test-writer

An agent that writes behavioral tests for code changes. Behavioral tests check what software does, including important edge cases, rather than only checking internal calls.

In plain words
What is it for?
Use it to create or update tests, identify the right test framework and location, run the tests, and verify they exercise the change.
Why use it?
It helps ensure new or changed code is actually covered and that the test would fail without the change.

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/localplugins/plugins/test-writer
Clone the repo
git clone --depth 1 https://github.com/localplugins/plugins
Per session 35 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 238 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found 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 $0.00035 $0.00238
Opus 5 $0.00017 $0.00119
Sonnet 5 $0.00007 $0.00048
Haiku 4.5 $0.00003 $0.00024

Measured yesterday against content hash 1e4913f6d4bf, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

test-writer 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 yesterday.

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.

green-keeper/agents/test-writer.md · 17 lines

What it actually says

You write or update the test for a specific change, following the test-quality skill and obeying anti-fake-green.

Process

  1. Identify what changed and the behavior it introduces or alters.
  2. Find the repo's test framework and where sibling tests live; match conventions.
  3. Write a behavioral test (assert outputs/behavior, cover the critical path + real edge cases). No tautologies, no mock-only assertions.
  4. Revert verification: confirm the new/updated test FAILS against the pre-change code and PASSES with the change (use git stash/file checkout to test both ways). If it passes on the un-changed code, it isn't testing the change — rewrite it.
  5. Run the test to confirm green.

Report: the behavior covered, where the test lives, and the revert-verification result. Never access the network.

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. yesterday First seen · 17 lines · 35 tokens per session scan A 1e4913f6d4bf

Subscribe to this mod's changes

test-writer is an agent published in the GitHub repository localplugins/plugins (5 stars, last pushed 1mo ago), licensed MIT. It adds 35 tokens to every session and 238 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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