Tarn README

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/.

Table of Contents

• Test File Format
• Request Body
• Assertions
• Variables
• Cookies
• Form URL-Encoding
• Multipart / File Upload
• Includes
• GraphQL
• Polling
• Lua Scripting
• CLI Reference
• Output Formats
• Performance Testing
• MCP Server
• Claude Code Plugin
• Claude Code Skill
• Troubleshooting
• GitHub Action
• Configuration
• Step Options
• JSON Schema
• VS Code Extension
• Zed Extension
• Shell Completions
• Development

Test File Format

Test files use .tarn.yaml and can be organized in any directory structure.

Minimal Test

name: Health check
steps:
  - name: GET /health
    request:
      method: GET
      url: "http://localhost:3000/health"
    assert:
      status: 200

request.method accepts standard verbs and custom tokens such as PURGE or PROPFIND.

Full Format

version: "1"
name: "User CRUD Operations"
description: "Tests complete user lifecycle"
tags: [crud, users, smoke]

env:
  base_url: "http://localhost:3000/api/v1"

defaults:
  headers:
    Content-Type: "application/json"
  timeout: 5000
  retries: 1

tests:
  create_and_verify:
    description: "Create a user, then verify it exists"
    tags: [smoke]
    steps:
      - name: Create user
        request:
          method: POST
          url: "{{ env.base_url }}/users"
          body:
            name: "Jane Doe"
            email: "jane.{{ $random_hex(6) }}@example.com"
        capture:
          user_id: "$.id"
        assert:
          status: 201
          body:
            "$.name": "Jane Doe"
            "$.id": { type: string, not_empty: true }

      - name: Verify user
        request:
          method: GET
          url: "{{ env.base_url }}/users/{{ capture.user_id }}"
        assert:
          status: 200
          body:
            "$.id": "{{ capture.user_id }}"

Setup and Teardown

setup runs once before all tests. teardown runs after all tests even if tests fail.

name: "CRUD with auth"

setup:
  - name: Login
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "{{ env.admin_email }}"
        password: "{{ env.admin_password }}"
    capture:
      auth_token: "$.token"

teardown:
  - name: Cleanup
    request:
      method: POST
      url: "{{ env.base_url }}/test/cleanup"

tests:
  my_test:
    steps:
      - name: Authenticated request
        request:
          method: GET
          url: "{{ env.base_url }}/users"
          headers:
            Authorization: "Bearer {{ capture.auth_token }}"
        assert:
          status: 200

Request Body

The most common case — POST (or PUT/PATCH) an endpoint with a JSON body. Write the body inline as YAML; Tarn serializes it as JSON and sets Content-Type: application/json automatically.

name: Create a user
steps:
  - name: POST /users
    request:
      method: POST
      url: "{{ env.base_url }}/users"
      body:
        name: "Jane Doe"
        email: "jane@example.com"
        role: "editor"
        tags: ["content", "marketing"]
    assert:
      status: 201
      body:
        "$.id": { type: string, not_empty: true }

body accepts any JSON value — object, array, string, number, boolean, or null. Templates resolve inside it, type-preservingly ("{{ capture.count }}" becomes the captured number, not a string):

request:
  method: POST
  url: "{{ env.base_url }}/orders"
  body:
    customer_id: "{{ capture.user_id }}"
    idempotency_key: "{{ $uuid }}"
    quantity: "{{ capture.qty }}"     # stays a JSON number

Body from a file (body_file)

For larger or shared payloads, keep the JSON in its own file and reference it with body_file. The path resolves relative to the test file, the content is parsed as JSON, and it is interpolated exactly like an inline body (so {{ env }}, {{ capture }}, and builtins work inside the file). body_file is mutually exclusive with body.

request:
  method: POST
  url: "{{ env.base_url }}/users"
  body_file: "payloads/create-user.json"

// payloads/create-user.json
{
  "name": "Grace Hopper",
  "email": "grace.{{ $random_hex(6) }}@example.com",
  "role": "admin"
}

A missing file or invalid JSON fails that step with failure_category: parse_error (the run continues; it does not abort). Runnable end-to-end example: examples/post-json.tarn.yaml.

Sending a non-JSON body? Use form for URL-encoded data, multipart for file uploads, or graphql for GraphQL queries.

Assertions

Status

assert:
  status: 200              # exact match
  status: "2xx"            # any 2xx status
  status:                  # set of allowed codes
    in: [200, 201, 204]
  status:                  # range
    gte: 400
    lt: 500

Body (JSONPath)

All body assertions use JSONPath expressions.

Equality:

body:
  "$.name": "Alice"              # string
  "$.age": 30                    # number
  "$.active": true               # boolean
  "$.deletedAt": null            # null
  "$.field": { eq: "value" }     # explicit
  "$.field": { not_eq: "bad" }   # inequality

Numeric comparisons:

body:
  "$.age": { gt: 18, lt: 100 }
  "$.count": { gte: 1, lte: 50 }

String assertions:

body:
  "$.email": { contains: "@example.com" }
  "$.id": { starts_with: "usr_", matches: "^usr_[a-z0-9]+$" }
  "$.name": { not_empty: true }
  "$.notes": { empty: true }
  "$.code": { length: 6 }
  "$.msg": { not_contains: "error" }

Format assertions:

body:
  "$.request_id": { is_uuid: true }
  "$.created_at": { is_date: true }
  "$.client_ip": { is_ipv4: true }
  "$.server_ip": { is_ipv6: true }

Integrity assertions:

body:
  "$": { bytes: 15 }                                # raw response body length
  "$.payload": { sha256: "2cf24dba5fb0a30e..." }    # matched value digest
  "$.legacy": { md5: "5d41402abc4b2a76..." }

Type checks:

body:
  "$.name": { type: string }
  "$.tags": { type: array, length_gt: 0 }
  "$.meta": { type: object }

Existence:

body:
  "$.id": { exists: true }
  "$.internal": { exists: false }

Combined (AND logic):

body:
  "$.id": { type: string, not_empty: true, starts_with: "usr_" }

Headers

assert:
  headers:
    content-type: "application/json"                    # exact match
    content-type: contains "application/json"           # substring
    x-request-id: matches "^[a-f0-9-]{36}$"            # regex

Header names are case-insensitive.

