iii - Cross-Language Backend Engine

SDK 0.3.0-alpha Breaking Change (verified 2026-02-18): The registerTrigger field is trigger_type (NOT type). HTTP triggers use trigger_type: "api" (NOT "http"). Using type: "http" causes silent 404s on all routes. This was confirmed in the tac-4-go rewrite from motia to direct iii-sdk.

SDK Note: The iii.dev docs reference @iii-dev/sdk with a Bridge class — this package does not exist on npm (404 as of 2026-02-17). Always install iii-sdk (no scope). The docs SDK uses different conventions (dot separators, trigger_type, function_path). See references/cross-language.md for translation guide.

Core Concepts

iii has two fundamental primitives:

1. Register - Make a function callable by the engine
2. Call - Invoke any registered function regardless of language or process

Services connect to the iii engine over WebSocket. The engine handles routing, retries, observability, state, streams, cron, and HTTP API exposure.