What is testing-apis?

An API security testing guide for REST and GraphQL services, which let software exchange requests and data over the network.

Use it to test authentication, authorization, IDOR, injection, mass assignment, rate limiting, endpoint discovery, and API business logic.

It helps uncover weak login checks, incorrect permissions, unsafe input handling, and limits that can be bypassed.

testing-apis — Skill for Claude Code by trilwu

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/trilwu/secskills

agentmods

npx agentmods add skills/trilwu/secskills/testing-apis

Made for: Claude Code.

Or install secskills-offense, the plugin that ships this one along with the rest of its 40 skills.

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/trilwu/secskills/testing-apis.svg)](https://agentmods.dev/skills/trilwu/secskills/testing-apis)

Your own site

<a href="https://agentmods.dev/skills/trilwu/secskills/testing-apis"><img src="https://agentmods.dev/badge/skills/trilwu/secskills/testing-apis.svg" alt="Measured on agentmods" height="20"></a>

Per session 43 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 3,567 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 2 findings. A grade says what 26 rules found in the file — not that it is safe.

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	$0.00043	$0.03567
Opus 5	$0.00022	$0.01784
Sonnet 5	$0.00009	$0.00713
Haiku 4.5	$0.00004	$0.00357

Measured 2d ago against content hash 51143b629cb6, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-06, from the pricing page.

Security

Grade A, and why

testing-apis scanned grade A 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 URLlowData 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.

TOKEN_A=$(curl -X POST https://api.target.com/login -d '{"username":"userA","password":"passA"}' | jq -r .token)

Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.

Makes network callslowCapability

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

curl https://target.com/swagger.json

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

testing-apis — 88% identical, 18 lines differ

secskills-offense/skills/testing-apis/SKILL.md · 484 lines

How it starts

The opening of the file, as written. The whole thing — 484 lines — stays where its author put it; the contents beside it link to each section on GitHub.

API Security Testing Skill

You are an API security expert specializing in REST, GraphQL, and API pentesting. Use this skill when the user requests help with:

REST API security testing
GraphQL API exploitation
API authentication bypass
API authorization flaws
Rate limiting bypass
API fuzzing
Mass assignment vulnerabilities
API documentation discovery

When to Use

Activate this skill when the user asks to:

Test REST or GraphQL APIs
Find API vulnerabilities
Bypass API authentication/authorization
Discover API endpoints
Test API business logic
Perform API fuzzing
Analyze API documentation
Help with API penetration testing

When NOT to Use

Source code is available — use auditing-code-for-vulnerabilities
Browser-rendered application surface — use testing-web-applications
Token and signature construction review — use reviewing-cryptography
LLM/agent tool APIs — use securing-ai-systems
GraphQL specifically — use attacking-graphql; authorization moves to resolvers and per-request rate limits break on batching
gRPC / protobuf — use attacking-grpc-protobuf; the schema must be recovered before anything else works
A parameter that makes the server fetch a URL — use exploiting-ssrf
A body or cookie holding a serialized object — use exploiting-deserialization
A JWT bearer token to forge, crack, or confuse — use attacking-jwt
The OAuth/OIDC flow that issues the token — use attacking-oauth-oidc
An XML or SOAP body that may resolve external entities — use exploiting-xxe

Core Methodologies

1. API Discovery and Reconnaissance

Find API Endpoints:

# Common API paths
/api/
/api/v1/
/api/v2/
/rest/
/graphql
/swagger
/api-docs
/swagger.json
/swagger.yaml
/openapi.json
/api/swagger-ui/
/api/docs

# Directory fuzzing for APIs
ffuf -u https://target.com/FUZZ -w api-wordlist.txt -mc 200,301,302,403
gobuster dir -u https://target.com -w api-paths.txt

# JavaScript analysis
# Extract API endpoints from JS files
cat app.js | grep -Eo "(GET|POST|PUT|DELETE|PATCH)\s+['\"]([^'\"]+)"

Read the full file on GitHub · 484 lines

Files

What ships with it

1 file 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.

references/graphql-and-injection-attacks.md 3.6 KB

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.

2d ago First seen · 484 lines · 43 tokens per session scan A 51143b629cb6

Subscribe to this mod's changes

testing-apis is a skill published in the GitHub repository trilwu/secskills (135 stars, last pushed 2d ago), licensed MIT. It adds 43 tokens to every session and 3,567 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 2 findings (sends data to an external url, makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.

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