WIT Types

WIT (WebAssembly Interface Types) is the interface definition language of the WebAssembly Component Model. It describes the function signatures that components export and import.

Declaring accurate types matters for two reasons:

Interoperability. WIT is language-neutral — a JS workflow can call a Rust activity and vice versa without any glue code. The synthesized interface is the shared contract regardless of implementation language. This is the common pattern: download a pre-built Rust (or any WASM) activity and wire it from a JS workflow or webhook.
Runtime behaviour. Obelisk uses the declared return type to interpret results and drive execution. For example, when an activity returns the err variant of a result type, Obelisk treats it as a failure and schedules a retry; an ok variant marks the execution as finished.

For JS components, Obelisk synthesizes WIT interfaces from the ffqn, params, and return_type fields in deployment.toml — no WIT files needed. Rust components define their own WIT files and use the bindgen! macro to generate bindings. See JS components and Rust components for details.

WIT identifiers must use kebab-case (e.g. sleep-millis, not sleep_millis).

Primitive types

WIT type	Description
`bool`	Boolean
`u8`	Unsigned 8-bit integer
`u16`	Unsigned 16-bit integer
`u32`	Unsigned 32-bit integer
`u64`	Unsigned 64-bit integer
`s8`	Signed 8-bit integer
`s16`	Signed 16-bit integer
`s32`	Signed 32-bit integer
`s64`	Signed 64-bit integer
`f32`	32-bit float
`f64`	64-bit float
`char`	Unicode scalar value
`string`	UTF-8 string

Compound types

WIT syntax	Description
`option<T>`	Optional value
`result`	Success/failure with no payloads
`result<T>`	Success with value, failure with no payload
`result<T, E>`	Success with value, failure with error
`list<T>`	Variable-length sequence
`tuple<T1, T2, ...>`	Fixed-length heterogeneous sequence
`record { field-name: T, ... }`	Object with named fields
`variant { case-name(T), ... }`	Tagged union
`enum { case-name, ... }`	Enumeration (one of a fixed set)
`flags { flag-name, ... }`	Bit-flag set