Effect-aware isolation#

This is one of the patterns that feels unfamiliar if you came from “hand-roll mocks.” HoBo puts side-effect intent on the contract. Tests read that metadata and choose isolation automatically.

EffectSpec in one minute#

A BoundaryContract may declare:

• Composition is union of callee effects.
• Subtyping: safer (fewer/purer) effects may be used where more effects are expected.
• Widening (pure → network, etc.) is a breaking can-deploy reason when effects are declared.

Harness modes#

import { createBoundaryContract, pureEffects } from "hobo/contracts";
import { createEffectIsolation } from "hobo/testing";

const pure = createBoundaryContract({
  id: "pure-math",
  effects: pureEffects(),
});
createEffectIsolation(pure);
// → isolationMode: "zero-infra", mocks: []

const net = createBoundaryContract({
  effects: {
    purity: "varies",
    capabilities: ["decides", "suspends"],
    domains: ["network", "externalCall"],
  },
});
createEffectIsolation(net);
// → isolationMode: "domain-mocked", mocks per domain

Resilience fault injection#

Attach a ResilienceProfile (secondary metadata — not a new top-level primitive):

• Deterministic retry schedules (stable seed + budget + attempt facts).
• Never retry policy.Decided deny, terminal failures, or cancellation.
• Inspection is reconstructed from facts, not process memory.

// Fault-injection harness: one policy deny, no blind retries
await runResilienceFaultInjection({
  contract: netWithProfile,
  failures: [{ attempt: 1, failureClass: "policy.Decided", message: "deny" }],
});
// attempts === 1 — policy denies are never retried

Capture without re-execution#

Effect capture records contract.EffectDeclared / contract.EffectInvoked. Replay inspects those facts — it does not re-invoke handlers. That matches the product rule: inspection is event-sourced.

Export#

CLI and MCP can describe a contract or export an effect manifest for policy and SRE tooling.

Next#

• Reference: effects
• Durable EventStore tests