---
title: "Print a few lines from the test harness and log the rest to a file"
description: "Test output for an agent stays short, puts each error on one searchable line, and includes precomputed summaries."
url: https://starsling.dev/ci-playbooks/techniques/agent-readable-test-output
canonicalUrl: https://starsling.dev/ci-playbooks/techniques/agent-readable-test-output
---

# Print a few lines from the test harness and log the rest to a file

Test output for an agent stays short, puts each error on one searchable line, and includes precomputed summaries.

## 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.

- **[Anthropic](https://starsling.dev/ci-playbooks/anthropic#agent-readable-test-output)**: Use it when An agent reads CI logs or test output to decide what to fix, and that output is long or free-form. Check before adopting: The full detail stays in a log file the agent opens on demand, uploaded as an artifact so people read the same file.

## Apply this technique

````text
Evaluate and apply this CI technique to my repository: Print a few lines from the test harness and log the rest to a file.

Test output for an agent stays short, puts each error on one searchable line, and includes precomputed summaries.

Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/agent-readable-test-output.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 Anthropic used this technique

Read what Anthropic changed and follow the original sources.

### Anthropic: Print a few lines from the test harness and log the rest to a file

In Anthropic's C compiler project, the test harness prints at most a few lines and logs important information to a file, so the agent's context window holds no thousands of useless bytes. Log files put ERROR and the reason on the same line, so grep finds them, and aggregate summary statistics are precomputed. The harness prints incremental progress infrequently and has a default --fast option that runs a 1% or 10% random sample, deterministic per agent and random across machines.

Evidence: Documented technique

Use it when: An agent reads CI logs or test output to decide what to fix, and that output is long or free-form.

Before applying it: The full detail stays in a log file the agent opens on demand, uploaded as an artifact so people read the same file.

[Read more in the Anthropic playbook](https://starsling.dev/ci-playbooks/anthropic#agent-readable-test-output)

[Primary source](https://www.anthropic.com/research/building-c-compiler)

## 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