api-testing

A set of rules for writing API tests with Playwright, a browser-testing tool that can also send HTTP requests. It covers response checks with Zod, request validation, reusable test fixtures, and handling path parameters.

In plain words
What is it for?
Use it when creating or updating Playwright API tests. It helps test response shapes, missing or wrongly typed fields, invalid URL parameters, shared setup, and API failures that need tracking.
Why use it?
It prevents tests from hiding invalid responses, hardcoded environment details, incomplete request validation, or undocumented API behavior. It also makes multi-request tests easier to follow.

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/idavidov13/agentic-playwright/api-testing
Any agent
npx skills add idavidov13/agentic-playwright --skill api-testing
Clone the repo
git clone --depth 1 https://github.com/idavidov13/agentic-playwright

Made for: Claude Code, Codex.

Per session 132 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 5,007 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. 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 $0.00132 $0.05007
Opus 5 $0.00066 $0.02504
Sonnet 5 $0.00026 $0.01001
Haiku 4.5 $0.00013 $0.00501

Measured 2d ago against content hash 8682a35635d2, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

api-testing scanned grade A with 1 finding 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.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

- **Do NOT** curl the endpoint first to "see what it returns" and then write schemas that match reality.
.claude/skills/api-testing/SKILL.md · 373 lines

How it starts

The opening of the file, as written. The whole thing — 373 lines — stays where its author put it; the contents beside it link to each section on GitHub.

API Testing

Critical

These rules are non-negotiable. Violating any of them breaks the scaffold's contract.

  • NEVER hardcode API URLs, access tokens, emails, passwords, or endpoint paths. The only allowed sources of truth are process.env.* (for URLs and credentials) and the enums/{area}/* enums (for endpoint paths, e.g. ApiEndpoints.LOGIN).
  • ALWAYS validate API response bodies with Zod using the exact assertion pattern expect(SchemaName.parse(body)).toBeTruthy();. Type generics alone are not enough, and schema.parse(body) without the expect(...).toBeTruthy() wrapper is not enough.
  • ALWAYS wrap each API call in test.step() when a test contains more than one API call.
  • NEVER silently drop a test because the API misbehaves. Write the test as the spec says, wrap it in test.skip, and add a // FIXME: <ticket-url> comment.
  • NEVER stop at {} empty-body validation. Every request-body endpoint requires per-field omission and per-field invalid-type tests.
  • ALWAYS fuzz path parameters with the invalid-format data-driven loop — regardless of whether OpenAPI mentions it.
  • ALWAYS use the apiRequest fixture directly in tests. Only promote to a helper fixture when the same setup/teardown is reused across 3+ test files.

Instructions

Phase 1: Source the contract (documentation first, exploration only as fallback)

The API contract — not observed behavior — is the source of truth for schemas and tests.

  1. If OpenAPI / Swagger / equivalent documentation exists (the normal case):
    • Build schemas and tests strictly from the documented contract: field names, types, required vs optional, nullability, status codes, error shapes.
    • Do NOT curl the endpoint first to "see what it returns" and then write schemas that match reality.
    • If, during test execution, the actual response disagrees with the documentation (missing field, wrong type, wrong status code, extra field), that is a bug to report, not a reason to loosen the schema. Handle it via Phase 7 (test.skip + // FIXME: <ticket-url>).

Read the full file on GitHub · 373 lines

Files

What ships with it

5 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. 2d ago First seen · 373 lines · 132 tokens per session scan A 8682a35635d2

Subscribe to this mod's changes

api-testing is a skill published in the GitHub repository idavidov13/agentic-playwright (134 stars, last pushed 5d ago), licensed MIT. It adds 132 tokens to every session and 5,007 once invoked, about $0.0007 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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