Tarn
API tests an AI agent can write, run, and debug.
Docs · Get started · MCP integration · Agent loop demo
Tarn is a CLI-first API testing tool written in Rust. Tests are .tarn.yaml files. Output is structured JSON with categorized failures and remediation hints, so an agent — Claude Code, Codex, opencode, Cursor, Windsurf, pi — can write a test, run it, read what broke, and fix it without scraping logs.
# tests/health.tarn.yaml
name: Health check
steps:
- name: GET /health
request:
method: GET
url: "{{ env.base_url }}/health"
assert:
status: 200$ tarn run
TARN Running tests/health.tarn.yaml
● Health check
✓ GET /health (4ms)
Results: 1 passed (15ms)When something breaks, --format json returns the same run as machine-readable data with failure_category, error_code, and the offending request/response. The tarn-mcp companion exposes a tarn_fix_plan tool that turns that report into actionable suggestions an agent can apply directly.
Why Tarn?
- Structured failures, not log scraping — every failure carries a stable category, error code, and remediation hints. Agents branch on taxonomy, not regex.
- MCP-native —
tarn-mcpexposeslist,validate,run, andfix_planas structured tools for Claude Code, Codex, opencode, Cursor, and Windsurf (and pi via the skill + CLI). See AI agent integrations. - YAML the model already knows — no DSL to teach, no test framework to bootstrap. An LLM writes a
.tarn.yamland ships. - One static binary —
curl | shinstall, no runtime, drops into any CI image. - Batteries included — REST + GraphQL, captures, cookies, multipart, includes, polling, Lua, parallel execution, 7 output formats.