Duration

assert:
  duration: "< 500ms"
  duration: "<= 1s"

Redirects

assert:
  redirect:
    url: "https://api.example.com/health"
    count: 2

redirect.url checks the final response URL after following redirects. redirect.count checks how many redirects were actually followed.

Variables

Environment Variables

Captures (Chaining)

Capture values from responses to use in subsequent steps. Captured values preserve their original JSON types (numbers stay numbers, booleans stay booleans).

steps:
  - name: Login
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "admin@example.com"
        password: "password123"
    capture:
      token: "$.token"              # JSONPath capture from body
      user_id: "$.user.id"          # nested path

  - name: Use token
    request:
      method: GET
      url: "{{ env.base_url }}/users"
      headers:
        Authorization: "Bearer {{ capture.token }}"

Header capture — capture values from response headers with optional regex:

capture:
  session_token:
    header: "set-cookie"
    regex: "session_token=([^;]+)"
  request_id:
    header: "x-request-id"

Cookie capture — capture a response cookie by name from Set-Cookie:

capture:
  session_cookie:
    cookie: "session"

Status capture — capture the HTTP status code as a number:

capture:
  status_code:
    status: true

Final URL capture — capture the final response URL after redirects:

capture:
  final_url:
    url: true

JSONPath with regex — extract a sub-match from a body field:

capture:
  user_id:
    jsonpath: "$.message"
    regex: "ID: (\\w+)"

Whole-body regex — extract from the full response body string:

capture:
  body_word:
    body: true
    regex: "plain (text)"

Transform-lite in interpolation — reshape captured arrays and collections without dropping into Lua:

request:
  form:
    first_tag: "{{ capture.tags | first }}"
    last_tag: "{{ capture.tags | last }}"
    tag_count: "{{ capture.tags | count }}"
    joined_tags: "{{ capture.tags | join('|') }}"
    words: "{{ capture.message | split(' ') | count }}"
    normalized: "{{ capture.message | replace(' response', '') }}"
    status_code: "{{ capture.status_text | to_int }}"
    payload: "{{ capture.user | to_string }}"

first and last expect arrays. count works on arrays, objects, and strings. join(...) joins array items after converting each item to its string form. split(...) and replace(..., ...) operate on strings. to_int parses integer strings, and to_string stringifies any captured value.

Built-in Functions

# UUIDs
"{{ $uuid }}"                    # UUID v4 (alias for $uuid_v4)
"{{ $uuid_v4 }}"                 # random UUID v4
"{{ $uuid_v7 }}"                 # time-ordered UUID v7 (Unix-ms prefix)

# Random primitives
"{{ $random_hex(8) }}"           # 8-char hex string
"{{ $random_int(1, 100) }}"      # random integer in range

# Wall-clock
"{{ $timestamp }}"               # unix timestamp
"{{ $now_iso }}"                 # ISO 8601 datetime

# Faker (EN locale)
"{{ $email }}"                   # random email
"{{ $first_name }}" "{{ $last_name }}" "{{ $name }}" "{{ $username }}"
"{{ $phone }}"                   # random phone number
"{{ $word }}" "{{ $words(3) }}" "{{ $sentence }}" "{{ $slug }}"
"{{ $alpha(8) }}"                # n lowercase letters
"{{ $alnum(8) }}"                # n lowercase alphanumerics
"{{ $choice(red, green, blue) }}"
"{{ $bool }}"                    # "true" or "false"
"{{ $ipv4 }}" "{{ $ipv6 }}"

Reproducible runs. Set TARN_FAKER_SEED=<u64> (or faker.seed: <u64> in tarn.config.yaml) to freeze every RNG-backed built-in for the process. Wall-clock values ($timestamp, $now_iso, the timestamp prefix of $uuid_v7) stay real-time.

UUID Version Assertions

body:
  "$.id":        { is_uuid: true }    # any UUID version
  "$.legacy_id": { is_uuid_v4: true } # must be random v4
  "$.event_id":  { is_uuid_v7: true } # must be time-ordered v7

Cookies

Tarn automatically captures Set-Cookie headers and sends stored cookies on subsequent requests. This is enabled by default.

name: Auth flow
steps:
  - name: Login
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "admin@test.com"
        password: "secret"
    # Set-Cookie from response is automatically stored
    assert:
      status: 200

  - name: Access protected resource
    request:
      method: GET
      url: "{{ env.base_url }}/profile"
    # Cookie header is automatically sent
    assert:
      status: 200

Disable automatic cookies per file:

cookies: "off"

Or reset the default jar between named tests in a file so IDE subset runs and flaky suites never see session state from a prior test. Setup and teardown still share the file-level jar. Named jars (multi-user scenarios) are untouched.

cookies: "per-test"

The --cookie-jar-per-test CLI flag forces per-test isolation regardless of the file's declared mode (except when the file sets cookies: "off", which always wins).

Auth

Tarn supports first-class bearer and basic auth helpers, while keeping explicit Authorization headers as the escape hatch:

request:
  auth:
    bearer: "{{ env.token }}"
  headers:
    X-API-Key: "{{ env.api_key }}"

Basic auth:

request:
  auth:
    basic:
      username: "{{ env.username }}"
      password: "{{ env.password }}"

You can also set defaults.auth once per file. If headers.Authorization is already present, Tarn leaves it unchanged.

