Fan-Out parallel block
:	Type declarations, key-value pairs
,	Argument separator
&	Policy rule combinator
.	Property access, method calls

String Literals

• Double-quoted: "hello world"
• Single-quoted: 'no escape needed'
• Escape sequences: \", \\, \n, \t

Model References (MODEL_REF)

Provider/name identifiers with dashes and slashes. Must contain at least one - or /.

Example	Provider
openai/gpt-4o-mini	OpenAI
anthropic/claude-sonnet-4	Anthropic
gemini-3-1-pro	Google