Tarn

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-mcp exposes list, validate, run, and fix_plan as 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.yaml and ships.
• One static binary — curl | sh install, no runtime, drops into any CI image.
• Batteries included — REST + GraphQL, captures, cookies, multipart, includes, polling, Lua, parallel execution, 7 output formats.

Install

# macOS / Linux
curl -fsSL https://raw.githubusercontent.com/NazarKalytiuk/tarn/main/install.sh | sh

# from source
cargo install --git https://github.com/NazarKalytiuk/tarn.git --bin tarn

Pre-built binaries for macOS (Intel + Apple Silicon), Linux (amd64 + arm64), and Windows (amd64 zip) are on the releases page, each with a tarn-checksums.txt for SHA-256 verification and a generated tarn.rb Homebrew formula artifact. The installer also lays down tarn-mcp and tarn-lsp when present in the archive. Set TARN_INSTALL_DIR to install elsewhere. Container path: ghcr.io/<owner>/tarn:<tag> from the release workflow. Manual verification works with shasum -a 256 -c tarn-checksums.txt.

Quick Start

The 60-second path:

tarn init                              # scaffold tests/ + tarn.env.yaml + advanced templates
# edit tarn.env.yaml so base_url points at your API
tarn run                               # runs every .tarn.yaml under tests/

Layer on the flags you actually need:

tarn run --format json --json-mode compact   # structured output for agents and CI
tarn run --env staging                       # use a named environment
tarn run --only-failed                       # quiet down a noisy run
tarn run --watch                             # rerun on file changes
tarn run --parallel                          # run files in parallel
tarn list --tag smoke                        # what would run, without running
tarn fmt --check                             # canonical YAML, CI-gateable

Debugging a failed run

Default to the failures-first loop — it keeps agents and humans off the megabyte-scale full report until they actually need it:

tarn validate <path>                  # syntax/config before running
tarn run <path>                       # writes .tarn/runs/<run_id>/
tarn failures                         # root-cause groups; cascades collapsed
tarn inspect last FILE::TEST::STEP    # full context for ONE failure
# patch tests or application code
tarn rerun --failed                   # replay only failing (file, test) pairs
tarn diff prev last                   # confirm fixed / new / persistent

tarn failures groups by root-cause fingerprint and collapses skipped_due_to_failed_capture cascades into their upstream entry — one failing step with five downstream skips surfaces as one entry with cascades: 5, not six. tarn inspect supports run-id aliases last / latest / @latest / prev and drills into one record via FILE[::TEST[::STEP]]. tarn rerun --failed stamps rerun_source onto the new report. tarn diff prev last buckets failure fingerprints into new / fixed / persistent so you can confirm a patch without re-reading the full report.

Reach for .tarn/runs/<run_id>/report.json only when failures + inspect cannot answer the question. See plugin/skills/tarn-api-testing/SKILL.md (Failures-First Loop) and docs/TROUBLESHOOTING.md for the canonical agent-facing guidance, including a worked example of a mutation endpoint whose response shape changed from {"uuid": "..."} to {"request": {"uuid": "..."}} and the $.uuid → $.request.uuid fix.

Hello World

A fully local demo with no external network dependency:

PORT=3000 cargo run -p demo-server &
cargo run -p tarn -- run examples/demo-server/hello-world.tarn.yaml

More local scenarios — redirects, cookies, forms, error responses, authenticated CRUD — live in examples/demo-server/.

Documentation

Full guides, CLI reference, AI workflow walkthroughs, and editor setup live on the docs site:

https://nazarkalytiuk.github.io/tarn/

In-repo docs to start with:

• docs/INDEX.md — canonical map of all in-repo docs
• docs/AI_WORKFLOW_DEMO.md — end-to-end agent loop walkthrough
• docs/MCP_WORKFLOW.md — MCP server usage patterns
• docs/TARN_PRODUCT_STRATEGY.md — product direction
• docs/TARN_VS_HURL_COMPARISON.md and docs/HURL_MIGRATION.md — comparison and migration
• docs/TARN_LSP.md — tarn-lsp Language Server (LSP 3.17) for Neovim, Helix, Zed, and other compatible clients
• docs/VSCODE_EXTENSION.md — VS Code extension in editors/vscode
• editors/zed/README.md — Zed extension, published via zed-industries/extensions

The reference sections below mirror what's on the docs site — useful when reading on GitHub directly.

AI agent integrations

Tarn drives any agent that speaks MCP or has a shell. tarn-mcp exposes list / validate / run / fix_plan (plus the failures-first tools); the tarn-api-testing skill teaches the loop. This table is the canonical supported-agents list — per-agent setup kits live under editors/.

A reproducible write → run → read-failure → fix loop for any of these lives in examples/agent-loop/.

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