Step-Level Cookie Control

Use cookies: false on a step to bypass the cookie jar entirely. No cookies are sent and no Set-Cookie headers are captured:

steps:
  - name: Login
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "admin@test.com"
        password: "secret"
    assert:
      status: 200

  - name: Test unauthenticated access
    cookies: false
    request:
      method: GET
      url: "{{ env.base_url }}/profile"
    assert:
      status: 401

Named Cookie Jars

For multi-user scenarios, use named jars to maintain separate cookie sessions:

steps:
  - name: Login as admin
    cookies: "admin"
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "admin@test.com"
        password: "secret"

  - name: Login as viewer
    cookies: "viewer"
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      body:
        email: "viewer@test.com"
        password: "secret"

  - name: Admin can manage users
    cookies: "admin"
    request:
      method: GET
      url: "{{ env.base_url }}/admin/users"
    assert:
      status: 200

  - name: Viewer cannot manage users
    cookies: "viewer"
    request:
      method: GET
      url: "{{ env.base_url }}/admin/users"
    assert:
      status: 403

Each named jar is independent — cookies captured in "admin" are never sent with "viewer" requests. Steps without a cookies: field (or with cookies: true) use the default jar.

Persist Cookie Jars

Use tarn run --cookie-jar .tarn-cookies.json to preload jars from disk and write back the updated state after the run. The file stores named jars too, so multi-user sessions can survive across runs.

--cookie-jar currently works only with sequential execution. Combine it with --parallel only after jar sharing becomes deterministic.

CSRF Protection

When the cookie jar sends cookies automatically, frameworks with CSRF protection (e.g., Better Auth) may reject requests that lack an Origin header. Add it to defaults to fix:

defaults:
  headers:
    Content-Type: "application/json"
    Origin: "http://localhost:3000"

If your app derives the expected origin from the request URL, set Origin to match env.base_url:

defaults:
  headers:
    Origin: "{{ env.base_url }}"

Form URL-Encoding

Send application/x-www-form-urlencoded payloads with form::

steps:
  - name: Login form
    request:
      method: POST
      url: "{{ env.base_url }}/auth/login"
      form:
        email: "user@example.com"
        password: "{{ env.password }}"
    assert:
      status: 200

Tarn URL-encodes the fields and auto-sets Content-Type: application/x-www-form-urlencoded unless you override it explicitly.

Note: form cannot be combined with body, graphql, or multipart on the same step.

Multipart / File Upload

Send multipart form data for file uploads using the multipart: field:

steps:
  - name: Upload photo
    request:
      method: POST
      url: "{{ env.base_url }}/api/photos"
      headers:
        Authorization: "Bearer {{ capture.token }}"
      multipart:
        fields:
          - name: "albumId"
            value: "{{ capture.album_id }}"
          - name: "title"
            value: "Upload {{ $random_hex(8) }}"
          - name: "description"
            value: "A test upload"
        files:
          - name: "photo"
            path: "{{ env.fixtures }}/test.jpg"
            content_type: "image/jpeg"
          - name: "thumbnail"
            path: "./fixtures/thumb.png"
            filename: "custom-name.png"
    assert:
      status: 201

Every string in multipart — field names and values, plus each file's path, filename, and content_type — is interpolated exactly like a JSON body. {{ env.* }}, {{ capture.* }}, and builtins such as {{ $random_hex(8) }} all resolve before the request is sent, so multipart fields can chain captured IDs and randomized values for test isolation. (Capture values are coerced to strings, since multipart fields are inherently textual.)

Note: multipart cannot be combined with body, form, or graphql on the same step.

Includes

Reuse shared step sequences across test files with include: directives:

name: User tests
setup:
  - include: ./shared/auth-setup.tarn.yaml
steps:
  - name: Get users
    request:
      method: GET
      url: "{{ env.base_url }}/users"
    assert:
      status: 200

The included file's setup and steps are inlined at the include point. Includes work in setup, teardown, steps, and tests.*.steps. Circular includes are detected and rejected.

Includes also support lightweight parametrization and deep overrides for reusable step packs:

steps:
  - include: ./shared/user-pack.tarn.yaml
    with:
      tenant: "acme"
      user_id: 42
    override:
      request:
        headers:
          X-Tenant: "acme"

Inside the included file, use {{ params.tenant }} and {{ params.user_id }} placeholders.

GraphQL

Native GraphQL support with the graphql: block. Automatically sets Content-Type: application/json and constructs the standard GraphQL JSON body.

steps:
  - name: Get user
    request:
      method: POST
      url: "{{ env.base_url }}/graphql"
      graphql:
        query: |
          query GetUser($id: ID!) {
            user(id: $id) {
              id
              name
              email
            }
          }
        variables:
          id: "{{ capture.user_id }}"
        operation_name: "GetUser"
    assert:
      status: 200
      body:
        "$.data.user.name": "Alice"
        "$.errors": { exists: false }

Polling

Re-execute a step until a condition is met. Useful for async workflows where you need to wait for a state change.

steps:
  - name: Create export
    request:
      method: POST
      url: "{{ env.base_url }}/exports"
    capture:
      export_id: "$.id"
    assert:
      status: 202

  - name: Wait for completion
    request:
      method: GET
      url: "{{ env.base_url }}/exports/{{ capture.export_id }}"
    poll:
      until:
        body:
          "$.status": "completed"
      interval: "2s"
      max_attempts: 15
    assert:
      status: 200
      body:
        "$.status": "completed"

poll.until uses the same assertion syntax. The step re-executes every interval until the until condition passes or max_attempts is reached.

