A guide to test-driven development (TDD), a way of writing a failing test before the code that makes it pass. It focuses on choosing a small test, proving a specific behavior, and checking the change in a bounded loop.

Use it when adding behavior, fixing bugs, or changing tests to decide the right test level, define the regression risk, and run focused verification.

It helps prevent tests that only repeat existing coverage or check internal implementation details. It also makes bug fixes and new behavior easier to verify against a clear expected result.

common-tdd — Skill for Claude Code and Codex by HoangNguyen0403

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.

Any agent

npx skills add HoangNguyen0403/agent-skills-standard --skill common-tdd

Clone the repo

git clone --depth 1 https://github.com/HoangNguyen0403/agent-skills-standard

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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/hoangnguyen0403/agent-skills-standard/common-tdd/github.svg)](https://agentmods.dev/skills/hoangnguyen0403/agent-skills-standard/common-tdd)

Your own site

<a href="https://agentmods.dev/skills/hoangnguyen0403/agent-skills-standard/common-tdd"><img src="https://agentmods.dev/badge/skills/hoangnguyen0403/agent-skills-standard/common-tdd/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/hoangnguyen0403/agent-skills-standard/common-tdd"><img src="https://agentmods.dev/badge/skills/hoangnguyen0403/agent-skills-standard/common-tdd.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 46 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. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector pass 7 Sept 2026

How audits are shown

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.

Model	Per session	Once invoked
Fable 5.1	$0.00046	$0.00822
Opus 5.5	$0.00018	$0.00329
Sonnet 5.5	$0.00009	$0.00164
Haiku 4.5	$0.00005	$0.00082

Measured 19d ago against content hash 2902d3a26ef2, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

common-tdd 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 19d 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.

.agents/skills/common/common-tdd/SKILL.md · 75 lines

How it starts

The opening of the file, as written. The whole thing — 75 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Quality-First TDD

Priority: P0 (CRITICAL)

A passing test is insufficient; the test must prove an owned behavior and a distinct plausible fault.

Choose the mode

New behavior: strict RED -> GREEN -> REFACTOR. Do not write production code before the expected RED.
Legacy or bug fix: characterize only when needed, then reproduce the intended change as a failing regression (RED). Preserve unrelated existing code; do not delete it merely because it predates the test.

Before writing a test

Compare against the existing suite before adding or removing cases. Ensure the scenario is not already covered upstream or in nearby layers.

Create one Test Intent Record per behavior/risk:

contract: observable contract — an application-owned result or outward side effect
fault: distinct plausible fault this test catches
layer: smallest honest unit, component, contract, integration, or E2E layer
cases: minimal distinct equivalence classes; use a parameterized test for equivalent inputs
command: exact focused single-run command

Reject tests that duplicate existing suite coverage, assert internal mock choreography over outward side effects, depend on time/network/order, or invent numeric four-pillar scores without measured evidence.

Bounded loop

Run configured lint/type checks, inspect nearby tests, and derive the smallest command.
RED: add one intent group and run it in the foreground, sequentially, single-run mode.
Classify RED as expected_red, invalid_red, unexpected_green, or verification_infra_failed. If it is unexpected_green, inspect existing coverage and remove a redundant or weak case before implementing production code.
GREEN: implement only enough to satisfy expected_red; rerun the same command.
REFACTOR: improve structure without changing behavior; rerun the same command.
Escalate only when evidence requires it: related unit target, integration/contract target, then explicit release/full-suite gate.

Read the full file on GitHub · 75 lines

Files

What ships with it

9 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.

19d ago Changed · +1 lines 2902d3a26ef2
1mo ago First seen · 74 lines · 46 tokens per session scan A e46965cdbbd9

Subscribe to this mod's changes

common-tdd is a skill published in the GitHub repository HoangNguyen0403/agent-skills-standard (583 stars, last pushed today), licensed MIT. It adds 46 tokens to every session and 822 once invoked, about $0.0002 per session on Opus 5.5. 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.

React TDD Testing best-practices coding-standards flutter nestjs nextjs

server-side-calls

Call tRPC procedures directly from server code using t.createCallerFactory() and router.createCaller(context) for integration testing, internal server logic, and custom API endpoints. Catch TRPCError and extract HTTP status with getHTTPStatusCodeFromError(). Error handling via onError option.

trpc/trpc · 61 tokens

common-tdd