---
title: "Transpile test sources once per run"
description: "Compile the TypeScript that test subprocesses load once, before the tests start."
url: https://starsling.dev/ci-playbooks/techniques/compile-test-sources-once
canonicalUrl: https://starsling.dev/ci-playbooks/techniques/compile-test-sources-once
---

# Transpile test sources once per run

Compile the TypeScript that test subprocesses load once, before the tests start.

## How to choose an approach

This technique currently has one documented company example. Validate its prerequisites, tradeoffs and original CI environment against your repository before adapting it.

- **[OpenAI](https://starsling.dev/ci-playbooks/openai#compile-test-sources-once)**: Use it when A test suite starts many subprocesses that each transpile the same sources. Check before adopting: The compiled snapshot belongs to one run; never restore it across runs.

## Apply this technique

````text
Evaluate and apply this CI technique to my repository: Transpile test sources once per run.

Compile the TypeScript that test subprocesses load once, before the tests start.

Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/compile-test-sources-once.md

Inspect this repository’s CI workflows, build and test commands, dependencies, and recent run timings. Identify where this technique applies and explain the expected benefit using this repository’s evidence.
Measure a baseline from recent comparable runs: elapsed time, queue time, runner minutes and failures where available. If data is unavailable, say what cannot be measured.
Compare relevant company approaches, checking prerequisites, platform differences and tradeoffs. Rank applicable options by evidence, fit and measurable opportunity; do not present company results as predictions for this repo.
Recommend the smallest safe change, show what checks and fallback paths remain, and ASK ME WHICH CHANGE TO APPROVE BEFORE EDITING ANY FILES.
Only after approval, work on a new feature branch, make one reversible change, and open a reviewable pull request. Never merge or push to a default or protected branch.
Keep required checks, test coverage, deployment and release behavior intact. Validate the change and compare equivalent runs where possible; report limitations and what should be monitored after merge.
````

## How OpenAI used this technique

Read what OpenAI changed and follow the original sources.

### OpenAI: Transpile test sources once per run

The Node SDK's tests start hundreds of child processes. Each used to load a TypeScript compiler and transpile the same sources; the suite now compiles the example, CLI and SDK sources once per Vitest invocation and points the subprocesses at that snapshot.

Evidence: Source PR/commit evidence

Node 22 job for the whole pull request, which also enables Node's compilation cache, bounded concurrency for package downloads and type checks, and an invariant JSON parse hoisted out of a property-test loop: 5m37s to 3m47s (1m32s before the regression on main), one successful run per revision ([Source](https://github.com/openai/openai-node/pull/2713))

Use it when: A test suite starts many subprocesses that each transpile the same sources.

Before applying it: The compiled snapshot belongs to one run; never restore it across runs.

[Read more in the OpenAI playbook](https://starsling.dev/ci-playbooks/openai#compile-test-sources-once)

[Primary source](https://github.com/openai/openai-node/pull/2713)

## CI optimization is continuous work.

Apply this technique yourself, or use StarSling Runners for faster GitHub Actions and optional agent-driven optimization PRs, which teams on eligible paid plans can turn on.

[Get self-improving CI](https://starsling.dev/products/runners)

## Company Playbooks

### [Anthropic](https://starsling.dev/ci-playbooks/anthropic)

Run only affected tests

### [DoorDash](https://starsling.dev/ci-playbooks/doordash)

Run build and test in parallel, sharded across CI agents

### [Linear](https://starsling.dev/ci-playbooks/linear)

Benchmark the same job on faster hardware

### [Mastra](https://starsling.dev/ci-playbooks/mastra)

StarSling runners for the slowest test workflow

### [OpenAI](https://starsling.dev/ci-playbooks/openai)

Rank CI fixes by the minutes they save

### [Ramp](https://starsling.dev/ci-playbooks/ramp)

Move a CPU-bound test suite to a faster CPU

### [Shopify](https://starsling.dev/ci-playbooks/shopify)

Let an agent iterate against one CI metric

### [Spotify](https://starsling.dev/ci-playbooks/spotify)

Build arm64 on native arm64 runners

### [Stripe](https://starsling.dev/ci-playbooks/stripe)

Run only the tests a change can reach

### [Uber](https://starsling.dev/ci-playbooks/uber)

Build the merge-queue speculations most likely to be needed