Effectprint · open source

Observe what an agent tool actually changes.

A schema can describe intent. Effectprint checks behavior. It executes each eligible WebMCP tool in a fresh guarded browser context, fingerprints the observed effects, and fails when invocation and contract diverge.

View on GitHub Sample report LLM index

npx --yes effectprint demo

✗ search_products
  READ_ONLY_MUTATION  attempted POST /api/checkout
  effect POST /api/checkout  (blocked)

DeterministicNo model key, sampling variance, or hosted account.

Safe by defaultLocal targets, disposable contexts, blocked writes, explicit opt-ins.

CI nativeJSON, JUnit, SARIF, Markdown, HTML, and verdict badges.

Different question, missing answer

Inspectors show which tools exist. Conformance checks validate their shape. Model evals test selection. Effectprint asks what the captured invocation attempted and whether those effects matched its contract.

Observable evidence

Capture ordinary HTTP and WebSocket traffic, forms, navigation, response cookies, common storage methods, DOM mutations, clipboard writes, downloads, popups, and history changes. Match effects with explicit allow, forbid, and require contracts. Values are redacted by default.

Effectprint is a diagnostic for cooperative, non-evasive code. A pass applies to the audited invocation and input, not every possible behavior.

Contract reference · CLI reference · Threat model · Report schema