Lua Scripting

For logic that goes beyond declarative assertions, use inline Lua scripts. Scripts run after the HTTP response is received and have access to response and captures.

steps:
  - name: Validate complex logic
    request:
      method: GET
      url: "{{ env.base_url }}/users"
    script: |
      -- Access response
      assert(response.status == 200, "Expected 200")

      -- Work with the response body (Lua table)
      local users = response.body.users
      assert(#users > 0, "Expected at least one user")

      -- Cross-field validation
      for _, user in ipairs(users) do
        assert(user.email:find("@"), "Invalid email for " .. user.name)
      end

      -- Set captures for subsequent steps
      captures.first_user_id = users[1].id
    assert:
      status: 200

Available in Lua:

• response.status — HTTP status code
• response.headers — response headers table
• response.body — response body as Lua table (auto-parsed from JSON)
• captures — read/write captures table
• assert(condition, message) — assertion (collected, not thrown)
• json.decode(string) — parse a JSON string into a Lua table
• json.encode(value) — serialize a Lua value to a JSON string

Shell Command Steps

Some test suites need to prep, stamp, or transform fixture files before running HTTP tests — for example, generating XML fixtures from a template or bumping a <ClassLibrary version> to a per-run unique value. A command: step runs an arbitrary shell command via sh -c (Unix) or cmd /C (Windows), with captured outputs feeding {{ capture.x }} like any HTTP step. Each step is either request: (HTTP) or command: (shell), never both.

setup:
  - name: Bump fixture version
    command:
      run: "python3 bin/bump-version.py --version {{ $timestamp }}"
      pass_env: [PATH, PYTHON]
      workdir: "scripts/fixtures"
      capture:
        bumped_version:
          stdout_regex: "version=([^\\s]+)"
        exit_status:
          exit_code: true

Security: inert by default

Shell commands inside a .tarn.yaml could otherwise be a supply-chain attack vector — a malicious test file could exfiltrate secrets from the environment if cloning and running a freshly checked-out project executed shell automatically. Tarn refuses to spawn command: children unless one of these is set:

• CLI flag: tarn run --allow-exec (preferred for ad-hoc runs, CI, and reruns)
• Project config: allow_exec: true in tarn.config.yaml (for trusted, project-owned repos)
• MCP: pass "allow_exec": true in the tarn_run / tarn_run_agent / tarn_rerun_failed tool params

Without an opt-in, every command: step is recorded with failure_category: skipped_by_policy and passed: true. The child process is never spawned. A freshly cloned repo therefore never executes shell on a default tarn run.

Env scrubbing — pass_env

The child process gets a tiny baseline env: PATH, HOME (USERPROFILE on Windows), and TMPDIR / TEMP / TMP. Tarn's own {{ env.x }} chain is never implicitly forwarded — secrets in tarn.env.local.yaml stay scoped to template interpolation. To forward an additional parent-process variable to the child:

command:
  run: "deploy --token $API_TOKEN"
  pass_env: [API_TOKEN]

If you need a Tarn env variable in the shell, either interpolate it into command.run directly (run: "TOKEN={{ env.api_token }} deploy") or list the corresponding parent-process variable in pass_env.

Captures from commands

Each entry under command.capture: must use exactly one of:

• stdout_regex: "PATTERN" — capture group 1 (full match if no group). A miss fails the step under failure_category: command_failed unless optional: true.
• exit_code: true — capture the literal exit code as an integer.

assert: and poll: are HTTP-only and rejected at parse time on command: steps.

Failure surfaces

• failure_category: skipped_by_policy — benign skip; the run did not opt in. Step passed: true, exit code unaffected.
• failure_category: command_failed — non-zero exit, signal kill (Unix), or a non-optional stdout_regex that did not match. Step passed: false, the run exits with 1.

CLI Reference

tarn run [PATH] [OPTIONS]          Run test files
tarn bench <PATH> [OPTIONS]        Benchmark a step
tarn validate [PATH] [--format]    Validate YAML (--format human|json)
tarn fmt [PATH] [--check]          Normalize Tarn YAML
tarn list                          List all tests
tarn summary <PATH|->              Re-render a prior JSON report (llm/compact)
tarn import-hurl <PATH>            Convert common-case Hurl files to Tarn
tarn init                          Scaffold a new project
tarn update                        Update to the latest version
tarn update --check                Check for updates without installing
tarn completions <SHELL>           Generate shell completions

tarn run Options

Examples

tarn run                                        # all tests
tarn run tests/auth.tarn.yaml                   # specific file
tarn run --tag smoke                            # filter by tag
tarn run --env staging                          # staging env
tarn run --var base_url=http://localhost:8080    # override var
tarn run --format json                          # JSON for LLM/CI
tarn run --format json --json-mode compact     # smaller JSON for automation loops
tarn run --format html                          # HTML dashboard
tarn run --format curl                          # failed requests as curl
tarn run --format curl-all=reports/replay.sh    # full suite replay script
tarn run --format human --format json=reports/run.json --format junit=reports/junit.xml
tarn run --format human,json=reports/run.json   # comma-separated also works
tarn run --watch                                # re-run on changes
tarn run --parallel --jobs 4                    # parallel execution
tarn run -v                                     # verbose
tarn run --dry-run                              # preview only
tarn run --only-failed                          # hide passing tests, show failures only
tarn run --no-progress                          # disable streaming, batch dump at end
tarn run --only-failed --format json            # CI-friendly: only failed items in JSON
tarn run --select tests/users.tarn.yaml::login   # run just the "login" test in one file
tarn run --select tests/users.tarn.yaml::login::2   # run just step index 2 of login
tarn run --select "a.tarn.yaml::login" --select "b.tarn.yaml::checkout"  # union across files
tarn fmt tests/                                  # rewrite a directory in place
tarn fmt tests/auth.tarn.yaml --check            # CI-style formatting check

LLM-friendly output (--format llm)

--format llm emits a grep-friendly summary line followed by only the failed steps, each expanded with request, response, and the assertion that failed. It strips ANSI colors automatically when stdout is piped and omits boxed/colored headers entirely.

tarn run tests/                    # emits llm format when piped (auto-selected)
tarn run tests/ --format llm       # force llm format explicitly
tarn summary .tarn/last-run.json   # re-summarize a prior run as llm

tarn summary reads a prior JSON report (.tarn/last-run.json is written after every run, or whatever you produced with tarn run --format json) and re-renders it as the llm format without re-running the tests. Use - to stream from stdin:

tarn run tests/ --format json > run.json
tarn summary run.json              # render run.json as llm
cat run.json | tarn summary -      # same, piped
tarn summary run.json --format compact  # render as compact instead

The sibling --format compact format is a shorter human-ish variant for quick console scanning — one line per file, inline expansion of failed tests, and a trailing HTTP 500: 3 | JSONPath mismatch: 18 tally of failure categories. Both formats strip colors in non-TTY output.

