Workflow authoring reference

A workflow structures work across many subagents — to be comprehensive (decompose and cover in parallel), to be confident (independent perspectives and adversarial checks before committing), or to take on scale one context can't hold (migrations, audits, broad sweeps). The script is where you encode that structure: what fans out, what verifies, what synthesizes.

When you do call the Workflow tool, the right move is often hybrid: scout inline first (list the files, scope the diff, find the modules) to discover the work-list, then call Workflow to pipeline over it, passing the list via args. You don't need to know the shape before the task — only before the orchestration step.

Common single-phase workflows you can chain across turns:

• Understand — parallel readers over relevant subsystems → structured map
• Design — judge panel of N independent approaches → scored synthesis
• Review — dimensions → find → adversarially verify
• Research — multi-modal sweep → deep-read → synthesize
• Migrate — discover sites → transform each → verify

For larger work, run several in sequence — read each result before deciding the next phase. You stay in the loop; each workflow is one well-scoped fan-out.

Every script must begin with export const meta = {...}:

export const meta = {
  name: 'find-flaky-tests',                       // kebab-case, required
  description: 'Find flaky tests and propose fixes',  // required
  phases: [                                       // one entry per phase() call
    { title: 'Scan', detail: 'grep test logs for retries' },
    { title: 'Fix', detail: 'one agent per flaky test' },
  ],
}
// script body starts here — use agent()/parallel()/pipeline()/phase()/log()

The meta object must be a PURE LITERAL — no variables, function calls, spreads, or template interpolation.

What a subagent is

Every agent() call is one agent-free ephemeral conversation linked to the invoking parent agent (is_subagent: true, named after the call's label). It starts with nothing but its prompt: no memory, no conversation context, no skills. Put ALL context a stage needs in the prompt — file paths, the rule it should apply, what shape to return.

Subagents are told their final text IS the return value (not a human-facing message), so they return raw data. Tools default to read-only (Read, Grep, Glob); widen with allowedTools for stages that must write. For stages whose input is entirely in the prompt (synthesis, judging, scoring) pass allowedTools: [] — a model that can still read files tends to wander, and a subagent that re-issues an identical tool call three times is stopped and resolves to null.

Their model defaults to the invoking conversation's model. opts.model (or the tool's model input) accepts any handle or alias listed by letta model list; an unknown value resolves that call to null. Use a cheaper model for mechanical stages only when you know a valid handle.

Workflow subagents require the API backend.

Script body hooks

• agent(prompt, opts?) → Promise. Spawn one subagent. Resolves to its final text, or with json: true to the parsed JSON value (say in the prompt what shape to return — nothing validates it; a reply that is not JSON resolves to null). Resolves to null on any failure — filter with .filter(Boolean). Options: label (display name), phase (progress group — use this inside pipeline()/parallel() stages to avoid races on the global phase() state), json, model, effort ('low' for mechanical stages, higher for the hardest verify/judge stages), allowedTools, systemPrompt (extra system prompt for this subagent), timeoutMs (default 10 minutes), maxToolCalls (positive safe integer; default 1000 unique tool calls for this subagent).
• pipeline(items, stage1, stage2, ...) → run each item through all stages independently, NO barrier between stages. Item A can be in stage 3 while item B is still in stage 1. This is the DEFAULT for multi-stage work. Wall-clock = slowest single-item chain, not sum-of-slowest-per-stage. Every stage callback receives (prevResult, originalItem, index) — use originalItem/index in later stages to label work without threading context through stage 1's return value. A stage that throws drops that item to null and skips its remaining stages.
• parallel(thunks) → run zero-arg functions concurrently. This is a BARRIER: it awaits all thunks before returning. A thunk that throws resolves to null in the result array — the call itself never rejects, so .filter(Boolean) before using the results. Use ONLY when you genuinely need all results together.
• phase(title) — start a new phase; subsequent agent() calls are grouped under this title in progress output.
• log(message) — emit a progress message to the user.
• args — the value passed as the tool's args input, verbatim. Pass arrays/objects as actual JSON values, NOT as a JSON-encoded string.

Scripts are plain JavaScript, NOT TypeScript — type annotations, interfaces, and generics fail to parse. The script body runs in an async context — use await directly and return the final result. Standard JS built-ins (JSON, Math, Array, etc.) are available; the hooks are the only globals provided. The script runs inside the CLI process with the CLI's own privileges (the vm context is a scope, not a security boundary), and the user approves it by reading it. Keep the script to orchestration: decide what runs and combine results. All reading, searching, and writing belongs in subagents, where the tool allowlist applies.

Pipeline vs barrier

DEFAULT TO pipeline(). Only reach for a barrier (parallel between stages) when stage N needs cross-item context from all of stage N−1:

• Dedup/merge across the full result set before expensive downstream work
• Early-exit if the total count is zero ("0 bugs found → skip verification")
• Stage N's prompt references "the other findings" for comparison

A barrier is NOT justified by:

• "I need to flatten/map/filter first" — do it inside a pipeline stage: pipeline(items, stageA, r => transform([r]).flat(), stageB)
• "The stages are conceptually separate" — that's what pipeline() models. Separate stages ≠ synchronized stages.
• "It's cleaner code" — barrier latency is real. If 5 finders run and the slowest takes 3× the fastest, a barrier wastes 2/3 of the fast finders' idle time.

