Single-writer parallel join

When using concurrency/parallel subagents, enforce synchronization and determinism:

1) Parallelize independent work

• Start independent steps concurrently (e.g., multiple subagents), each operating on immutable inputs.

2) Join before dependent stages

• Wait for all parallel steps to complete before moving to any stage that depends on their outputs.

3) Single-writer rule for shared state

• Make workers/subagents return structured results (digests/messages) only.
• Ensure exactly one “lead/orchestrator” performs side effects that mutate shared state (e.g., writing research.md).

4) Preserve deterministic ordering

• If outputs must be committed “in plan order” or another specific sequence, have the lead commit in that order after the join.

Example (orchestrated parallel workers with single writer):

async function runPlan(planDims: Dimension[], runWorkers: (d: Dimension) => Promise<Digest>) {
  // 1) parallel: gather all digests for a phase
  const digests = await Promise.all(planDims.map(runWorkers));

  // 2) single-writer: commit sequentially in required order
  for (const d of planDims) {
    const digest = digests[planDims.indexOf(d)];
    researchMd.append(renderDimensionSection(d, digest));
  }
}

Apply this especially when multiple concurrent agents could otherwise race on files, database rows, or shared in-memory structures.