Structured Validation (tarn validate --format json)

tarn validate --format json emits a machine-readable report so editors and CI can surface parse errors inline. The schema:

{
  "files": [
    {
      "file": "tests/users.tarn.yaml",
      "valid": false,
      "errors": [
        { "message": "found unexpected end of stream ...", "line": 14, "column": 7 }
      ]
    }
  ]
}

• line and column are populated for YAML syntax errors (derived from serde_yaml's error location).
• Parser semantic errors (unknown fields, shape mismatches) surface message only when the underlying error does not carry a location.
• Exit code is 0 when every file is valid, 2 otherwise. The human format (--format human, the default) is unchanged.

Environment Discovery (tarn env --json)

tarn env --json prints the project's named environments in a stable schema so editors can populate pickers and previews:

{
  "project_root": "/path/to/project",
  "default_env_file": "tarn.env.yaml",
  "environments": [
    {
      "name": "staging",
      "source_file": "tarn.env.staging.yaml",
      "vars": {
        "base_url": "https://staging.example.com",
        "api_token": "***"
      }
    }
  ]
}

Inline vars from tarn.config.yaml are redacted when the key matches redaction.env (case-insensitive), so tarn env --json never prints literal secrets. Environments are sorted alphabetically by name.

Parallel Execution

tarn run --parallel dispatches test files across multiple rayon workers. The parallelism unit is the file: all setup, teardown, captures, and cookie jars stay file-scoped, but files may execute concurrently. That's unsafe when tests share mutable state (DB rows, singletons, filesystem fixtures, rate-limited upstreams). Tarn ships three isolation primitives to close the gap:

• serial_only: true on a TestFile (top-level) or on an individual named test under tests: pins the file onto a single worker that runs sequentially after every parallel bucket completes. A single serial_only test escalates its whole file to the serial bucket so per-file isolation (setup/teardown, cookie jars) stays intact.
• group: "postgres" on a TestFile buckets files by resource name. Files sharing a group run on the same worker (serialized within the group), while different groups run in parallel. Use this to serialize "all the postgres tests" without giving up concurrency across unrelated resources.
• parallel_opt_in: true in tarn.config.yaml silences the startup warning once the suite has been audited. While this flag is absent (or set to false), running tarn run --parallel emits a one-line stderr warning: warning: --parallel enabled without parallel_opt_in: true in tarn.config.yaml. Tests without serial_only may share state. Pass --no-parallel-warning on the CLI to suppress it for one-off CI runs.

# tarn.config.yaml
parallel: true
parallel_opt_in: true  # opts in once the suite has been audited

# any .tarn.yaml that shares DB state
name: Users CRUD
serial_only: true

# or bucket by resource so postgres tests serialize while S3 tests run in parallel
name: Postgres integration
group: postgres

Streaming Progress

By default tarn run streams per-test output as each test finishes instead of dumping everything at the end. The behaviour adapts to how stdout is used:

• Sequential (default) — each test is printed the moment it completes. You see progress live as the suite runs.
• Parallel (--parallel) — each file is printed atomically when it completes, so output from concurrently running files never interleaves.
• Stdout is human — streaming writes directly to stdout and the final emit prints only the summary line (no duplication).
• Stdout is a structured format (json, junit, tap, html, curl) — progress streams to stderr so stdout stays pure and parseable.

Pass --no-progress to disable streaming entirely and restore the old "batch at end" behaviour (useful for CI logs that already capture per-line timestamps).

NDJSON Streaming (--ndjson)

tarn run --ndjson streams machine-readable events to stdout, one JSON object per line. Designed for editor integrations (live Test Explorer updates), MCP clients, and CI pipelines that want structured progress without post-processing the final report.

Event types, in order:

• file_started — a test file has begun running
• step_finished — one step finished (with phase: "setup" | "test" | "teardown"). On failure, also carries failure_category, error_code, and assertion_failures[]
• test_finished — a named test finished, with per-step counts
• file_finished — a file finished, with its own summary
• done — emitted once at the very end, carrying the aggregated summary for the whole run

--ndjson composes with file-bound --format targets, so you can stream live progress and write a final report at the same time:

# Stream NDJSON to stdout, final JSON report to disk
tarn run --ndjson --format json=reports/run.json | jq '.event'

# Pure NDJSON (default human output is silently dropped on stdout)
tarn run --ndjson

--ndjson collides with any other structured format writing to stdout (e.g. --format json). Route the other format to a file, or pick one of the two streams.

In parallel mode (--parallel), each file's event stream is emitted atomically on file_finished so events from concurrently running files never interleave.

--only-failed works with both streaming and batch modes: passing tests and steps are omitted everywhere, but the final summary still reports total passed/failed counts.

Exit Codes

Output Formats

You can emit multiple formats in one run. Keep at most one bare non-HTML format for stdout and send the rest to files:

tarn run \
  --format human \
  --format json=reports/run.json \
  --format junit=reports/junit.xml \
  --format html=reports/run.html \
  --format curl=reports/failures.sh \
  --format curl-all=reports/replay.sh

JSON (--format json)

Structured JSON with versioned schema. Key design:

• schema_version: 1 for forward compatibility
• Full request/response included only for failed steps
• failure_category on failures: assertion_failed, response_shape_mismatch, connection_error, timeout, parse_error, capture_error, unresolved_template, skipped_due_to_failed_capture, skipped_due_to_fail_fast
• Stable error_code and remediation_hints are included on failed steps for automation-friendly diagnostics
• response_status and response_summary on all executed steps (passed and failed) — AI agents can see what a passed step returned
• captures_set on steps listing which capture variables were set; captures map on test groups showing all resolved values
• --json-mode compact keeps the same top-level schema but drops passed assertion details and truncates response bodies to ~200 chars
• Sensitive headers are redacted by default and can be customized per file with top-level redaction:
• request is present for failed executed steps; response is omitted for connection/setup failures where no response exists

