What is api-contract-testing?

A way to check that an API still follows the agreed request, response, data-type, and behavior rules between connected services.

Use it to test microservices, check API version compatibility, validate OpenAPI or Swagger specifications, and catch contract violations in continuous integration.

It catches changes that could break another service without requiring a full end-to-end test of the whole system.

api-contract-testing — Skill for Claude Code and Codex by aj-geddes

Install

Getting it into your agent

It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.

Clone the repo

git clone --depth 1 https://github.com/aj-geddes/useful-ai-prompts

agentmods

npx agentmods add skills/aj-geddes/useful-ai-prompts/api-contract-testing

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/aj-geddes/useful-ai-prompts/api-contract-testing/github.svg)](https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/api-contract-testing)

Your own site

<a href="https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/api-contract-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/api-contract-testing/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/aj-geddes/useful-ai-prompts/api-contract-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/api-contract-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 36 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 710 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

Socket pass 16 Sept 2026
Snyk pass 15 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	$0.00036	$0.00710
Opus 5.5	$0.00014	$0.00284
Sonnet 5.5	$0.00007	$0.00142
Haiku 4.5	$0.00004	$0.00071

Measured 2mo ago against content hash d12944381add, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

api-contract-testing 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 2mo ago.

The scan reads SKILL.md. This mod also ships 2 executable files (scripts/scaffold-tests.sh, templates/test-template.js), 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.

skills/api-contract-testing/SKILL.md · 103 lines

How it starts

The opening of the file, as written. The whole thing — 103 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Overview

Contract testing verifies that APIs honor their contracts between consumers and providers. It ensures that service changes don't break dependent consumers without requiring full integration tests. Contract tests validate request/response formats, data types, and API behavior independently.

When to Use

Testing microservices communication
Preventing breaking API changes
Validating API versioning
Testing consumer-provider contracts
Ensuring backward compatibility
Validating OpenAPI/Swagger specifications
Testing third-party API integrations
Catching contract violations in CI

Quick Start

Minimal working example:

// tests/pact/user-service.pact.test.ts
import { PactV3, MatchersV3 } from "@pact-foundation/pact";
import { UserService } from "../../src/services/UserService";

const { like, eachLike, iso8601DateTimeWithMillis } = MatchersV3;

const provider = new PactV3({
  consumer: "OrderService",
  provider: "UserService",
  port: 1234,
  dir: "./pacts",
});

describe("User Service Contract", () => {
  const userService = new UserService("http://localhost:1234");

  describe("GET /users/:id", () => {
    test("returns user when found", async () => {
      await provider
        .given("user with ID 123 exists")
        .uponReceiving("a request for user 123")
        .withRequest({
          method: "GET",
          path: "/users/123",
          headers: {
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the references/ directory:

Guide	Contents
Pact for Consumer-Driven Contracts	Pact for Consumer-Driven Contracts
OpenAPI Schema Validation	OpenAPI Schema Validation
JSON Schema Validation	JSON Schema Validation
REST Assured for Java	REST Assured for Java
Contract Testing with Postman	Contract Testing with Postman
Pact Broker Integration	Pact Broker Integration

Read the full file on GitHub · 103 lines

Files

What ships with it

8 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.

2mo ago First seen · 103 lines · 36 tokens per session scan A d12944381add

Subscribe to this mod's changes

api-contract-testing is a skill published in the GitHub repository aj-geddes/useful-ai-prompts (376 stars, last pushed 8mo ago), licensed MIT. It adds 36 tokens to every session and 710 once invoked, about $0.0001 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.

Backend and APIs Testing

api-contract-testing next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
api-contract-testingaj-geddes/useful-ai-prompts	Skill	376	8mo ago	MIT	A	36	Claude Code, Codex
mem0-oss-to-platformmem0ai/mem0	Skill	68k	2d ago	Apache-2.0	A	217	Claude Code, Codex
azure-mgmt-botservice-dotnetmicrosoft/skills	Skill	3.2k	yesterday	MIT	A	78	Claude Code
fastapi-router-pymicrosoft/skills	Skill	3.2k	yesterday	MIT	A	46	Claude Code
dev-runtimelangwatch/langwatch	Skill	5.0k	yesterday	Apache-2.0	A	172	Claude Code
api-designericrisco/rsc-harness	Skill	503	2d ago	MIT	A	105	Claude Code, Codex
migrate-segw-to-raparc-mcp/arc-1	Skill	252	yesterday	MIT	A	106	Claude Code

api-contract-testing