Concurrent agent() calls are capped per run (maxConcurrent, default 16) — excess calls queue and run as slots free up, so passing 100 items is fine. Total agent count across a run is capped at 1000 — a runaway-loop backstop. A single parallel()/pipeline() call accepts at most 4096 items.

When a barrier IS correct — dedup across all findings before expensive verification:

const all = await parallel(DIMENSIONS.map(d => () => agent(d.prompt, {json: true})))
const deduped = dedupeByFileAndLine(all.filter(Boolean).flatMap(r => r.findings ?? []))  // needs ALL at once
const verified = await parallel(deduped.map(f => () => agent(verifyPrompt(f), {json: true})))

Loop-until-count pattern — accumulate to a target:

const bugs = []
let rounds = 0
while (bugs.length < 10 && rounds++ < 5) {
  const result = await agent('Find bugs in this codebase. Reply {"bugs": [{file, line, summary}]}', {json: true})
  bugs.push(...(result?.bugs ?? []))
  log(`${bugs.length}/10 found`)
}

Always bound a loop with a round counter as well as the target: a subagent that keeps returning nothing would otherwise run to the 1000-agent cap.

Composing patterns — exhaustive review (find → dedup vs seen → diverse-lens panel → loop-until-dry):

const seen = new Set(), confirmed = []
let dry = 0
while (dry < 2) {                                              // loop-until-dry
  const found = (await parallel(FINDERS.map(f => () =>          // barrier: collect all finders this round
    agent(f.prompt, {phase: 'Find', json: true})))).filter(Boolean).flatMap(r => r.bugs ?? [])
  const fresh = found.filter(b => !seen.has(key(b)))           // dedup vs ALL seen — plain code, not an agent
  if (!fresh.length) { dry++; continue }
  dry = 0; fresh.forEach(b => seen.add(key(b)))
  const judged = await parallel(fresh.map(b => () =>           // every fresh bug judged concurrently...
    parallel(['correctness','security','repro'].map(lens => () =>   // ...each by 3 distinct lenses
      agent(`Judge "${b.desc}" via the ${lens} lens — real? Reply {"real": boolean}`, {phase: 'Verify', json: true, allowedTools: []})))
      .then(vs => ({ b, real: vs.filter(Boolean).filter(v => v.real).length >= 2 }))))
  confirmed.push(...judged.filter(v => v.real).map(v => v.b))
}
return confirmed
// dedup vs `seen`, NOT `confirmed` — else judge-rejected findings reappear every round and it never converges.

Quality patterns — pick by task and compose freely

• Adversarial verify: spawn N independent skeptics per finding, each prompted to REFUTE. Kill if ≥majority refute. Prevents plausible-but-wrong findings from surviving.

  const votes = await parallel(Array.from({length: 3}, () => () =>
    agent(`Try to refute: ${claim}. Default to refuted=true if uncertain. Reply {"refuted": boolean}`, {json: true})))
  const survives = votes.filter(Boolean).filter(v => !v.refuted).length >= 2
  

• Perspective-diverse verify: when a finding can fail in more than one way, give each verifier a distinct lens (correctness, security, perf, does-it-reproduce) instead of N identical refuters — diversity catches failure modes redundancy can't.

• Judge panel: generate N independent attempts from different angles (e.g. MVP-first, risk-first, user-first), score with parallel judges, synthesize from the winner while grafting the best ideas from runners-up. Beats one-attempt-iterated when the solution space is wide.

• Loop-until-dry: for unknown-size discovery (bugs, issues, edge cases), keep spawning finders until K consecutive rounds return nothing new. Simple counters (while count < N) miss the tail.

• Multi-modal sweep: parallel agents each searching a different way (by-container, by-content, by-entity, by-time). Each is blind to what the others surface; useful when one search angle won't find everything.

• Completeness critic: a final agent that asks "what's missing — modality not run, claim unverified, source unread?" What it finds becomes the next round of work.

• No silent caps: if a workflow bounds coverage (top-N, no-retry, sampling), log() what was dropped — silent truncation reads as "covered everything" when it didn't.

Scale to what the user asked for. "find any bugs" → a few finders, single-vote verify. "thoroughly audit this" or "be comprehensive" → larger finder pool, 3–5 vote adversarial pass, synthesis stage. When unsure, lean toward thoroughness for research/review/audit requests and toward brevity for quick checks.

Diagnosing a run

Every run persists its script, args, and a journal.jsonl with one line per completed subagent call (prompt, outcome, conversation id) under ~/.letta/workflows/executions/<id>/; the tool result names the paths. Before diagnosing why a workflow returned an empty or unexpected result, read that journal — it records each agent's actual return value and, for a null, which guard or error produced it. A failed run is not resumable: fix the script and launch it again.