Schema files:

• test files: schemas/v1/testfile.json
• JSON report output: schemas/v1/report.json

{
  "schema_version": 1,
  "summary": { "status": "FAILED", "steps": { "total": 5, "passed": 4, "failed": 1 } },
  "files": [{
    "tests": [{
      "captures": { "user_id": "usr_123", "token": "abc" },
      "steps": [{
        "name": "Create user",
        "status": "PASSED",
        "response_status": 201,
        "response_summary": "201 Created: Object{3 keys}",
        "captures_set": ["user_id"]
      }, {
        "name": "Update user",
        "status": "FAILED",
        "response_status": 400,
        "response_summary": "400 Bad Request: name required",
        "failure_category": "assertion_failed",
        "error_code": "assertion_mismatch",
        "remediation_hints": ["..."],
        "assertions": {
          "failures": [{ "assertion": "status", "expected": "201", "actual": "400", "message": "..." }]
        },
        "request": { "method": "POST", "url": "..." },
        "response": { "status": 400, "body": { "error": "name required" } }
      }]
    }]
  }]
}

Curl (--format curl, --format curl-all)

curl exports only failed executed requests. curl-all exports every executed request in run order, including setup and teardown.

tarn run --format human --format curl=reports/failures.sh
tarn run --format curl-all=reports/replay.sh

Also supports: Human (colored terminal), JUnit XML, TAP, HTML (self-contained dashboard).

Example:

redaction:
  headers:
    - authorization
    - x-session-token
  env:
    - api_token
  captures:
    - session_token
  replacement: "[redacted]"

Performance Testing

Reuses your existing test files for benchmarking.

tarn bench tests/health.tarn.yaml -n 1000 -c 50
tarn bench tests/health.tarn.yaml -n 500 -c 25 --ramp-up 5s
tarn bench tests/health.tarn.yaml --format json     # for CI thresholds
tarn bench tests/health.tarn.yaml --format csv --export json=reports/bench.json
tarn bench tests/health.tarn.yaml --fail-under-rps 200 --fail-above-p95-ms 80

 TARN BENCH  GET http://localhost:3000/health — 200 requests, 20 concurrent

  Requests:      200 total, 200 ok, 0 failed (0.0%)
  Throughput:    3125.0 req/s

  Latency:
    min        1ms
    p50        2ms
    p95        43ms
    p99        45ms
    max        45ms

MCP Server

Tarn includes an MCP (Model Context Protocol) server for direct integration with AI coding tools.

Setup

The simplest approach is a project-level .mcp.json in the repo root (works with Claude Code and other MCP-compatible tools):

{
  "mcpServers": {
    "tarn": {
      "command": "tarn-mcp",
      "args": []
    }
  }
}

Alternatively, add to your Claude Code project settings (.claude/settings.json):

{
  "mcpServers": {
    "tarn": {
      "command": "tarn-mcp",
      "args": []
    }
  }
}

For Cursor, add to .cursor/mcp.json:

{
  "mcpServers": {
    "tarn": {
      "command": "tarn-mcp"
    }
  }
}

Available Tools

The MCP server lets your AI agent write .tarn.yaml tests, execute them, parse structured results, and iterate — all without leaving the editor.

Typical agent loop:

1. tarn_list to discover tests and steps
2. tarn_validate after generating YAML
3. tarn_run to get structured failures
4. tarn_fix_plan to turn the latest report into machine-friendly next steps
5. inspect failure_category, error_code, assertions.failures, and optional request/response
6. patch the test or application code
7. rerun until summary status is PASSED

See docs/MCP_WORKFLOW.md, docs/AI_WORKFLOW_DEMO.md, and docs/CONFORMANCE.md.

Claude Code Plugin

Tarn ships two Claude Code plugins from a single marketplace. They solve different problems and can be installed independently or together:

1. tarn (top-level plugin/) — bundles the tarn-mcp MCP server and the tarn-api-testing skill. Gives your agent structured API testing capabilities: tarn_run, tarn_validate, tarn_list, tarn_fix_plan.
2. tarn-lsp (editors/claude-code/tarn-lsp-plugin/) — registers the tarn-lsp language server with Claude Code's LSP plugin system so you get full .tarn.yaml language intelligence (diagnostics, hover, completion, code lens, code actions, quick fix, rename, go-to-definition, and the JSONPath evaluator) while editing in Claude Code.

Both plugins live in the same marketplace (the repo root .claude-plugin/marketplace.json). Register it once, then install either or both:

# 1. Register the marketplace (once)
claude plugin marketplace add NazarKalytiuk/tarn

# 2a. Install the MCP + skill plugin
claude plugin install tarn@tarn

# 2b. Install the LSP plugin (project scope — see caveat below)
claude plugin install tarn-lsp@tarn --scope project

After installing tarn, Claude Code can write, run, and debug .tarn.yaml tests directly via the bundled MCP server and skill. See MCP Server and Claude Code Skill for what each component provides.

tarn — MCP + skill plugin

Manual setup

If you prefer manual configuration, add the MCP server to a project-level .mcp.json in the repo root:

{
  "mcpServers": {
    "tarn": {
      "command": "tarn-mcp",
      "args": []
    }
  }
}

This is equivalent to configuring the MCP server in .claude/settings.json but is portable across editors and tools that support MCP.

Plugin metadata

The tarn plugin configuration lives in .claude-plugin/:

• plugin.json — name, version, description, author, and repository URL
• marketplace.json — marketplace listing with owner info and the plugin registry (both tarn and tarn-lsp)

tarn-lsp — language server plugin

Separate install from the MCP plugin above (same marketplace though). This one registers tarn-lsp for .tarn.yaml / .yaml / .yml via Claude Code's LSP plugin system so every feature documented in docs/TARN_LSP.md is available while you edit in Claude Code.

