write-failing-test

write-failing-test is a skill for Claude Code, Codex from djscheuf/agentic-dev-ecosystem-template. It costs 32 tokens per session (822 once invoked), scanned A, original, MIT.

A test-first workflow for writing one test that should fail before the related code is implemented. TDD, or test-driven development, is a practice of writing a failing test, then code to pass it.

In plain words
What is it for?
Selecting the next test case, setting up its data and mocks, performing one action, checking the expected result, and running the test to verify it fails correctly.
Why use it?
It makes the expected behavior explicit and confirms that the new test detects the missing behavior for the right reason.

Skill for Claude CodeCodex

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 skills/djscheuf/agentic-dev-ecosystem-template/write-failing-test
Any agent
npx skills add djscheuf/agentic-dev-ecosystem-template --skill write-failing-test
Clone the repo
git clone --depth 1 https://github.com/djscheuf/agentic-dev-ecosystem-template

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for write-failing-test

README.md
[![agentmods](https://agentmods.dev/badge/skills/djscheuf/agentic-dev-ecosystem-template/write-failing-test.svg)](https://agentmods.dev/skills/djscheuf/agentic-dev-ecosystem-template/write-failing-test)
Your own site
<a href="https://agentmods.dev/skills/djscheuf/agentic-dev-ecosystem-template/write-failing-test"><img src="https://agentmods.dev/badge/skills/djscheuf/agentic-dev-ecosystem-template/write-failing-test.svg" alt="Measured on agentmods" height="20"></a>
Per session 32 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 822 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 $0.00032 $0.00822
Opus 5 $0.00016 $0.00411
Sonnet 5 $0.00006 $0.00164
Haiku 4.5 $0.00003 $0.00082

Measured 5d ago against content hash 2a195ecddc4d, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

write-failing-test 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 5d ago.

The scan reads SKILL.md. This mod also ships 3 executable files (reference/test_example.py, reference/test_example.ts, verify.sh), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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.

.devin/skills/write-failing-test/SKILL.md · 120 lines

How it starts

The opening of the file, as written. The whole thing — 120 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Step 1: Select Next Test Case

From your test plan, identify:
- The highest priority unimplemented test
- The specific behavior being tested
- The expected outcome

Step 2: Write the Test Shell

Create the Test with the proper structure. Identify the appripriate language for the component and review an example in one of these files:

  • C#: /reference/test_example.cs
  • Python: /reference/test_example.py
  • TypeScript: /reference/test_example.ts

Step 3: Implement Arrange Section

Set up the test preconditions:
- Create test data using factories/builders
- Configure mocks for dependencies
- Initialize the system under test (SUT)

Keep setup minimal - only what's needed for THIS test.

Step 4: Implement Act Section

Execute the behavior under test:
- Single method call or action
- Capture the result if needed
- Should be ONE line in most cases

Step 5: Implement Assert Section

Verify the expected outcome:
- Use specific assertions (not just assertTrue)
- One logical assertion per test
- Assert on behavior, not implementation

Step 6: Verify Test Fails

Run the test and confirm:
✓ Test executes without setup errors
✓ Test fails for the RIGHT reason
✓ Failure message is clear and helpful
✗ Test should NOT pass yet

If test passes → Something is wrong:
- Test may not be testing new behavior
- Code already exists
- Test has a bug

// TODO - Add Run-Test Skill specific to the project's testing framework, supporting that projects structure

Step 7: Document the Failure

Update the test plan and mark the test as "failing".

Step 8: Write the Sentinel File

  • create a sentinel file in the .process directory, named {skill-name}.done.json.
  • the sentinel file will follow @/schema/sentinel.schema.json.
  • set the task field to "{skill-name}".
  • the verify_params of the sentinel file will follow @/schema/verify-params.schema.json.
  • set the verify_params as follows:
    • set "test_cases_path" to the path of the test cases file relative to repo root.
    • set "failing_test_path" to the path of the failing test file relative to repo root.
    • set "failing_test_name" to the name of the failing test.

Read the full file on GitHub · 120 lines

Files

What ships with it

6 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

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. 5d ago First seen · 120 lines · 32 tokens per session scan A 2a195ecddc4d

Subscribe to this mod's changes

write-failing-test is a skill published in the GitHub repository djscheuf/agentic-dev-ecosystem-template (12 stars, last pushed today), licensed MIT. It adds 32 tokens to every session and 822 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 2026-08-30.