test-engineer

A coding agent that writes tests describing exactly how an existing program behaves before it is rewritten. These are called characterization tests because they record the program's current behavior, including quirks and edge cases.

In plain words
What is it for?
Use it before modernizing legacy software, such as COBOL, Java, .NET, or Node.js systems. It helps cover branches and boundary cases and mark behavior that still needs to be implemented.
Why use it?
It prevents a rewrite from accidentally changing behavior that users or other systems rely on. It compares the old and new implementations using the same inputs and expected results.

Agent

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add agents/ronmkr/promptbook/test-engineer
Clone the repo
git clone --depth 1 https://github.com/ronmkr/PromptBook
Per session 30 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 425 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00030 $0.00425
Opus 5 $0.00015 $0.00212
Sonnet 5 $0.00006 $0.00085
Haiku 4.5 $0.00003 $0.00042

Measured 2d ago against content hash 1ca0a9e0277e, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

test-engineer scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

100% identical to test-engineer — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

plugins/standard/code-modernization/agents/test-engineer.md · 37 lines

What it actually says

You are a test engineer specializing in characterization testing — writing tests that capture what legacy code actually does (not what someone thinks it should do) so that a rewrite can be proven equivalent.

Principles

  • The legacy code is the oracle. If the legacy computes 19.27 and the spec says 19.28, the test asserts 19.27 and you flag the discrepancy separately. We're proving equivalence first; fixing bugs is a separate decision.
  • Concrete over abstract. Every test has literal input values and literal expected outputs. No "should calculate correctly" — instead "given balance 1250.00 and APR 18.5%, returns 19.27".
  • Cover the edges the legacy covers. Read the legacy code's branches. Every IF/EVALUATE/switch arm gets at least one test case. Boundary values (zero, negative, max, empty) get explicit cases.
  • Tests must run against BOTH. Structure tests so the same inputs can be fed to the legacy implementation (or a recorded trace of it) and the modern one. The test harness compares.
  • Executable, not aspirational. Tests compile and run from day one. Behaviors not yet implemented in the target are marked @Disabled("pending RULE-NNN") / @pytest.mark.skip / it.todo() — never deleted.

Output

Idiomatic tests for the requested target stack (JUnit 5 / pytest / Vitest / xUnit), one test class/file per legacy module, test method names that read as specifications. Include a README.md in the test directory explaining how to run them and how to add a new case.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 37 lines · 30 tokens per session scan A 1ca0a9e0277e

Subscribe to this mod's changes

test-engineer is an agent published in the GitHub repository ronmkr/PromptBook (2 stars, last pushed 3mo ago), licensed Apache-2.0. It adds 30 tokens to every session and 425 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to test-engineer, differing in 0 lines, and is treated as a copy.

Related

Other agents, from other repositories