Prerequisites:

• Claude Code 2.0.74+
• tarn-lsp binary available on $PATH — install with cargo install --path tarn-lsp from this repo, or symlink a workspace build (ln -s $(pwd)/target/release/tarn-lsp /usr/local/bin/tarn-lsp)

Install (from inside a Claude Code session):

/plugin marketplace add NazarKalytiuk/tarn
/plugin install tarn-lsp@tarn --scope project
/reload-plugins

Already registered the marketplace for the tarn plugin? Skip the add line — both plugins share the same marketplace now. Substitute /absolute/path/to/repo for NazarKalytiuk/tarn if you want to install from a local checkout instead.

Compound-extension caveat: Claude Code's LSP plugin system only supports simple file extensions, so the tarn-lsp plugin claims all .yaml and .yml files in any project it is installed in (not just .tarn.yaml). Always install with --scope project in Tarn-focused repos only — do not install it globally if you also edit unrelated YAML in Claude Code.

See editors/claude-code/tarn-lsp-plugin/README.md for the full spec, troubleshooting, and the list of supported LSP features.

opencode

opencode supports Tarn through config only — there is no plugin installer or marketplace, so integration is three files checked into your repo:

your-repo/
├── opencode.jsonc                          # MCP + LSP registration
└── .opencode/skills/tarn-api-testing/      # agent-visible skill
    └── SKILL.md

This repo ships exactly this layout at opencode.jsonc and .opencode/skills/tarn-api-testing/ (the skill is a symlink to the canonical plugin/skills/tarn-api-testing/). Clone the repo, install tarn-mcp and tarn-lsp on $PATH, run opencode inside — MCP tools, .tarn.yaml diagnostics/hover/completion, and the tarn-api-testing skill light up immediately.

To mirror the setup in your own repo, copy the snippet from editors/opencode/opencode.example.jsonc into your own opencode.jsonc:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "tarn": { "type": "local", "command": ["tarn-mcp"], "enabled": true }
  },
  "lsp": {
    "tarn": { "command": ["tarn-lsp"], "extensions": [".yaml", ".yml"] }
  }
}

Compound-extension caveat: opencode's LSP matcher uses path.parse(file).ext, so the tarn LSP entry claims every .yaml / .yml file in the workspace (not just .tarn.yaml) — the same limitation as Claude Code. Keep this in project-level opencode.jsonc, not your global config.

See editors/opencode/README.md for prerequisites, troubleshooting, and the full skill-install flow.

Claude Code Skill

The skills/tarn-api-testing/ directory contains a Claude Code skill that teaches AI agents how to write, run, debug, and iterate on Tarn tests. The skill is automatically loaded when an agent encounters API testing tasks.

What the skill provides:

• Core workflow (write → validate → run → inspect → fix → rerun)
• Complete command reference with all CLI flags
• Test file format with minimal and full-featured examples
• Environment variable resolution chain
• Capture formats (JSONPath, headers, cookies, URL, status, body)
• Assertion operator quick reference
• JSON output schema and failure category taxonomy
• Diagnosis loop for structured failure triage
• MCP integration setup for Claude Code, opencode, Cursor, and Windsurf

Reference docs in skills/tarn-api-testing/references/:

The skill triggers on keywords like "API test", "tarn", ".tarn.yaml", "test this endpoint", "smoke test", and "integration test for API".

Troubleshooting

See docs/TROUBLESHOOTING.md for the full guide, including the NestJS-style route ordering trap that Tarn flags automatically.

Common cases:

• connection_error: server is down, wrong host/port, DNS issue, TLS/connect failure
• timeout: step timed out before receiving a complete response
• assertion_failed: request succeeded, but status/header/body/duration check failed
• capture_error: the step passed assertions, but extraction failed afterward
• parse_error: invalid YAML, invalid JSONPath, or invalid config surface

Agent diagnosis loop:

1. run tarn validate first for syntax/config errors
2. run tarn run --format json
3. read failure_category before reading the message text
4. if a failed status assertion carries hints, follow the first hint before second-guessing the test
5. if response exists, inspect it before editing assertions
6. if request.url still contains {{ ... }}, fix env/capture interpolation before retrying

Non-JSON bodies:

• Tarn preserves plain text / HTML responses as JSON strings in the structured report
• use body: { "$": "plain text response" } to assert the whole root string when needed

Intentional Gaps

Tarn does not aim for full Hurl parity. The main intentionally unclosed gaps are:

• XPath / HTML assertions and captures
• full Hurl-style filter DSL
• exotic auth and libcurl-specific transport features
• OpenAPI-first generation workflows

GitHub Action

- uses: NazarKalytiuk/tarn@v1
  with:
    path: tests/
    format: junit
    env: staging

Inputs:

Configuration

tarn.config.yaml (optional)

test_dir: "tests"
env_file: "tarn.env.yaml"
timeout: 10000
retries: 0
parallel: false
parallel_opt_in: false  # set to `true` once tests have been audited for cross-file state sharing; silences the --parallel warning
defaults:
  connect_timeout: 1000
  follow_redirects: true
redaction:
  headers: ["authorization", "cookie"]
environments:
  staging:
    env_file: "env/staging.yaml"
    vars:
      base_url: "https://staging.example.com"

Behavior:

• test_dir sets the default discovery directory for tarn run, tarn validate, and tarn list
• env_file changes the root env file name; Tarn also checks .{name} and .local variants
• defaults acts as project-wide request policy for headers/auth/timeouts/retries/redirects/delay
• redaction provides a project-wide default report sanitization policy
• environments makes named --env profiles first-class and powers tarn env
• parallel: true makes parallel file execution the default for tarn run
• parallel_opt_in: true acknowledges the isolation tradeoff (see Parallel Execution) and silences the --parallel warning; pair it with serial_only: and group: markers on files that share mutable state

File-level defaults

defaults:
  headers:
    Content-Type: "application/json"
  timeout: 5000
  retries: 1
  delay: "100ms"    # default delay before each request

Step Options

Retries

retries: 3    # retry up to 3 times on failure (exponential backoff)

Timeout

timeout: 30000    # 30 seconds for this step

Delay

delay: "2s"    # wait before executing

Debug

Mark an individual step so the report always records its response body, response headers, and captures — even when the step passes. Equivalent to running with --verbose-responses but scoped to a single step:

- name: fetch user
  debug: true    # keep response in the report for this step
  request:
    method: GET
    url: "{{ env.base_url }}/users/42"
  assert:
    status: 200

The global --verbose-responses flag plus --max-body <BYTES> give the same behavior for every step in the run; debug: true is a targeted override for one-off debugging. Bodies exceeding the --max-body cap (8 KiB by default) are truncated with a "...<truncated: N bytes>" marker.

