Writing Scenarios

Scenarios are plain Lua files that return a table. sigil, expect, and invariant are pre-injected globals — do not require('sigil'), it will error.

File layout

.sigil/scenarios/<service>/
  visible/
    auth/
      login.lua
      logout.lua
    billing/
      checkout.lua
  holdout/
    auth/
      password-reset.lua.age     # age-encrypted
  lib/
    auth.lua                      # shared helpers — require('lib.auth')

Scenario ID is derived from the file path (visible/auth/login.lua → auth/login). Never declare it in the file.

Minimum scenario

return {
  priority = "P0",
  run = function()
    local res = sigil.get("/health")
    expect(res.status == 200)
  end,
}

Full metadata

return {
  title     = "Login with valid and invalid credentials",  -- optional human label
  priority  = "P0",                                         -- required: P0/P1/P2
  tags      = {"auth", "login"},
  endpoints = {"POST /api/login"},
  budget    = { max_seconds = 30 },
  policy    = { capabilities = {"http", "judge"} },        -- linted statically
  run       = function() ... end,
}

expect(expr) — power assertions

expect() is source-rewritten before execution. It captures both sides of comparisons and every step of dotted chain accesses, then renders an Ariadne code-frame diagnostic on failure with value labels.

expect(res.status == 200)
-- on failure:
-- expected res.status == 200
--          |          |   |
--          |          |   200
--          |          false
--          404

invariant(name, opts) — property testing

invariant("email normalization is idempotent", {
  cases = 100,
  for_all = { email = sigil.gen.email() },
  check = function(case)
    local a = sigil.post("/api/normalize", { email = case.email })
    local b = sigil.post("/api/normalize", { email = a.json.normalized })
    expect(a.json.normalized == b.json.normalized)
  end,
})

Seeds are derived deterministically: BLAKE3(scenario_seed ‖ invariant_name ‖ case_index). On failure, Sigil shrinks the counterexample (ints toward 0, strings toward shorter).

sigil.judge(response, opts) — LLM judge

The --- doc comment directly above the call becomes the rubric.

--- Response includes a valid session cookie that is HttpOnly and Secure.
--- The cookie expiration is between 1 hour and 30 days.
sigil.judge(res, { min_score = 0.8 })

Supports thinking models (falls back to the reasoning field when content is empty). Requires [judge] config with a provider.

sigil.intent(opts) — agentic executor

--- Log in to the dashboard and find the organization name.
local result = sigil.intent({
  capabilities = { "browser" },
  context      = { api_key = "test-key-1" },
  capture      = { org_name = "string: the org name shown after login" },
  max_steps    = 15,
})
expect(result.completed)
expect(result.org_name == "default")

The LLM drives scenario tools (http, browser, exec) via tool-use to accomplish the --- objective. Capture fields with type prefixes become part of the complete tool’s schema.

sigil.browser.* — first-class browser

Shells out to agent-browser with automatic session isolation per scenario ID.

sigil.browser.open("/login")
sigil.browser.fill("#email", "[email protected]")
sigil.browser.fill("#password", sigil.env("ALICE_PASSWORD"))
sigil.browser.click("button[type=submit]")
sigil.browser.wait({ text = "Dashboard" })
expect(sigil.browser.url():match("/dashboard"))

Getters return strings; actions return nil or error. Key methods: open, click, fill, wait, text, html, title, url, screenshot, eval, cookies, snapshot, visible.

Capabilities

The policy.capabilities field is static metadata. sigil scenario lint rejects scenarios that use a capability they didn’t declare:

Capability	Grants access to
http	sigil.get/post/put/patch/delete
judge	sigil.judge
intent	sigil.intent
property	invariant
exec	sigil.exec
browser	sigil.browser.*
db	sigil.db (Phase C+)

Sandbox rules

• Lua 5.4 via mlua, sandbox(true) enabled.
• math.random / math.randomseed neutered — use sigil.gen.*.
• load, loadstring, loadfile disabled.
• require('sigil') → error (it is a pre-injected global).
• require('lib.X') resolves only inside .sigil/scenarios/<service>/lib/.

Editor support

Run sigil generate-types to emit .sigil/types/sigil.lua — a LuaLS type stub that gives you autocomplete, hover docs, and inline type errors in any LSP-aware editor.