api-tester

A guide for testing web APIs, which are interfaces that let software exchange requests and responses. It covers REST and GraphQL APIs, authentication, curl, and scripted tests.

In plain words
What is it for?
Use it to read an API specification, test endpoints with curl or httpie, work with Postman collections, debug authentication, and validate response data without exposing credentials.
Why use it?
It provides a repeatable way to check successful requests as well as errors, edge cases, response formats, headers, and status codes.

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/librefang/librefang-registry/api-tester
Any agent
npx skills add librefang/librefang-registry --skill api-tester
Clone the repo
git clone --depth 1 https://github.com/librefang/librefang-registry

Made for: Claude Code, Codex.

Per session 19 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 745 The whole file, excluding the scripts and references it only reads on demand.
Security scan B 2 findings. Scan, not verified.
Origin 91% copy Near-identical to another mod 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 $0.00019 $0.00745
Opus 5 $0.00010 $0.00373
Sonnet 5 $0.00004 $0.00149
Haiku 4.5 $0.00002 $0.00075

Measured 2d ago against content hash 4a0ba2ba6f06, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade B, and why

api-tester scanned grade B with 2 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 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.

Sends data to an external URLmediumData exfiltration

A POST to an outside endpoint may be telemetry or may be exfiltration; either way the mod talks to somewhere, and you should know where.

- POST with JSON: `curl -s -X POST -H "Content-Type: application/json" -d '{"name":"test"}' https://api.example.com/users`

Makes network callslowCapability

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

description: API testing expert for curl, REST, GraphQL, authentication, and debugging
Origin

This is a copy

91% identical to api-tester — 3 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/api-tester/SKILL.md · 58 lines

How it starts

The opening of the file, as written. The whole thing — 58 lines — stays where its author put it; the contents beside it link to each section on GitHub.

API Testing Expert

You are an API testing specialist. You help users test, debug, and validate REST and GraphQL APIs using curl, httpie, Postman collections, and scripted test suites. You cover authentication, error handling, and edge cases.

Key Principles

  • Always start by reading the API documentation or OpenAPI/Swagger spec before testing.
  • Test the happy path first, then systematically test error cases, edge cases, and boundary conditions.
  • Validate response status codes, headers, body structure, and data types — not just whether the request "works."
  • Keep credentials out of command history and scripts — use environment variables.

curl Essentials

  • GET: curl -s https://api.example.com/users | jq .
  • POST with JSON: curl -s -X POST -H "Content-Type: application/json" -d '{"name":"test"}' https://api.example.com/users
  • Auth header: curl -s -H "Authorization: Bearer $TOKEN" https://api.example.com/me
  • Verbose mode: curl -v to see request/response headers and TLS handshake details.
  • Save response: curl -s -o response.json -w "%{http_code}" https://api.example.com/endpoint
  • Follow redirects: curl -L, timeout: curl --connect-timeout 5 --max-time 30.

Testing Methodology

  1. Authentication: Verify that unauthenticated requests return 401. Verify expired tokens return 401. Verify wrong roles return 403.
  2. Input validation: Send missing required fields (expect 400), invalid types, empty strings, overly long strings, special characters.
  3. Pagination: Test first page, last page, out-of-range page, zero/negative limits.
  4. Idempotency: Send the same POST/PUT request twice — verify correct behavior.
  5. Rate limiting: Send rapid requests — verify 429 responses and Retry-After headers.
  6. CORS: Check Access-Control-Allow-Origin and preflight OPTIONS responses from a browser context.

GraphQL Testing

  • Use introspection queries ({ __schema { types { name } } }) to discover the schema.
  • Test query depth limits and complexity limits to verify protection against abuse.
  • Test with variables rather than inline values for parameterized queries.
  • Verify that mutations return the updated object and that subscriptions emit events correctly.

Read the full file on GitHub · 58 lines

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 · 58 lines · 19 tokens per session scan B 4a0ba2ba6f06

Subscribe to this mod's changes

api-tester is a skill published in the GitHub repository librefang/librefang-registry (11 stars, last pushed 9d ago), licensed MIT. It adds 19 tokens to every session and 745 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it B with 2 findings (sends data to an external url, makes network calls). It is 91% identical to api-tester, differing in 3 lines, and is treated as a copy.

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