JSON Schema

Add to the top of your .tarn.yaml files for IDE autocompletion:

# yaml-language-server: $schema=https://raw.githubusercontent.com/NazarKalytiuk/tarn/main/schemas/v1/testfile.json
name: My test
steps: ...

The schema is bundled at schemas/v1/testfile.json in the repository. The structured report schema is bundled at schemas/v1/report.json.

VS Code Extension

A full-featured Tarn extension lives in editors/vscode and is published from tagged releases to both the VS Marketplace (nazarkalytiuk.tarn-vscode) and Open VSX via .github/workflows/vscode-extension-release.yml. Current version: 0.6.1.

Running tests from the editor

• Test Explorer discovery — .tarn.yaml files are indexed into a file → test → step tree. Run and Dry Run profiles, cancellable runs, and live streaming via tarn run --ndjson keep the UI in sync with long runs.
• CodeLens above every test and step for Run, Dry Run, and Run step — no Test Explorer navigation required.
• Rich failure peek view — on failure you get a unified diff of expected vs actual, plus the full request, response, remediation hints, failure_category, and error_code pulled straight from Tarn's JSON report.
• Tag filter command and an "Install / Update Tarn" helper command, both surfaced in the command palette.
• Output channel streams tarn stdout/stderr for each run.

Environment management

• Environment picker with a status-bar entry, persisted per workspace.
• tarn.defaultEnvironment setting for the initial pick.
• Status bar entries summarize active environment, tag filter, and last-run status.

Language features

• Tarn file association for *.tarn.yaml and *.tarn.yml.
• Full JSON schema validation for both test files and tarn-report.json via the redhat.vscode-yaml extension dependency.
• Snippet library for test skeletons, polling, multipart, GraphQL, form requests, and includes. Prefix and coverage details live in editors/vscode/README.md.
• Experimental LSP client (off by default) — the window-scoped tarn.experimentalLspClient setting spawns the tarn-lsp server alongside the extension's in-process providers. This is Phase V scaffolding; no feature has migrated to the LSP path yet, so leave it disabled unless you are testing the handoff.

Workspace trust and remote development

• Trusted / Untrusted workspace aware. In untrusted workspaces, Tarn features run in a read-only mode — discovery and schema validation still work, but commands that would execute tarn are disabled.
• Remote Development audited end-to-end: Dev Container, GitHub Codespaces, WSL, and Remote SSH all work without additional configuration. See docs/VSCODE_REMOTE.md.

Public API

The extension exports a TarnExtensionApi for other extensions to consume:

const tarn = vscode.extensions
  .getExtension('nazarkalytiuk.tarn-vscode')
  ?.exports as TarnExtensionApi | undefined;

See editors/vscode/docs/API.md for the surface and stability guarantees.

Reference

• Canonical user docs: editors/vscode/README.md
• Design / internals: docs/VSCODE_EXTENSION.md
• Changelog: editors/vscode/CHANGELOG.md

Zed Extension

A Zed extension lives in editors/zed and is published to the zed-industries/extensions registry under the id tarn. The extension wraps the same tarn-lsp binary used by the VS Code extension — installing from Zed's Extensions panel auto-downloads the matching tarn-lsp release on first activation.

Coverage:

• Syntax highlighting for .tarn.yaml / .tarn.yml, backed by tree-sitter-yaml.
• Full tarn-lsp language intelligence: diagnostics, completion, hover, code actions, code lens, formatting, symbols, rename, references.
• Snippet library ported from the VS Code extension (tarn-test, tarn-step, tarn-capture, tarn-poll, tarn-form, tarn-graphql, tarn-multipart, tarn-lifecycle, tarn-include).
• Runnable tasks: tarn: run file, tarn: dry-run file, tarn: validate file, plus whole-workspace variants. Accessible from the task picker or the gutter runnable at the top of each file.
• Settings passthrough via lsp.tarn-lsp.settings in Zed's settings.json, forwarded to tarn-lsp as workspace/configuration.

Zed has no custom UI surface for extensions, so the VS Code-only features (Test Explorer tree, environment picker, run-history panel, HTML report viewer, walkthrough) are not ported. Users who need them stay on VS Code; Zed users rely on LSP-driven feedback and the task runner.

See editors/zed/README.md for install and configuration.

Shell Completions

tarn completions bash > /etc/bash_completion.d/tarn
tarn completions zsh > ~/.zsh/completions/_tarn
tarn completions fish > ~/.config/fish/completions/tarn.fish

Development

git clone https://github.com/NazarKalytiuk/tarn.git
cd tarn

cargo build                    # build
cargo test --all               # test suite
cargo clippy                   # lint
cargo fmt                      # format
bash scripts/ci/smoke.sh       # release-path smoke test

# Run demo server + examples
PORT=3333 cargo run -p demo-server &
cargo run -p tarn -- run examples/ --var base_url=http://localhost:3333

See docs/RELEASE_VERIFICATION.md for the broader release-candidate checklist, including watch-mode and installer verification.

Architecture

Pipeline: parse YAML → resolve env → interpolate → execute HTTP → assert → report

License

MIT